vacuum ultraviolet photoionization and photodissociation of molecules and clusters || photoelectron...
TRANSCRIPT
43
PHOTOELECTRON SPECTROSCOPY OF SHORT-LIVED MOLECULES
M.C.R. Cockett, J.M. Dykea and H. Zamanpour
Department of Chemistry, The University,
Southampton S09 5NH, U.K.
CONTENTS
I. Introduction II. The Background
III. PES of Short-Lived Molecules Generated by Rapid Atom-Molecule Reactions
IV. A Case Study: A PES Study of the CH3CHF and FCH2CHF Radicals Prepared by Fluorine Atom Abstraction Reactions IV. 1. Introduction IV.2. Experiment
IV.2.a. F + QHsF IV.2.b. F + FCH2CH2F
IV.3. Hel PES of FCH2CH2F IV.4. Computational Details
IVAa. F + QH5F IV.4.D. F + FCH2CH2F
IV.5. Results and Discussion IV.5.a. F + QHsF IV.5.b. F + FCH2CH2F
V. Possible Directions for the Future Acknowledgments References Appendix
a) To whom correspondence should be addressed
PHOTOELECTRON SPECTROSCOPY OF SHORT-LIVED MOLECULES
M.C.R. Cockett, J.M. Dykea and H. Zamanpour
Department of Chemistry, The University,
Southampton S09 5NH, U.K.
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I. INTRODUCTION
Ultraviolet photoelectron spectroscopy (u.v.p.e.s.) was first used to study the electronic structure of short-lived molecules in the gas-phase approximately twenty years ago [1-4], Since that time progress in this area has been achieved steadily, and a number of reviews have been written which summarise the developments made and illustrate the interests of the main research groups in the field [5-12]. Each review deals with four basic areas which must be taken into account in the use of u.v.p.e.s. to study short-lived molecules, namely (i) the instrumental factors which must be considered in order to obtain spectra, (ii) the experimental methods used to generate short-lived molecules in the gas-phase
for p.e.s. study, (iii) the theoretical methods used to assign the spectra obtained, and (iv) the information obtainable from the experimental spectra.
This present review attempts to minimize duplication of material that has been presented in earlier reviews and starts with the following objectives: (i) to outline briefly the principles involved in producing a radical in the gas phase
by a rapid atom-molecule reaction for study by p.e.s. Other methods of making short-lived molecules have been used (e.g. pyrolysis or photolysis) but they will not be described in this article. High temperature photoelectron spectroscopy, for example, can now be regarded as a separate research area and could be the subject of a separate review.
(ii) to consider in detail two related examples which illustrate the information obtained from the p.e.s. study of an atom-molecule reaction. The radicals chosen are CH3CHF and FCH2CHF respectively.
(iii) to list the main p.e.s. studies, on short-lived molecules, which have been performed since 1980, as investigations performed before that time have been adequately described elsewhere [5-8]. This tabulation of more recent p.e.s. studies is presented in the Appendix.
(iv) to indicate possible directions for future work.
II. THE BACKGROUND
Since its birth in the early 1960's, u.v.p.e.s. has become an established spectroscopic technique notable in its ability to provide detailed information relating to the electronic structure of both a neutral molecule and the cationic states obtained on one-electron ionization. P.e.s. provides a direct probe of molecular bonding and the relationship between the neutral molecule and a cationic state resulting from the photoionization process. This relationship can be defined in terms of the directly measurable ionization energy and by the change in molecular geometry that occurs on ionization. For diatomics, the change in equilibrium bond length on ionization can be calculated from the resolved vibrational envelope of a photoelectron band by making use of Franck-Condon calculations [7]. Also, ab-initio molecular orbital calculations can be used to compute the minimum energy geometries of both the neutral molecule and a cationic state obtained on ionization, and the results can be combined with experimental information obtained
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from the photoelectron spectrum to assist in spectral assignment. Examples of this occur in the u.v. photoelectron spectra of CH3CHF and FCH2CHF which will be described later.
The primary quantity measured by photoelectron spectroscopy is the ionization energy of the molecule (or atom) under study. At the simplest level, for closed-shell molecules, measured vertical ionization energies can be directly related to the negative of orbital energies obtained from an ab initio calculation at the Hartree-Fock limit. This is Koopmans' Theorem [13], which was responsible for much of the early success of u.v.p.e.s. in that a u.v. photoelectron spectrum of a closed-shell molecule could be used to yield a diagram of the energies of the occupied valence orbitals in the molecule. However, it was recognised that Koopmans' theorem is only an approximation and molecular vertical ionization energies do not necessarily follow the same order as the Hartree-Fock molecular orbital energies [14]. Also, Koopmans' theorem is usually inapplicable to short-lived molecules as many of these molecules are open-shell species (e.g. CH3) or have closed-shell ground states which are not adequately represented by a single determinant (e.g. 03). Other methods therefore have to be used to assign the photoelectron spectra of such molecules.
In principle, as well as bands associated with ionizations to different cationic states, vibrational and rotational structure will also be observed in each band. In practice, it is very rare for any rotational structure to be observed in a photoelectron spectrum because the spectral resolution in u.v.p.e.s. (« 160 cm'1) is usually too low. In any case the working resolution in a p.e.s. study of a short-lived molecule is often lower than 160 cm"1
because the conditions used to generate the molecule of interest are invariably contaminating and corrosive. Nevertheless, vibrational structure is commonly resolved in the photoelectron spectra of small short-lived molecules and measurement of the separation of vibrational components observed in a photoelectron band can be used to calculate the vibrational constant, coe, for the vibrational mode excited in the ion on ionization. Although u.v.p.e.s. is a low resolution spectroscopic method, it has the ability to detect all molecular and atomic species that are present in the photon beam at concentrations greater than the sensitivity limit of the method (~ 1010 molecules cm"3). Bands associated with a short-lived intermediate can be observed and evidence as to the identity of the intermediate can be obtained by observing bands associated with the reagents that produce it and reaction products that arise from it, as a function of reaction time.
The methods that have been used to prepare molecules in the gas-phase for p.e.s. study include microwave discharge of a flowing gas mixture, a method which is rather indiscriminate and lacking in control, and use of a rapid atom-molecule reaction, a method which is more selective and controllable [5-8]. The other main methods which have been used are pyrolysis, photolysis and gas-solid reactions. Clearly the aim of all these methods is to generate the species of interest in high enough concentration in the photon beam to be detected. The lifetime of the molecule of interest under the conditions used should also be considered. Short-lived molecules should either be generated in the photon beam or at a short distance above the photon beam and then pumped rapidly into the photoionization region. In practice, the current lower lifetime limit for a molecule to be detected in the gas-phase by p.e.s. is ~ 0.1 msec.
The Appendix lists most of the short-lived molecules to have been studied by u.v.p.e.s. since 1980. The ionization energies listed are in all cases obtained from the first
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photoelectron band although in a large number of the studies higher bands were observed. The adiabatic ionization energies quoted refer to the first vibrational component of the first band, where vibrational structure was resolved, or to the onset of the first band where no structure was resolved. Similarly, the vertical ionization energy is taken either as the most intense vibrational component in a vibrationally resolved band or the maximum in an unresolved band. The tabulated vibrational wavenumber values refer either to an average vibrational separation where a short or incomplete vibrational progression was resolved, or to a derived vibrational constant (cSj in the ion where a more complete progression was recorded. The method used to analyze the structure obtained has been included in each case in the Appendix. Further details can obviously be found in the original references.
III. PES OF SHORT-LIVED MOLECULES GENERATED BY RAPID ATOM-MOLECULE REACTIONS
Probably the most controllable means of generating short-lived molecules in the gas-phase for study by single photon p.e.s. is via a rapid atom-molecule reaction. Fluorine atom abstraction reactions satisfy the sensitivity requirements of u.v.p.e.s. in that these reactions often have room temperature rate constants that approach the collision frequency limit. Experimentally determined rate constants at room temperature relative to that of F + CH4, have been tabulated for a large number of fluorine atom abstraction reactions [15-17] and these provide a convenient way of choosing a suitable reaction to make a selected radical. Unfortunately, for fluorine atom reactions, secondary reactions are usually also fast and experimental conditions have to be optimized to maximize the signal intensity arising from the short-lived molecule of interest. Nevertheless, this method of generating short-lived molecules is now an established way for making radicals for p.e.s. investigation and examples of the use of this method can be found in the Appendix. The design details of the photoelectron spectrometer used in experiments of this type have been presented elsewhere [5,6,8] and will not be described here. It would, however, be valuable to describe briefly the inlet system and pumping system used.
The inlet system and ionization region of a single detector photoelectron spectrometer used for fluorine atom abstraction studies is shown schematically in Figures 1 and 2.
