Infrared laser transient absorption spectroscopy of the ethyl radical

The observation and analysis of the high resolution spectrum of the CH2 rocking fundamental of the ethyl radical, C2H5, at wavelengths close to 18.9 mu m is reported. The band origin is found to be at 528.1 cm(-1). The spectrum shows evidence for a very low barrier to internal rotation, or torsion, in this species. A simple model Hamiltonian, based on an assumed structure with G(12) symmetry qualitatively reproduces the observations and implies a torsional barrier of approximately 20 cm(-1) in both the zero point and excited vibrational states. The Hamiltonian cannot reproduce the observations to the level of the experimental accuracy and the importance of several neglected terms is tested and discussed. The observed torsional splittings imply that, within the confines of the model, the a-inertial and internal rotation axes in the molecule are coincident to within 1 degrees. In addition to their intrinsic interest, the spectra will be useful for future state resolved studies of the kinetics of chemical reactions involving the radical. (C) 1996 American Institute of Physics.