a photoionization mass spectrometric investigation of ch3cn and cd3cn
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A photoionization mass spectrometric investigation of CH3CN and CD3CND. M. Rider, G. W. Ray, E. J. Darland, and G. E. Leroi Citation: The Journal of Chemical Physics 74, 1652 (1981); doi: 10.1063/1.441306 View online: http://dx.doi.org/10.1063/1.441306 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/74/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photoionization mass spectrometric studies of the isomeric transient species CH2SH and CH3S J. Chem. Phys. 97, 1818 (1992); 10.1063/1.463169 Photoionization mass spectrometric study of CH3OF J. Chem. Phys. 95, 7957 (1991); 10.1063/1.461325 Photoionization mass spectrometric studies of the isomeric transient species CD2OH and CD3O J. Chem. Phys. 95, 4033 (1991); 10.1063/1.460758 Threshold electronphotoion coincidence mass spectrometric study of CH4, CD4, C2H6, and C2D6 J. Chem. Phys. 58, 3800 (1973); 10.1063/1.1679733 Mass Spectrometric Study of CH3D. Dissociation Probabilities of C–H and C–D Bonds by Electron Impact J. Chem. Phys. 14, 701 (1946); 10.1063/1.1724089
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A photoionization mass spectrometric investigation of CH3CN and C03CN
D. M. Rider,a) G. W. Ray,b) E. J. Darland,C) and G. E. Leroi
Department of Chemistry. Michigan State University. East Lansing. Michigan 48824 (Received 25 July 1980; accepted 14 October 1980)
A photoionization mass spectrometric investigation of CH,CN and CD,CN from the onset of ionization to - 20 e V is reported. Photoionization efficiency (PIE) curves of the acetonitrile parent ions and of the fragments C2H,N+, C,HN+, CH,N+, CHt, CHi and their deuterated analogs have been measured. The parent ion curves display isotope-dependent direct ionization as well as autoionization structure. Excitation of one quantum of the doubly degenerate CCN bending mode (v 8) in the ground state of CH,CN+ is observed, indicating the presence of Iahn-Teller coupling. The measured ionization potentials are l2.194±0.OO5 eV (CH3CN) and 12.235 ± 0.005 e V (CD3 CN). Autoionizing Rydberg series converging to the first and second excited electronic states have been observed and assigned. At their thresholds, C,H,N+ and C,D,N+ are produced exclusively from parent ion states which are populated by autoionization. Appearance potentials of the fragment ions for which PIE curves were obtained are reported, and their heats of formation have been calculated where more reliable literature values are not available. The observed fragment ion appearance potentials and relative ion yields indicate that H-atom migrations in the molecular ion are important in the fragmentation mechanisms.
INTRODUCTION
Photoionization mass spectrometry has been applied to the study of a wide variety of molecules; however, as a class of compounds, aliphatic nitriles have received little attention. The ionization potential (IP) of acetonitrile, the simplest aliphatic nitrile, has been determined by photoionization with1 and without2,3 mass analysis and by photoelectron spectroscopy. 4-6 The range of reported first IP's, 12.18 5 to 12. 22 eV, 3 is not great, but it is considerably larger than the error with which the IP can be measured, thus making the value somewhat uncertain. The only reported photoionization mass spectrometric study of the fragmention of acetonitrile is that of Dibeler and Liston, 1 who reported appearance potentials for C2H2N., C2HN., and CH;;. No photoionization efficiency (PIE) curves of the parent ions or of the fragment ions have been published.
The present study of acetonitrile was initiated to measure the PIE curves of the parent and daughter ions, to determine accurate ionization and appearance potentials, and to investigate the fragmentation pathways of the mo-1ecular ion. CH3CN has been detected in the comet Kohoutek 7 and in interstellar clouds8
; thus, in addition to being of interest as a model molecule, acetonitrile is of potential astrophysical importance. 9
EXPERIMENTAL
The photoionization mass spectrometer has been previously described. 10 Briefly, the vacuum UV radiation from a Hinterregger -type window les s discharge lamp is dispersed and focused with a near-normal incidence monochromator (McPherson 225) into a static-
a)Present address: Department of Chemistry, stanford Univer sity, Stanford, CA 94305.
b)Present address: Molecular PhysiCS and Chemistry Section, Jet Propulsion Laboratory, Pasadena, CA 91103.
c) Present address: Hewlett-Packard Laboratories, 3500 Deer Creek Road, Palo Alto, CA 94304.
type ion source into which the sample gas flows. The monochromator is equipped with a 1200 l/mm, MgF 2 -
over coated aluminum concave diffraction grating and provides 0.84 and 0.42 A (FWHM) photon band widths when used with 100 and 50 11m slit widths, respectively. The transmitted photon intensity is measured by monitoring the fluorescence of a sodium salicylate coated quartz disk with an RCA 8850 photomultiplier. The signal from the photomultipler is amplified with a picoammeter, and digitized with a voltage-to-frequency converter and computer-interfaced counter. The Hopfield helium continuum served as the photon source for the acetonitrile experiments.