The design of the inlet system needs to satisfy the requirement that reaction products can be observed as a function of reaction time. As shown in these figures, the inlet designed for this purpose consists of a T-shaped piece of glassware, fitted to the ionization chamber top flange at an angle of 15° to the vertical and parallel to the analyzer entrance slits with the end placed as close as possible to the point of photoionization. A second glass tube is placed inside the tube attached to the ionization chamber. The end of the inner tube consists of a small spherical glass bulb, which is perforated with a number of small holes, through which vapour from the target species can pass and mix with a mixture of helium and atomic fluorine passed down the outer tube. The mixing distance of the reagents relative to the photon beam can be varied simply by raising or lowering the central tube and, if the flow rate of the reagents into the ionization region can be estimated, measured mixing distances can be converted into
III. PES OF SHORT-LIVED MOLECULES GENERATED BY RAPID ATOM-MOLECULE REACTIONS
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rig. l . scneraatic diagram of an Inlet system used for fluorine atom abstraction.
Fig. 2. Schematic diagram of an ionization chamber used on a photoelecti*on spectrometer to study fluorine atom abstraction reactions,
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approximate reaction times. In this way spectra can be obtained as a function of reaction time. As shown in Figure 2, fluorine atoms can be produced in an alumina discharge tube by a microwave discharge (2.45 GHz) in a flowing mixture of 5% molecular fluorine in helium. Also, the inside of the glass inlet system is coated with teflon to prevent fluorine atoms reacting with the glass. A high pumping speed is required and this is achieved by pumping the ionization chamber with a large diffusion pump.
An inlet system of this type has been used in the study of the F + CH3CH2F and F + FCH2CH2F reactions described later.
IV. A CASE STUDY: A PES STUDY OF THE 1-FLUOROETHYL AND 1,2-DIFLUOROETHYL RADICALS PREPARED BY
FLUORINE-ATOM ABSTRACTION REACTIONS
VI. I. Introduction
A number of fluorine-atom abstraction reactions using haloalkanes as target molecules have been studied in some detail in the Southampton p.e.s. group. The earlier work concentrated on reactions between fluorine atoms and the halosubstituted methanes [19-22] (i.e. F + CH3X and F + CH2X2, where X = F, Cl, Br, and I) whereas the more recent work has involved fluorine atom reactions with the halosubstituted ethanes (23) (i.e. F + QHsX and F + Cy^X^ where X = Cl and Br). This case study extends the work of reference [23] in that the reactions F + QHjF and F + FCH2CH2F are investigated by photoelectron spectroscopy.
For the F+C2H5F reaction, three primary reaction products can be produced by direct hydrogen or fluorine abstraction; CH2CH2F, CH3CHF and C2H5. Preliminary evidence as to which primary reaction route dominates can be obtained from bands observed in the photoelectron spectrum of the products from this reaction (e.g. by observation of bands assigned to HF, F2 or secondary products derived from the intermediates). Unfortunately, the reaction enthalpies cannot be calculated for all three reactions because the heat of formation of CH2CH2F has not been determined experimentally, although the heats of formation, AHf(298), of CH3CHF [24] and QHs [25] are both known [(-74.1 ± 4.6 kJmol1
and (+116.3 ± 4) kJmol1 respectively]. However, all three radicals, CH2CH2F, CH3CHF and CJ159 have been the subject of a number of theoretical and experimental investigations [26-33].
In practice, it should be possible to calculate relative energies of CH3CHF and CH2CH2F in their electronic ground states at their equilibrium geometries via molecular orbital theory. Indeed, ab-initio molecular orbital calculations have previously been performed for both CH3CHF(X2A) and CH2CH2F(X2A) at their computed minimum energy geometries [27] using a 6-31G basis set at the unrestricted Hartree-Fock (UHF) level. Allowance for electron correlation via fourth order M0ller-Plesset perturbation theory at the SCF optimized geometries was also carried out. At the UHF SCF level it was found that the CH2CH2F radical was the more stable by 0.02 eV but when electron correlation was accounted for via fourth order M0ller-Plesset perturbation theory, the CH3CHF radical was found to be more stable by 0.06 eV. It is clear from this evidence
IV. A CASE STUDY: A PES STUDY OF THE 1-FLUOROETHYL AND 1,2-DIFLUOROETHYL RADICALS PREPARED BY
FLUORINE-ATOM ABSTRACTION REACTIONS
VI.I. Introduction
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that both radicals are very close in energy and, although the CH3CHF radical appears to be the more stable of the two from the evidence of reference [27], no firm conclusions can be drawn at this stage.
Ab-initio Hartree-Fock SCF molecular orbital calculations have also been performed on the ground cationic states resulting from one-electron ionization of the two radicals, CH3CHF and CH2CH2F [28]. SCF total energies were computed for CH3CHF(X1A) and C H J C H J F V A ) at their minimum energy geometries using a 4-31G basis set and it was found that CH3CHF(X1A) was more stable than C H . C H . F ^ A ) by 0.80 eV.
The fluoroethyl radicals have been studied experimentally by a number of spectroscopic techniques [34-37]. For example, e.s.r. spectra have been recorded for CH3CHF in solution and the equilibrium geometry of the radical estimated by comparing measured proton and fluorine hyperfme splittings with those computed using semiempirical molecular orbital methods [34].
In a series of kinetic investigations, Tschuikow-Roux and co-workers [30,36] have studied the photochlorination of some fluoroethanes. The reaction of Cl^P), generated photochemically from Cl2, with fluoroethane has been investigated by a competitive photochlorination method using CH4 as a primary standard [30]. Quantitative product analysis for CH3CHFC1 and C1CH2CH2F, arising from a or (J hydrogen atom abstraction from CH3CH2F, followed by chlorine atom addition, was carried out by means of flame ionization gas-chromatography. The conclusion made from these studies was that the rate of oc-hydrogen abstraction is about ten times greater than the rate of (3-hydrogen abstraction in the C1(2P) + CH3CH2F reaction at 298 K.
The second reaction described in this case study is the F + FCH2CH2F reaction. In this reaction, there are only two possible primary reaction products; FCH2CHF from hydrogen abstraction and CH2CH2F from fluorine atom abstraction. Unfortunately, it is not possible to estimate which route is thermodynamically more favourable because the heats of formation of FCH2CH2F, FCH2CHF and CH2CH2F are not available. However, a kinetic study of the related reaction C1+C1CH2CH2C1 [38], has concluded that, at room temperature, hydrogen abstraction is much more rapid than chlorine atom abstraction.
The photoelectron spectrum of 1,2-difluoroethane has not previously been recorded, probably because this compound is unavailable commercially. However, it can be prepared from the reaction of l,2-bis(p-toluenesulfonyl) ethane with potassium fluoride in a suitably involatile solvent [39,40]. The details of this preparation together with the Hel photoelectron spectrum of 1,2-difluorethane will be presented later.
VI.2. Experiment
VI.2.a. F + C2H5F
The aim of the F + QHjF experiment was to record at least the first photoelectron band of the primary product from this reaction.
The commercial sample of gaseous Cy^F used (obtained from Columbia Organic Chemicals) was found to be contaminated with vinyl fluoride and consequently had to be purified in situ on the photoelectron spectrometer by freezing the cylinder containing the ethyl fluoride to 236 K in a chloroform/liquid nitrogen slush bath and pumping off the
VI.2.a. F + C2H5F
VI.2. Experiment
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vinyl fluoride. Fluorine atoms were generated in a microwave discharge (2.45 GHz) of 5% molecular fluorine in flowing helium.
The experiments were performed on the single detector transient photoelectron spectrometer described in ref [5]. The typical operating resolution as measured for argon (f.w.h.m.) using Heloc radiation was 25-30 meV. Energy calibration of the spectra obtained was carried out using the Heloc photoelectron spectrum of methyl iodide, which was introduced into the ionization region of the spectrometer via an inlet tube on the side of the ionization chamber.
For the F + C2H5F reaction, an optimum mixing distance above the photon beam of 7 mm was used to generate the short-lived reaction product at maximum concentration, as estimated from the spectra obtained. A detailed mixing distance study for this reaction will be presented later in the Results and Discussion section.
IV.2.b. F + FCH2CH2F
Much of this section will be devoted to a description of the preparative details used to make 1,2-difluoroethane, as most of the other experimental details are similar to those described above for the F + C2H5F reaction. The two main differences are the calibrants used, which in the F + FCH2CH2F case was water and the band associated with the He(II) ionization of helium, and the optimum mixing distance above the photon beam for generation of the short-lived reaction product, which was 15 mm. Again, the results of a mixing distance study for this reaction will be presented in the Results and Discussion section.
The method of preparation of 1,2-difluoroethane used in this work was that reported by Edgell and Parts [39] and modified by Butcher et al. [40]. It involved the reaction of l,2-bis(p-toluenesulfonyl) ethane with potassium fluoride in PP'-dihydroxyethyl ether, a solvent chosen for its involatility since the reaction occurs at temperatures up to 453 K.
