1

PHOTOIONIZATION MASS SPECTROMETRIC STUDIES OF FREE RADICALS

J. BERKOWITZ AND B. RUSCIC

Chemistry Division Argonne National Laboratory

Argonne, IL 60439, USA.

CONTENTS

I. Introduction II. Experimental Arrangement

A. The Photoionization Mass Spectrometric Method B. Generation of Transient Species for PIMS Studies

IE. Survey of Experimental Results A. The Pnicogen and Chalcogen Hydrides (Group V and VI Hydrides) B. The Nitrogen and Phosphorus Fluorides C. The Group IV Hydrides

1. The CH^ SiH,,, Gel^ Sequence a. The SiH^ System b. The GeH^ system

2. Comparisons of CH^ SiH^ GeK^ a. Step-wise Bond Energies b. Jahn-Teller Distortion in CH4

+, SiH4+, GeH4

+

3. C2H3 and C2H5: C-H Bond Energies of Ethylene and Ethane. Some Observations on the C-H Bond Energy in Acetylene

D. The Boron hydrides 1. BH3 2. B2H5 3. B2H4

E. COOH - an Important Reaction Intermediate F. SO and P2; Some General Observations on Transient Species

IV. Concluding Remarks Acknowledgment References

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I. INTRODUCTION

The techniques for performing vacuum ultraviolet studies of photoionization in the gas phase were introduced about three decades ago. Two general methods have been employed:

a) analyzing the kinetic energy spectrum of photoelectrons (PES), and

b) measuring the intensities of mass-selected ions as a function of incident photon energy (PIMS).

These methods are often complementary. The photoelectron spectrum obtained with fixed incident wavelength yields information about the first and higher ionization potentials of the molecule. With vibrationally resolved spectra, it is possible to infer the change in geometrical structure that occurs upon ionization, by utilizing the Franck-Condon approximation. Tunable wavelength PIMS is sensitive to the onset of photoionization (the adiabatic ionization potential), sometimes more so than PES. However, the same process which may favor PIMS for determining the adiabatic onset (i.e. autoionization) may hinder this technique in detecting higher ionization potentials. Both methods have by now been applied to a large class of stable molecules.

The above considerations also apply to the study of free radicals. However, most methods of generating transient species do not yield a unique product. Often, some fraction of the precursor survives, and other transient species may be produced by parallel or consecutive reactions. A photoelectron spectrum, usually obtained without concomitant mass analysis, will be a superposition of photoelectrons from all species present as target materials. One must then identify the portion of the photoelectron spectrum relevant to the desired species. In case of accidental overlap, subtractions must be performed, which contribute to the uncertainty of weak onsets. Mass analysis (as implied by PIMS) isolates the ionization of the desired species from other components in the target gas. However, the difficulty of establishing higher ionization potentials remains. Clearly, PES performed in coincidence with mass analysis would offer the best of both techniques. Such experiments are routinely performed on stable molecular species (where the superposition problem is not significant), but are more difficult to perform on transient species, since the density of the latter in the target region is usually substantially lower. Coincidence measurements involving threshold photoelectrons and photoions are feasible, but the threshold photoelectron spectrum is also often plagued by autoionization effects.

In the absence of autoionization effects, an idealized photoionization spectrum would consist of a sequence of step functions.1 The derivative of such a spectrum would then be comparable to a photoelectron spectrum. Since an idealized photoionization spectrum is not frequently encountered, a vibrationally resolved photoelectron spectrum provides a more reliable source of data for Franck-Condon analysis, and therefore a measure of the geometry change that occurs when the neutral species is ionized. The PIMS method, in addition to providing an accurate adiabatic ionization potential for the transient species, can be utilized to determine the

3

appearance energy of the radical cation from a stable molecular progenitor. Thus, two threshold measurements,

A + hv -> A+ + e

AB + hv -> A+ + B + e

can be algebraically summed to determine the bond energy D0(AB). In summary, PES of free radicals is more advantageous for determining higher ionization potentials, and deducing structural changes. PIMS of free radicals is generally free of contamination from unwanted species, can detect low abundance species in the presence of high concentrations of other species, and is more directly applicable to the determination of thermochemical energies.

In this chapter, we shall describe the experimental methods employed in PIMS for the investigation of transient species, illustrate the type of data obtained with some examples, and summarize the results.

H. EXPERIMENTAL ARRANGEMENT

A. The Photoionization Mass Spectrometric Method

In principle, the requirements for this investigation are:

a) a tunable vacuum ultraviolet source; b) a method of monochromatizing this radiation; c) an ionization region, where the monochromatized radiation passes through

the gaseous target; and d) mass analysis of any photoions created.

At each wavelength, the photoion and light intensity and the precise wavelength must be recorded. The tunable VUV source can be a laboratory discharge lamp, a synchrotron or a VUV laser. Each has been used for conventional PIMS of stable species, but to the best of our knowledge, only the laboratory discharge light source has thus far been used for the study of transient species. (Here, we disregard multiphoton ionization experiments). For mass analysis, magnetic sector or quadrupole mass spectrometers have been used. A time-of-flight mass spectrometer is suitable for pulsed sources, such as a pulsed VUV laser or certain coincidence experiments. Since photoion signals are typically weak in free radical studies, care must be taken in extracting the photoions from the ionization region and focussing them onto the entrance of the mass analyzer. Longer counting times are usually required for free radical studies, but a compromise is often dictated by the lifetime of the free radical source. In a long duration scan, the radical abundance may be changing with time. To compensate for this effect, shorter wavelength ranges are scanned with long counting

A + hv -> A+ + e

AB + hv -> A+ + B + e

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times and finely spaced points; a coarsly-spaced scan is then made of the entire wavelength region, and the shorter wavelength portions are overlapped and matched, using the coarse scan as an overall guide. In modern studies, the scans are controlled by a dedicated computer, which can be programmed for a desired counting time. A stepping motor is then activated to select the next wavelength for study.

Photoionization studies of free radicals are typically performed in rather limited wavelength domains. Even with mass analysis, there are usually photodissociative ionization processes at higher photon energies which generate from some precursor an ion of the same mass as is produced by photoionization of the free radical. Subtraction or, in some cases, coincidence techniques can be used at these energies, but they become very uncertain when the "contaminant" gives rise to a much larger ion intensity than the free radical. This criterion usually limits the scanning range to ~ 2-4 eV beyond the ionization threshold of the free radical. Some radicals such as CH3 and OH can be generated in abundance, and consequently a larger energy range can be scanned.

Many free radicals have low ionization potentials. It may be convenient for some purposes to isolate the light source from the remainder of the apparatus with a LiF window. Radiation with X t 1100 A will not be transmitted by this material. Some radicals (e.g. OH) have ionization thresholds well below 1100 A. In such cases, windowless operation is mandatory. In other cases, windowless operation is recommended, since LiF crystals form color centers after prolonged exposure to VUV radiation, and lose their transmission quality.

B. Generation of Transient Species for PIMS Studies

Free radicals are usually chemically reactive on surfaces, and sometimes with other gaseous species such as their precursors. Unlike stable species, which can be introduced into a relatively tight (enclosed) ionization chamber with apertures for incoming and outgoing photon beam and exiting photoions and photoelectrons, free radical studies are best conducted with a molecular beam of free radicals crossed by the photon beam. The interaction should occur in a region of well-defined electrical potential, so that the photoions and photoelectrons can be efficiently extracted and focussed. This can be achieved by making large openings in a conventional enclosed chamber, so that the beam of free radicals passes through the chamber and enters the throat of a pumping system.

Ideally, one would like to prepare as intense a beam of free radicals as possible, since the in-beam density will be less than the density achievable in an enclosed source of stable molecules. With a cw beam of free radicals, this condition places a premium upon high pumping speed and a large fractional content of free radicals in the beam. In principle, photodissociation of a progenitor in the beam, just before it enters the ionization zone, would appear to be ideal. However, most precursor molecules have large photoabsorption and photodissociation cross sections at wavelengths X < 2000 A. An intense laser source is required to obtain large fractional conversion. The ArF excimer laser (X ~ 1935 A) has a convenient wavelength and is intense, but it is pulsed

5

and has a very poor duty cycle. It could be utilized with a pulsed VUV (laser) source, and time-of-flight detection. Such an approach is envisioned in our future plans. Another problem with such a source is that the free radicals may be generated in a range of vibrational and rotational (and conceivably electronic) states which are not necessarily characteristic of a single Boltzmann temperature. With a VUV source which is cw, the optimum free radical source should also be continuous. Three types of such free radical sources have found application in our studies - pyrolysis, microwave discharge and in-situ chemical reaction. A fourth, sublimation of a chosen compound whose vapor contains the desired species, might also be included. The free radicals or transient species we have succeeded in generating and studying thus far, and their methods of production, are listed in Table I.