Ions are accelerated from the ion source at 90 0 to the photon beam, focused by an ion lens system, mass selected with a quadrupole mass filter, and detected with a continuous channel electron multiplier. The ion pulses from the electron multiplier are amplified and discriminated,l1 and counted with another computer-interfaced counter.
The data collection employs a variable integration time technique that permits the data to be acquired with a constant signal-to -noise ratio in a minimum amount of time. 12 Photon and ion counts are accumulated at each wavelength until the desired signal-to -noise ratio is attained; the monochromator wavelength is then changed by a predetermined amount via a computer-controlled stepping motor, and the process is repeated. The relative photoionization efficiency (PIE = ion count/transmitted photon count) is calculated at each wavelength and plotted as a function of photon wavelength or energy to produce a PIE curve. The acetonitrile data were acquired at 0.25 and 0.18 A intervals when 100 and 50 11m slit widths were used, respectively. PIE curves were recorded with a signal-to -noise ratio of at least 100, which required integration times of 20-600 s per point. Data collection times varied from 12-48 continuous hours per experiment. The PIE curves were corrected10 for sample pressure and instrumental drift, as well as for stray light.
1652 J. Chem. Phys. 74(3).1 Feb. 1981 0021-9606/81/031652·09$01.00 © 1981 American Institute of Physics
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Rider, Ray, Darland, and Leroi: Photoionization of CH 3CN 1653
TABLE I. CH,CN and CD,CN photoionization mass spectra. a (Ionization energy = 21. 2 eV.)
CH 3CN CD,lCN
Intensity, Intensity, relative relative to C2H3N' to C2D"N'
m/e (m/e=4I) Assignment (m/e"'44) Assignment
14 18.2 CHi
15 4.4 CHj < 1 CHD'
16 27.6 CO2
17 < 1 CHDi
18 6.8 CDj
26 <1 eN+, C2H;
27 <1 HCN', C2H~
28 5.2 CH2N', Ni < 1 DCN', Ni, C2D2 30 8.1 C~N', C2D:
38 1.2 C2N' 1.2 C2N'
39 23.0 C2HN'
40 78.7 C2H2N' 22.7 C2DN'. (C2H2N')
41 100 C2H3N' 2.3 C2HDN', (C 2H3N')
42 13.3 C2H,N' 80.4 C2~N', (C 2H,N')
43 6.0 C2HD2N'
44 100 C2D3N'
45 3.2 C2HD3N'
46 19.0 C2D,N'
aUncorrected for small variations in the transmission efficiency of the quadrupole mass filter with m/ e.
CHsCN with a stated purity of 99% was obtained from the Aldrich Chemical Co. and CDsCN (99.5 at, %D) from Stohler Isotope Chemicals. The deuterated compound was found to be contaminated with DzO and was dried over calcium hydride. Both compounds were thoroughly degassed by repeated freeze -pump -thaw cycles. The ion source sample pressure was maintained at 5.0 (± O. 5)x10-4 Torr, as monitored by a capacitance manometer.
RESULTS AND DISCUSSION
A. Mass spectra
With undispersed (He continuum) radiation, small amounts of CN- and C2H~- were detected. However, the intensities of these negative ions were much too low to permit PIE curves to be recorded.
The relative positive ion intensities observed in the 584 A (21. 2 eV) photoionization mass spectra of CH3CN and CD3CN, and their compositional assignments, are listed in Table I. The electron impact mass spectrum of CHsCN shows slightly more fragmentation13- 15 than we observe: however, the overall agreement between the two types of mass spectra is good, and the small differences can be attributed to the higher ionizing energies (50-80 eV) employed in the electron impact experiments.
Assignment of most of the peaks in the tabulated mass spectra is straightforward, despite the slight proton con-' tamination of the deuterated compound. It has been sug-
gested13 that m/e = 15 from CH3CN might contain some contribution from NW since a small amount of NH; was detected in that electron impact study. We observe no peak at m/ e = 16. This is consistent with thermochemicai calculations which show the thresholds for NH+ and NH; to lie considerably above those for CHi and CHi, and we conclude that NW(ND+) and NH;(NDi) do not contribute significantly to the 21 eV photoionization mass spectra. For m/e values with relative intensity greater than 1% of the parent ions (neglecting contributions from 13C isomers and H contamination in the deuterated compound), only m/e = 28 from CH3CN and m/e = 30 from CD3CN are of multiple composition. The m/e = 28 signal was found to have a small contribution from Ni. The photoionization efficiency for CH~+ was obtained by subtracting an appropriately scaled N; PIE curve. Although m/e = 30 from CD3CN may be composed of CD2N' and C 2D;, the low intensity of m/ e = 27 (HCN', C 2Hi) from CH3CN suggests that the contribution from C2D; is negligible.
