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J. Phys. Chem. 1996, 100, 8280-8284
State-Selected Photodissociation of Acetaldehyde Molecular Ion: Hydrogen Scrambling and the Product Branching Ratio Seung Koo Shin,*,†,‡ Byungjoo Kim, Jessie G. Haldeman, and Seung-Jin Han Department of Chemistry, UniVersity of California, Santa Barbara, Santa Barbara, California 93106-9510 ReceiVed: September 6, 1995; In Final Form: December 15, 1995X
Acetaldehyde molecular ions were produced in an ICR cell using (2 + 1) resonance-enhanced multiphoton ionization of a pulsed molecular beam of acetaldehyde via a two-photon resonant 3s r n Rydberg transition in the wavelength range 364-354 nm. Mass-selected REMPI spectra of CH3CHO are presented. The power dependencies of ion intensities were examined to help establish the mechanisms of REMPI dissociation processes. The product branching ratios from REMPI dissociations of CH3CHO and CH3CDO were determined. A multireference configuration interaction method was employed to obtain structural and energetic ˜ 2A′′, and B ˜ 2A′ states of acetaldehyde molecular ion. Comparisons of theory information about the X ˜ 2A′, A with the experiments lead us to conclude that acetaldehyde molecular ions are excited to the B ˜ 2A′ state and undergo both R and β cleavage processes. The R C-C cleavage yields the formyl ion without hydrogen scrambling. The product branching ratio of [HCO+]/[CH3CO+] remains constant at 2.1 ( 0.2 over the entire wavelength range. CH3CDO yields the branching ratio of R C-D to β C-H cleavage, [CH3CO+]/[CH2DCO+] ) 4.7. An aldehydic deuterium substitution reduces the branching ratio of R C-H(D) to R C-C cleavage.
Introduction Photodissociation of the acetaldehyde molecular ion has attracted much experimental and theoretical interest for the purpose of elucidating the dissociation mechanisms at selected energy levels.1-6 Typically, acetaldehyde molecular ions were generated by using either vacuum-ultraviolet (VUV) photoionization3-5 or multiphoton ionization (MPI).6 For instance, Baumga¨rtel et al. reported photoionization mass spectrometric (PIMS) studies of acetaldehyde using a synchrotron radiation source.3 They determined appearance potentials (APs) of 10.22 (CH3CHO+), 10.90 (CH3CO+), 12.03 (HCO+), 12.61 (CH4+), and 14.08 eV (CH3+) from CH3CHO. The hydrogen-scrambled formyl ions were observed at higher APs from the C-C bond cleavage of partially deuterated aldehydes: 12.67 eV (HCO+) from CH3CDO and 12.65 eV (DCO+) from CD3CHO. On the contrary, no hydrogen-scrambled acetyl ions were observed from the C-H(D) bond cleavage. The photoelectron-photoion coincidence (PEPICO) spectroscopy was also employed by Bombach et al.4 and Johnson et al.5 In the X ˜ photoelectron band, the parent ions appear in the low vibrational levels,5 while the acetyl ions appear in the high vibrational levels.4 In the A ˜ photoelectron band above 13 eV, the CH4+ ions appear in addition to the acetyl and formyl ions,5 while the metastable parent ion signals fall off near the onset of the A ˜ band.4 The appearance of CH4+ was considered as a characteristic of the A ˜ band. In the B ˜ photoelectron band above 14.1 eV, the CH3+ ions emerge with increasing abundance as the CH4+ ions fall off at the onset of CH3+.4,5 The CH3+ ions were the most abundant products in the photoelectron bands above 15 eV.5 The MPI dissociation of acetaldehyde was studied by Fisanick et al.6 Extensive fragmentations with hydrogen scrambling were observed from CD3CHO. Both the C-C and C-H bond * To whom correspondence should be addressed. † National Science Foundation-Young Investigator, 1994-1999. ‡ Arnold and Mabel Beckman Foundation-Young Investigator, 19941996. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
