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

Abstract

We have used a tandem pair of supersonic nozzles to produce clean samples of CH3OO radicals in cryogenic matrices. One hyperthermal nozzle decomposes azomethane (CH3NNCH3) to generate intense pulses of CH3 radicals, while the second nozzle alternately fires a burst of O2/Ar at the 20 K matrix. The CH3/O2/20 K argon radical sandwich acts to produce target methylperoxyl radicals: CH3 + O2 → CH3OO. 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 CH3OO, X̃ 2A‘ ‘, 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 CH3 rocking mode, ν11, nor the torsion, ν12. The infrared spectra of CH318O18O, 13CH3OO, and CD3OO 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 CH3OO radical are recommended. See also http://ellison.colorado.edu/methylperoxyl.