B. Photo ionization efficiency curves
PIE curves of the molecular ions and all other 'ions with an intensity greater than 2% of the parent ions were recorded. They are depicted for C2H3N' and five protonated daughter ions in Fig. 1. Portions of the PIE curves of C2D~+ are given in later figures; the remaining portions and the curves of C2DN', CD~+, CD;, and CD; are not shown here since they are identical in appearance to their protonated counterparts. 16 The intensity of all ions having m/ e::S parent ion was found to depend linearly on the sample pressure.
Pressure-dependence measurements show that the ion at P + 1 (P + 2 for CD3CN) is the product of an ionmolecule reaction. An ion cyclotron resonance study13 has shown the reaction to be an exothermic hydrogenatom abstraction by the parent ion from the neutral molecule. The PIE curve of this ion is not presented here since it is identical to the parent ion curve over the range of our observations.
1. Parent ions
The C2HsN'PIE curve (600-1040 A) is shown in Fig. 1. The threshold is quite sharp and. is followed to higher energy by an autoionizing Rydberg series converging to the first excited state of the ion. The sharp rise at - 950 A correlates very closely with the origin of the second band system of the photoelectron spectrum and appears to be the threshold for the first excited state of the ion. The prominence of the step at - 950 A. the broadness of the autoionization structure, and the absence of ionic fragmentation in this energy region indicate that the autoionizing Rydberg states are undergoing fast predissociation into neutrals. The hump between 820-940 A falls in a blank. region of the photoelectron spectrum and is therefore attributable to autoionization. Indeed, some weak autoionization structure is observed. The lowest fragmentation threshold is at - 885 A. This accounts for the leveling off of the molecular ion PIE at - 885 A and the subsequent decrease to shorter wavelengths. All five fragments for which PIE curves were measured have thresholds below 15.5 eV and one would
J. Chern. Phys., Vol. 74, No.3, 1 February 1981
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1654 Rider, Ray, Darland, and Leroi: Photo ionization of CH 3CN
ENERGY (Electron Volts) 21 20 19 18 17 16 15 14 13 12
o -- \".
600
'. '. \
-- 0
-- 0
-- 0
-- 0
700 800 900
WAVELENGTH (Angstroms) 1000
FIG. 1. Photoionization efficiency curves for parent and daughter ions frpm CH3CN •. (Arbitrarily scaled, see Table I for true relative intensities.)
expect any parent ions formed with 3.3 eV or more internal energy to be completely dissociated. Thus the region between 600 -800 A in the parent ion PIE curves reflects the behavior of the cross sections for direct ionization to the ground and first excited states of these ions.
In its general features, the deuterated parent ion PIE curve is identical to that of the protonated compound. However, there are some differences. The autoionization in the 820-900 A region is conspicuously absent and the thresholds are also perceptibly different. These two points are discussed in the following paragraphs.
The threshold regions of the C2HsN+ and C2DSW PIE curves, acquired with 0.42 A bandpass (FWHM), are displayed in Fig. 2. Both curves exhibit a sharp rise at the onset of ionization; however, the C~sN+ threshold is at a shorter wavelength and does not show the second narrow step observed in the C2HsN+ curve. The structure in the 1000-1010 A region appears to be due to weak autoionization. However, two considerations lead to the conclusion that the steplike structure at the thresh-01ds corresponds to direct ionization to the ground vi-
brational state of the ion. First, the photoelectron spectrum shows a strong 0 - 0 transition to the ground state of the ion. Therefore the direct ionization cross section should exhibit a prominent step at the adiabatic IP. Second, members of two Rydberg series are observed in the region above threshold (Fig. 3). An analysis of these series shows that no members fall at the thresholds. Further, members of the series just above the thresholds appear to be strongly predissociated.