S0022-3654(95)02607-4 CCC: $12.00
cleavage processes yield the hydrogen-scrambled formyl and acetyl ions that show strong dependence on laser power. However, the mechanisms of hydrogen scrambling in the state-selected photodissociation of the acetaldehyde molecular ion have not been well established yet. In the present study, acetaldehyde molecular ions were prepared in an ion cyclotron resonance (ICR) cell by (2 + 1) resonance enhanced multiphoton ionization (REMPI)6-12 and excited to the B ˜ state. The REMPI process involves a two-photon resonant 3s r n Rydberg transition followed by one-photon ionization of the resonant Rydberg state in the wavelength range 364-354 nm. The stateselected photodissociation was carried out at the origin of Rydberg transition with a high photon flux to ensure the saturation of resonant two-photon transition. In the saturation regime, essentially all molecules were ionized, and the resultant molecular ions were immediately excited to the B ˜ state.6,9 The dissociation of molecular ion from the B ˜ state may follow direct pathways governed by the state-to-state electron correlations1 and/or proceed through vibrational predissociations with extensive intramolecular rearrangements, thereby scrambling hydrogens.2 A Fourier transform ion cyclotron resonance (FT-ICR) technique13 was employed to detect all product ions at the same time. The mass-resolved detection of all ions provides the product branching ratio and the propensity for the intramolecular rearrangement. The FT-ICR technique was chosen because of its high mass resolution with a near-unity ion trapping efficiency.14 The FT-ICR detection allows the accurate measurement of product branching ratio under a collision-free condition.14 Mass-selected REMPI spectra of acetaldehyde are presented with photodissociation mass spectra of CH3CHO and CH3CDO. The laser power dependence of ion intensity was examined to help establish the mechanisms of REMPI dissociation. Dissociative pathways are listed below for the processes having thermochemical threshold less than the photon energy in the wavelength range 364-354 nm. The heats of formation (in electronvolts) that were used to evaluate the above enthalpy © 1996 American Chemical Society
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changes were taken from the literature.15 CH3CHO f CH3CHO+ + e- ∆H ) 10.23, ∆∆H ) 0.0
(1)
f CH3CO+ + H + e- ∆H ) 10.74, ∆∆H ) 0.51 (2) f HCO+ + CH3 + e- ∆H ) 11.78, ∆∆H ) 1.55 (3) f CH4+ + CO + e- ∆H ) 12.32, ∆∆H ) 2.09 (4) f CH3+ + HCO + e- ∆H ) 13.51, ∆∆H ) 3.28 (5) Though all channels are energetically accessible with four photons, no detectable signals are observed from the reactions 4 and 5. To test for hydrogen scrambling, experiments were carried out with CH3CDO. Specifically, the detection of HCO+ and CH2DCO+ would provide direct evidence for intramolecular rearrangements. Very little HCO+ was found, while a significant amount of CH2DCO+ was observed. Structural and energetic information about the electronically excited states of acetaldehyde molecular ion were obtained from the multireference configuration interaction calculations. The mechanisms of fragmentations from state-selected photodissociation of acetaldehyde molecular ions are discussed in light of comparisons with theory and previous experiments.
Figure 1. Mass-selected ion yield spectra of CH3CHO from the (2 + 1) REMPI via a 3s r n Rydberg transition in the wavelength range 364-354 nm: (a) HCO+ and (b) CH3CO+. A pulsed molecular beam of neat acetaldehyde was used. A laser power was ∼15 mJ/pulse. Ion yield spectra are not normalized for the dye tuning curve.
