lOlS0
J. Phys. Chem. 1992,96, 10150-10154
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Electronic Spectrum of Allyl and AllyCds Radicals. The B[l2A1] C[22Bl] X[12A2], and D[12B2] X[1*A2] Band Systems
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X[12A2],
Joel A. Blush, David W.Minsek, and Peter Chen*.' Mallinckrodt Chemical Laboratory. Harvard University, Cambridge, Massachusetts 02138 (Received: May 21, 1992: In Final Form: July 8, 1992)
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Three electronic band systems, D[12B2] X[l2A2],C[2'Bl] X[l2A2]. and B[12Al] X[lZA2],are found and analyzed for both C3H5and C3D5between 238 and 250 nm by resonant 1+1 multiphoton ionization. Partially-resolved rotational structure is simulated and fit to determine the upper-state vibronic symmetry of each band. Assignment of each band was then done using symmetry, isotope shift, and normal-mode information. The appearance of nominally forbidden bands in the B[ 12Al] X[12A2]system is rationalized by the proposed nonplanarity of the upper state of the transition. The resulting double-well potential causes large inversion doubling of B-state levels, which explains anomalies in both the 1+1 and 2+2 MPI spectra of allyl radical.
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We report the rotational and vibrational analysis, and assignments, for three overlapping electronic hnds of the C3H5and C3D5 radicals, observed by resonant 1+1 multiphoton ionization (MPI) in the 234-250-nm region. An analysis of the origin band of the C[22Bl] X[12A2] system was the subject of a preliminary report.2 In this work, we complete the analysis and assignments of all the large bands between 238 and 250 nm. Allyl radical, C3H5,is the simplest hydrocarbon *-radical and is the prototype for extended conjugated systems with an odd number of electrons. Pedagogically, the structure, thermochemistry, and excited-state manifold of the allyl radical are often used as examples for molecular orbital treatments, from simple Hiickel to ab initio, of delocalized systems. It is surprising, in this context, that there is not much more spectrosoopicdata for this fundamental molecule. Structural parameters, e.g., bond lengths and angles, have been extracted from ESR3 and gas-phase electron diffraction4studies of the radical. Vibrational frequencies have been measured in matrix-isolation infrared5v6and gas-phase resonance Raman experiments.' An absorption spectrum around 404 nm was reported by Currie and Ramsay,* following flash photolysis of allyl precursors, and assigned to the lowest energy valence excitation between the bonding and nonbonding r-orbitals. Subsequently, a more intense band system, near 224 nm, was found by Callear and Leeg in a similar experiment. The integrated absorption cross section of the latter band system was measured by Nakashima and Yoshihara.lo While resolvable vibrational structure was observed in each of these studies, no vibronic assignments were made because the spacings and intensities were irregular. One further excited state, which we have designated the B[ f2Al]state, has been ~ b s e r v e d ~by' . ~2+2 ~ resonant MPI and assigned to the 3s Rydberg state forbidden in onephoton absorption. The origin of that band system was reported to be 40056.8 cm-', equivalent to a onsphoton wavelength of 249.65 nm, and several vibrational intervals in that excited state were extracted from the spectrum." Molecular orbital calculations for allyl radical have also been reported.13 Our preliminary work2on the electronic spectrum of allyl radical identified the sharp feature at 248.15 nm as the origin band of the C[22Bl] X[12A2]system, allowed in one photon. We had also realized that, counting from the C-state origin band to the blue, there were far too many vibronic bands than could be accounted for by the number of low-frequency vibrations in the molecule. Given the close proximity of the C[22Bl]state to at least one other known state, the B[l2A1]state, and several other predicted we anticipated that the rich vibronic structure in our spectrum could result from interactions between levels nominally associated with several electronic states. Using rotational contour simulations, isotopic shifts, comparisons to predicted spectra, and symmetry arguments, we have deconvoluted and assigned bands nominally belonging to three electronic transitions in the region between 238 and 250 nm: D[l2B2] X[12A2],
SCHEME I: 1+1 Multiphotoa Idzrtion Scheme for AUyl Radicala
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H
H observed stntes
OAb initio vertical excitation energies in eV (1 eV = 8065 cm-l) are taken from ref 13.