To determine the IP's as accurately as possible, the thresholds were fit to an integrated Gaussian transition probability centered at the IP, i. e. ,
JEp
PIE(Ep ) = a exp[ - b(E - IP)2] dE , o
where E is the energy above the ground state of the neutral, Ep is the photon energy, IP is the ionization pot en -tial, b determines the half -width and a is a scaling factor. IP, a, and b were adjusted to obtain the best fit to the data. For the C2HaN+ curve the sum of two Gaussians
ENERGY (Electron Volts) 12.4 12.3 12.2 12.1 12.0
--- 0
--- 0
1000 1010 1020 1030
WAVELENGTH (Angstroms)
FIG. 2. Threshold regions of the PIE curves for C2H j N+ and C2D3N+. (Spectral bandwidth=O.42 A; the noise at the foot of the C 2H3N+ PIE arises from the short integration times employed to obtain those data points.) The arrows indicate ionization potentials calculated (see texd by fitting Gaussian transition probabilities to each step: 12.194 eV (A), 12.247 eV (B), and 12.235 eV (CL The two-step fit calculated for C2H3N+ is shown by the solid line.
J. Chern. Phys:. Vol. 74. No.3. 1 February 1981
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Rider, Ray, Darland, and Leroi: Photoionization of CH 3CN 1655
TABLE II. First ionization potentials of CH3CN and CD3CN.
Reported value (eV) Methoda Reference
CH3CN 12.194±0.005 PIlVIS This work 12.19± O. 01 PIlVIS 1 12.205±0.004 PI 2 12.22±0.01 PI 3 12.20±0.01 PE 4 12.21 PE 6
12.18 PE 5
CD3CN 12.235±0.005 PIMS This work 12.23±0.01 PE 4
apIMS, photoionization mass spectrometry; PI, photoionization without mass analysis; PE, photoelectron spectroscopy.
was used, one for each step. This fitting procedure makes the implicit assumptions: (1) that the direct ionization cross section for each rovibronic component within the rotational envelope of a vibronic direct ionization transition is a step function; (2) that the transition probability within the rotational envelope, when convoluted with the instrument function, is adequately represented by a Gaussian distribution; and (3) that the rotationless transition is at the center of the Gaussian distribution. The reliability of the first two assumptions can be judged by the quality of the fits. The third assumption is more difficult to test. However, a survey of ordinary optical absorption spectra indicates this to be valid within the limits of resolution of the PIMS experiments, and this same assumption is used in photoelectron spectroscopy when the maximum of a resolved vibronic peak is assigned as the adiabatic IP.
The two-Gaussian fit obtained for C2HsN+ is shown in Fig. 2; the ionization potentials calculated by the fitting procedure are indicated therein by arrows, and their values are given in Table II along with those determined by other investigators. The IP for C2HSW reported here is slightly lower than most of the previously reported values, with the exception of Refs. 1 and 5. The photoelectron studies were unable to resolve the transition corresponding to the second step in the PIE curve and this may account for the higher values reported in Refs. 4 and 6. The IP determined by Dibeler and Liston, 1
within the error limits of their value, is in good agreement with ours. The value reported in Ref. 5 is clearly too low. Although the photoelectron investigators4- 6
indicated that they recorded the spectrum of C 2DSN+, only Lake and Thompson4 reported an IP. Their value is in good agreement with ours. The difference in the IP's of CHsCN and CDsCN must reflect a dissimilarity in the difference of the zero point energies of the neutral and the ion in the two cases.
The second step in the C2H3W threshold PIE curve is 53 meV (427 cm-1
) above the IP. An analysis of the autoionizing Rydberg states(vide infra) shows that this step is not due to autoionization. The spacing, too small to be resolved in, the previous photoelectron work, 4-6 corresponds most closely to the single excitation of the doubly degenerate CCN bending vibration va of the neutral molecule (362 cm-1).17 Within the assumptions of the Franck-Condon principle the single excita-
tion of V8 in the ion is symmetry forbidden. However, the ground state of the ion, which corresponds to the removal of an electron from the 3e orbital (11 bonding) of the neutral, 4-6 is electronically doubly degenerate and is susceptible to Jahn-Teller vibronic coupling. 18 Vibronic coupling of VB with the ground electronic state of the ion would allow the transition to the singly excited va level, and we assign the second step in the PIE of C 2HSN + accordingly. (The analogous interpretation has been applied to the photoelectron spectra of HCN and DCN . 19) The higher frequency of the va fundamental in the ground state of the ion with respect to-the neutral is consistent with the analysis of Jahn-Teller coupling in ions by Leach and coworkers. 20 The single excitation of va in C2DsN+ is estimated to be 48 meV (390 cm-1) above the ground ionic state and the transition should be resolvable in the PIE curve. The corresponding step would be at 1009.4 A and appears to be obscured by autoionization; both parent ion PIE's exhibit a rise having a midpoint at 1010.3 A, due to autoionization. A more detailed discussion of the threshold curves is given in Ref. 16.
