43) Analogous to those in the ' n r * states of ketones, studied via the circular polarization of the fluorescence, see: Schippers, P. H.; Dekkers, H. P. J. M. Chem. Phys. Lett. 1982.88, 512-516. Dekkers, H. P. J. M.; Moraal, P. F. InAbstract: The first Raman spectra of the allyl radical have been obtained. The intensities of the observed Raman spectra indicate excited-state dynamics consistent with a disrotary photoisomerization of the allyl radical to form a cyclopropyl radical. Prior to this work, direct examination of the photoisomerization pathway was not possible due to limitations of the techniques applied. The ground-state vibrational frequencies observed are found to be in excellent agreement with recent theoretical calculations suggesting a reassignment of the literature infrared frequencies. This work demonstrates that resonance Raman spectroscopy is a powerful method for examination of gas-phase free radicals.

The nature of the allyl radical excited states has been examined by Raman spectroscopy. Fundamentals of non-totally symmetric normal modes are observed in the Raman spectra. The occurrence and intensiQ in non-totally symmetric fundammtalp is indicative of B-term scattering due to vibronic coupling. Analysis of the vibronic coupling is performed in terms of the Kramers-Heisenberg-Dirac formalism. The Raman spectra indicate that the vibronic coupling is from the weakly allowed 12B2 state to either a molecular state arising from a combination of 3p atomic carbon orbitals or a valence state via non-totally symmetric vibrational modes. The vibrational frequencies for v9 and ul2 in the 12Bz state have been determined to be 596 and 564 cm-' respectively.

Resonance Raman spectra of the allyl-d5 radical have been obtained with excitation between 247 and 223 nm. Analysis of the spectra yields the first observation of fundamental frequencies, ν4, ν5, and ν7 and overtone frequencies 2ν9, 2ν10, and 2ν12. The new vibrational data are combined with previously observed frequencies of allyl-h5 and allyl-d5 radical to produce the force field analysis for the allyl radical. This study suggests reassignment of several previously observed infrared (IR) bands. Experimental frequencies and assignments for allyl-h5 and allyl-d5 are compared with results from ab initio calculations. Force constants obtained in the present work are compared with the force constants of other sp2 hybridization molecules such as benzene, allene, and ethylene.

The resonance Raman spectrum of the ␤-methylallyl ͑2-methyl-allyl͒ radical is detected with excitation from 223 nm to 243 nm. The first observation of four fundamentals, the symmetric CH 2 rock, the symmetric CCC stretch, the C-CH 3 stretch, and the CCC bend, as well as an overtone, the symmetric CH 2 twist of the ␤-methylallyl radical are reported. The assignments of the resonance Raman spectra are based upon the previous force field calculation, and comparisons with molecules having similar characteristic vibrations. Results from the resonance Raman spectra are compared with frequencies obtained by infrared spectroscopy using matrix isolation and our previous work on the allyl radical.

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