Experimental Section Experiments were performed on an FT-ICR spectrometer interfaced with an IonSpec Omega-386 FT data system. Details of the pulsed molecular beam ICR setup have been described elsewhere.14,16,17 Experimental procedures are briefly outlined below. The ICR chamber is operated under vacuum at a base pressure of 5 × 10-8 Torr. A molecular beam of neat acetaldehyde or a 5% acetaldehyde/helium mixture is pulsed into a differentially pumped vacuum chamber for ∼100 µs. The vapor pressure of acetaldehyde at 35 °C provides the backing pressure for a pulsed valve. A skimmed beam enters the ICR chamber. A peak pressure in the ICR chamber from the molecular beam monitored with an ion gauge was kept below at ∼6 × 10-6 Torr. An output from a dye laser (Lambda Physik FL 2002) pumped by a XeCl excimer laser (Lambda Physik EMG 202) is focused onto the center of a 1.85 in. cubic ICR cell by a 5 cm focal length lens. The laser beam intersects with the pulsed molecular beam near the center of cell. The delay between the pulsed valve and pump laser was optimized to give the best signalto-noise ratio in a mass spectrum while minimizing ionmolecule reactions. Ions trapped by an electrostatic trapping potential undergo cyclotron motion due to an external magnetic field. The magnetic field axis is perpendicular to both the molecular beam and laser propagation axes. The magnetic field strength can be varied in the 0.2-1.2 T range. The trapped ions were accelerated by either an impulse, a radio-frequency (rf) chirp, or a single-resonant rf excitation. The image charge signals induced on the receiver plates were processed and digitized by a 2 MHz, 32K data point, 9 bit transient digitizer. The REMPI spectrum was obtained by scanning the dye laser with a 0.15 cm-1 increment. A DMQ dye was used to scan the wavelength in the range 350-365 nm. A laser bandwidth is ∼0.35 cm-1, and the maximum energy reaches 30 mJ. At each wavelength, each transient ICR signal from a single laser shot was accumulated four times and averaged to yield a mass
spectrum. A typical shot-to-shot delay was 3 s only limited by the pumping speed. Both CH3CHO and CH3CDO samples were purchased from Aldrich Chemicals Inc. and used after several freeze-pump-thaw cycles. Computational Details. Since the spin state of the acetaldehyde molecular ion is doublet, the restricted open-shell Hartree-Fock (ROHF) wave function, which is an eigenfunction of the spin-squared operator, is used as a starting configuration. With the core orbitals kept frozen, a set of nine reference configurations, which is of the full optimized reaction space type, is selected by allowing a single excitation within the valence orbital space, designated as ROHF*Sval. The ROHF*Sval wave function is also the eigenfunction of the spin. A multireference configuration interaction (MRCI) wave function is composed of 5598 configuration state functions in the C1 symmetry that result from the single excitation out of the nine reference configurations to all the virtual orbitals. The virtual orbitals are constructed by optimizing the ROHF wave function for a doubly-charged cation. The resultant MRCI wave function is denoted [ROHF*Sval]*Sall. The variational determination of the CI coefficients allows the overall wave function to relax so that the [ROHF*Sval]*Sall wave function represents an excited state rather than an ionized state. Allowing both positive and negative combinations of R and β excitations from the valence to virtual orbitals helps conserve the spin state. Geometries for the first three low-lying electronic states were optimized by minimizing the total energy at the [ROHF*Sval]*Sall level with a standard 6-311G** basis set using a GAMESS program.18 Zero-point vibrational energies are not corrected for. Results REMPI Spectrum. The mass-selected MPI spectra of the 3s r n Rydberg transition in acetaldehyde are presented in Figure 1. Both HCO+ and CH3CO+ ions yield identical REMPI spectra over the entire wavelength range, suggesting that both
8282 J. Phys. Chem., Vol. 100, No. 20, 1996
Shin et al.
Figure 2. Power dependences of REMPI dissociation of CH3CHO at the origin of the 3s r n Rydberg transition. A partial pressure of acetaldehyde is 5 × 10-7 Torr. The relative errors are ∼4% at the maximum laser power and ∼10% at the minimum laser power.