The 1,2-bis(p-toluenesulfonyl) ethane (Aldrich Chemicals Ltd.) was recrystallized from ether and stored in a desiccator. It was then mixed with a ten fold molar excess of potassium fluoride (Aldrich Chemicals Ltd.) in a suitable volume of the solvent, PP'-dihydroxyethyl ether (Aldrich Chemicals Ltd.), in a round bottomed flask which was fitted to a glass vacuum line and evacuated. The flask was immersed in a silicone oil bath and heated slowly over a period of two hours up to 438 K using a thermostatic hot plate fitted with a magnetic stirrer. The temperature was carefully monitored throughout using a chrome/alumel thermocouple placed in the oil bath. Dry ice/acetone and liquid nitrogen traps were used to collect the product which was then further distilled by passing it several times between two liquid nitrogen traps via a C02/acetone trap. The product was finally collected in a sample tube which was later attached directly to the spectrometer.
Each preparation yielded about 3 ml of the colourless, volatile liquid product from 13.6 g of [CH3C6H4S03CH2]2 and 18.7 g of KF. This volume was sufficient for 3-4 hours running time on the spectrometer. The product was characterized by mass spectrometry, infra-red spectroscopy and by comparison of the photoelectron spectrum with the known photoelectron spectra of 1,2-dichloroethane [41] and 1,2-dibromoethane [41].
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IV3. The Hel Photoelectron Spectrum of 12-difluorethane
The Hel photoelectron spectrum of 1,2-difluorethane is presented in Figure 3. This spectrum shows six bands in the 12.0 to 19.0 eV ionization energy
Fig. 3. Hel photoelectron spectrum of 1,2-difluorethane.
region having vertical ionization energies of (12.76 ± 0.03), (13.26 ± 0.03), (14.82 ± 0.03), (16.38 ± 0.03), (17.38 ± 0.03) and (18.11 ± 0.03) eV, respectively. The vertical ionization energies of these bands, as measured from thirteen expanded spectra, were calibrated using C02 (an impurity present in the sample) and Mel (added from the side of the ionization chamber). The assignment of the bands in Figure 3 was achieved by comparison of the experimental vertical ionization energies with computed vertical ionization energies, which were obtained in this work from ab-initio SCF molecular orbital calculations performed on FCH2CH2F at a computed equilibrium geometry similar to that obtained from ab-initio calculations in reference [42]. In fact the parameters quoted in reference [42] did not include the dihedral angles necessary to specify the geometry fully. As a result, the geometry was re-optimized using the same basis set as reference [42] (6-311G**) by freezing all the variables except the dihedral angles. This geometry was then used to perform an SCF calculation on the electronic ground state of FCH2CH2F using the same 6-311G** basis as used in reference [42]. Vertical ionization energies were obtained by applying the Koopmans' approximation to the SCF orbital energies. Both the computed and experimental vertical ionization energies are tabulated in Table 1 and the computed minimum energy geometry of FCH2CH2F(X1 A) is presented
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Table 1
Experimental and Computed Vertical Ionization Energies of 1,2-difluoroethane
Band Experimental(a)
V.LE.(eV) Koopmans' theorem00
V.LE.(eV) Ionization(c)
1 1 12.76 ± 0.03 13.78 (9a)"' 1
1 2 13.26 ± 0.03 14.37 14.75
(8a)"' (8b)-'
1 3 14.82 ± 0.03 16.94 (7b)'1
1 4 16.38 ± 0.03 18.34 18.49
(6b)' (7a)"'
1 5 17.38 ± 0.03 19.27 (6a)"'
1 6 18.11 ±0.03 20.08 20.08
(5b)"' (5a);' |
(a) See Figure 3 for the numbering of the observed bands in FCH2CH2F. 00 See text for details of calculation. (c) The electronic ground state configuration of cis FCH2CH2F(X2A) can be written as:
...(5a)2(5b)2(6a)2(7a)2(6b)2(7b)2(8b)2(8a)2(9a)2.
in Table 2, together with an experimental geometry, derived from electron diffraction studies [43].
The electronic configuration of FCH2CH2F(X1A), as obtained from the results of the ab-initio calculations performed in this work, can be written as:
...(5a)2(5b)2(6a)2(7a)2(6b)2(7b)2(8b)2(8a)2(9a)2
As shown in Table 1, the six observed photoelectron bands can be assigned, in order of increasing ionization energy, to one electron ionization of the (9a), (8a) and (8b),(7b), (6b) and (7a), (6a), and the (5a) and (5b) molecular orbitals, respectively. The spectrum in Figure 3 compares well with the known photoelectron spectra of the related molecules, 1,2-dichloroethane and 1,2-dibromoethane, which both have two intense, closely spaced low ionization energy bands separated by a 2 eV energy window from a group of progressively less intense bands spanning a 4-5 eV region to higher ionization energy [41]. In the case of 1,2-dichloroethane, the intense features to low ionization energy have been assigned to ionization from the highest occupied molecular orbitals which have predominantly chlorine (3p) character, and hence are non-bonding. In contrast, the group of bands to higher ionization energy have been assigned to ionization from bonding and anti-bonding molecular orbitals involving chlorine (3p) orbitals, carbon (2p) orbitals, and hydrogen (Is) orbitals. However, examination of the eigenvectors obtained from the
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Table 2
Minimum Energy Geometry of FCH2CH2F(X2A)(,)
Computed Geometry(aXb) Experimental Geometry(cXd)
r(C-C) 1.501 A r(C-C) 1.535 A r(C-F) 1.364 A r(C-F) 1.394 A r(C-H) 1.083 A r(C-H) 1.130 A
<CCF 110.33° <CCF 108.30° <CCH 110.02° <CCH 108.30° <FCH1 108.17° < FCH2 108.29° < HCH 109.98° < FCCF 70.40° <FCCF 74.50° < FCCH1 170.33° < H1CCH2 70.24°
Computed in this work at the SCF level using a 6-311G** basis set. w Calculations performed in reference [42] indicated that the gauche conformer is more stable than the trans
conformer by 0.01 eV. (c) See reference [43] <d) Determined in the gas phase by electron diffraction; only the gauche conformer was found.
Computed SCF Total Energy : -276.99789239 au
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molecular orbital calculations performed in this work for 1,2-difluoroethane, reveal that substitution of fluorine atoms for chlorine atoms in C1CH2CH2C1 has a significant effect on the bonding in the molecule when compared with 1,2-dichloroethane. For example, the highest occupied (9a) molecular orbital in FCH2CH2F has both bonding Ojwycap) character and bonding aC(2p)_C(2p) character whilst the equivalent molecular orbital in C1CH2CH2C1 is predominantly composed of chlorine (3p) atomic orbitals with very little contribution from the carbon (2p) orbitals and is thus non-bonding in character. However, as the molecular orbitals become more negative in energy, the fluorine atoms start to dominate the character of the orbitals. For example, the (7a) molecular orbital in FCH2CH2F is chiefly composed of fluorine (2p) atomic orbitals with smaller contributions from the carbon (2p) atomic orbitals, resulting in a molecular orbital with strongly bonding TCCF character and weak antibonding TCcc character.
IVA. Computational Details
IVA.a. F + Cjti^F
In this work ab-initio molecular orbital calculations were performed using the CADPAC suite of programmes [44] for both the CH3CHF and CH2CH2F radicals and their low-lying cationic states. The basis set used was a 6-311G** split valence type basis set [45] and all geometries for the neutral and ionic states considered were optimized using the analytical gradient method incorporated into the CADPAC programmes. The optimized geometries obtained for the neutral ground states and lowest-lying ionic states of both CH3CHF and CH2CH2F are presented in Tables 3 and 4, respectively. In addition, Mulliken analyses were performed on the converged SCF wavefunctions of the neutral and ionic states of both radicals. Electron correlation was accounted for by performing single point configuration interaction calculations (all single and double excitations included), using the ATMOL DIRECT-CI programme [46], on the neutral ground states of both CH3CHF and CH2CH2F, at their respective computed SCF minimum energy geometries, and the ground ionic states at their computed SCF minimum energy geometries to yield ASCF+CI adiabatic ionization energies. Additionally, CI calculations were performed on the ground ionic states of both radicals at the computed SCF minimum energy geometry of the respective neutral molecules to enable ASCF+CI vertical ionization energies to be calculated. The basis set used was the same 6-311G** basis [45] used for the geometry optimization calculations.
The computed electronic ground state configurations for the two radicals, CH3CHF and CH2CH2F are:
...(9a)2(10a)2(l l a f tUaf tna ) 1 CH3CHF(X2A)
...(9a)2(10a)2(l la)2(12a)2(13a)1 CH2CH2F(X2A)
For both CH3CHF(X2A) and CH2CH2F(X2A), ionization from the (13a) molecular orbital results in a *A cationic state, whilst ionization from the (12a) molecular orbital results in two cationic states, !A and 3A. Following Hartree-Fock SCF molecular orbital
IVA. Computational Details
IVA.a. F + CMsF
...(9a)2(10a)2(l la)2(12a)2(13a)1 CH3CHF(X2A)
...(9a)2(10a)2(l la)2(12a)2(13a)1 CH2CH2F(X2A) Vac
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Table 3.