Table I

Transient Species Studied by PIMS, and Their Methods of Preparation

A. Sublimation, vaporization (I, Te)

B. Pyrolysis (NF2, PF2, BH3, C ^ , CH3, PH2)

C. Microwave discharge (H, 0, N, F, Cl, Br, P2, SO)

D. Chemical abstraction reactions involving H and F atoms (S, Se, P, As, OH, SH, SeH, PH, PH2, AsH, AsH2, NH2, SiH3, SiH2 (3Bj and 1A1), SiH, C ^ , C2ri5, COOH, B2xi5, B2H )

A quartz pyrolysis source, such as the one depicted in Figure 1, can conveniently achieve temperatures up to ~ 1000° C. Often an organometallic species can serve as the species to be pyrolyzed. Thus, dimethyl mercury and divinyl mercury are convenient sources for CH3 and C2H3. We have also utilized N2F4, P2F4 and benzyl phosphine as sources of PF2, NF2 and PH2, respectively. The free radicals thus generated are usually thermalized, albeit at a rather high temperature. Obviously, the generality of this methods is limited by the available precursors. Lossing,2 in his extensive studies of free radicals by electron impact methods, has explored a wide range of species which pyrolyze to form free radicals.

Since free radicals are typically unstable thermochemically and kinetically, their production often requires a disruptive, non-equilibrium process. Electrical discharges and flames are convenient sources that satisfy this requirement. Various ac and dc electrode discharges and rf or microwave electrodeless discharges have been used in the past. In our experience, a microwave discharge in an Evenson cavity is as effective as the other methods, and the electrical noise is more confined to the region of

6

Figure 1. Pyrolysis source for free radicals.

7

excitation. Due to the mechanical constraints, the microwave discharge is situated about 30 cm from the photoionization zone in our apparatus, although it could conceivable be closer. If the free radicals generated in the discharge were to effuse to the region of ionization, the loss by the inverse square would be unacceptable. In our design, the radicals are directed through an inner glass or pyrex tube toward an effusion orifice that is located about 1 cm above the ionization chamber. The bulk of the discharged gas continues past this orifice into an annular region, in a fast flow system, and is either pumped away or trapped by a cryopump. (See Figure 2).

The inner glass tube improves the solid angle subtended between effusing free radicals and the entrance to the ionization chamber enormously, but introduces a problem of its own. The free radicals undergo numerous collisions on the surface of this tube, and also some with other gaseous species. The surface collisions are particularly destructive of transient species. Some type of coating is necessary to make the surface relatively inert to the reactive species. Various types of coating have been suggested in the literature, including Dryfilm (a chlorinated, methylated silane), Teflon, metaphosphoric acid and haloform wax. Although we have not tested the various coatings exhaustively, we have found that metaphosphoric acid is useful for, Cl and Br atoms, P2 and SO molecules. The haloform wax is more effective for F atoms. Probably less that 50% of the radicals formed in the discharge region are lost due to wall collisions when a "good" coating is deposited. The metaphosphoric acid coating is notoriously irreproducible, in our experience. It will be noted that only two molecular species are included in group C in Table I. Both are relatively stable. Other, more reactive molecular species which have been generated were lost, either by wall or gas phase reactions.

The atomic species generated by direct microwave discharge can be utilized for subsequent chemical reactions, in a small reaction chamber. (See Figure 2). The reaction rates here must be very fast, in order to yield measurable signals. The advantage is that these reactions are free of electrical noise, and few collisions ensue before the reaction products enter the ionization region. We have also benefitted from two pleasant, unexpected surprises.

1. The products of successive reactions can be observed. Thus, the reaction of H atoms with PH3 proceeds by successive hydrogen abstractions to generate PH2, PH and even P. All three species are created with enough abundance to be investigated by PIMS. Similarly, the reaction of F atoms with SiH4 yields SiH3, SiH2 (in two electronic states) and SiH.

2. The reaction products appear to be thermalized, to a temperature of -300 K. In most cases, this conclusion is based on the absence of "hot bands". In the F atom studies, we observe the low lying F(2P1/2) excited state, as well as the ground (2P3/2) state, with a relative abundance corresponding to a Boltzmann distribution temperature of -300 K.

Most of the free radicals described in the subsequent section have been generated by the chemical reaction method. Even with the relatively long distance between

8

Figure 2. Microwave discharge and chemical reaction source for free radicals.

9

microwave discharge and ionization chamber, large backgrounds of charged species (electrons and ions) are sometimes encountered. To minimize these contributions, we have incorporated an inner mesh (located at the system dividing the atomic source from the chemical reaction chamber) and an outer mesh, surrounding the cylindrically symmetric free radical tube. In practice, the potentials on these meshes are empirically adjusted to minimize the extraneous charged species, presumably emanating directly from the microwave discharge region.

HI. SURVEY OF EXPERIMENTAL RESULTS

A. The Pnicogen and Chalcogen Hydrides (Group V and VI Hydrides)

Goddard and Harding3 made a semi-empirical estimate of the successive bond energies in these systems. They assumed each bond pair to be covalent. The naive expectation might be that the three bond energies in the pnicogen hydrides (and the two in the chalcogen hydrides) would be comparable. One might anticipate a slight decrease in bond energies upon successive hydrogenation, due to bond-bond repulsions. Goddard and Harding show that this is not the case. The stabilization due to the p-p' exchange interactions is greatest in Pn-H, least in PnH3. Thus, D0(Pn-H) < D0(HPn-H) < D0(H2Pn-H), the difference in successive bond energies being equal to 1/2 the p-p' exchange integral. With two prescribed parameters (the experimental atomization energy, and the magnitude of the p-p' exchange integral, derived from atomic spectra), they were able to predict bond energies "in most cases ... probably more accurate than the current (1978) experimental data." In recent years, we have generated and studied the free radicals NH2, PH2, PH, AsH2, AsH, SH and SeH. The adiabatic ionization potentials obtained from these species, together with appearance potentials from the stable pnicogen trihydrides and chalcogen dihydrides, enable us to deduce the corresponding bond energies, by combining e.g.

PnH3 + hv -> PnH2+ + H + e

PnH2 + hv -> PnH2+ + e.

These data, when supplemented by the heats of formation of the stable neutral species,4

the heat of formation of OH4 and the ionization potential of NH,5 enable us to construct the tables of bond energies of pnicogen hydrides (Table II) and chalcogen hydrides (Table III). Also shown are the predicted bond energies of Goddard and Harding,3 and the results of modern ab initio calculations.63"*1 We can now see that the qualitative conclusions of Goddard and Harding are correct. There is an increase in the bond energies of these hydrides upon successive hydrogenation. There is even fairly good agreement quantitatively. The poorest agreement occurs for the first row N-Hn and 0-Hn sequence. One can show from the differences in electronegativity (N-H, O-H) that these bonds have more ionic character than the corresponding second and third row bonds, and therefore the assumption of pure covalency is least valid for the

HI. SURVEY OF EXPERIMENTAL RESULTS

A. The Pnicogen and Chalcogen Hydrides (Group V and VI Hydrides)

PnH3 + hv -> PnH2+ + H + e

PnH2 + hv -> PnH2+ + e.

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Table II

Comparison of Experimental Pn-H,, Bond Energies (Pn =N, P, As) with Semi-empirical Calculations and Ab Initio Calculations (in kcal/mol at 0 K)

ab initio" Semi-empirical PIMS°

A. N-HJJ Compounds D0(H2N-H) 105.8 101.39 106.710.3 D0(HN-H) 91.2 92.23 91.0+0.5 D0(N-H) 77.2 83.07 79.0±0.4

B. P-I^ Compounds D0(H2P-H) 82.4 81.11 82.5±0.5 D0(HP-H) 75.4 75.69 74.5±0.5 D0(P-H) 67.6 70.27 70.1±0.5

C. As-P^ Compounds D0(H2As-H) ~73.0(74.6)d 74.89 74.910.2 D0(HAs-H) -9.2(69. l)d 69.75 66.510.2 D0(As-H) ~62.3(62.4)d 64.60 64.610.7

a N-H,, from ref. 6b; P-H,, from ref. 6a; As-H,, from ref. 6c. b Ref. 3. c N-H„ from S. T. Gibson, J. P. Greene and J. Berkowitz, J. Chem. Phys. 83 (1985) 4319; P-H,, from J.