ions are derived from a common excited state. The REMPI spectra are in excellent agreement with the previously reported spectra obtained in the wavelength range 364-358 nm.7,8 The most intense feature in the spectrum is the 000 band at 363.48 nm. The deuterium substitution in place of aldehydic hydrogen shifts the Rydberg transition origin to the blue by 70 cm-1. Other prominent features to the blue include the ν10 in-plane CCO deformation (1010), the ν9 symmetric methyl rock (910), the ν8 symmetric C-C stretch (810), the ν7 symmetric methyl deformation (710), the ν6 in-plane CH wag (610), and the ν5 symmetric methyl deformation (510). Some combinations with the ν15 methyl torsions are also active. Combinations with 1520 appear to be quite active due to the vibrational symmetry. The vibrational assignments are based on the previous VUV spectral assignments by Creighton and Bell.19 All the active in-plane modes indicate the planar structural changes in going from the ground state to the Rydberg state. The ab initio calculation using a single-excitation correlation interaction (CIS) method12 has shown that the CCO angle increases by 5°, the C-C bond extends by 0.07 Å, and the CCH angles decrease by 4-6°, which are in good agreement with the active modes observed in the REMPI spectra. Power Dependence of REMPI Process in the FT-ICR. The power dependence of ion intensity was examined to help establish the mechanisms of photodissociation.6,9 First, the ICR detection linearity was tested by examining the pressure dependence of ion intensity at the origin of Rydberg transition with a pulse energy of 7 mJ. Neat acetaldehyde was introduced into the ICR chamber through a leak valve. Ion intensities vary linearly with the pressure in the range 2.0 × 10-7-2.0 × 10-6 Torr. Further increase of pressure led to a logarithmic increase of ion intensity due to the collisional damping of ICR trajectories. The power dependence of ion intensity was then examined with neat acetaldehyde at the linear detection regime of 5.0 × 10-7 Torr. Figure 2 shows the power dependence from CH3CHO at the origin of Rydberg transition in the pulse energy range 0.8-27 mJ. The parent ion shows a slope of ∼1, indicating the saturation of two-photon resonant Rydberg transition at the band origin. The daughter ions show the slopes of ∼1.4, implying that one-photon excitation of the parent ion is very efficient. The rate equation model6,9 suggests that such power dependencies may arise at high photon fluxes if the ionization is the rate-determining step and if one-photon dissociation is dominant. The parent ion is not the favored species in all parts of a saturated volume under the present experimental condition.
Figure 3. MPI mass spectra of (a) CH3CHO and (b) CH3CDO taken at their origins of two-photon resonant 3s r n Rydberg transition. A pulsed molecular beam of a 5% acetaldehyde/He mixture was used with a laser power of 4 mJ/pulse.
Figure 4. Temporal variations of ion intensities of CH3CO+ and the m/z ) 44 ion (CH2DCO+) in the presence of neat acetaldehyde. Slow increases of ion intensities are due to the slow drift in the pulsed valve condition.
Photodissociation Mass Spectra. The MPI mass spectra of CH3CHO and CH3CDO at their origins of resonant two-photon transitions are shown in Figure 3. The parent ion dissociates on a time scale faster than our detection limit of 1 µs.14 None of the CH2+, CH3+, and CH4+ ions were observed with CH3CHO, which is in sharp contrast to the previous observation by Fisanick and Eichelberger.6 No such ions were observed with CH3CDO. The small peaks at m/z ) 14.5 and 15 in Figure 3a,b are the second harmonics of HCO+ (m/z ) 29) and DCO+ (m/z ) 30) ICR frequencies, respectively. Very little HCO+ ions were produced from CH3CDO, suggesting little hydrogen scrambling during the C-C bond cleavage. In contrast, Fisanick and Eichelberger6 observed the hydrogen-scrambled product, DCO+, from the MPI dissociation of CD3CHO. On the other hand, the C-H bond cleavage yielded both CH3CO+ (m/z ) 43) and the m/z ) 44 ion. To determine the structural identity of the m/z ) 44 ion, its bimolecular reactivity with the parent neutral was examined. Figure 4 shows the temporal variations of the m/z ) 44 ion from the β C-H bond cleavage and those of the CH3CO+ ion from the R C-D bond cleavage in the
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Figure 5. He(I) photoelectron spectrum of acetaldehyde taken from the ref 22 and the potential energy level diagram.