Minimum Energy Geometries of CH3CHF(X2A) and CH3CHF*(X1A)(%)
Computed Geometry*** Experimental Geometry**0
CH3CHF(X2A) CH3CHFf(X1A) CH3CHF(X2A)
SCF Total Energy -177.49501 au -177.22388 au SCF+CI+Q Total Energy -177.87298 au -177.59621 au -
r(C-C) 1.490 A 1.446 A 1.52 A r(C-Hl) 1.085 A 1.080 A 1.08 A r(C-H2) 1.085 A 1.090 A 1.08 A r(C-H3) 1.089 A 1.095 A 1.08 A r(C-F) 1.339 A 1.229 A 1.35 A r(C-H4) 1.077 A 1.083 A 1.08 A
<CCF 114.03° 121.43° 119.0° <CCH4 121.00° 124.19° 121.0° <CCH1 110.39° 112.62° <CCH2 110.04° 108.02° <CCH3 111.06° 105.89° < H1CH2 108.78° 112.75° 109.0° < H1CH3 108.09° 111.51° 109.0° <H4CF 111.61° 114.38° 119.0° < H2CH3 108.41° 105.56° 109.0° < H1CCF 55.26° 4.97° < H1CCH4 167.15° 175.88°
(,) Computed in this work at the SCF level using a 6-311G** basis set. 00 See text for details. (c) Determined from an e.s.r. study in the liquid phase[341.
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Table 4.
Minimum Energy Geometries of CH2CH^O^A) and CH3CH2P(X1A)(,)
Computed Geometrv(aXb) Computed Geometry(c)
CH2CH2F(X2A) CH.CH.F^X'A) CH2CH2F(X2A)
SCF Total Energy -177.49069 au -177.17229 au -177.4850 au SCF+CI+Q Total Energy--177.86589 au -177.54979 au -177.8480 au
r(C-C) 1.490 A 1.446 A 1.498 A r(C-H2) 1.082 A 1.074 A r(C-F) 1.378 A 1.528 A 1.439 A r(C-Hl) 1.075 A 1.074 A
<CCF 110.61° 61.78c > 108.0° <CCH1 119.56° 120.24° <CCH2 111.34° 120.24° < H2CH2 109.28° 118.76° 110.0° <FCH2 107.04° 107.79° < H1CH1 117.61° 118.76° 118.0° < H1CCH2 161.55° 169.83° < FCCH1 79.56° 95.10
a i 197.70° 171.29° 190.0° « 2 128.96° 171.29° 130.0°
(,) Computed in this work at the SCF level using a 6-311G** basis set. 00 See text for details. (c) Large scale MRD-CI calculations performed in reference [56] using a [4s,2p,ld] contracted Gaussian basis
set for both fluorine and carbon with a [2s] contracted basis for hydrogen.
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calculations on the 3A ionic state arising from ionization from the (12a) orbital and the lA state arising from ionization from the (13a) orbital at the equilibrium geometry of the respective neutral molecules computed in this work, it was found that, for both CH3CHF* and CH2CH2F*, the closed-shell singlet state, (*A), was lower-lying energetically than the 3A open-shell state. This difference in energy is quantified in Table 5 for both radicals in terms of the vertical ionization energy to their respective 3A and lA ionic states computed at the SCF level only.
Table 5
Computed ASCF and ASCF+CI Adiabatic and Vertical Ionization Energies of CH3CHF(X2A) and CH2dl^(X2A)w
Ionization CH3CHF<-CH3CHF CH2CH2F<-CH2CH2F | Ionization
X'A^-X^ 3A<-X2A X'A*-X2A 3A<-X2A 1
ASCF | A.LE.(eV) 7.38 - 8.66 -
ASCF V.LE.(eV) 8.43 12.76 9.11 12.23 |
ASCF+CI A.I.E.(eV) 7.50 - 8.63 1
ASCF+CI V.LE.(eV) 8.44 - 9.52 II ASCF+CI+Q00
A.LE.(eV) 7.53 - 8.60 1 ASCF+CI+Q00
V.LE.(eV) 8.41 - 9.58 1 Experiment^
A.LE.(eV) 7.8110.03 - - 1 Experiment^ V.LE.(eV) 8.39±0.03 - - 1
(t) See text for details of calculations. ^ Including Davidson's correction for quadruple excitations. <c) This work.
Having obtained the equilibrium geometries of CH3CHF(X2A), CH3CHF(X1A), CH2CH2F(X2A) and CH2CH2F+(X1A), adiabatic and vertical ionization energies could be calculated at both the ASCF level and the ASCF+CI level (having performed the single point configuration interaction calculations described above). In addition, the effects of quadruple excitation were accounted for by making use of Davidson's correction. In each case, the coefficient of the reference configuration in the CI expansion was greater than 0.94. Consequently, adiabatic and vertical ionization energies were also calculated at the ASCF+CI+Q level, where Q represents the Davidson correction. It is interesting to note at this stage that comparison of the total energies at the SCF+CI+Q level for both CH3CHF(X2A) and CH2CH2F(X2A) at their respective computed equilibrium geometries, reveals that CH3CHF(X2A) is more stable than CH2CH2F(X2A) by 0.19 eV.
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IVA.b. F + FCH2CH^
In the case of the F + FCH2CH2F reaction, only one possible primary reaction product will result from hydrogen abstraction. Since fluorine abstraction will produce the CH2CH2F radical considered for the F + CH3CH2F reaction, it is only necessary to perform calculations on the FCH2CHF radical. As with the calculations performed on CH3CHF and CH2CH2F, the minimum energy geometries for the neutral ground state and lowest-lying ionic state (in this case the *A state resulting from ionization from the highest occupied MO) were computed using the analytical gradient method incorporated into the CADPAC programmes. The optimization procedure was initially carried out on three possible rotamers (two staggered gauche-type structures and one staggered anti-type structure) to establish which represented the minimum energy conformation. The three rotamers are depicted in Figure 4 [labelled (a), (b), and (c)] together with the total energies at their representative minimum energy geometries. Clearly, at the SCF level,
Figure 4. The three rotamers of FCH2CHF(X2A) considered in the ab-initio calculations performed in this work. (a) staggered gauche, (b) staggered anti and (c) staggered gauche. The SCF total energies are shown above each rotamer
rotamer (a) is the lowest-lying in energy being 0.03 eV more stable than rotamer (b) and 0.04 eV more stable than rotamer (c). As with the parent molecule, 1,2-difluoroethane (which has a staggered gauche conformation), the minimum energy geometry of FCH2CHF is a staggered gauche-type structure at the SCF level. The minimum energy geometries obtained for both FCH2CHF(X2A) and FCH2CHF(X1 A) are presented in Table 6. In addition, Mulliken analyses were performed on the converged SCF wavefunctions of the neutral ground state and lowest-lying ionic state. Electron correlation has been accounted for by performing single point configuration interaction calculations (with all single and double excitations included and the two lowest occupied molecular orbitals held frozen), using the ATMOL DIRECT-CI programme [46], on the neutral ground state of FCH2CHF at its computed minimum energy geometry, and the ground ionic state, FCH2CHF(X1A), at its computed minimum energy geometry to yield the ASCF+CI adiabatic ionization energy. Additionally, CI calculations were also performed on the
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Table 6
Minimum Energy Geometries of FCH2CHF(X2A) and FCHzCHFCX'A)^
Computed Geometry00
FCH2CHF(X2A) FO^CHFCX'A)
SCF Total Energy -276.36693 -276.07207 SCF+CI+Q Total Energy -276.79840 -276.49847
r(C-C) 1.483 A 1.474 A r(C-Hl) 1.082 A 1.091 A r(C-H2) 1.081 A 1.091 A r(C-Fl) 1.373 A 1.329 A r(C-H3) 1.075 A 1.085 A r(C-F2) 1.329 A 1.225 A
<CCF1 111.01° 108.64° < CCH1 109.86° 107.75° <CCH2 110.21° 107.75° <CCF2 115.06° 119.11° <CCH3 121.09° 124.36° < F1CH1 107.71° 111.98° < F1CH2 107.91° 111.99° < H1CH2 110.08° 108.56° < F2CH3 112.82° 116.53° < F1CCF2 77.79° 179.98° < F1CCH3 63.65° 0.03° < H1CCF2 163.18° 58.46°
(,) Computed in tis work at the SCF level using a 6-311G** basis set.