The autoionization structure between 940 A and threshold in the C2HSW PIE is shown in Fig. 3 and the peak locations are listed in Table III. Members of two Rydberg series are apparent in this region and are found to be isotope independent16; these two series have also been observed in the electron impact energy loss spectrum of acetonitrile. 21 The expected position of the next lower member of each series is indicated by a dashed vertical line in Fig. 3. The 4da peak, observed by Fridth21 at 12.180 eV, falls below the adiabatic ionization potential. The n= 5 member of the sa series is expected at about 12.3 eV, and indeed a very weak peak is observed near 1006 A in the PIE curves of both C2HSW and C~3W (see Fig. 2) which may be attributable to autoionization from this Rydberg level. The weakness of this feature, and the absence of other series members above the ionization threshold are attributed to small Franck-Condon factors or competitive predissociation. We fit the autoionization peaks of more intense series to the Rydberg equation:
13.2 13.0 ..L ___ ~_l_ __ L
ENERGY (Electron Volts)
12.8 -1
12.6 I
12.4 12.2 '--~-:-~
CD 12109 8 7 6 (5) nsCT !
6
o ,----- r-
940 960 980 1000
WAVELENGTH (Angstroms)
(4)ndCT
-----r-- i
1020
FIG. 3. The PIE curve of C2H3N+ between the 940 A and threshold. The Rydberg series converge to the first excited electronic state of the ion.
J. Chern. Phys., Vol. 74, No.3, 1 February 1981
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1656 Rider, Ray, Darland, and Leroi: Photoionization of CH 3CN
TABLE III. Rydberg series converging to the first excited state of CH3CN+.
n
nsu 5 6 7 8 9
10 11 12
ndu 4 5 6
Series limit (eV) = 13.133 ± 0.004" (, (nsu) = O. 93± O. 04
Energy (eV) (,
[12.2951 b
12.601 0.94 12.775 0.84 12.864 0.89 12.924 0.93 12.964 1. 03
13.020 1. 03
[12.1801b
12.542 0.20e 12.724 0.23
aErrors are linear estimates of the standard deviation. ~rom Ref. 21; not observed in this investigation. "Quantum defects for the ndu series were calculated from limits of the nsu series.
where Ep is the transition energy, SL is the ionization limit to which the series converges, R is the Rydberg constant (13.605 eV), n is the quantum number of the Rydberg orbital, and 0 is the quantum defect. SL and o were taken as adjustable parameters. The calculated limit was 13.133± 0.004 eV (944.0± O. 3 A) and the quantum defect was 0.93 ± 0.04; the values were determined by minimizing the sum of the squares of the residuals between the observed and calculated values of Ep. Within experimental error, the same Rydberg equation parameters result from fitting the more intense series in the deuterated compound. 16 The two members of the less intense Rydberg series have an average 0 = O. 2, where the derived limit of the more intense series was used as the convergence limit.
The Rydberg series converge to the first excited state of the ion and correspond to excitation of an electron in the 5a1 orbital (nitrogen lone pair). The derived Rydberg series limit falls at the midpoint of the step beginning at - 946 A in the PIE curve. This indicates that the derived series limit is an accurate measure of the 5a 1
orbital IP, probably more accurate than values obtained from the photoelectron spectra. 4-6
The more intense series (0 = 0.93) was previously assigned as a pa -type series and the less intense series (0 = o. 2) as a da type. 21 We assign the series with o = 0.93 as sa. The quantum defect would be unusually large for a p -type series. More important, the pa assignment is inconsistent with the Rydberg series converging to the ground state of the ion, measured in the electron impact energy loss spectrum, where both an sa series (0 = 0.97) and a pa series (6 = 0.76) are observed. 21 Rydberg series converging to excited ionic states usually have quantum defects very close to the analogous series converging to the ground ionic state, 22
and we therefore prefer an sa assignment for the series with 0 = 0.93. (Our results suggest that the energy loss
data have a small systematic error of - - 10 meV which might have contributed to a misassignment. 16)
The 800-880 A region of the parent ion PIE curves is shown in Fig. 4. Weak, but distinctive autoionization structure is readily apparent in the C2H3N+ ion curve; however, only hints of such structure can be discerned in the C 2D3N+ PIE. The same autoionization features are observed in the C 2H2N+ fragment PIE (see Fig. 1), but are absent in the curve for C 2D~+. Five members of a Rydberg series converging to the second excited state of the ion, with accompanying vibrational structure, can be identified if one adopts the ionization limits and vibrational assignments given for the second excited state of CH3CN+ by Turner et al. 6 The average 0 is 1.06 and we have assigned this as an sa series. The vibrational structure of this band system in the photoelectron spectrum of CD3CN is broader and more overlapped than the corresponding structure in the CH3CN spectrum. One would expect the vibrational structure of Rydberg series converging to the corresponding ionic state of C2D3N+ to also be more overlapped, and this might explain the absence of autoionization structure in the 800-880 A region for the deuterated molecule.