C[2*Bl]+X[12A2], and B[12Al]+X[12A2], for both C3H5and C3D5. This is shown pictorially in Scheme I. Experimental Seetion The production of C3H5and C3D5radicals in a supersonic jet expansion by flash pyrolysis of 3-iodopropme and 3-iodopropenad5 has been described in detail in our previous reports2 The nozzle daigd4and the molecular beam timesf-fight mas spectrometer have also been previously described.ls The 1+1 resonant MPI spectra were obtained piecewise with an injection seeded Nd3+:YAG-pumpddye laser (Spectra-Physics GCR-3G, PDL-2, PDL-3) operating with DCM and Rhodarmn ' e 640 laser dyes. The dye laser output was doubled and mixed with the YAG fundamental (Spectra-Physics WEX-ld) to obtain tunable f a r - W with either 0.5 cm-' (survey scans) or 0.1 cm-l (close-up) spectral
0022-365419212096-101 50$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10151
Electronic Spectrum of Allyl and Allyl-d5 Radicals
234
236
238
240
242
244
246
248
250
wavelength (nm) Fiopre 1. Survey scan of C3H5from 234 to 250 nm. Bands are dcsignated as by [state $mode] as described in the main text. The 234238-nm region is a vertically expanded by a factor of 10. Laser bandwidth is 0.5 cm-'.
245.5 nm
246.1 nm
246.7 nm
Figure 5. C3H5 bands near 246 nm at 0.1 cm-I laser bandwidth, and simulation as a composite of three bands, marked a (type A), b (type B), andc(typeA),withLCCC = 118.8', 119.95O,and 118.8°,respsCtively. The three bands are assigned to B b12, B b9, and C b7. The "F' bandheads are associated with B i9.
experimental spectrum n
c
wavelength (nm) Figure 2. Survey scan of C3D5from 238 to 250 nm. Bands are dcsig
nated as by [state $mode] as described in the main text. Laser bandwidth is 0.5 cm-'.
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n
I
I
I
242.6 nm
243.4 nm
244.1 nm
Figure 6. C3Hs bands near 243 nm at 0.1 cm-I laser bandwidth, and simulation as a composite of three bands, marked a, b, and c, all type A, with LCCC = 119.4', 118.8', and 119.5', respectively. The three bands am uigned to B bll, C i7,and B ;lo. The marked QPbandheads belong to the B A1 1 and B A10 bands.
y.)
experimental spectrum
experimental
dL I
specrum
"
Kn
w h
-rInaf-"~=
spectrum
I
I
249.2 nm
249.6 nm
4
C-type simulated spectrum
I
250.0 nm
Flgore 3. C3Hsscan over the Bt band region at 0.1 cm-'laser bandwidth and simulation as a type A band with LCCC = 115.0'.
K'
0
n
A-type
simulated
spectrum I
241 0 nm
242.2 nm
242.7 nm
Fiplm 7. C3Hs bands near 242 nm at 0.1 cm-l laser bandwidth, and simulation of the two component bands, marked a and b. Band a is type A with LCCC = 119.6O and QPbandheads as indicated. Band b & type C with LCCC = 119.55' and "F' bandheads as indicated. The two bands arc assigned to C and B b17, respectively.
A
bandwidth. The far-UV was separated from the otha wavelengths with an INRAD four-prismharmonic separator. The spectra were r d e d for m / e = 41 or 46 ion peab. Rotational contours wcrc simulated with the ASYROT F C program.lb
H 1%
.-.-
247.7 nm
240.2 nm
240.7 nm
Figm 4. C3H5scan over the c",band region at 0.1 cm-' laser bandwidth and simulation as a type A band with LCCC = 117.5'.
R d b and Discussion Survey spectra for C3HSand C3DS, constructed as composites of several shortcrscans, arcshown in F@ra 1 and 2. The bands arc deaignated accofdinB to the nominal electronic and vibratiohal assignment of the upper state of the transition. For convenience,
Blush et al.