Berkowitz, L. A. Curtiss, S. T. Gibson, J. P. Greene, G. L. Hillhouse and J. A. Pople, J. Chem. Phys. 84 (1986) 375, slightly modified in J. Berkowitz and H. Cho, J. Chem. Phys. 90 (1989) 1; As-H,, from J. Berkowitz, J. Chem. Phys. 89 (1988) 7065.

d Quantities in parentheses are from ref. 9a.

first row compounds. The ab initio calculations agree with the experimental results to 12 kcal/mol in most cases. When the photoion yield curves are analyzed, some structural information may be deduced. Figure 3 shows the photoion yield curve of NH2

+ from NH2, while Figure 4 is the analogous curve of PH2+ from PH2. The ground

state of neutral NH2 has a bond angle of 103.2°, while that of PH2 is 91° 42'7. The calculated bond angles8 of NH " are 150.9° ^B^ and 110.0° ^A^; the corresponding PH2

+ bond angles8 are 121.4° $B{) and 94.4° ^A^. According to the Franck-Condon principle, transitions in which the change in geometry is slight should be sharp, while large changes should lead to a vibrational progression in the excited normal mode(s), resulting in a broad structure. For both NH2 and PH2, the sharp structure should correspond to formation of lAv and the broad structure to 3BX (assuming that the bond distances don't change substantially, which can be shown to be the case). In Figure 3, we can see a sharp series of autoionizing peaks, corresponding to the excitation of members of a Rydberg series. Each of these members is a Rydberg state with little

11

t I 't I I I

12

Table m

Comparison of Experimental Ch-Hn Bond Energies (Ch = O, S, Se)with Semi-empirical Calculations and Ab Initio Calculations (in kcal/mol at 0 K)

ab initioa Semi-empiricalb Expt.c

A. O-K^ Compounds D0(HO-H) D0(O-H)

116.8 100.5

115.32 104.04

117.91±0.29 101.44±0.29

B. S-H^ Compounds D0(HS-H) D0(S-H)

89.8 81.9

90.03 83.56

90.6310.69 82.5410.70

C. Se-Hp Compounds D0(HSe-H) D0(Se-H)

78.6 72.7

77.75 71.58

78.8910.18 74.2710.23

a O-Hn from ref. 6b; S-I^ from ref. 6a; Se-J^ from ref. 6c. b Ref. 3. c O-HJJ is inferred from ref. 13; S-F^ from unpublished data obtained in our laboratory; Se-F^ from S.

T. Gibson, J. P. Greene and J. Berkowitz, J. Chem. Phys. 85 (1986) 4815.

Figure 4. Photoion yield curve of PH2+ from PH2.

change in structure from the ground state, and hence the limit of this Rydberg series is a state of NH2

+ resembling the ground state. We conclude that this limit corresponds to the lAx state of NH2

+. The gradually rising portion beneath the autoionizing structure is characteristic of a large geometry change, and hence the ground state of NH2

+ is 3B1. For PH2+ (PH2) (Figure 4), the structure is quite different. The region of

13

adiabatic onset is relatively abrupt. Between -1259 and -1180 A, the intensity remains roughly the same, although there is weak autoionizing structure superposed. Below -1180 A, the intensity increases monotonically to H 1040 A. We interpret the abrupt increase between 1265 - 1259 A to represent the transition to the ionic state most closely resembling the neutral ground state, i.e. lAx of PH2

+ . The gradually increasing portion below H 1180 A involves the opening of channels to the geometrically different 3 B 1 state. Note that we do not identify the onset of the gradually increasing portion with the adiabatic I.P. of PH2

+, 3BV A channel in this context could be a Rydberg member of a series converging to 3 B 1 ? which is energetically capable of autoionizing. We conclude from this analysis that the ground state of NH2

+ is 3BX (as in CH2), while that of PH2+ is lAr (as in SiH2, to be discussed

later). The ordering in AsH2+ and GeH2 remains the same as in the second row.9

B. The Nitrogen and Phosphorus Fluorides

The behavior of the bond energies in the NFn and PFn compounds provides an interesting contrast to that of the corresponding hydrides. In the latter, we have seen that bond-bond repulsions and differences in electronegativity were minor effects compared to the p-p' exchange integral. Here, we shall find that they are the dominant effects.

The NF 2 and PF 2 radicals were prepared by pyrolysis of N 2 F 4 and P 2 F 4 , respectively. Their adiabatic ionization potentials were determined by PIMS.10 The adiabatic ionization potentials of NF11 and PF1 2 were taken from PES measurements. The heats of formation of the corresponding ions were obtained from PIMS appearance potentials.10 The resulting bond energies are given in Table IV.

Table IV

Bond Energies (at 0 K) of N-Fn and P-Fn compounds* (kcal/mol)

A. N-Fn Compounds D0(F2N-F) 57.0±0.2 D0(FN-F) 65.7±0.5 D0(N-F) 75.4+0.5

B. P-Fn Compounds D0(F2P-F) 131.710.5 D0(FP-F) <125.6±0.6 D0(P-F) >100.6

aRef. 10.

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The comparison of the NFn bond energies with those of PFn, and with the previously discussed hydrides, is revealing. The NFn bond energies are much weaker than the PFn energies, and even lower than the NH^ bond strengths. Furthermore, the N-F bond energies diminish with each additional N-F bond, just the reverse of the N-H, P-H and P-F bonding behavior. Finally, the P-F bond energies are much larger than the P-H bond strengths. These patterns can be explained qualitatively in the following manner. Using the electronegativity criterion, the N-F bonds have ~22% ionic character, the P-F bonds H 60% ionic character. Hence, the N-F bonds are mostly covalent, as are the N-H bonds. In fact, Do(NH)=79.0±0.4 kcal/mol is not very different from Do(NF)=75.4±0.5 kcal/mol. The significant difference between N-H and N-F bonding occurs with addition of H or F atoms. The step-wise diminution of N-F bond energies is very likely due to repulsion between the charge clouds surrounding the F atoms in the relatively short N-F bonds. This repulsion should be significantly diminished with the longer P-F bonds, and outweighed by the strong ionic contribution to the energy of these bonds. In fact, since the ionization potentials decline from P to PF and PF2, the ionicity increases, as do the corresponding bond energies.

C. The Group IV Hydrides

1. The CHn, SiHn and GeHn Sequence

Prior to our studies of these systems, the heats of formation and the molecular structures of the C-U^ species were rather well established, as judged by the agreement among some standard compilations,4'7,13 and the error limits. The successive bond energies and the ground state molecular structures are summarized in Table V. For the corresponding Si-Hn species, significant controversy existed, particularly with regard to the second and third bond energies. Much less was known about the Ge-F^ species. Hence, we have concentrated our efforts on the silicon-hydrogen and germanium-hydrogen systems.

a. The SiHn System.

As mentioned earlier, the reaction of F atoms with SiF^ generated SiH3, SiH2 in two states, and SiH. The photoion yield curve of SiH3 near threshold14 consists of a series of rounded steps. This is an indicator of direct ionization. The derivative of such a curve should be similar to a conventional photoelectron spectrum. It is interpreted as a vibrational progression in the inversion mode; thus, SiH3

+ (ground state) is much less pyramidal than SiH3, and may in fact be planar. The intensity of the vibrational progression wanes as one approaches the adiabatic ionization potential. If the intensity of the 0-0 band is 1-2% of the largest vibrational band, it becomes difficult to distinguish this onset from a vibrational hot band at 300K. We noted14 "a

C. The Group IV Hydrides

1. The CHn, SiHn and GeHn Sequence

a. The SiHn System.