2. Fragment ions
Also shown in Fig. 1 are the fragment ion PIE curves from CH3CN. The experimental appearance potentials (AP's) were determined by extrapolating a straight line, drawn through the linear post -threshold region of the curves, to the base,line. The intersection of the extrapolated line with the base line was taken as the experimental AP. Table IV contains a summary of the measured AP's. In all cases the AP of the deuterated fragment is higher than its protonated counterpart; this may be due to disparities in the relevant zero point energy differences for the fragmentation processes, as well as to the larger kinetic shift23 expected for deuterated molecules. 24 In addition, calculated heats of formation are tabulated for those ions for which accurate literature
w a..
800 820
ENERGY (Electron Volts)
", ~ .... , "
840 860
WAVELENGTH (Angstroms)
880
FIG. 4. The 800-880 A region of the parent ion PIE curves. The Rydberg series observed for C2H:lN+ converge to the second excited electronic state of the ion.
J. Chem. PhY5., Vol. 74, No.3, 1 February 1981
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Rider, Ray, Darland, and Leroi: Photoionization of CH 3 CN 1657
TABLE IV. Summary of fragment ion appearance potentials and heats of formation. a
From CH3CN Appearance potential (eV) Heat of formationC (kJ/mole)
Ion Neutral This work PI literatureb ~ ~f298
C,H2N+ H 13.99±0.01 14.01 ± O. 02 1 1235 ± 1d
C2HN+ H2 15.25 ± O. 02 15.1±0.1 1 < 1551 1,26
CHi HCN 15.29±0.02 14. 94± O. 02 1 1398 [14.92]"
CH; CN 15.34±0.O4 [14.85]" 1095
CH2N+ CH 15.56 ± O. 04 S 1009d
From CnjCN
C2D2N+ D 14.15±O.01 1234± 1d
C2DN+ D2 15.36±0.O2 s 1565d
CO2 DCN 15. 40± 0.02 s 139sct
CD1 CN 15.55±O.O4 s l103d
CD2N+ CD 15.61±O.04 s997d
aLiterature values for heats of formation are listed when they are considered to be more reliable. bphotoionization literature values. cBased on heats of formation of neutrals from Ref. 27 except for CD3CN, DCN, and CD, for which heats of formation were estimated from those of the protonated counterparts by using differences in zero point energies. ~his work. eC alculated from literature heats of formation, Ref. 27.
values are not available. The heats of formation were calculated assuming the lowest energy neutral products, which were always more than an electron volt lower than other possible products.
The C 2H~+ (C 2D2W) fragment ion has the lowest AP, which falls in a blank region of the photoelectron spectrum. The C2H2N+ PIE curve, shown in Fig. 5, has a small low energy tail and rises sharply and linearly beyond the threshold. Some autoionization structure, which correlates with structure in the parent ion curve, is evident between 820-860 A. The sharp rise commencing at - 820 A corresponds closely with the origin of the second excited state of the parent ion, which has been observed in the photoelectron spectrum (15.13 eV, 819 A). The C2D~+ PIE curve (also shown in Fig. 5) is nearly identical to that of the protonated ion; however, the structure in the 820-860 A region is absent, consistent with the observations in the parent ion, and the appearance potential is significantly higher than that of C2H2N+.
The linearity of the C 2H2N+ and C 2D~+ PIE curves above the onsets and the weak. intensity of the low energy tails suggest that the thresholds for these ions may exhibit negligible kinetic shift. Accordingly, we have approximately corrected the experimental AP's for the internal thermal energy of the neutral by the procedure of Chupka, 25 and Guyon and Berkowitz, 26 to arrive at o oK AP's. This entails the addition of the internal thermal energy of the neutral at the experimental temperature to the AP determined from the extrapolation. When the statistical mechanics expression for uncoupled harmonic oscillators is employed with the vibrational frequencies from Shimanouchi17 for the thermal vibrational
energy and t kT for the thermal rotational energy, the correction is 62 meV for CH3CN and 68 meV for CD3CN. The onset of the linear ascending portion of the PIE curve for both first daughter ions occurs at an energy equal to the sum of the thermal energy and the experimental AP, in accordance with the assumptions of the procedure. 25 The corresponding heats of formation are easily calculated from the 0 oK AP 's. They are 1235 ± 1 kJ/mole (C2H~+) and 1234± 1 kJ/mole (C2D~+); no literature values have been found for these hydrogen-loss ions.