10152 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
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experimental spectrum
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zzz sss
g
overlapping
!g
0
0
1
8 8$,$9
r.-NO1LD-
h h
A-type and C-type bands
simulated
n
241.8 nm
242.2 nm
246.2 nm
242.7 nm
Figure 8. C3H5 bands near 242 nm at 0.1 cm-I laser bandwidth, and simulation as a composite of the C A6 and B A17 bands in Figure 7.
246.8 nm
fi
2 :
c)
m
CIN
$ 5
sub-bondheads
experimental
247.4 nm
Figure 11. C3D5bands near 246.5 nm at 0.5cm-l laser bandwidth, and simulation as a compositeof three bands, marked a (type A), b (type B), and c (type A), with LCCC = 118.8O,119.95', and 118.8O,respectively. The three bands are assigned to B Q12,B b9, and C b7. Each group of P bandheads is associated with the vibronic origin to their blue. 0
t
N N--
mmm0D
0 0 0 0
t t t t
22
'P-branch
h WL
spectrum
sub-bondheads
-h 3 1 0h
o s
experimental spectrum
simulated spectrum
A
2 I
I
.-mc '5
2
w
9 0
D
I
9
241.2 nm
FIppe 9. C3H5scan over the D8 band region at 0.1cm-'laser bandwidth and simulation as a type C band with LCCC = 119.45'.
K'd
0)
O n
1
240.7 nm
240.1 nm
.-c .-0
simulated spectrum
w I
t
t
t
244.2 nm
244.7 nm
245.3 nm
Figure 12. C3D5bands near 245.5 nm at 0.5cm-l laser bandwidth, and simulation as a composite of three bands, marked a, b, and c, all type A, with LCCC = 118.8O,119.4O,and 118.8O,respectively. The threc bands are assigned to B ill, B ;IO, and C i7. experimental -7
sz ;I
f?Qf'91?
9- c! ?
7
c.
RSZNGZZ 8t 1g IP vI**Y)Y.IY)YIY)ttttt
-------------
ttttttttttttt
experimental
'P-branch
spectrum
-c)
simulated spectrum
=--z=
sub-bondheads
K:
I
I 248.0 nm
248.3 nm
248.6 nm
band region at 0.1 cm-l laser bandwidth and simulation as a type A band with LCCC = 117.6O.
Figure 10. C3D5scan over the
a labeling scheme using ground-state normal-mode designations for ref 7 mode) is used even though it is evident that substantial vibronic mixing of levels associated with the three zero-order excited electronic states could spoil the labeling scheme. Higher resolution scans over -1-nm intervals are shown in Figures 3-13. The figures include both the experimental spectrum and a rotational contour simulation for the partially resolved rotational structure of each band. For overlapping bands, the simulation is a composite of those for the individual components of the convolution. Figure 7 shows a typical example of a badly overlapping pair of bands pulled apart into two components. The composite simulation for the same region is shown in Figure 8. Allyl is a near-prolate asymmetric top, K = 4 . 9 , so Kais an approximate rotational quantum number. The ground-state rotational constants from the electron diffraction study' are A = 1.803, B = 0.328,and C = 0.278 cm-'.LWar branch designations are used in the figures. The underlying J structure of the K,
G,
I
I
240.5 nm
240.9 nm
I
241.2 nm
Figure 13. C3H5scan over the D: band region at 0.5 cm-'laser bandwidth and simulation as a type C band with LCCC = 119.45O.