15

Table V

Successive Bond Energies in the C-Hn and Si-Y^ Systems (kcal/mol at 0 K), and the Ground State Molecular Structure, Taken from Standard Compilations

Glushko N B S a JANAFb etal . c Structure of Parent0

D0(H3C-H) 103.2+0.3 103.2+0.2 103.310.2 Td, r(C-H)=l.0939810.00001 A

D0(H2C-H) 109 108.2±1.0 109.1±1.0 D3h, r(C-H)=1.079±0.005A

DQ(HC-H) 100.6 100.6±4.3 100.4±1.0 (100.0±0.7)d

X3B1,C2v, $=136+8° r(C-H)=1.078±0.005A

D0(CH) 80.0 80.4±4.2 79.6±0.3 X2n,re=1.1199A

D0(H3Si-H) 87.7 - 92.413.6 Td,re=1.455±0.00lA (re=1.4806±0.00lA)b

D0(H2Si-H) 73.5±2.5 - 59.2±5.1 C3v,r(Si-H)=1.49±0.03A *=112°+3°

D0(HSi-H) 72.2±2.8 - 80.3±4.1 X ^ . - ^ ^ ' + S ' r(Si-H)=1.5163±0.005A

D0(SiH) 68.0±2.8 68.6±2.8 70.5±2.7 X2n,re=1.520lA

For D0(H3Ge-H), D0(H2Ge-H), D0(HGe-H) and D0(GeH), no data are available in the three standard compilations cited here.

aRef. 4. ^ef. 13. 'Kef. 7, Volume 2. dR. K. Lengel and R. N. Zare, J. Am. Chem. Soc. 100 (1978) 7495.

distinct rise above the background level at -1527 A = 8.12 eV" but "an even smaller step (about 3-4 times smaller) which begins at -1547 A = 8.01 eV". PES measurements of SiH3,15 also produced by the F+SiH4 reaction, exhibited the expected vibrational progression, masked near the adiabatic onset by the presence of impurity peaks. By judicious subtraction, these authors15 obtained 8.14±0.01 eV for the adiabatic onset. More recently, Johnson et al.16 have observed a Rydberg series in SiH3 using multiphoton ionization, and obtained an extrapolated value of 8.135 to!oo2 eV for the adiabatic ionization potential of SiH3.

16

Figure 5. Photoion yield curve of SiH+ from SiH. The adiabatic onset, corresponding to formation of X1!*, occurs at 1570 A = 7.897 eV. The extrapolated limit of the Rydberg series, corresponding to a3n, occurs at 1214 A s 10.21 eV.

For SiH2, we obtained two distinct onsets, at 8.24^ ± 0.025 eV and <9.15 eV, corresponding to ionization of neutral SiH2 in the 3BX and states, respectively. The difference in these ionization potentials, -0.906 eV = 21.0 kcal/mol, is a direct measure of the energy of excitation of 3BX above X XAX. With a very small flow rate of SiH4 and an excess of fluorine, it was possible to observe SiH+ from SiH. The signal was so weak that only the light peaks in the light source (hydrogen lamp) could be utilized to obtain statistically significant data. Nonetheless, a satisfactory, structured spectrum was obtained (see Figure 5). A clear adiabatic onset at 1570 A = 7.897 eV can be discerned, corresponding to SiH+(X1Z+). At least one Rydberg series could be identified among the autoionizing bands. The extrapolated limit corresponds to 10.21 eV, the ionization threshold for the a3Il state of SiH+.

By combining these adiabatic ionization potentials with appearance potentials obtained in the same study, and with relatively well-established auxiliary thermochemical data, one can deduce the step-wise bond energies in the silicon-hydrogen system. These are summarized in Table VI, and compared with recent ab initio calculations and with Walsh's17 compilation. Comparing Table VI with Table V, we see that the level of uncertainty has been substantially reduced. The NBS compilation4 implies that D0(H2Si-H) * D0(HSi-H). The Russian compilation7 implies that D (H2Si-H) is about 20 kcal/mol less than D0(HSi-H). The results of the PIMS study14 lead to D0(H2Si-H) less than D0(HSi-H) by about 8 kcal/mol.

17

Table VI

Recently Reported Bond Energies (kcal/mol at 0 K) in the SiH,, Sequence

Pople et al.a Hoetal.b Walshc PIMSd

D0(Si-H) 69.8 67.4 69.2 68.7±0.7 D0(HSi-H) 76.2 74.1 82.3 75.6±1.4 D0(H2Si-H) 66.8 71.0 61.5 >67.3 D0(H3Si-H) 91.7 90.1 88.3 <91.1

AHfo atomiz. 304.5 302.6 301.3 302.7 (SiH4)

a Ref. 6a,b. For a slightly corrected value, see also: L. A. Curtiss, J. A. Pople, Chem. Phys. Lett. 144 (1988) 38.

b P. Ho, M. E. Coltrin, J. S. Binkley, C. F. Melius, J. Phys. Chem. 89 (1985) 4647. c Ref. 17. d Ref. 14. Here, D0(H3Si-H) is based on AP(SiH3

+)< 12.086 eV from ref. 14 and the recently determined n>(SiH3)=8.135!§;8[g eV ref. 16 rather than that of ref. 17.

b. The GeHn System

PIMS studies of germanium species in low abundance is exacerbated by the fact that naturally occurring germanium is distributed among five isotopes. This not only reduces the intensity in any given mass, but poses the problem of distinguishing between e.g. 72GeH4

+ 73GeH3+ and 74GeH2

+ A judicious choice of masses can be made to minimize the confusion and maximize the sensitivity. Nevertheless, the results could be substantially improved with a more nearly monoisotopic source of germanium.

In our studies,18 the reaction of F atoms with GeH4 generated GeH3, and a very small abundance of GeH2. The most reliable thresholds from these studies were the adiabatic ionization potential of GeH3 (< 7.948 ± 0.005 eV) and the appearance potential of GeH2

+ from GeH4 (10.772 ± 0.009 eV). Upper limits were established on other quantities, e.g. GeH3

+ from GeH4 (< 11.657 ± 0.01 eV), GeH++H2 from GeH3 (£ 9.02 ± 0.03 eV) and GeH2

+ from GeH2 (< 9.25 eV). Consequently, a complete description of the step-wise bond energies in the Ge-Hjj system could not be deduced from PIMS studies alone. However, the combination of the measured values with auxiliary thermochemical data, some ab initio calculations and plausibility arguments (presented in the original paper) enable us to construct a set of most probable bond energies, summarized in Table VII.

18

Table VII

Successive Bond Energies in the Ge-Hn System (kcal/mol, 0 K)

PIMS study*

Setser and co-workersb

Walsh and co-workersc

Klynning and Lindgrend

Calc.

D0(H3Ge-H) <85.5 (82+2)

78.0±1.0 82.7±2.4 - 84.8e, 85.2*

D0(H2Ge-H) >56.4 (59)

- 56.9±2.9 - 58.0e

56.9g

D0(HGe-H) <68.9 (66)

- - - 70.2e[68.5] 69.2«

D0(Ge-H) >53.7 (63)

- - <76.1 63.2±lf

64.9e[63.8]

a Ref. 18. Quantities in parentheses are more probable values. b B. S. Agrawalla and D. W. Setser, J. Chem. Phys. 86 (1987) 5421; J. Phys. Chem. 90 (1986) 3450;

and earlier work from this laboratory. c M. J. Almond, A. M. Doncaster, P. N. Noble and R. Walsh, J. Am. Chem. Soc. 104 (1982) 4717; P.

N. Noble and R. Walsh, Int. J. Chem. Kinetics 15 (1983) 547. d L. Klynning and B. Lindgren, Ark. Phys. 32 (1966) 575. e R. C. Binning and L. A. Curtiss, J. Chem. Phys. 92 (1990) 1860. Quantities in square brackets

include an estimated spin-orbit correction. f K.BalasumbramanianandJ.Li,J.Mol. Spectrosc. 128(1988)413. « K. K. Das and K. Balasubramanian, J. Chem. Phys. 93 (1990) 5883. These values may be De, rather

thanD0.

2. Comparisons ofCHn, SiHn, GeHn

a. Step-wise Bond Energies

The sequential bond energies are best compared by normalizing each bond energy to the average bond energy for that system. When this is done, one obtains the patterns shown in Figure 6. The sequential bond energies in the Sil^ (second row) and GeHn (third row) systems track one another quite closely, whereas the CHn (first row) pattern is markedly different. The strongest bond in the CF^ system is the weakest one in the SiH^ and GeHn systems. Clearly, some dramatic change occurs between the first and second row. This can be rationalized by arguments introduced by Goddard and

2. Comparisons ofCHn, SiHn, GeHn

a. Step-wise Bond Energies

19

Harding.3 In the first row elements, the 2s and 2p orbitals are more correlated, leading to favorable "lobe orbitals", or hybridization.

Figure 6. The ratio of a particular bond energy to the average bond energy, plotted against the sequential bonds M-H, HM-H, H2M-H and H3M-H, where M = C(o), Si([]) and Ge(A).