The remaining fragment ions, C2HN., CH2N+, CHi, CH~, and their deuterated counterparts, have AP's very near or just above the third photoelectron band system origin. The AP's of these fragments are more than 1 eV higher than the C2H2N+ (C2D2N+) AP and are very close to one another, which suggests that there is a substantial kinetic shift.23 This is also implied by the PIE curves which with decreasing energy slowly and asymptotically approach the baseline. The low energy tails have more intensity and extend over a larger range than can be accounted for by the thermal energy of the neutral.
The extrapolated, room temperature appearance potential for the second daughter ion from acetonitrile is 15.25±0.02 eV (H2loss) or 15.36±0.02 eV (D2 loss). The previously reported1 AP of C2HN+ is 150 meV lower than the value we measure; most likely the experimental AP's were determined by different procedures. The literature value of the heat of formation of this ion (1551 kJ/mole27) was based on Dibeler and Liston's1 AP. However, our data indicate that this thermochemical value should be considered an upper limit.
J. Chern. Phys., Vol. 74, No.3, 1 February 1981
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1658 Rider, Ray, Darland, and Leroi: Photoionization of CH 3CN
ENERGY (Electron Volts)
600 700 800
WAVELENGTH (Angstroms)
FIG. 5. PIE curves for C2H2W and C2D2N+.
0:
oi 900
Because of the additional competition from lower energy fragmentation pathways, the threshold regions of the remaining daughter ions display significant curvature (see Fig. 1), and the experimental AP's listed in Table IV will be considerably higher than the thermochemical values. In several cases the ionic heats of formation have been accurately determined by other means, and these values can be used in a thermochemical cycle to calculate the expected AP's. In this manner the kinetic shifts in the experimental appearanc e potentials may be evaluated. For example, the heat of formati on of CHi (1398 kJ/mole27) is well established. 28-30
The thermochemical threshold for the formation of CHi from CH3CN calculated from this value is 14.92 eV, which leads to an estimate of 370 meV for the kinetic shift in the experimental AP. Similarly, the thermochemical threshold for CHi from CH3CN is calculated to be 14.85 eV, which is almost half an electron volt below the experimental AP. The heats of formation listed in Table IV for CD; and CD; are approximately corrected for kinetic shift by the corresponding values for CHi and CHi. Unfortunately accurate heats of formation are not available for C2HW and CH2N+ or the deuterated fragments; the heats of formation calculated from the AP 's of these ions and given in Table IV should be considered as upper limits, only.
The calculated thermochemical AP's of CHi and CHi reveal an interesting anomaly. The thermochemical AP of CHi is 70 meV lower than that of CHi, yet the experimental CHi AP is 50 meV higher than that of CHi. Also, the intensity of CHi in the mass spectrum (Table I) compared to the intensity of CHi is unusually high for two fragments for which the AP' s are so close in value. These observations might be interpretable in terms of a mechanism involving competitive dissociation of a single, loosely bound precursor parent ion to form several daughter ions, as has been suggested for the difluoroethylenes. 31 However, we prefer to rationalize the anomalous CHi/CHi AP's as the result of rapid H-atom migrations from the terminal carbon to the nitrile group in the parent ions in the energy range near and above the CHi and CHi fragmentation thresholds.
One can conceive alternative mechanisms by which the decomposition CHsCN+ - CHi + HCN could proceed.
The HC N bond could be formed simultaneously with the concerted rupture of the CC bond and a CH bond, a onestep process31
; or isomerization involving transfer of a hydrogen atom to the cyanide group might occur, followed by rupture of the CC bond, a two-step process. Since the AP of C H~ lies more than 3 e V above the ground state of the parent ion, it seems reasonable to assume that H-atom lability would be energetically feasible in the range of the CH~ threshold. Migrations of hydrogen atoms are common for gas phase ions, 32
as well as for ions in solution, where they are observed at thermal energies. 33 CH3CN+ is a radical ion containing a half -filled orbital to which a hydrogen could easily migrate. Parent ions formed upon photon impact must initially have a methyl cyanidelike structure; the potential barrier to isomerization could preclude rapid hydrogen migration near the ionization threshold. However, in the energy range of interest -the thermochemical AP of CHi is more than 2.6 eV above the ground state of the ion (see Table IV)-the molecular ions posses significant internal energy and proton transfer should be facile. (The activation energy for isomerization of the ground state cyclo-octatetraene ion to the styrene ion, which involves considerable atomic rearrangement, has been found to be about 1 eV. 34) Isomerization prior to fragmentation, or in competition with direct dissociation, is common for organic ions. 35
Rapid migration of H atoms in the parent ion could have two effects on the CHi fragmentation channel. First, since CHi must be produced from parent ions with the methyl group intact, the intensity of CHi would be reduced. Second, rapid migration could also shift the apparent (experimental) CHi threshold because the fragmentation channel would be in competition with the isomerization; CHi would not be detected until an energy was reached at which the fragmentation rate to form CHi is comparable to the rate of rearrangement. As a corollary, H atom migration should enhance the competitive rate of fragmentation to form CHi, which is in accord with the experimental data.
