Infrared spectrum of the CH2 out-of-plane fundamental of C2H5

Measurements of the ethyl radical -CH2 out-of-plane rocking vibrational fun damental by transient diode laser absorption spectroscopy are reported. The new data provide information on the structure of, and the barrier to inter nal rotation in, the radical, and how these quantities change on vibrationa l excitation. We find that the effective barrier decreases from approximate ly 17 cm(-1) in the zero point level to 10 cm(-1) in the excited vibrationa l level. The assigned data set now contains approximately 450 rotation-tors ional transitions and has been fit to a model effective Hamiltonian. The de rived molecular parameters generally reproduce the measured line positions to better than 0.01 cm(-1), but this does not approach the estimated measur ement accuracy of 0.001-0.002 cm(-1). (C) 1999 American Institute of Physic s. [S0021-9606(99)00144-0].