Manifestations of torsion-CH stretch coupling in the infrared spectrum of CH₃OO

With a step-scan Fourier-transform spectrometer we recorded temporally resolved infrared absorption spectra of CH₃OO radicals that were produced upon irradiation of CH₃COCH₃ and O₂ at 193 nm in a flowing mixture. At a resolution of 0.15 cm^-1, the rotational structure of the ν₂ band of CH₃OO near 2954.4 cm^-1 is partially resolved and shows an unexpectedly broadened, and somewhat distorted, Q-branch. A 4D model Hamiltonian, consisting of three CH stretches and the methyl torsion, was developed to explore the origins of this broadening. The vibrational progressions predicted by the model Hamiltonian and the rotational contours of the ν₂ band, based on experimental ground-state rotational parameters and their values scaled by their calculated ratios for the upper state, produced simulations in satisfactory agreement with the observed spectrum. These results provide new insight into the vibrational couplings in CH₃OO.