subbands is unrtsolved. Most of the asymmetrically-shaped "peaks*cormpond to P-branch bandheads for a series of subbands differing in K,. The extended P-branchesare indicative of a large decrease in the A rotational constant upon electronic excitation. The shading to the blue of each P-branch subbandhead indicates that (B C)/2 increases going from the ground state to the excited state. The resolution of the spectrum is insufficient to allow a complete determination of the upper- and lower-state rotational constants. For the simulations, we employ the same model that we had used previously:2 the ground-state geometry is fured at that reported by Vajda et al.' by electron diffraction, and the excited-state geometry is taken to differ only in the central CCC bending angle. A sensitivity analysis of the rotational
+
Electronic Spectrum of Allyl and Allyl-d, Radicals
TABLE I: Nominal B d Assignments d Positions for CjHs and
cp,. C3H5 nominal assignment
4 D7
cBg517 C)5 B 10 cy7
B$l
3; B+ By12 B7
4
vibronic band origin maximum (cm-I) (nm) 238.39 41946.6* 240.60 41557.8 241.31 41451.0* 241.92 41317.6 242.05 41324.4 41145.9 243.09 41080.6 243.49 243.63 41056.4 245.03 40816.8* 245.80 40691.5 246.09 40647.3 40628.2. 246.19 247.34 40435.7* 40305.5 248.15 249.68 40056.5
ClDS vibronic band origin maximum (cm-I) (nm) 41860.9 238.87 240.77 41532.1 41165.0* 242.98 41033.6* 243.69 243.24 41120.9* 40878.0 244.67 40934.1 244.34 40837.7 244.92 245.52 40734.4* 40609.1 246.30 40516.9 246.87 40492.0 247.01 40415.1. 247.46 40286.7 248.26 40095.9* 249.43
'Vibronic origins are accurate to *0.5 cm-' in absolute calibration. Entries marked with an asterisk are h2.0 cm-l. The band designations do nor include explicit specification of inversion doublet components. Extraction of vibrational intervals from this table would require additional information to complete the extended designations (see text).
constants to geometric distortions fmds that, while most distortions cause modest changes in rotational constants, the A constant varies from =0.3 to =5 cm-l as the CCC angle goes from 60° to 180O. Chemical intuition also finds that angle to be strongly affected by electronic excitation; e.g., allyl is isoelectronic with NOz. We therefore simply varied the excited-state CCC bending angle until a good fit to peak positions and intensities in the rotational contour was obtained. With only one adjustable angle, known rotational line-strength factors, known nuclear statistical weights, a line width, and a temperature, the simulation is completely determined. Type A, B, and C bands look different, and we can be confident that deconvolution of those regions where there are badly overlapping bands can be done in a nonarbitrary fashion. A sensitive test of the simulations comes from comparison of the C3H5to C3D, spectra. Even an approximate knowledge of the excited-state normal modes,[' based on analogy to those in the ground state, yields isotope shifts for each band, and tentative correlations between spectra for different isotopmers. Deuteration also c a w a large decrease in the A rotational constant, but almost no change in the B and C constants. Consequently, K changes, which affects both the rotational level spacings and the rotational line-strength factors. For a given geometry, the band for C3D5 does not necessarily look like a scaled version of the same band for C3H5. Fit of the rotational contours of a C3Dsband, and the corresponding C3HSband, should give the same geometry. The good agreements, where possible, testify to the reliability of our model and assignments. For some of the other bands, the signal-to-noise ratio for one isotopomer or the other is insufficient to allow detailed fits for both. We assume, then, that the geometry obtained from the fit on one isotopomer can be used for the other to estimate the frequency difference between an observed band maximum and the vibronic origin. A similar procedure can be used to estimate the frequency difference between the band maxi" and vibronic origin of one band using the geometry from another related band within one isotopic series. The vibronic origins estimated by either of these procedures are marked with an asterisk in Table I. Assuming Cb symmetry for both the ground and excited states of allyl radical, the rotational contours of each band tell us the vibronic symmetry of the upper state (because the ground state is Az). Taking a coordinate system in which a planar radical lies in the y z plane and the Cz axis is the z axis, a transition moment oriented along the x, y, and z axes gives type C, A, and B bands, which correspond to B2, BI, and Az vibronic symmetries, respectively, for the upper state. Using the symmetries of the normal modes, the band types, and isotope shifts, each band was assigned