It was concluded in a rec ent photo ion -photoelectron coincidence investigation of the fragmentation of C4HsN+ ions produced from several isomeric neutrals that the most stable structure for C2HsN+ is the ketenimine ion, H2CCNH'; that result was supported by theroretical calculations. 36 It has also been shown, in a study where C 2H3N+ ions were generated from several different precursors via collision-induced dissociation (cm), that parent ions formed from acetonitrile retain the general structure of the neutral (H3CCN+). 37 Our data are consistent with both of these results, provided that there is a barrier for the isomerization of the acetonitrile parent ion to the ketenimine structure. Molecular ions formed with insufficient energy to fragment on the time scale of the cm experiment retain the acetonitrile structure' whereas those formed with sufficient internal energy to fragment to produce CHi or CHi daughter ions may also isomerize to form the ketenimine structure. The precise energy of the barrier to the rearrangement is not revealed by our results. However, it appears that hydrogen migration is energetically feasible at the CHi thermochemical threshold (14.85 eV). The rear-
J. Chem. Phys., Vol. 74, No.3, 1 February 1981
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Rider, Ray, Darland, and Leroi: Photoionization of CH 3CN 1659
rangement may even be possible at energies as low as the C2H2N' threshold (13.99 eV, 1.8 eV above the ionization potential); this forms the lower limit because parent ions having only the acetonitrile structure were found in the cm experiment. S7 The detection of CH2W in our experiments suggests that parent ion structures other than HsCCW and H2CCNW may also be possible in the energy range near 15.6 eV, since two hydrogens must be transferred to the CN moiety to form this daughter ion. Moreover, this further isomerization must occur at a rate comparable to that for rupture of the carboncarbon bond in the acetonitrile or ketenimine parent ions.
The relative intensities of CH;, CH2N+, and CHi in the photoionization mass spectrum (Table I) are in the ratio 1. 0: 0.29: 0.24; similar relative intensities are measured for the corresponding ions from CDsCN. At first glance these results defy chemical intuition. One would not predict CHi (two bonds broken) to be four times as intense as CH; (one bond broken), and CH~+ would be expected to be quite weak. However, the results are consistent with the proton lability inferred from the experimental PIE curves. The fact that C Hi and C H2N + are more intense than CHi means that hydrogen migration must be more rapid than CH; fragmentation, even at 21 eV.
CONCLUSIONS
The acetonitrile PIE curves yield a remarkable amount of information. From the parent ion curves, accurate ionization potentials for the 3e (C=N bonding) and 5a l
(nitrogen lone pair) orbitals of CHsCN and CDsCN have been measured, and it was found that the former is isotope dependent. Previously unresolved vibrational structure was observed in the CHsCN+ PIE curve, and it was concluded that the Jahn-Teller effect is operable in the ground state of the ion. Autoionization makes a significant contribution to parent and fragment ion formation. Several Rydberg series, converging to the first and sec-0nd excited electronic states of the ion, have been observed and assigned. The appearance potentials of the five most intense ion fragments have been measured, and an accurate 0 cK heat of formation for C 2H2N+ (C2D~+) has been determined. Although heats of formation calculated from the experimental AP's for other daughter ions must be regarded as upper limits, the data should prove useful to scientists interested in the ion chemistry of acetonitrile. Finally, our results indicate that hydrogen (deuterium) atom migrations in the parent ion exert an important influence on the fragmentation mechanism. Unfortunately, mass spectrometric experiments provide little direct structural information, and one can only speculate about the geometries of the rearranged parent and daughter ions16; perhaps matrix isolation experimentsS8 would reveal the structures of these interesting species.
ACKNOWLEDGMENTS
We are indebted to the U. S. Office of Naval Research for initial support, and to the National Science Founda-
tion for continued funding. D. M. R. was a 1979 General Electric Foundation Summer Fellow. We thank Dr. T. V. Atkinson for his considerable advice with computer graphics, and Mike Baska for helping maintain the photoionization mass spectrometer. Helpful discussions with Professor W. A. C hupka are gratefully acknow ledged.
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J. Chem. Phys., Vol. 74, No.3, 1 February 1981
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J. Chem. Phys., Vol. 74, No.3, 1 February 1981
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