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ground ionic state of FCH2CHF at the computed minimum energy geometry of the neutral molecule to enable a ASCF+CI vertical ionization energy to be calculated. Again, the 6-311G** basis set [45] was used for all calculations.
The ground state electronic configuration of FCH2CHF(X2A) computed in this work is:
...(14a)2(15a)2(16a)2(17a)!
Ionization from the (17a) orbital will give rise to a 1A cationic state whilst ionization from the (16a) orbital will result in both *A and 3A cationic states. Hartree-Fock SCF molecular orbital calculations were performed, at the equilibrium geometry of the neutral molecule computed in this work, on the *A state arising from ionization of the (17a) orbital and on the 3A state arising from ionization of the (16a) orbital. In practice, the ab-initio SCF calculations on the triplet cationic state exhibited slow convergence characteristics and were not completed due to the prohibitive amount of computer time required. Nevertheless, the convergence tolerance was sufficiently small to conclude, from the total energies obtained, that the closed-shell singlet ionic state was energetically lower-lying than the triplet ionic state at the computed equilibrium geometry of the neutral molecule and the singlet state was therefore taken as being the ground state of FCF^CHF* for the purpose of the calculations carried out in this work. For puiposes of comparison, the vertical ionization energy to the (3A) ionic state of FCH2CHF, computed at the SCF level only, is presented later in Table 9.
Having obtained the computed equilibrium geometries of FCH2CHF(X2A) and FCH2CHFf(X1A), the adiabatic and vertical ionization energies were then calculated at both the ASCF level and the ASCF+CI level (having performed the single point configuration interaction calculations described earlier). The effects of quadruple excitations were again accounted for by making use of Davidson's correction. Again, in each case, the coefficient of the reference configuration in the CI expansion was greater than 0.94. Consequently, adiabatic and vertical ionization energies were also calculated at the ASCF+CI+Q level, where Q represents the Davidson correction.
IV.5. Results and Discussion
IV.5.a. F + C2H5F
The Hel photoelectron spectrum recorded for the F+C2H5F reaction over the ionization energy range 4.5-13.5 eV at a mixing distance of 7 mm above the photon beam, is presented in Figure 5. In this spectrum, the broad band centred at 12.43 eV ionization energy has been assigned to the first photoelectron band of the unreacted parent molecule C2H5F [41] and the band centred at 10.56 eV vertical ionization energy has been assigned to the secondary reaction product, vinyl fluoride [53]. The band in the 8.0-9.0 eV ionization energy region shows a maximum intensity at a mixing distance of 7 mm above the photon beam.
An expanded scan of the 7.5-10.5 eV ionization energy region recorded at a mixing distance of 7 mm above the photon beam is shown in Figure 6. The vertical ionization
..(14a)2(15a)2(16a)2(17a)!
IV.5. Results and Discussion
IV.5.a. F + C2H5F
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Fig. 5. Hel photoelectron spectrum recorded for the F + QHsF reaction over the ionization energy range 5.0-13.5 eV, at a reagent mixing distance of 0.7 cm above the photon beam.
Fig. 6. The first photoelectron band of CH3CHF, corresponding to the ionization CH3CHF(X1A) <r- CH3CHF(X2A), recorded with Hel radiation.
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energy of the band labelled as CH3CHF in this figure, as measured over sixteen spectra and calibrated using the spin orbit components of the first band of methyl iodide, is (8.39 ± 0.03) eV. Regularly spaced vibrational components have been observed in this band and the average spacing measured over sixteen spectra was (1010 ± 95) cm1. The band onset was measured as (7.70 ± 0.03) eV and the adiabatic ionization energy (which was taken as being the position of the first vibrational component) was measured as (7.81 ±0.03) eV.
Progressive increases in the mixing distance to 52 mm above the photon beam at constant reagent partial pressures resulted in gradual loss in intensity of this band to a point where it could no longer be detected. At mixing distances greater than 52 mm, only bands associated with vinyl fluoride [53] and hydrogen fluoride [41] were observed as reaction products. The results of a mixing distance study performed for this reaction at constant reagent partial pressures are summarized in Figure 7. It is reasonable to
Fig. 7. Results of a mixing distance study of the F + QHjF reaction performed at constant reagent partial pressure.
assume, on the basis of this evidence, that the band centred at (8.39 ± 0.03) eV, must be associated with a short-lived intermediate produced as a primary product of the F + C2H5F reaction (i.e. the QH4F radical) since, as QF^F initially decreases, so the band at (8.39 ± 0.03) eV increases and when this band maximizes, vinyl fluoride was observed to increase.
No higher bands of the QH4F radical were observed because they are probably overlapped by more intense bands associated with other reaction products (e.g. HF and vinyl fluoride) and the residual parent molecule (QI^F).
For the F + CyH F reaction, it is probably safe to conclude from the theoretical evidence and from thermodynamic calculations performed on the analogous F + Qt^X (X=C1, Br) reactions [23], that hydrogen abstraction will be more exothermic than fluorine
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abstraction and that the a-hydrogen abstraction route is more rapid than the P-hydrogen abstraction route. It is still necessary, however, to decide positively whether- the band seen in the photoelectron spectrum centred at (8.39 ± 0.03) eV ionization energy, can be asigned to either CH3CHF or CH2CH2F resulting from a or (3 hydrogen abstraction from C2H5F.
It would be useful at this stage to briefly summarize the thermodynamic evidence referred to above before describing the results of the ab-initio SCF molecular orbital calculations performed in this work.
There are three possible primary reaction products for the F + C2H5F reaction:
F + CH3CH2F -> CH3CHF + HF (1)
F + CH3CH2F -> CH2CH2F + HF (2)
F + CH3CH2F -> CH3CH2 + F2 (3)
When the most recent and presumably most reliable value for the heat of formation of the CH3CHF radical of (-74.1 ± 4.5) kJmol"1 [24] is combined with the heats of formation of F[51], HF[51] and Cyi5F[52], the heat of reaction (1) can be calculated as (-162.3 ± 5.0) kJmol"1. Similarly, the heat of reaction (3) can be calculated as (+301.0 ± 7.0) Umol"1. Clearly, reaction (1) is more exothermic than reaction (3) and presumably more rapid. Unfortunately, the heat of formation of CH2CH2F does not appear to have been determined, although, on the basis of the ab-initio calculations performed in this work, it can be assumed to be «18 kJ mol"1 more positive than that of CH3CHF(X2A). Also, some evidence as to the exothermicity of reaction (2) relative to reactions (1) and (3) can be obtained by looking at the thermodynamic trends exhibited in the analogous reactions involving other ethyl halides. For both the F + C ^ B r reaction and the F + C2H5C1 reaction, available heats of formation can be used to calculate the heats of reaction of the equivalent routes (l)-(3) [23]. In both cases, route (1) is shown to be marginally more exothermic than route (2). If this argument is extended to the F + C2H5F reactions, then it seems reasonable to assume that route (1) is the most exothermic in this case and that reactions (1) and (2) are both significantly more exothermic than reaction (3). This evidence seems to be supported by the kinetic and theoretical evidence presented earlier in the Introduction.
The results of the computed ASCF, ASCF+CI and ASCF+CI+Q first adiabatic and vertical ionization energies for both CH3CHF(X2A) and CH2CH2F(X2A) obtained in this work are presented in Table 5. For comparison, the ASCF vertical ionization energies to the respective lowest triplet ionic states are also included in Table 5. Clearly, the computed ASCF+CI+Q first adiabatic and vertical ionization energies for the CH3CHF radical of 7.53 and 8.41 eV, respectively, are in very good agreement with the experimental values of (7.81 ± 0.30) and (8.39 ± 0.03) eV, respectively. This compares with the computed ASCF+CI+Q first adiabatic and vertical ionization energies for the CH2CH2F radical of 8.60 and 9.58 eV, respectively.