presence of CH3CDO. Both ions were unreactive toward the parent neutral. The product branching ratio of [m/z ) 44]/[CH3CO+] ) ∼0.21 from CH3CDO is in good agreement with the previous observation from the MPI dissociation of CD3CHO.6 The branching ratio of the C-C to C-H bond cleavage processes, [HCO+]/[CH3CO+], remains constant at ∼2.1 ( 0.2 over the entire wavelength range. A deuterium substitution on the R-position decreases the ratio of the daughter ion yield of the R C-D bond cleavage to that of the R C-C bond cleavage relative to the branching ratio from CH3CHO. The relative R cleavage branching ratio from CH3CDO was independent of laser power in the range 1-20 mJ/pulse. Discussion To establish the mechanisms of photodissociation of acetaldehyde molecular ion, the present results are combined with the previous PIMS,3 PEPICO,4,5 and photoelectron spectroscopic (PES) studies.20-22 Structural and energetic information are obtained from ab initio calculations. Figure 5 shows a He I photoelectron spectrum22 and the energy levels involved in REMPI dissociation processes of acetaldehyde. The π* r n excitation of acetaldehyde with the band origin at 3.69 eV (335.9 nm) is outside the present excitation wavelength range.23 Acetaldehyde molecular ions are produced from the resonant two-photon transition combined with one-photon ionization. The vertical ionization potentials of acetaldehyde to the X ˜, A ˜ , and B ˜ bands of the parent ion are 10.23, 13.15, and 14.1 eV, respectively.20-22 The adiabatic ionization potentials are estimated to be 10.229, 12.63, and 13.6 ˜ and B ˜ ionic states are ( 0.1 eV, respectively.20-22 Both the A energetically accessible from the ionic ground state in the photon energy range 3.41-3.50 eV employed in the present study. Other closely lying product channels are also included in Figure 5. The previous PEPICO and PIMS studies have shown that the CH3+ ion appears at the onset of 14.08 eV and becomes the most abundant species above 15.0 eV.3-5 Since the formation of CH3+ requires at least two-photon absorption, its intensity in Figure 3 suggests that the two-photon absorption by the parent ion is negligible. Meanwhile, the absence of ˜ state, CH4+, the formation of which is a characteristic of the A implies that the A ˜ rX ˜ transition is not involved. No CH3D+ ions were observed from CH3CDO. The absence of both CH3+ and CH4+ ions indicates that the parent ion is excited to the B ˜ state by one-photon absorption. The three low-lying electronic states of the acetaldehyde ˜ 2A′′, and B ˜ 2A′ states. The molecular ion are the X ˜ 2A′, A
Figure 6. Geometries of acetaldehyde. Geometries of the neutral states are taken from a CI singles calculation by Buntine et al. (ref 12). Geometries of the ionic states are optimized at the [ROHF*Sval]*Sall level with the 6-311G** basis set.
calculated energy levels for the A ˜ and B ˜ states are 2.68 and 3.43 eV above the ground state at the [ROHF*Sval]*Sall level, which are in excellent agreement with the difference in adiabatic IPs of 2.40 and 3.4 eV, respectively. Figure 6 shows geometries of acetaldehyde. The neutral geometries are taken from the CI singles calculation by Buntine et al.12 The geometries in the Rydberg and ionic ground states are quite similar. The ground state of the parent ion has an eclipsed conformation (one methyl hydrogen eclipsed with oxygen), while both the A ˜ and B ˜ states of ion are in the staggered conformations (one methyl hydrogen eclipsed with aldehydic hydrogen). The electronic transitions to both the A ˜ and B ˜ states may result in a free internal rotor molecule. The most significant structural changes in going from the X ˜ state to the A ˜ state via an n r nσ transition are the increase of C-O bond length by 0.138 Å (12%) and the decrease of C-C bond length by 0.141 Å (9%). The B ˜ rX ˜ excitation via an n r πCH3 transition accompanies the decrease of C-C bond length by 0.081 Å (5%) and the concomitant