Polarized Infrared Absorption Spectra of Matrix-Isolated Allyl Radicals

We have measured the polarized infrared absorption spectrum of the allyl radical, CH 2 CHCH 2 (X̃) 2 A 2, in an argon matrix at 10 K. The experimental CH 2 CHCH 2 frequencies (cm -1 ) and polarizations follow: a 1 modes, 3109, 3052, 3027, 1478, and 1242; b 1 modes, 983, 801, and 510; b 2 modes, 3107, 3020, 1464, 1390, and 1182. Two modes (ν 6 and ν 18 ) are very weak and could not be detected; the lowest frequency a 1 mode (the CH 2 −CH−CH 2 bending mode ν 7 ) is estimated to be beyond the wavelength range of our MCT infrared detector. Infrared absorption spectra of two deuterated isotopomers, CH 2 CDCH 2 and CD 2 CDCD 2, were recorded in order to compare experimental frequency shifts with calculated [UB3LYP/6-311-G(d,p)] harmonic frequencies. Linear dichroism spectra were measured with photooriented samples in order to establish experimental polarizations of most vibrational bands. True gas-phase vibrational frequencies were estimated by considering the gas-to-matrix shifts and matrix inhomogeneous line broadening. The allyl radical matrix frequencies listed above are within ±1% of the gas-phase vibrational frequencies. A final experimental set of all the vibrational frequencies for the allyl radical are recommended. See also: http://ellison.colorado.edu/allyl .