It seems then that all of the available evidence indicates that the band centred at (8.39 ± 0.03) eV in Figure 6 can be assigned to one-electron ionization of CH3CHF(X2A). Having established the assignment of this band, attention can now be turned to the
F + OLCH.F -> CH,CHF + HF
F + CH3CH2F -> CH2CH2F + HF
F + CH3CH2F -> CH3CH2 + F2
(1)
(2)
(3)
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analysis of the structure seen in the band. The results of Mulliken population analyses of the converged SCF wavefunctions of
CH3CHF(X2A) and CH3CHFf(X1A) are presented in Table 7. These results show that the main loss in electron density occurs from the C2 and F atoms on ionization (where the C2 and F atoms are bonded (see Table 3)). Inspection of the eigenvectors of the converged SCF wavef unction for CH3CHF(X2A) shows that the half-filled molecular orbital in CH3CHF(X2A) is essentially composed of a C ^ - F ^ antibonding atomic orbital linear combination. Ejection of an electron from this molecular orbital should cause a decrease in the C-F equilibrium bond length in the ion and an increase in vibrational frequency in the ion relative to that in the neutral molecule. Indeed, the main computed change in geometry that occurs on ionization (see Table 3) was found to be in the C-F bond length which reduced from 1.34 A in the neutral molecule, CH3CHF(X2A), to 1.23 A in the ion, CH3CHFf(X1A). This is all consistent with the experimental evidence since the average vibrational separation in the band in Figure 6 was measured as (1010 ± 95) cm1, which is a typical separation for a C-F stretch. Although the C-F stretching frequency for CH3CHF(X2A) has not been measured, the C-F stretching fundamental in CH2CH2F(X2A) has been computed as 1006 cm"1 from force constant calculations at the SCF level using a split-valence 4-31G basis set [47]. It is well established that calculated vibrational frequencies obtained from such SCF calculations are about 10-15% higher than the experimental values [48]. When this computed value is reduced by 10-15%, the C-F stretching frequency of CH2CH2F can be estimated as 850-900 cm'1. This reasoning is also supported by the C-F stretching frequency in CH3CH2F which has been measured as 880 cm"1 [49]. If this value is taken to be a reasonable estimate of the C-F stretching mode in CH3CHF(X2A), then it is clear that an increase in vibrational frequency has occurred on ionization as expected from the above arguments.
The results of Mulliken population analyses on the converged SCF wavef unctions of CH2CH2F(X2A) and CH2CH2Ff(X1A), summarized in Table 8, show only a significant loss in electron density from the C2 atom (see Table 4 for atomic numbering), and excitation of the C-F stretching mode would not be expected to accompany the CH2CH2F+(X1A) <— CH2CH2F(X2A) ionization. However, the first photoelectron band of CH2CH2F(X2A) might exhibit vibrational structure associated with excitation of an out-of-plane CH2 deformation mode on ionization as is observed in the case of the methyl radical [50]. In fact, on the basis of the computed geometries shown in Table 4, CH2CH2F(X2A) would undergo a dramatic change in geometry on ionization to the ground state ion, from a classical neutral structure to a cyclic structure with the C-C bond being bridged by the fluorine atom. This has the effect of bringing the two CH2 groups almost into a plane orthogonal with the plane containing the C-C bond and the bridging F atom (see diagram accompanying Table 4), and it seems reasonable to expect that this might excite a CH2 rocking mode in the ion. In the previously recorded photoelectron spectrum of QHj [54], the observed vibrational spacing of «400 cm"1 was assigned to a CH2 pyramidal bending mode in the ion, since a transition occurs from a non-classical bridged structure in the ion (at the adiabatic ionization energy) to a classical eclipsed ionic structure (at the vertical ionization energy). However, in a combined theoretical and experimental study using photoionization mass spectrometry [55], Berkowitz et al. argued that the path between the classical and non-classical ionic structures of C2H5 would occur via a vibrational motion in the ion that involved a hydrogen atom on the CH3 moiety. Whichever argument is
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Table 7
Results of Mulliken Population Analysis of the Converged SCF Wavefunctions of CH3CHF(X2A) and CH3CHF(X*)W
Centre CH3CHF(X2A) CH3CHFf(X1A) Differences
HI 0.88876 0.78682 0.10194 H2 0.90174 0.78330 0.11844 H3 0.89619 0.78171 0.11448 H4 0.91025 0.77114 0.13911 Cl 6.26964 6.32774 -0.05810 C2 5.83303 5.52077 0.31226 F 9.30038 9.02852 0.27186
Total 25.00000 24.00000 1.00000
(a) Performed at the respective minimum energy geometries computed in this work using a 6-311G** basis set.
Table 8
Results of Mulliken Population Analysis of the Converged SCF Wavefunctions of CH^FCX'A) and CH2CH2F(X,A)(*)
Centre CH2CHF(X2A) CHJCH^QOA) Differences
HI 0.92314 0.75931 0.16383 H2 0.92314 0.75931 0.16383 H3 0.87996 0.75934 0.12062 H4 0.87996 0.75934 0.12062 Cl 5.77402 5.87318 -0.09916 C2 6.26255 5.87314 0.38941 F 9.35723 9.21636 0.14087
Total 25.00000 24.00000 1.00000
(,) Performed at the respective minimum energy geometries computed in this work using a 6-311G** basis set.
applied to the present work, it is certain that any resolved vibrational structure observed in the first photoelectron band of CH2CH2F would have a value substantially smaller than the observed value of (1010 ± 95) cm"1 and probably less than the 400 cm"1 spacings
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observed in the first photoelectron band of QHj [54], It is, nevertheless, satisfying to note that the geometry calculated in this work for
CH2CH2F(X2A) is in reasonable agreement with that calculated in a recent paper by Peyerimhoff et al. [56], who performed large scale MRD-CI calculations using a [4s,2p,ld] contracted Gaussian basis set for both fluorine and carbon and a [2s] contracted basis for hydrogen (see Table 4 for a comparison with the geometry obtained in this present work).
All of the above evidence points to an assignment of the band at (8.39 ± 0.03) eV, observed in the photoelectron spectrum recorded for the F + Q H F reaction, to the ionization, CH3CHFf(X1A) <- CH3CHF(X2A). A possible reaction scheme for the F + QHsF reaction which would be consistent with all the experimental observations is:
F + CH3CH2F -> CH3CHF + HF
CH3CHF + F -> [CH3CHFJ*
[CH3CHF2f -> CH2=CHF + HF
IV.5.b. F + FCH2CH2F
The Hel photoelectron spectrum recorded for the reaction of fluorine atoms with 1,2-difluorethane at a mixing distance of 15mm above the photon beam, is presented in Figure 8. The bands in the 12.0-14.0 eV ionization energy region can be assigned to the
F + CH3CH2F -> CH3CHF + HF
CH3CHF + F -> [CH3CHFJ*
[CH.CHFJ* -> CH,=CHF + HF
IV.5.b. F + FCH2CH2F
Fig. 8. Hel photoelectron spectrum recorded for the F + FCHjCty7 reaction over the ionization energy range 5.0-14.0 eV, at a reagent mixing distance of 1.5 cm above the photon beam. The positions of vibrational components in the first photoelectron band of CF are shown in this diagram with vertical dashed lines, although CF makes a negligible contribution to the band shown, assigned to FCH2CHF.
first two bands of 1,2-difluorethane whereas the bands centred at 10.64 eV [57] and 11.50 eV [57] can be assigned to 1,2-difluoroethylene and fluoroacetylene, respectively, which are secondary products of the reaction. However, the main feature of interest in Figure
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8 is the band centred at 9.36 eV ionization energy which showed a maximum in intensity at a mixing distance of 15 mm above the photon beam.
An expanded scan of the 5.0-10.0 eV ionization energy region recorded at a mixing distance of 15 mm above the photon beam is shown in Figure 9. At fixed reagent partial pressures, the intensity of the band at 9.36 eV maximized at 15 mm mixing distance and decreased progressively, as the mixing distance was increased above 15 mm, to a point where it disappeared at approximately 40 mm mixing distance. At mixing distances greater than 40 mm, only bands associated with CF, 1,2-difluoroethylene and fluoroacetylene were observed. The adiabatic and vertical ionization energies of this band were measured as (8.86 ± 0.04) and (9.36 ± 0.03) eV, respectively when averaged over twelve expanded spectra. Regularly spaced vibrational structure was observed in this feature and the average vibrational spacing was measured as (1070 ± 30) cm"1. It is reasonable to assume on the basis of the mixing distance behaviour, shown in Figure 10, that this band must be associated with a short-lived intermediate generated by the F + FCH2CH2F reaction.
Fig. 9. The first photoelectron band of FCH2CHF recorded with Hel radiation. Positions of vibrational components in the first photoelectron band of CF are shown as vertical dashed lines, although CF makes a negligible contribution to the band shown, assigned to FC^CHF.
The plot of normalized band intensities versus mixing distance for the F+FCH2CH2F reaction is presented in Figure 10. As can be seen, the band at (9.36 ± 0.03) eV (labelled as FCH2CHF) was only observed at mixing distances less than 40 mm above the photon beam. The first band of the CF radical [58] was observed at larger mixing distances and has a maximum in intensity at 40 mm above the photon beam. It appears from Figure 10 that, as the parent molecule, FCH2CH2F, is depleted, a short-lived species, FCH2CHF, is generated which reacts further at larger mixing distances to produce CF, 1,2-difluoroethylene and fluoroacetylene.
Initially it was difficult to distinguish the band at (9.36 ± 0.03) eV from the first band of CF (first V.I.E. 9.55 eV [58]) since their first vertical ionization energies lie
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Fig. 10. Results of a mixing distance study of the F + FCH2CH2F reaction performed at constant reagent partial pressure.
quite close to each other. However, it was possible to record the band at (9.36 ± 0.03) eV with very little contribution from the first band of CF by optimizing the relative reagent concentrations at short mixing distances. Although Figures 8 and 9 are thought to contain very little CF, for convenience the known positions of the vibrational components of the first band of CF are shown in these diagrams by dotted lines.