increase of eclipsed methyl C-H bond by 0.143 Å (13%) as well as the increase of dihedral angle between staggered methyl hydrogens from 117.2° to 153.8°. A calculated transition moment for the A ˜ r X ˜ excitation is close to zero. A nonzero transition moment is obtained for the B ˜ r X ˜ excitation, which supports our conclusion that the parent ions are excited dominantly to the B ˜ state. The absent formation of hydrogen-scrambled HCO+ from ˜ state CH3CDO suggests that the R C-C cleavage from the B does not involve a long-lived intermediate that scrambles hydrogens. The appearance of HCO+ from the VUV photo˜ ionization of CH3CDO at 12.67 eV may be related to the A state.3 Both the CH3D+ and HCO+ ions observed in the PIMS studies of CH3CDO might have been derived from a common reaction intermediate, such as a CH3D+‚‚‚CO complex formed after an intramolecular rearrangement from the A ˜ state. The appearance of the m/z ) 44 ions from CH3CDO raises an interesting question about their origins and identities. The sample contamination by CH3CHO is most unlikely because
8284 J. Phys. Chem., Vol. 100, No. 20, 1996 the REMPI of CH3CDO is optically selected by tuning the wavelength at the origin of CH3CDO. If any contaminated CH3CHO were present, the nonresonant MPI would have produced a sizable HCO+ peak. No such ions were detected. The thermoneutral electron transfer from CH3CDO+ to CH3CHO is also precluded as a source because the electron-transfer reaction is too slow to be observed under the present experimental condition. Thus, the m/z ) 44 ions are solely derived from the REMPI dissociation of CH3CDO. The identity of the m/z ) 44 ion is also intriguing. The formation of the formylmethyl ion (CH2CDO+) is one possibility, but it is not probable because a direct bond dissociation requires at least 3.93 eV to break the methyl C-H bond.24 Interestingly, the bimolecular reactivity of the m/z ) 44 ion resembles that of the acetyl ion in the presence of acetaldehyde as shown in Figure 4. The acetyl ion is unreactive toward acetaldehyde because the proton affinity (PA) of ketene (PA ) 198 kcal mol-1) to form an acetyl ion is 12 kcal mol-1 higher than that of acetaldehyde (PA ) 186 kcal mol-1).25 If the m/z ) 44 ion were the formylmethyl ion (CH2CDO+), it would have reacted with the parent neutral to form deuteronated acetaldehyde (CH3CDOD+) and ketene. The double-resonance reactivity studies reveal that the CH3CDOD+ ions are originated from DCO+. The identical bimolecular chemical reactivities of the m/z ) 44 ion with those of CH3CO+ in the presence of CH3CDO lead us to conclude that the m/z ) 44 ion is the acetyl ion, CH2DCO+. The structural identification of the β C-H bond cleavage product as the acetyl ion implies an intramolecular rearrangement of hydrogens. Radom and co-workers24 have reported corroborative theoretical results that the transition state for the 1,2-hydrogen migration from the formylmethyl ion to the acetyl ion is lower in energy than the formylmethyl ion itself. Since the B ˜ state has one elongated C-H bond in the methyl group and a wide dihedral angle of 154° between the other two C-H bonds in the methyl group, it is conceivable that the cleavage of eclipsed β C-H bond may be assisted by the in-plane CCO deformation and C-D bending vibrations, that results in activating the 1,2-deuterium migration. However, the β C-H bond cleavage process may not involve any intermediates. There are several pieces of experimental evidence against the formation of long-lived ion-molecule complexes, such as CH3CO+‚‚‚H. First, the branching ratio of C-H(D) bond cleavage, [CH3CO+]/[CH2DCO+] ) 4.7, is far from being statistical, [D]/[H] ) 0.33 in CH3CDO. Second, the primary isotope effect is observed in the R cleavage branching ratio: [HCO+]/[CH3CO+] ) 2.1 Vs [DCO+]/[CH3CO+] ) 5.7. Lastly, the ionmolecule interaction between atomic hydrogen and the acetyl ion is too weak to influence the exit channel reaction dynamics. Fisanick et al.6 have carried out the MPI mass spectrometric studies of CD3CHO at a low photon flux using a quadrupole mass spectrometer. They have observed extensive hydrogen scramblings. Since their branching ratios were affected severely by the kinetic shift and the spatial filtering of the ion collection optics,6,9 direct comparisons cannot be made. Conclusion State-selected photodissociations of acetaldehyde molecular ion reveal the mechanisms of photodissociation and hydrogen scrambling. The B ˜ state ion dissociates to three channels: (i)