This favors the formation of CH2 in a less bent state (H-C-H angle = 136±8°). The relatively short bond lengths in the first row hydrides also increase the repulsion between ligands, again favoring a larger angle. Both of these factors favor the formation of the less bent 3BX over the more bent XAV For SiH2 and GeH2, the outermost orbital available for bonding is 3p and 4p, respectively. There is less s-p correlation and the bond lengths are longer; both of these factors favor a smaller bond angle near 90°, and hence energetically favor lAx over 3B1 . From this perspective, the formation of SiH3 by addition of a hydrogen atom to SiH2 requires energy to unpair the spins of the lAx state. The resultant bond energy is weaker due to this effect, which also exists for GeH2+H. However, an unpaired orbital is immediately available in CH2

( Bx). The formation of CH3 radical, by minimizing ligand repulsion in the planar configuration then corresponds to the strongest bond in the C-U^ system.

The analogous behavior of Si l^ and GcH^ in Figure 6 suggests that Snl^ would conform to the same pattern. The heats of formation of the Sn-U^ free radicals are not known, but that of SnH4 is known. From this datum, and the pattern of S i ^ and GeF^ in Figure 6, one can predict18 the step-wise bond energies, and hence the unknown heats of formation of SnH, SnH2 and SnH3.

20

b. Jahn-Teller Distortion in CH4+, SiH4

+, GeH4+

Methane, silane and germane are not free radicals, but their ionization behavior is relevant to this discussion, and illustrates another difference between PES and PIMS. For each of these molecules, the uppermost occupied orbital is triply degenerate. Removal of one electron from this filled orbital (t1) gives rise to a triply degenerate state of the cation, subject to Jahn-Teller distortion. The case of methane had been extensively studied prior to our work. The ground state of CH4

+ has been shown to have C2v symmetry.19 The distortion from tetrahedral symmetry is relatively mild. Both PES and PIMS yield the same adiabatic ionization potential (Figure 7).

The distortion in SiH4+ and GeH4

+ is much more severe. It manifests itself experimentally in several ways. The difference in structure between the neutral species and the cation expresses itself as very poor Franck-Condon factors near threshold. In PES, the true ionization is difficult to distinguish from background, but the mass selection implicit in PIMS enables one to detect this weak ionization. Figures 8 and 9 display PES and PIMS studies, placed on the same energy scale, for SiH4 and GeH4, respectively. The weak SiH4

+ and GeH4+ signals extend considerably below the

apparent adiabatic onsets in PES. The ab initio structures calculated for SiH4+ and

GeH4+ (Figures 10a and 10b) are consistent with a large departure from tetrahedral

geometry, poor Franck-Condon factors, and a much lower adiabatic ionization potential than determined by PES (~ 0.6 eV lower for SiH4, H 0.8 eV lower for GeH4). These structures can be roughly described as H2 molecules weakly adhering to SiH2

+ and GeH2+ In fact, the dominant ions in the mass spectra of these systems are

SiH2+ and GeH2

+ The parent ions begin to decompose at about the energies where the Franck-Condon factors become more significant.

3. C2H3 and C2H5: C-H Bond Energies ofEthylene and Ethane; Some Observations on the C-H Bond Energy in Acetylene

The C-H bond energies of ethane, ethylene and acetylene are believed to be prototypical of C-H bond energies in alkanes, alkenes and alkynes, respectively. Recently, there has been controversy regarding the magnitudes of these bond energies. One way of arriving at these quantities is by the combination of appearance potentials of radical cations and the adiabatic ionization potentials of the radicals, as discussed earlier.

Shiromaru et al.20 reported on a variation of this procedure for ethylene. In the photodissociative ionization of ethylene, two of the possible processes are:

C2H4 + hv->C2H3+ + H + e (a)

->C2H3 + H+ + e (b)

3. C2H3 and C2H5: C-H Bond Energies ofEthylene and Ethane; Some Observations on the C-H Bond Energy in Acetylene

C2H4 + hv -> CJA? + H + e

->C2H3 + H+ + e

(a)

(b)

21

Figure 7. a. Hel PES of CH4 in the threshold region, from J. N. Rabalais, Phys. Scripta 3 (1971) 13; b. PIMS of CH4 in the threshold region, with the gas sample at 78°K and 300°K, from W. A. Chupka and J. Beikowitz, J. Chem. Phys. 54 (1971) 4256.

22

Figure 8. a. Hel PES of SiH4, displaying an adiabatic threshold at 11.6 eV. From A. W. Potts and W. C Price, Proc. Roy. Soc. (London) A326 (1972) 165; b. PIMS of SiH4 (from ref. 14), displaying an adiabatic threshold at 11.0 eV.

23

Figure 9. a. Hel PES of GeH4, displaying an adiabatic threshold at 11.3 eV. From A. W. Potts and W. C. Price, Proc. Roy. Soc. (London) A326 (1972) 165; b. PIMS of GeH4 (from ref. 18), displaying an adiabatic threshold at £10.53 eV.

Figure 10. a. Ab initio (HF/6-31G*) calculated structure of SiH4+. From ref. 8; b. Ab initio

(MP2/DZP) calculated structure of GeH4+. From T. Kudo and S. Nagase, Chem. Phys.

Lett. 148 (1988) 73.

24

The threshold for process (a) has been measured by several groups; perhaps the best value is 13.22 ± 0.02 eV.21 To arrive at a bond energy, utilizing this appearance potential, one would have to determine the adiabatic ionization potential of vinyl radical. Shiromaru et al. chose to determine the threshold for process (b), since the true thermochemical onset for this process could immediately be related to a bond energy, utilizing the very well known ionization potential of atomic hydrogen. Their measured threshold for process (b) was 18.66 ± 0.05 eV, which leads to a bond energy of 5.06 ± 0.05 eV = 116.7 + 1.2 kcal/mol. The appearance potential of process (b) is more than 5 eV higher than for process (a). It is often observed in dissociative ionization studies that higher energy processes are retarded, a phenomenon sometimes referred to as a kinetic shift. It is a consequence of competing rates. In the language of statistical theories, e.g. RRKM or QET, the phase space for the lower energy process increases with excess available energy, and dominates the higher energy process at its threshold. For the higher energy threshold to be measurable, the available energy must exceed this latter threshold. Thus, the appearance potential for process (b) determined by Shiromaru et al. might be too high.

To test this hypothesis, we22 embarked on a project to measure the adiabatic ionization potential of the vinyl radical, C2H3. This value could then be combined with the lower threshold of process (a) to arrive at the C-H bond energy. Vinyl radical was prepared in two ways - the hydrogen abstraction reaction of F atoms with ethylene, and the pyrolysis of divinyl mercury. The results were similar, although there was some evidence that the pyrolysis reaction produced radicals characterized by a higher temperature. Figure 11 (a,b) displays the photoion yield curve obtained from vinyl radicals which were generated by hydrogen abstraction. The approach to threshold is gradual, indicative of poor Franck-Condon factors, and hence a geometrical change between neutral C2H3 and its cation. At shorter wavelengths, prominent structure attributable to autoionization is observed. The interpretation is considerably simplified with the aid of ab initio calculations. The structure of the vinyl radical predicted by these calculations23 is rather similar to that of ethylene, with a missing hydrogen atom. For the cation, a non-classical hydrogen bridged structure is calculated to be most stable; the classical structure, also having C2v symmetry but with two hydrogen atoms bonded to one carbon, is not a minimum on this cation's potential surface, but rather a saddle point. Hence, the gradual approach to threshold in the photoion yield curve can be attributed to poor Franck-Condon factors connecting C2H3 with C2H3

+. By contrast, the first excited state of C2H3

+, a triplet, is calculated to have a structure rather similar to that of neutral C ^ , with a somewhat larger C-C distance. The sharp structure at shorter wavelength is identified as an autoionizing Rydberg series, whose convergence limit is the excited triplet state of C2H3

+. From Figure 1 lb, the adiabatic ionization potential obtained for C2H3 is < 8.59 ±

0.03 eV = 1443 ± 5 A. When combined with the appearance potential of process (a), this yields a lower limit to the C-H bond energy of 4.63 ± 0.04 eV s 106.8 ± 0.8 kcal/mol. Other recent experimental values utilizing different methods,24 as well as ab initio calculations,23 now point to a value of 107-109 kcal/mol, so our lower limit is close to the true value. Hence, the high value of Shiromaru et al.20 may be attributable to the aforementioned kinetic shift.

25

Figure 11. Photoion yield curve of C2H3+ (C2H3), where C2H3 is produced by the F + C2H4

reaction, a. Overview, revealing autoionization structure of Rydberg states converging to a triplet excited state of C2H3

+; b. An expanded view of the threshold region. The lowest distinct signal above background occurs at 1443 ± 5 A a 8.59 ± 0.03 eV.