The arguments presented in the Introduction indicate that hydrogen abstraction is almost certainly the preferred primary route for the F+FCH2CH2F reaction. In order to confirm the assignment of the band at (9.36 ± 0.03) eV in Figures 8 and 9 to ionization of FCH2CHF, it would be valuable to compare the measured vertical ionization energy with the first vertical ionization energy of FCH2CHF(X2A), computed via ab-initio SCF molecular orbital calculations performed on FCH2CHF(X2A) and FCH2CHFf(X1A).
Ab-initio restricted Hartree-Fock SCF molecular orbital calculations have been performed in this work for CH2CH2F(X2A) and CH2CH2FH"(X1A) at their respective computed minimum energy geometries with a 6-311G** basis set [45], and the ASCF+CI+Q adiabatic and vertical ionization energies have been computed at 8.60 and 9.58 eV, respectively (see Table 6). Additionally, geometry optimization calculations were performed for both FCH2CHF(X2A) and FCH2CHFf(X1A) with the same basis set, as described earlier, followed by single point configuration interaction calculations, to obtain ASCF+CI+Q adiabatic and vertical ionization energies for the FCF^CHF* (X*A) <— FCH2CHF(X2A) ionization. These values are tabulated in Table 9 together with the ASCF vertical ionization energy to the triplet state of the ion as a means of comparison. This procedure yielded ASCF+CI+Q adiabatic and vertical ionization energies to the ground singlet state of the ion of 8.16 and 9.19 eV, respectively (see Table 9). Although the computed adiabatic and vertical ionization energies for FCH2CHF(X2A) show good agreement with the experimental values it would be imprudent to positively assign the photoelectron band centred at (9.36 ± 0.03) eV in Figures 8 and 9 to the FCH2CHF
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Table 9
Computed ASCF and ASCF+CI Adiabatic and Vertical Ionization Energies of CH2FCHF(X2A)(a)
Ionization CH2FCHF<-CH2FCHF(X2A)|
X!Af-X2A 3A<-X2A |
ASCF A.LE.(eV) 8.02 -
ASCF V.LE.(eV) 9.34 15.27 |
ASCF+CI A.LE.(eV) 8.13 1
ASCF+CI V.LE.(eV) 9.25 -
ASCF+CI+Q^ A.LE.(eV) 8.16 -
ASCF+CI+Q^ V.LE.(eV) 9.19 1 Experiment(c)
A.LE.(eV) 8.86±0.04 " Experiment(c)
V.LE.(eV) 9.36±0.03 -
(a) See text for details of calculations. ^ Including Davidson's correction for quadruple excitations. (c) This work.
radical on this evidence alone, particularly in view of the fact that the results of the configuration interaction calculations performed on CH2CH2F and CH2CH2F*" also give a vertical ionization energy which compares well with the experimental value. However, if fluorine abstraction was the favoured route, then it would be expected that F2 would be observed as a reaction product of the F+FCH2CH2F reaction and this was not the case. HF was, however, clearly observed as a reaction product and this evidence, together with the results of the calculations described above, allow assignment of the band at (9.36 ± 0.03) eV in Figures 8 and 9 to ionization of the radical, FCH2CHF.
The results of Mulliken population analyses of the converged SCF wavefunctions of FCH2CHF(X2A) and FCHjCHF^A) in Table 10, clearly show that a loss in electron density from the bonded C2 and F2 atoms in the CHF moiety occurs on ionization (see Table 5 for the atom numbering used). It might therefore be expected that the C-F stretching mode would be excited on ionization. Inspection of the eigenvectors of the converged SCF wavefunction for FCH2CHF(X2A) indicates that the half-filled molecular orbital in FCH2CHF(X2A) is essentially composed of a Q^-Fj^ antibonding combination. The ejection of an electron from this orbital would cause a decrease in the C-F equilibrium bond length and an increase in the vibrational frequency of the ion. This hypothesis is supported by the computed change in minimum energy geometry that occurs in FCH2CHF on ionization (see Table 5). The most significant changes all occur at the CHF end of the molecule with the CF bond length being reduced from 1.33 A in the
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Table 10
Results of Mulliken Population Analysis of the Converged SCF Wavefunctions of CH2FCHF(X2A) and CH^FCHFXX'A)^
Centre CH2FCHF(X2A) CH.FCHF^A) Difference
HI 0.91515 0.78889 0.12626 H2 0.90426 0.78888 0.11538 Fl 9.34609 9.23929 0.10680 Cl 5.79866 5.87133 -0.07267 C2 5.86287 5.54909 0.31378 F2 9.27966 9.01753 0.26213 H3 0.89331 0.74500 0.14831
Total 33.00000 32.00000 1.00000
(a) Performed at the respective minimum energy geometries computed in this work using a 6-311G** basis set.
neutral molecule to 1.22 A in the ion. Unfortunately, no experimental or theoretical estimates of the C-F stretching frequency in FCH2CHF appear to have been made. However, the C-F stretching mode for FCH2CH2F has been measured from infrared absorption spectroscopy as 1088 cm"1 [59], whilst the C-F stretching mode in CH2CH2F has been computed at the SCF level as 1006 cm"1 [47] and the C-F stretch in CH3CH2F has been measured as 880 cm"1 [49]. It seems reasonable to conclude, therefore, that the vibrational structure associated with the band centred at 9.36 eV in Figure 9, having an average spacing of (1070 ± 30) cm"1, can be assigned to a C-F stretching mode in FCHjCHF^X1^), increased from that in the ground state of the neutral molecule.
In the light of the evidence presented above and in view of the fact that CF, 1,2-difluoroethylene and fluoroacetylene were all observed as secondary reaction products, a possible reaction scheme for the F+FCH2CH2F reaction might therefore be:
F + FCH2CH2F -> FCH2CHF + HF
F + FCH2CHF -> [FCH2CHFJ*
[FCH2CHFJ* -> FCH=CHF + HF
F + FCH=CHF -> [F2CHCHF]*
[F2CHCHF]* -> FCHCF + HF
FCHCF + FCH2CH2F -» [FCH=CHF]* + FCH2CHF
F + FCH2CH2F -> FCH2CHF + HF
F + FCH2CHF -> [FCH2CHFJ*
[FCH2CHFJ* -> FCH=CHF + HF
F + FCH=CHF -> [F2CHCHF]*
[F2CHCHF]* -^ FCHCF + HF
FCHCF + FCH2CH2F -> [FCH=CHF]* + FCH2CHF
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[FCH=CHF]* -> HC^CF + HF
Although some CF presumably arises from further fluorine atom attack on the monofluoroacetylene followed by rupture of the carbon-carbon bond, the mixing distance study presented in Figure 10 shows that the CF intensity reaches a maximum significantly earlier than the fluoroacetylene intensity. There must therefore be an as yet unidentified alternative route for production of CF.
It is interesting to make a comparison between the results obtained in this work for the F + QHsF and F + FCH2CH2F reactions, with those obtained in reference [23] for the analogous F + C2H5X and F + XCH2CH2X reactions, where X = Cl and Br (see also the Appendix). Bands observed in the photoelectron spectra recorded for the F + C2H5C1 and F + C2H5Br reactions [23], have been assigned to the respective primary reaction products CH3CHCI and CH3CHBr, having vertical ionization energies of (8.18 ± 0.02) eV and (8.05 ± 0.01) eV, respectively. If the vertical ionization energy measured in this work for the CH3CHF radical of (8.39 ± 0.03) eV, is included with these results then a clear trend emerges in the experimental CH3CHX first vertical ionization energies. As the halogen group becomes heavier, so the vertical ionization energy of the corresponding haloethyl radical decreases. This trend is also exhibited in the measured vibrational spacings in the respective first photoelectron bands, which decrease from (1010 ± 95) cm"1
in CHaCHF* to (700 ± 30) cm1 in CH3CHBr+(X1A). Similarly, photoelectron bands observed in the spectra recorded for the analogous F
+ XCH2CH2X reactions, where X = Cl and Br, have been assigned to ionization of the respective C1CH2CHC1 and BrCH2CHBr radicals. Again, if the first vertical ionization energy obtained in this work for the FCH2CHF radical, of (9.36 ± 0.03) eV, is compared with the first vertical ionization energy of C1CH2CHC1 ((8.68 ± 0.01) eV[23]) and BrCH2CHBr ((8.20 ± 0.01) eV[23]), then a clear trend of decreasing ionization energy as a function of increasing atomic number of the halogen group becomes evident.
V. POSSIBLE DIRECTIONS FOR THE FUTURE
It is clear that although the study of short-lived molecules by single photon p.e.s. has developed considerably in the last ten years, interest has broadened during that time to include the use of lasers as ionization sources. The introduction of lasers to the field of small molecule p.e.s. has been accompanied by a clear improvement in the experimental resolution that can be achieved.
