Polarized Infrared Absorption Spectrum of Matrix-Isolated Methylperoxyl Radicals, CH₃OO X ²A‘ ‘

We have used a tandem pair of supersonic nozzles to produce clean samples of CH 3 OO radicals in cryogenic matrices. One hyperthermal nozzle decomposes azomethane (CH 3 NNCH 3 ) to generate intense pulses of CH 3 radicals, while the second nozzle alternately fires a burst of O 2 /Ar at the 20 K matrix. The CH 3 /O 2 /20 K argon radical sandwich acts to produce target methylperoxyl radicals: CH 3 + O 2 → CH 3 OO. The absorption spectra of the radicals are monitored with a Fourier transform infrared spectrometer. We report 10 of the 12 fundamental infrared bands of the methylperoxyl radical CH 3 OO, X̃ 2 A‘ ‘, in an argon matrix at 20 K. The experimental frequencies (cm -1 ) and polarizations follow: the a ‘ modes are 3032, 2957, 1448, 1410, 1180, 1109, 902, 492, while the a ‘ ‘ modes are 3024 and 1434. We cannot detect the asymmetric CH 3 rocking mode, ν 11, nor the torsion, ν 12 . The infrared spectra of CH 3 18 O 18 O, 13 CH 3 OO, and CD 3 OO have been measured as well in order to determine the isotopic shifts. The experimental frequencies, {ν}, for the methylperoxyl radicals are compared to harmonic frequencies, {ω}, resulting from a UB3LYP/6-311G(d,p) electronic structure calculation. Linear dichroism spectra were measured with photooriented radical samples in order to establish the experimental polarizations of most vibrational bands. The methylperoxyl radical matrix frequencies listed above are within ±2% of the gas-phase vibrational frequencies. A final set of vibrational frequencies for the CH 3 OO radical are recommended. See also http://ellison.colorado.edu/methylperoxyl .