The v2 = 1←0, 2←1, and 3←2 bands of the methyl radical were observed in the gas phase by infrared tunable diode laser spectroscopy at 606.4531, 681.6369, and 731.0757 cm−1. The observed vibration–rotation spectra were analyzed to derive precise values for the rotational constants, centrifugal distortion constants, and spin–rotation interaction constants. The absence of N = odd, K = 0 levels in the v2 = even states and of N = even, K = 0 levels in the v2 = odd states was confirmed, indicating that each vibrational state is nondegenerate. The observed band origins, when analyzed, led to a potential function for the out-of-plane bending vibration that has no potential hump at the planar configuration. The large negative anharmonicity of this potential was ascribed to a vibronic interaction with excited electronic states. The methyl radical was generated by a 60 Hz discharge in di-tert-butyl peroxide and was found to have a lifetime of about 1.4 msec; its concentration in the absorption cell was estimated to be about 1013 molecules/cm3, provided that a recombination reaction is the only route for eliminating methyl radicals.

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A new spectroscopic method of triple resonance is proposed for studying chirality of a molecule of C1 symmetry. Each enantiomer of such a molecule is of mixed parity and thus exhibits all three a-, b-, and c-types of rotational spectra. The present study concludes, by using time-dependent perturbation theory, that the transition probability between two of the three rotational levels under triple resonance differs for different enantiomer. This result can thus be of some significance for enantiomer differentiation.