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10153 to a vibronic transition. The results are shown in Table I. The frequencies do not correspond to the observed band maxima because the tabulated values are for the vibronic origins of each band extracted from the fits. All of the large bands and most of the small bands between 238 and 250 nm can be assigned to transitions terminating in levels built on three electronic origins: B[ 12Al], C[22Bl], and D[12Bz]at TO= 40056.5,40305.5, and 41 557.8 cm-], respectively, for C3H5. For C3D5the corresponding B[I2A1], C[22Bl], and D[l2B2] electronic origins are To = 40095.9, 40286.7, and 41 532.1 cm-'. The relative intensities of some bands, e.g., the $7 progression built on the C-state electronic origin, behave as one would expect from simple Franck-Condon considerations. As would be expected for allowed transitions, the totally symmetric vibrations, us, 4, and v7, are all active in the C[22Bl]state. Similarly, v7, at least, is active in the D[12Bz]state. However, the appearance of numerous bands assoCiated with the electronically forbidden B[lZAl] state is, at first glance, surprising. While there are differences in one and two-photon selection rules and rotational linestrength factors which make the band maximum in the present work not directly comparable to that in the two-photon spectnun by Sappey and Weisshaar,lZthe upper-state origin of their transition lies within a few wavenumbers of that reported here, and, almost certainly, is the same state. Vibronic coupling'* can rationalize some of the observed B-state levels. All three bl normal modes,19 vl0, vll, and v12, are active in the B state, and couple B-state levels to the C[22Bl] state origin from which oscillator strength is borrowed. The one active b2 mode, VI79 corresponds to a B[12Al] state vibronic level that lies only 240 cm-'to the red of the D [12Bz] state origin from which it borrows intensity. Upon deuteration, the B[12A1](~17=1) level shifts to lower energy much more than does the D[ 12Bz]origin level, increasing the energy gap, decreasing the vibronic coupling, and reducing the relative intensity of the B A17 transition of C3D5vis-&vis that for C3HS. Even with vibronic coupling, three anomalies remain unsolved. (1) A simple vibronic coupling mechanism cannot explain the observable B: band or its $7 progression. The rotational contours show type A band profiles, which would suggest that intensity is borrowed from nearby C[22Bl]state levels despite the absence of a coupling vibration of the right symmetry. (2) The B 19 band, identified by its rotational contour as type B (az vibronic symmetry), is formally allowed, but there is no nearby Az electronic state from which it can borrow intensity. (3) Sappey and Weisshaar,12 and Hudgens and Dulcey," in their 2+2 spectra, also find the B,: B $7, B 19, and B 112 bands. For only the B! band does their frequency match ours. The differences between B-state vibrational frequencies, derived from one-photon versus two-photon spectra, are 14 cm-'for v7, 62 cm-'for v9, and 81 cm-I for uI2 for C3H5. We do not find bands at their positions in our 1+1 MPI s p e c " , and they did not find bands at our positions in their 2+2 MPI spectra. We suggest a solution that rationalizes all three anomalies. If the B state is slightly nonplanar such that the symmetry at its potential minimum is reduced from Cb to C, ( u remaining), ~ then additional interactions occur. Two equivalent pictures can be used to identify the new interactions and allowed transitions. If we work with C, symmetry, al and b, correlate to a', while az and b2 correlate to a". The slight nonplanarity would induce a small mixing of the B and C states, giving oscillator strength to the B: and B $7 bands with type A profiles. The B and D states would be mixed by an a2vibration (a" in C,), rationalizing the B b9 band. A nonplanar B state would necessarily have a doublewell potential with inversion doubling of levels at or below the top of the hmer, so, more properly, we can work with C b labels for levels split by that inversion doubling.20 For a barrier that is several hundred cm-' high, the inversion splittings would be large for the first few bands associated with low-frequency vibrations in the B state. In complete analogy to the near-W band system of formaldehyde?' for which the spectroscopicconsequences of a planar-tenonplanar transition have been worked out in detail, an one-photon vibronic transition from the vibrationless planar ground state of allyl radical to the B state is allowed to only one component (the upper, "minus"
Blush et al.
