High-resolution infrared spectroscopy of jet-cooled allyl radical (CH2–CH–CH2): In-phase (ν1) and out-of-phase (ν13) antisymmetric CH2 stretching vibrations

Abstract

High-resolution infrared spectra of jet-cooled allyl (CH2–CH–CH2) radical in the CH2 stretching region are obtained in a slit jet discharge spectrometer. Over 200 transitions are observed in the 3100–3140 cm−1 region, which have been rigorously assigned to “in-phase” (ν1) and “out-of-phase” (ν13) coupling of antisymmetric CH stretch excitations on each of the equivalent CH2 subunits. Origins for the two bands are determined to be 3113.98 488(89) cm−1 (ν1) and 3110.59 857(36) cm−1 (ν13), respectively. The data are reasonably well fit to semirigid asymmetric rotor behavior for the Ka⩽2 levels populated in the slit jet at Trot≈20 K, though not to within the experimental precision, which provides some indication of perturbations in the upper states. The sign and magnitude of the +3.4 cm−1 splitting between“in-phase” and“out-of-phase” excitations are in good qualitative agreement with a simple electrostatic model of dipole–dipole coupling between the two “local mode” CH2 groups. Due to sub-Doppler resolution (Δν≈70 MHz) in the slit jet expansion, quantum-state-dependent excess broadening of the rovibrational transitions is observed, which can be ascribed to spin–rotation interactions. Based on a least squares analysis of the high-resolution line shapes, the data are consistent with a spin rotation constant of εaa≈−67(25) MHz.