As indicated earlier, perhaps the best working resolution that can be obtained with Hel radiation in a single photon p.e.s. study of an effusive gas mixture is 20 meV [60]. The factors which contribute to the working resolution in molecular p.e.s. have been discussed elsewhere [61] and include an instrumental contribution from the analyser, the linewidth of the photon source, broadening due to population of rotational levels in the target molecules and Doppler broadening. If the target molecules are cooled in a molecular beam before photoionization occurs, then contributions from rotational population in the neutral molecules and Doppler broadening can be significantly reduced. In this way, Shirley and coworkers [62-69] have obtained Hel photoelectron spectra of a number of small, stable molecules in the gas-phase at a resolution of « 12 meV and this has allowed vibrational separations in each photoelectron band to be measured with improved
[FCH=CHF]* -> HC^CF + HF
V. POSSIBLE DIRECTIONS FOR THE FUTURE
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precision. It should be possible to extend this approach to the study of short-lived molecules as the methods for preparing rotationally cold radicals using a discharge nozzle source or laser photodissociation of stable molecules in a seeded molecular beam are now well established and have been used in other areas of spectroscopy [70-73]. The major experimental difficulty will be the rather low number density of radicals in the ionization region prepared by such methods, although this problem could be alleviated by increasing the spectrometer collection efficiency using a multichannel detection system [18]. However, even when a molecular beam source is used, the instrumental contribution from the analyser, as well as the other contributing factors, limit the resolution in a single photon p.e.s. study to, at best, « 10 meV. Unfortunately, this has prevented the technique from being used as a general method to resolve rotational structure associated with a photoelectron band (although rotational structure has been resolved for a few molecules with large rotational constants, see for example, references [63] to [65]). However, a recently developed technique, zero kinetic energy photoelectron spectroscopy (ZEKE-PES), makes use of the fact that, in most energy analysers, the resolution improves as the electron kinetic energy decreases. Consequently, at zero kinetic energy, the resolution will simply be determined by the laser line width [74]. The experiment involves detection of only zero kinetic energy electrons whilst sweeping the energy of the ionization source. A lamp and monochromator or a synchrotron were used as photon sources in the original work but more recently molecular ionization was achieved using a laser via a MPI process. Using laser excitation and time-delayed extraction fields, Miiller-Dethlefs and coworkers [74-77] have been able to reduce the electron energy resolution to essentially the laser linewidth and hence have measured rotationally resolved ionization thresholds of nitric oxide and benzene to ~ 0.4 cm'1 (0.05 meV). If this method were to be applied to short-lived molecules, a number of conditions must be satisfied. Firstly, a convenient, non-contaminating source of the molecule of interest would be required, the first adiabatic ionization energy of the molecule must be known (preferably to within ± 0.01 eV (80 cm' *)) and an established MPI scheme must be available for the molecule under study. Hence, a single photon p.e.s. investigation and a laser MPI study of the short-lived molecule of interest would provide valuable information to assist in the planning of a ZEKE-PES experiment.
Three radicals have already been studied by laser MPI-PES, (CF, CC1 and CH [78,79]) at lower resolution than can be achieved in a ZEKE experiment. CF(X2II) and CCl(X2n) were prepared by photodissociation of CC12F2 and CC13F at 193 nm and ionized via a (1 + 1) mechanism [78]. Electron energy analysis was achieved using an electrostatic analyser at a resolution of =20 meV. From the vibrationally resolved spectra, spectroscopic constants coe and coexe were obtained for the ground state ion in each case. Also, from the spectra obtained, the difference between the first adiabatic ionization energies of CF(X2IT) and CC1(X2II) could be measured as (0.13 ± 0.01) eV. When this is combined with the first adiabatic ionization energy of CF, determined in a single photon p.e.s. study as (9.11 ± 0.01) eV [58], the first adiabatic ionization energy of CC1 can be derived as (8.98 ± 0.02) eV. In the CH work [79], the radical was prepared by photodissociation of ketene and ionized by a (2 + 1) MPI process. Vibrationally resolved photoelectron spectra were obtained using a time-of-flight photoelectron spectrometer at an excitation laser wavelength of 310.8 nm. The spectra obtained allowed the vibrational numbering of the intermediate resonant state to be established for the transitions observed
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and this in turn assisted in analysis of a previously recorded CH absorption.spectrum. These two studies show that in MPI-PES information is obtained for the final ionic state as well as for the resonant intermediate state.
It is clear that studies of other radicals will be made by this method in the near future. The advantages of such studies are: (i) In an MPI photoelectron spectrum, there is rarely an overlapping band problem
from ionic states of different molecules, as frequently occurs in single-photon p.e.s., as the laser is highly selective at the intermediate neutral molecule level and ionization will normally occur to one ionic state at a given laser wavelength.
(ii) The photoionization process can be used to selectively excite vibrations in a polyatomic ion by selecting the vibronic character of the intermediate state. This has already been demonstrated in some MPI-PES studies of stable molecules [80-82].
(iii) As can be seen in the Appendix, many radicals have low first ionization energies, often in the 6.0-9.0 eV range. This is a distinct advantage when designing a laser MPI experiment using photons in the visible or ultraviolet region, because when compared with a closed shell molecule with a first ionization energy of typically 12 eV, the overall number of photons required for ionization of a radical will be lower and hence the overall cross-section will be higher than that of the reference closed-shell molecule. Also, the selectivity of the MPI process should allow photoelectron spectra of low concentrations of radicals to be obtained in the presence of higher concentrations of stable molecules.
(iv) As has already been demonstrated in MPI experiments on a number of stable molecules [83,84], MPI-PES should allow the dynamics of the resonant intermediate state to be probed. Studies of this type will almost certainly increase as picosecond lasers become more readily available.
Because of the anticipated low concentration of the radicals of interest in the ionization region and the pulsed nature of the lasers used, a time-of-flight spectrometer with high collection efficiency and reasonable resolution will be needed. This requirement is satisfied by a magnetic bottle electron-energy analyser [85] which uses an inhomogeneous magnetic field to parallelize photoelectrons produced in a 2TC solid angle i.e. half of the electrons that are emitted are collected. This type of analyser has already been used to study a number of small stable molecules with MPI-PES with a working resolution of approximately 10 meV [86-88].
Inspection of the references listed in the Appendix shows that almost all previous u.v.p.e.s. studies of short-lived molecules have used an inert gas discharge lamp as the source of ionizing radiation. In fact most of the examples included in this tabulation used Hel (58.4nm) radiation. Synchrotron radiation is, however, an established photon source for probing the electronic structure of small molecules via photoelectron spectroscopy and it would be extremely valuable to use this type of radiation to investigate short-lived molecules.
Synchrotron radiation provides a number of advantages over discrete line sources. For example, because it is continuously tunable, measurements as a function of both photon energy and emitted electron energy provide valuable information about the way in which resonances decay into specific vibrational and electronic states of the ion. An autoionization resonance process, once identified, can give rise to extra structure over that
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seen in single photon ionization. Examples of this have been clearly demonstrated in the photoelectron spectra of Cl2 [89] and 02 [90]. If the photoelectron spectrum of a small unstable molecule is investigated in this way, the extra vibrational structure arising from autoionization will be very useful in probing regions of the ionic potential surface which are not accessed by direct ionization. Examples which could be studied in this way are the N3(X2n) [91] and CH2F(X2B2) [22] radicals, where only three vibrational components are clearly observed in their first photoelectron bands recorded with Hel radiation. Furthermore, as has been clearly demonstrated in synchrotron studies on stable molecules, relative photoelectron band intensities allow partial photoionization cross-sections to be derived and angular distribution measurements provide information concerning the dynamics of the photoionization process. The use of synchrotron radiation also means that the inner-valence region of most molecules can be probed and it is in this region that a complete breakdown of the one-electron ionization model often occurs [92].
A number of interesting experiments could be performed if a laser was used in combination with a synchrotron source. One possible experiment would be to produce short-lived molecules via laser photodissociation of a small stable molecule and then to study the photodissociation fragments with p.e.s. using a synchrotron as the photon source.
The obvious importance of these likely developments does not, however, preclude single photon gas-phase p.e.s. studies with inert gas discharge sources. Indeed such investigations are extremely useful in designing experiments of the type described. For example, as described earlier, for an atom-molecule reaction, Hel photoelectron spectra can be recorded as a function of reaction time. Bands associated with reactants, products and intermediates can be observed. This should allow the dominant reaction pathway to be identified and the reaction conditions to be optimized for the production of the shortlived intermediate of interest in the photoionization region.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the S.E.R.C. for financial support of this research. This work was also supported in part by the Air Force Office of Scientific Research (Grant No. AFOSR-89-0351) through the European Office of Aerospace Research (EOARD).
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