26

The photoionization behavior of ethyl radical is similar to that of the vinyl radical.25 The neutral species has a structure rather similar to ethane, with one hydrogen missing. The cation has been calculated to have a non-classical, hydrogen bridged structure. We25 have generated C2H5 by the reaction

F + C ^ - ^ C ^ + H F .

The photoion yield curve descends gradually to threshold, commensurate with the change in geometry upon ionization. The adiabatic ionization potential obtained is 8.117 ± 0.008 eV. The heat of formation of C2H5

+ can be obtained from appearance potential measurements on several species, including ethane and the ethyl halides. This heat of formation has undergone some revision in the last decade. Combining I.P.(C2H5) with AHfo(C2H5

+) yields 31.6 ± 0.5 kcal/mol for the heat of formation of ethyl radical. This quantity, together with well established values for AHfo(C2H6) = -16.4 kcal/mol (NBS) and AHfo(H) = 51.634 kcal/mol, yields 99.6 ± 0.6 kcal/mol for the C-H bond energy in ethane. Other recent experiments26 arrive at a similar value.

At this time, the C-H bond energy in acetylene is still controversial. Recent experimental results for D0(HC2-H) cluster around 132 kcal/mol20'27 or 126 kcal/mol.28,29 Several approaches to this problem involving photoionization exist, in principle. The basic approach is to determine the appearance potential of C2H+ from C2H2, and then measure the adiabatic ionization potential of C2H. Several studies of the appearance potential of C2H+(C2H2) have been reported. The two most thorough investigations, involving a) temperature dependence studies, and b) photoelectron-photoion coincidence measurements, yield 17.36 ± 0.01 eV30 and < 17.33 ± 0.05 eV31

at 0 K. There remains some question regarding the thermochemical significance of this threshold, due to possible symmetry restrictions to the dissociation process. Only one direct determination of the ionization potential of C2H has been reported.32 This measurement was performed by a relatively crude electron impact method on C2H in equilibrium with graphite and hydrogen at high temperatures, and yielded LP^C^H) = 11.6 ± 0.5 eV. Two indirect photoionization measurements, based on differences in appearance energies, produced values of 11.9833 and H 11.51 eV.34 We have made several attempts to generate C2H for photoionization studies, including the F + C2H2 reaction, the Li + C2HBr reaction, and laser photodissociation methods, thus far without success. Recent ab initio calculations of I.P.(C2H) hover around 11.6 eV.35

These values tend to favor D0(HC2-H) = 5.73 eV = 132 kcal/mol. Shiromaru et al20 measured the threshold for the process

C2H2 + hv->H+ + C2H + e

and obtained 19.35 ± 0.05 eV. This value, together with I.P.(H), yields D0(HC2-H) = 5.75 ± 0.05 eV = 132.6 ± 1.2 kcal/mol, in essential agreement with the thermochemical combination involving the C2H+ threshold. However, one must exercise caution with the H+ threshold determination, since it occurs at an energy - 2 eV higher than the C2H+ threshold. It will be recalled that in the C2H4 measurements, where the H+

F + C ^ - ^ C ^ + H F .

C2H2 + hv->H+ + C2H + e

27

threshold was found to be retarded by ~ 0.4 eV, the gap between thresholds was > 5 eV.

Two additional photoionization schemes are possible both based on ion-pair formation. Ono and Ng36 observed a weak, low energy tail for the C2H+ threshold. Shiromaru et al.20 suggested that this tail could be due to formation of C H"1" + H", but experimental proof of this process is lacking. We37 have attempted to observe the formation of C2H~ + H+, reasoning that the isoelectronic process

HCN + hv->H+ + CN"

is readily observed.38 Since the electron affinity of C2H is now well established,24

such an observation could lead to an alternative measurement of D0(HC2-H). This photoion-pair process turned out to be very weak, about 5000 times less intense than the primary photoionization in this energy range, and hence aion p a i r * 6 x 10"21 cm2. Nevertheless, it was possible to observe a relatively abrupt onset at 759 ± 1 A = 16.335 ± 0.02^ eV. Upon introducing I.P.(H) = 13.598 eV and E.A. (C2H) = 2.969 ± 0.010 eV,24 we infer D0(HCC-H) < 5.706 ± 0.023 eV = 131.6 ± 0.5 kcal/mol. At higher sample pressures, a C2H" signal is observed at longer wavelengths, which can be attributed to the second-order process

C2H2 + hv -> CjH^ + e (a)

e + C ^ - ^ H + H . (b)

The kinetic energy of the photoelectron e can be smoothly tuned by changing the photon energy from 11.40 eV (the ionization potential of C2H2) to higher values. In this way, a threshold is observed at 878 ± 2 A = 14.12 ± 0.03 eV, which corresponds to a photoelectron energy of 2.72 ± 0.03 eV. If we add the electron affinity of C2H (2.969 ± 0.010 eV)24 to our observed threshold for reaction (b), we deduce D0(HCC-H) = 5.69 ± 0.03 eV = 131.2 ± 0.7 kcal/mol, in very good agreement with the "higher" value for this quantity.

D. The Boron Hydrides

The simplest stable gaseous borohydride is diborane, B2H6, with its characteristic double hydrogen bridged structure. Although the trihalides of boron are stable in their monomeric forms (BF3, BC 3, etc.), BH3 is not. One of our research goals was to determine the photoionization behavior of BH3. As part of this project, we hoped to determine the dimerization energy of BH3, by combining measurements of I.P.(BH3) with the appearance potential of BH3

+(B2H6). In preliminary studies with B2H6,39 a threshold for BH3

+ was observed, but several other fragments appeared at lower energy, making the thermochemical significance of that threshold dubious. The most abundant fragment was B2H5

+, about 103 times larger than the parent ion. These observations provided the motivation for

HCN + hv->H+ + CN"

C2H2 + hv -> CjH^ + e

e + C ^ - ^ H + H .

(a)

(b)

D. The Boron Hydrides

28

ab initio calculations by Curtiss and Pople,40 which indicated that B2H5+ had an

unusual triple hydrogen bridge in its ground state. In order to cast some light on this prediction, we needed to prepare the B2H5 radical. Our studies of borohydride free radicals thus branched into two directions - the preparation and photoionization of BH3,39 and the study of some species containing two boron atoms, which were limited in our case to B2H5 and B2H4.41'42 Prior to the free radical studies, measurements were performed to determine the appearance potentials of the various fragments from B2H6. The values obtained, after isotopic unfolding, are summarized in Table VIII.

Table VIH

Ionization Potential and Appearance Potential* of Ionic Species from Photoionization of B2H6.

Process Threshold Potential, eV

B2H6 + hv -> B ^ * + e <11.37±0.05 ->B2H5

+ + H + e <11.40±0.05 -» B2H4

+ + H2 + e <11.415±0.04 -> B2H3

+ + H + H2 + e <14.15 -> B2H2

+ + 2H2 + e <13.25 -> BH3

+ + BH3 + e <14.31-14.39 ->BH2

+ + BH3 + H + e <14.84o±0.017

a Observed appearance potentials have been reduced to equivalent 0 K thresholds by adding the internal thermal energy of B2B6 at 298 K, 0.0618 eV.

7. BH3

BH3 was generated by the pyrolysis of B2H6 (1:1 dilution with He) at a temperature of H 530°C. The photoion yield curves of masses 11,12,13 and 14 were obtained. After isotopic unfolding, the photoion yield curves of BH3

+, BH2+ (+H) and

BH+ (+H2) were extracted, and are reproduced in Figure 12.

7. BH3

29

Figure 12. Photoion yield curves of BH3+, BH2

+ and BH+ from BH3. Isotopic corrections have been made, so that this figure represents the true relative abundances of these ions.

The photoion yield curve of BH3+ (BH3) increases gradually from a threshold of

1031 ± 2 A (12.026 ± 0.024 eV) to a maximum at ~ 974 A (12.73 eV), and then declines slowly. The implication of the gradual increase is that a geometry change accompanies the photoionization transition. Neutral BH3 has been calculated43 to be planar (D3h symmetry), with a B-H bond length of 1.191 A. BH3

+ is doubly degenerate in its ground state, and subject to Jahn-Teller distortion. The states of lower (C2v) symmetry formed are 2B2 and 2A1? with calculated44 I.P.'s of 12.11 and 12.27 eV, respectively. These states have bond angles and bond lengths significantly different from that of BH3, implying a broad Franck-Condon range, and hence a slowly increasing photoion yield curve. Both the experimental curve and the interpretation are reminiscent of the photoionization behavior of CH4. The BH3

+ ion may be the smallest molecule to exhibit Jahn-Teller distortion.44

The difference between the appearance potential of BH3+ from B2H6 and the

ionization potential of BH3 would imply a dimerization energy of 2.28-2.38 eV = 52.7-54.7 kcal/mol. A rather lengthy argument,39 too detailed to reproduce here, leads to the conclusion that the appearance potential of BH3

+ from B2H6 experiences a kinetic shift, and that the best estimate for AHQ (dimerization) is 34.3-39.1 kcal/mol.