Shin et al. an R C-C bond cleavage, (ii) an R C-H bond cleavage, and (iii) a β C-H bond cleavage. The R C-C bond cleavage yields predominantly the formyl ion; there is no indication of hydrogen scrambling and methyl ion formation. The isotope effect is very pronounced in the R C-H(D) bond cleavage. In light of ab initio theoretical calculations combined with bimolecular reactivity studies, it is suggested that the β C-H bond cleavage occurs with 1,2-hydrogen migration to form an acetyl ion. It is also suggested that the photodissociation from the B ˜ state occurs without involving any long-lived intermediates. Acknowledgment. This work was supported by the National Science Foundation Grant CHE-9302959 and the National Science Foundation Young Investigator Award CHE-9457668. Acknowledgment is made to the donors of the Petroleum Research Fund (Grant 25423-G3), administered by the American Chemical Society, for the partial support of this research. A new FT-ICR data system was purchased with the partial support from the Camille and Henry Dreyfus Foundation. This work was also made possible by the Santa Barbara Laser Pool under NSF Grant CHE-9413030. Special thanks go to Professor Mike Bowers for the use of the ICR setup and the computer time of an IBM/RISC-6000 workstation. References and Notes (1) Minot, C.; Anh, N. T.; Salem, L. J. Am. Chem. Soc. 1976, 98, 2678. (2) Yamashita, K.; Kato, S.; Yamabe, T.; Fukui, K. Theor. Chim. Acta 1978, 49, 25. (3) Jochims, H. W.; Lohr, W.; Baumga¨rtel, H. Chem. Phys. Lett. 1978, 54, 594. (4) Bombach, R.; Stadelmann, J.-P.; Vogt, J. Chem. Phys. 1981, 60, 293. (5) Johnson, K.; Powis, I.; Danby, C. J. Chem. Phys. 1982, 70, 329. (6) Fisanick, G. J.; Eichelberger, T. S. J. Chem. Phys. 1981, 74, 6692. (7) Heath, B. A.; Robin, M. B.; Kuebler, N. A.; Fisanick, G. J.; Eichelberger, T. S. J. Chem. Phys. 1980, 72, 5565. (8) Eichelberger, T. S.; Fisanick, G. J. J. Chem. Phys. 1981, 74, 5962. (9) Fisanick, G. J.; Eichelberger, T. S.; Heath, B. A.; Robin, M. B. J. Chem. Phys. 1980, 72, 5571. (10) Gordon, R. D. J. Chem. Phys. 1980, 73, 5907. (11) Gu, H.; Kundu, T.; Goodman, A. J. Phys. Chem. 1993, 97, 7194. (12) Buntine, M. A.; Metha, G. F.; McGilvery, D. C.; Morrison, R. J. S. J. Mol. Spectrosc. 1994, 65, 12. (13) Comisarow, M. C.; Marshall, A. G. Chem. Phys. Lett. 1974, 25, 282. (14) Shin, S. K.; Han, S.-J.; Kim, B. J. Am. Soc. Mass Spectrom., in press. (15) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys. Chem. Ref. Data 1977, 6 (Suppl. 1). (16) Beggs, C. G.; Kuo, C.-H.; Wyttenbach, T.; Kemper, P. R.; Bowers, M. T. Int. J. Mass Spectrom. Ion Processes 1990, 100, 397. (17) Wyttenbach, T.; Bowers, M. T. J. Phys. Chem. 1992, 96, 8920. (18) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. GAMESS IBM (AIX) Version ) 1 Feb 1995; J. Comput. Chem. 1993, 14, 1347. (19) Crighton, J. S.; Bell, S. J. Mol. Spectrosc. 1985, 112, 285. (20) Chadwick, D.; Katrib, A. J. Electron Spectrosc. 1974, 3, 39. (21) Cvitasˇ, T.; Gu¨sten, H.; Klasinc, L. J. Chem. Phys. 1976, 64, 2549. (22) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules; Halsted Press: New York, 1981. (23) Lee, E. K. C.; Lewis, R. S. AdV. Photochem. 1980, 12, 1. (24) Nobes, R. H.; Bouma, W. J.; Radom, L. J. Am. Chem. Soc. 1983, 105, 309. (25) Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1984, 13, 695.
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