10154 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
despite the near-monotonic increase of the intensities reported by Callaer and Lee9 going from 248 to 224 nm. Because the MPI spectrum is really a mass excitation spectrum, as opposed to an absorption spectrum, a fast excited process in competition with ionization can quench the MPI spectrum. Without more detailed information on the perturbing states and the nature of the interaction, one cannot predict with certainty which levels will remain observable by MPI. The relatively sharp bands to the blue of 238 nm could possibly correspond to weak transitions that are now seen only because stronger bands have been eliminated. The dynamical process whose onset decreases excited-state lifetimes can be plausibly attributed to a photochemical valence isomerivltion to cyclopropyl radical predicted” at various levels of theory. Photochemical pericyclic reactions of free radicals are largely unexplored and would be accessible now that the allyl electronic spectrum has been assigned.
Figure 14. Comparison of allowed one-photon (solid arrow) and two-
photon (dashed arrow) transitions from the vibrational ground state of a planar A2 electronic state to vibronic levels of a nominal AI electronic state with excitation of a bl vibration. The levels are labeled according to their uibronic symmetry, rel@ I’db. In case a, there is a double well in the upper state due to nonplanarity of the Al state, induced by a bl distortion. The v’ = 0 and v’ = 1 levels are split by inversion doubling into O+, 0-, 1+, and 1- components. In case b, the upper state is planar. component with bl vibronic symmetry) of each inversion doublet. The corresponding two-photon transition would be allowed only to the other component (the lower, ‘plus” component with a l vibronic symmetry). This also neatly explains why the v9 and vI2 fundamentals were seen at all in the 2+2 MPI spectrum where only totally symmetric modes should have been active.u Together, the one-photon and two-photon spectra have mapped out the inversion splittings and should enable a fit to a model potential function. Rigorously, the band designations should specify this, e.g., B k12 or B r 1 2 A7 instead of B i12 or B A7, where the supersuperscripts “-n indicate the antisymmetric components of the v l 2 = 1 or yIz = 0 inversion doublets for a double well in the vI2 coordinate. We do not use the extended specification in the figures, and we do not tabulate vibrational frequencies, because we do not know for sure that it is the vI2 coordinate, for example, in which there is a double well, ~d we do not yet know the barrier height above which the extended specification becomes superfluous. Figure 14 compares a vibronic transition (one coordinate of bl symmetry shown) from a single well to a double well to transition between two single wells, with allowed transitions indicated for one and two photons. An incidental consequence of nonplanarity would be a failure of the model we used in rotational contour fitting to extract a realistic CCC bond angle from certain bands. For the B! band, the CCC angle that gives the best fit is 115O, which is significantly less than the 118’ expected23 for a 3s Rydberg state built on a ground-state allyl cation core. It only remains to understand why a nominal 3s Rydberg state would be nonplanar at all, given the planarity of allyl cation. In hindsight, the preaence of the C[22Bl]state only 249 cm-I above the B[12AI]state should cause a peudo-Jahn-TellerZ4 distortion of the B[12Al]along a bl coordinate. Interaction of two states, mediated by motion along a vibrational coordinate, can induce a double-well minimum in the lower surface if the two states are close enough in energy. While this is simply a static collsequence of large vibmNc coupling, the designation ”pseudo-Jahn-Teller effect” is used because it results in a symmetry lowering associated with near electronic degeneracy. For the B[ 12AJ state of allyl radical, this produces exactly the right symmetry lowering. An independent confirmaton of the proposed nonplanarity will be the subject of future studies. We did not attempt to assign the large number of very weak bands to the blue of 238 nm because of the onset of a perturbation decreasing the excited-state lifetimes. The intensities of bands in 1+1 resonant MPI to the blue of 238 nm are more than an order of magnitude lower than those of bands to the red of 238 nm,
Acknowledgment. This study was greatly aided by discussions and exchanges of data with Professors P. B. Kelly (University of California, Davis) and J. C. Weisshaar (University of Wisconsin, Madison). We acknowledge funding from the National Science Foundation (CHE-8719587) for most of the lasers used in this work, and for a Presidential Young Investigator award. Support from the Department of Energy (DEFG02-90ER14132) and an additionalequipment supplement are also gratefully acknowledged.