30

More detailed photoion yield curves than those shown in Figure 12 have been obtained39 for BH2

+ and BH+. The appearance potential of BH+ from BH3 (< 13.372 ± 0.015 eV) can be combined with experimental values of D0(BH) = 3.426 ± 0.05 eV, LP.(BH) = 9.77 ± 0.05 eV and D0(H2) = 4.47813 eV to obtain the atomization energy of BH3, < 11.506 ± 0.072 eV = 265.3 ± 1.7 kcal/mol. Pople et al.6a have calculated 266.9 ± 2 kcal/mol for this quantity, and at a higher level,43 264.3 kcal/mol. Curtiss and Pople have calculated somewhat different values than the experimental ones we have used, i.e. D0(BH) = 81.5 kcal/mol45 and I.P.(BH) = 9.90 eV 43 With their values, our atomization energy becomes 264.8 ±1.7 kcal/mol. A similar calculation using experimental quantities yields < 8.080 ± 0.052 eV, or < 186.3 ± 1.2 kcal/mol for the process

BH3 -> BH + 2H ,

or using the calculated I.P.(BH), < 7.950 ± 0.052 eV = 183.3 ± 1.2 kcal/mol. This latter value is in better agreement with the calculational result, 181.6 kcal/mol.43

Without an experimental value for I.P.(BH2), it is not possible to further subdivide the incremental bond energies. However, we can relate the sum of I.P.(BH2) and D0(H2B-H) to the observed appearance potential for BH2(BH3) < 12.819 ± 0.020 eV. The sum of calculated values for D0(H2B-H) and I.P.(BH2) is 12.73 eV,43 within 0.09 eV of the experimental observation. Hence a plausible conclusion is that the first and second bond energies, i.e. D0(BH) and D0(HB-H), are both about 80 kcal/mol, while the third is significantly larger, H 105 kcal/mol. More recent calculations yield 82.4 ± 0.2 kcal/mol46 and 81.5 ± 0.5 kcal/mol47 for D0(BH).

2. B2H5

As mentioned earlier, the most stable structure of B2H5+ has been calculated40 to

have a triple hydrogen bridge, with doubly bridged and singly bridged structures lying 20.2 and 41.0 kcal/mol, respectively, above the triply bridged structure. The most stable form of neutral B2H5 has been calculated48,49 to be singly bridged, about 6 kcal/mol more stable than a doubly bridge structure, with a small potential barrier between these minima. No experiments leading to structural information are available for either species. If both ab initio calculations are correct, the drastic structural change accompanying ionization should manifest itself as very poor Frank-Condon factors near threshold.

B2H5 was prepared in situ by the abstraction reaction of F atoms with B2H6. The photoion yield curve for B2H5

+ (B2H5) is displayed in Figure 13. The calculated energies of the triply bridged, doubly bridged and singly bridged cation structures are delineated by arrows. The figure encompasses about four orders of magnitude in relative photoionization cross section. The value of the cross section near threshold is, indeed, very small. In fact, indirect means are necessary to deduce the adiabatic ionization potential.

At about 1282 A (near the peak intensity of B2H5+) the fragment ion B2H3

+

(+H2) begins to appear. More precisely, the 0 K threshold for this fragmentation

BH3 -> BH + 2H,

2. B2H5

31

i

i H 1 + 8

Ji IS

.a <a &

'I

*3

"1

I* afl « 'I <s I o ° 3

-

II |

|

a 1

32

process is 9.695 eV. From the earlier studies on B2H6 (see Table VIII) we had obtained < 14.15 eV for the appearance energy of B2H3

+ (+H2+H) from B2H6. The difference between these quantities, ~ 4.455 eV = 102.7 kcal/mol, is a measure of the B2H5-H bond energy.

Again referring to Table VIII we note that the appearance potential of B2H5+

(+H) from B2H6 is ^ 11.40 ± 0.05 eV. The difference between the latter two quantities, :> 6.945 eV, is the ionization potential of B2H5, ~ 1785 A. Both the bond

Figure 14. Photoion yield curve of B2H4+ from B2H4 (V). Also shown is the m/e=26 to m/e=27

intensity ratio (O), which should mimic the photoion yield curve.

energy and the adiabatic ionization potential are in good agreement with the ab initio calculations,49 which predict D0(B2H5-H) = 100.0 kcal/mol, I.P.(B2H5) = 6.94 eV. The satisfactory agreement between experiment and ab initio calculation for the energies, as well as the shape of the photoion yield curve, provide strong support for the calculated structures of both neutral B2H5 and its cation. The step-like structure in Figure 13 between ~ 1430 - 1600 A has been interpreted41 as evidence for a weak Boltzmann abundance of the doubly-bridged neutral B2H5, forming a doubly-bridged cation upon ionization.

3. B2H4

Although B2H4 had not previously been observed experimentally, it had been the subject of several ab initio calculations. Vincent and Schaefer50 and Mohr and

3. B2H4

33

Lipscomb51 had both concluded that the ground state structure consisted of two perpendicular H-B-H planes (D2d), with a barrier to rotation of about 12 kcal/mol. However, Mohr and Lipscomb showed that inclusion of correlation stabilized a non-planar, puckered doubly hydrogen bridged C^ structure, which was only 1.5 kcal/mol less stable than the D 2 d structure. Curtiss and Pople52 have re-examined these structures at the Gl level of theory, and concluded that they are essentially equal in energy (within 0.1 kcal/mol), but probably separated by a large potential barrier.

For the cation B2H4+ Curtiss and Pople52 found that an analogous doubly

bridged C2v structure was clearly the most stable, with triply bridged and singly bridged structures lying 11.4 and 19.0 kcal/mol higher.

Photoionization offers a means to distinguish between the two "accidentally degenerate" neutral B2H4 structures. If the doubly bridged structure is generated, the photoion yield curve should have a rather sharp onset, since the ground state of the cation has a similar structure. If the D2 d neutral structure is generated, the onset of the photoion yield curve should be much more gradual. We succeeded in producing B2H4

by successive hydrogen abstraction reactions using F atom as the reagent. The photoion yield curve of B2H4

+ (B2H4) is shown in Figure 14. The adiabatic ionization potential is found to be 9.70 ± 0.02 eV, to be compared with a calculated52 value of 9.64 eV. Some step structure is evident in the photoion yield near threshold. It is attributed42 to vibrational excitation in the puckered 4-membered ring, which is more nearly planar in the cation than in the neutral species. Thus, the shape of the photoion yield curve near threshold and the good agreement in ionization potential between experiment and ab initio calculation can be interpreted as evidence that at least a significant fraction of the B2H4 generated has the puckered C2v structure. At higher energies, the fragment B2H2

+ (+H2) begins to appear. Its threshold is found to be < 11 .53 5 ±0 .03eV. 4 2 Earlier, we found that the threshold for formation of B2H2

+

(+2H2) from B2H6 had a threshold < 13.25 eV at 0 K (see Table VIII). The difference between these thresholds corresponds to

^2^6 ~* B2H4 + ^ 2 »

which is endothermic by ~ 1.715 ± 0.03 eV = 39.5 ± 0.7 kcal/mol. Since D0 (H2) = 103.268 kcal/mol, the energy required to remove two hydrogen atoms from B2H6 is 142.8 kcal/mol. From our study of B2H5, we deduced that removing the first hydrogen atom required <: 102.7 kcal/mol. Therefore, the bond energy for the process

B2rlfj —> B2H4 + H

requires only ~ 40.1 kcal/mol. Curtiss and Pople52 have calculated 40.6 kcal/mol for this bond energy. There is some evidence42 from the intensity of the B2H2

+ fragment from B2H4 that the D 2 d structure of B2H4 may also have been generated in our experiment.

^2^6 ~* B2H4 + ^2»

B2rlfj —> B2H4 + H

34

In summary, the studies of these simple borohydrides has provided experimental evidence to support the predictions of ab initio calculations. A wide range of diverse structures is inferred, including a triple hydrogen bridge. Very different structures are accidentally close in energy. The B-H bond energies vary from ~ 40 to ~ 105 kcal/mol, the last B-H bond in both BH3 and BjH^ being the strongest.

£. COOH - an Important Reaction Intermediate

The reaction OH + CO-»H + C02

is the dominant source of C02 in the oxidation of hydrocarbons53 (and hence fossil fuels), is important in the chemistry of the upper atmosphere,54 and the formation of chemical smog.55 The reaction has been modelled56 to proceed through a collision complex COOH*, which can be stabilized to COOH by a third body. No direct experimental evidence of the thermochemical stability of COOH or of its isomeric form, HCOO, is available. Ab initio calculations have provided predictions of the geometrical structure of both isomers.57,58

By contrast, the geometrical structure of COOH+ is now fairly well established spectroscopically.59 Lines attributable to COOH+ have been identified in interstellar gas clouds.60 Recent work in our laboratory61 and elsewhere62 has established its heat of formation. The isomeric HCOO+ appears to be much less stable (by > 100 kcal/mol).63,64 Thus, in principle, a determination of the adiabatic ionization potential of COOH could determine AHfo (COOH), and the shape of the photoion yield curve could provide evidence regarding its geometrical structure.

We have prepared COOH by the F + HCOOH reaction.65 We have also examined the F + HCOOD, and F + DCOOH reactions. In the F + HCOOD experiment, the transient species formed has mass 46; in the F + DCOOH study, the free radical has mass 45. In both experiments, prominent autoionization structure at mass 44 characteristic of C02 is observed. Hence, it seems fairly certain that the species produced in our experiments is COOH, and not HCOO. The photoion yield curve of COOH+ (COOH) is presented in Figure 15. An adiabatic ionization potential of 8.486 ± 0.012 eV is obtained. When combined with the recent value59 for AHfo (COOH+) = 143.2 ± 0.5 kcal/mol, this leads to AHfo (COOH) = -52.5 ± 0.6 kcal/mol. Hence, COOH in its ground state is stable with respect to the lowest energy dissociation products (H + C02) by 10.2 ± 0.6 kcal/mol.

The photoion yield curve (Figure 15) exhibits step structure, indicative of at least one vibrational progression excited in the ionization process. Ab initio calculations57

have shown that COOH can exist in both a cis and a trans form, the latter being H 3 kcal/mol more stable. The calculated structures of COOH+ and trans COOH are displayed in Figure 16. Symmetry restrictions on Franck-Condon active frequencies confine the possible vibrational progressions to an OCO in-plane bending mode (~ 525 cm"1) and the two C-O stretching modes (~ 1245 cm"1 and ~ 2435 cm 1) . The experimental data show evidence for onsets of rounded steps at ~ 1430, ~ 1415, ~ 1365

£. COOH - an Important Reaction Intermediate

The reaction OH + CO-»H + C02

35

Figure 15. Photoion yield curve of m/e=45 (COOH+) obtained from the reaction of F atoms with HCOOH, which generates COOH, and not HCOO (see text).

Figure 16. Ab initio calculated structures of: a. COOH+ (ref. 61); b. trans COOH (Ref. 55).

36

and possibly ~ 1330 A, which roughly corresponds to three consecutive quanta of ~ 2300 cm'1, and one quantum of ~ 1500 cm"1. These could be identified with a progression in the higher frequency asymmetric C-O stretch, and a single quantum excitation of the lower frequency symmetric C-O stretch. The much lower OCO bending frequency would be much more difficult to observe, given the scatter in the data. A more definitive interpretation of the data, which could provide an experimental test of the COOH geometry, requires a rather involved calculation of the active Franck-Condon transitions and their relative intensities.

F. SO and P2; Some General Observations on Transient Species

In Section II.B. we remarked about the difficulty of observing transient molecular species emanating directly from the microwave discharge. One such species observed was SO, generated in a discharge through S0 2 vapor. SO is a relatively stable free radical; its re-oxidation to S0 2 is probably slow. A very complex autoionization structure was observed, which we have been unable to interpret. This is perhaps not too surprising, since the spectrum of the isovalent 0 2 molecule has not yet been completely interpreted.

In another series of experiments, we hoped to observe PF or PF2 in a microwave discharge through PF3. The desired species, although very likely formed in the discharge, could not be detected 30 cm downstream. We did, surprisingly, observe photoionization signals at mass 62, attributable to P2. This molecule can be expected to be fairly stable - it is isovalent with N2. However, the mechanism of formation is not at all clear. Reactions involving ground state species, such as

PF + PF3 -> P2 + 2F2

PF2 + PF2 -> P2 + 2F2

PF + PF -* P2 + F2

are all strongly endothermic. The formation of P2 very likely involves excited reactants.

A portion of this spectrum is shown in Figure 17, together with the densitometer trace of a photoabsorption spectrum obtained by Carroll and Mitchell.66 The latter authors have assigned five Rydberg series and one member of a sixth in this wavelength region. Most converge to v' = 0 of the A state of P2

+ The correspondence between the wavelength positions of the autoionization peaks and the photoabsorption peaks is generally quite good, but the relative intensities sometimes vary dramatically. Each quasidiscrete state in this energy region has some probability for predissociation, as well as autoionization. The relative rates of these processes determine the quantum yield of ionization, which will vary depending upon the proximity of predissociative curve crossings. A similar variation in ionization probabilities is observed in N2,67

although the ground and first excited states of N2+ and P2

+ are reversed.

PF + PF3 -> P2 + 2F2

PF2 + PF2 -> P2 + 2F2

PF + PF -* P2 + F2

37

Figure 17. a. Photoion yield curve of P+ from P2; b. Photoabsorption spectrum of P2. from ref. 66).

38

The weak structure observed at ~ 1185 A in the photoionization spectrum is most probably due to autoionization from v" = 1 of P2. The remainder of the spectrum is attributable to photoabsorption from v" = 0. The photo-absorption peak at ~ 1182 A does not appear in the photoionization spectrum, but the one at ~ 1179 A does. Clearly, the ionization threshold lies in this region. The value of the ionization potential deduced by Carroll and Mitchell66 from Rydberg series analysis is 10.567 ± 0.002 eV = 1173.3 ± 0.2 A. Bulgin et al.68 obtained 10.53 eV = 1177.4 A for X2II1/2, and 10.55 eV = 1175.2 A for X2n3/2 by PES. The photoion yield curve of Figure 17 implies that ionization has already begun at 1179 A = 10.516 eV, distinctly closer to the PES value.

IV. CONCLUDING REMARKS

The study of free radicals containing pnicogen and chalcogen hydrides has enabled us to determine the successive bond energies in these systems. The experimental results generally support the semi-empirical model of Goddard and Harding, and also agree well with modern ab initio calculations. However, the successive bond energies in NFn and PFn are dramatically different. The experiments with SiHjj have resolved the experimental discrepancies for D0(H2Si-H) and D0(HSi-H); those with GeHn have filled a void. It is now clear that there is a change in bonding pattern between the first row compounds (CHn) and the heavier isovalent analogs. This change is connected with the inversion of the energy ordering of 3BX and lAi of the corresponding dihydrides. The isoelectronic ions (NH2

+, PH2+, AsH2

+ ...) exhibit the same inversion of ordering between first and second rows. The Jahn-Teller splitting in SiH4

+ and GeH4+ is much more severe than in CH4

+. The C-H bond energies in ethane, ethylene and acetylene have been more clearly established as a result of the present research.

Most of the transient species discussed in this monograph have been generated by hydrogen abstraction reactions. One obvious requirement for achieving a sufficient number density in these studies is that the reaction rates should be very fast. A bonus in some cases was that successive reactions were rapid, enabling us to examine several species with the same reactants. Another fortuitous circumstance is that the transient species appeared to be thermalized (to room temperature) at the region of interaction with the photon beam. The only exception was the 3B t electronically excited state of SiH2. One disadvantage is that transient species which are highly reactive with precursors or at the walls may not survive.

Many species remain to be studied by the hydrogen abstraction method, including e.g. S i ^ , SiH2X and CH2X (X=halogen), H02 , HCO. We have briefly explored flame reactions involving Na and Li vapors reacting with halides of hydrocarbons (a classical method of preparation), thus far without success. A more careful experimental arrangement could be more fruitful. Pulsed laser photolysis followed by pulsed VUV laser radiation should open up new domains for photoionization mass spectrometic studies of free radicals, but some complications may result, due to a non-Boltzmann distribution of initial states. A more intense VUV

39

source, such as the undulator port at the new ALS (Berkeley), may ease the difficulty of studying species produced in very low abundance. Finally, photoelectron-photoion coincidence techniques may provide an unambiguous means of identifying the excited states of radical cations.

Acknowledgment

This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. W-3M09-ENG-38.

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