References and Notes (1) National Science Foundation Presidential Young Investigator, David and Lucile Packard Fellow, Camille and Henry Dreyfus Teacher-Scholar, Alfred P. Slonn Research Fellow. (2) Minsek, D. W.; Blush, J. A.; Chen, P. J. Phys. Chem. 1992,%, 2025. (3) Fasenden, R. W.; Schuler, R. H. J. Chem. Phys. 1963, 39, 2147. (4) Vajda, E.; Tremmel, J.; Rozsondai, B.; Hargittai, I.; Maltsev, A. K.; Kagramanov, N. D.; Nefedov, 0. M. J. Am. Chem. Soc. 1986,108,4352. Vajda et al. report CC = 1.428 CH = 1.069 A, CCC = 124.6O, and CCH = 120.9O for the ground state of allyl radical by gas-phaseelectron diffmction. (5) Mal’tsev, A. K.; Korolov, V. A.; Nefedov, 0. M. Izu. Akad. Nauk SSSR, Ser Khim. 1982,2415; Bull. Acad. Sci. USSR, Chem. Ser. 1982,31, 2131. (6) Maier, G.;Reisenauer H.P.; Rohde, B.; Dehnicke, K. Chem. Ber. 1983, 116, 732. (7) Getty, J. D.; Burmeister, M. J.; Westre, S.G.;Kelly, P. B. J. Am. Chem. Soc. 1991. 113. 801: see also followinn Dawr in this issue. (8) Currie, C..L.; Ramsay, D. A. J. C h e i . Phys. 1966,45,488. (9) Callear, A. B.; Lee, H. K. Trans. Faraday Soc. 1968,64, 308. (10) Nakashima, N.; Yoshihara, K. Laser Chem. 1987, 7, 177. (11) Hudgens, J. W.; Dulcey, C. S. J. Phys. Chem. 1985,89, 1505. (12) Sappey, A. D.; Weisshaar, J. C. J. Phys. Chem. 1987, 91, 3731. (13) Ha, T. K.; Baumann, H.; Oth, J. F. M.J. Chem. Phys. 1986,85, 1438, and referenccs therein. (14) Kohn, D. W.; Clauberg, H.;Chen, P. Reu. Sci. Imtrum. 1992,63, 4003. (15) Blush, J. A.; Park, J.; Chen, P. J. Am. Chem. Soc. 1989,111,8951. Miasek, D. W.; Chen, P. J. Phys. Chem. 1990,94,8399. Claubcrg, H.; Chen, P. J. Am. Chem. Soc. 1991,113,1445. Clauberg, H.; Minsek, D. W.; Chen, P. J. Am. Chem. Soc. 1992, I14,99. Zhang, X.; Chen, P. J. Am. Chem. Soc. 1992, 114, 3147. Blush, J. A.; Chen, P. J. Phys. Chem. 1992, 96, 4138. (16) Judge, R. H. Comput. Phys. Commun. 1987, 47, 361. (17) Ground-state frequencies for several htopomers of allyl are calculated by: Takada,T.; Dupuis, M. J. Am. Chem. Soc. l#u, 105,1713. Ratios of ground-state frequencies can be used as a first guess for isotope shifts of excited-state frequencies. (18) Herzberg, G. Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966; pp 137-141. (19) See ref 7 for a complete table of normal modes and observed frequenciea.
(20) For the symmetry and selection d e s related to components of inversion doublets, we: Herzberg, G. Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966; pp 22-23,17&173. (21) Waleb, A. D. J. Chem. Soc. 1953,2306. Brand, J. C. D.J. Chem. Soc. 1956,858. Herzberg, G. Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Pdyatomtc Molecules; Van Nostrand Reinhold: New York, 1966; pp 518-522. (22) This anomaly was noted, but not explained, in the Note Added in Proof of ref 12. (23) HF/6-31Gb geometry from: Wibcrg, K. B., private communication. (24) A good d h w i o n of pudo-Jahn-Teller distortions m a y be found in: Fischcr, G. Vibronic C a p l i n s Academic Preap: New York, 1984; pp 82-85. (25) Merlet, P.; Peyaimboff, S.D.; Buenker, R. J.; Shih,S . J. Am. Chem. Soc. 1974, 96, 959. Famell, L.; Richards, W. G. J. Chem. Soc., Chem. Commun. 1973, 334.
