Intramolecular vibrational energy redistribution and conformational isomerization in vibrationally excited 2-fluoroethanol:: High-resolution, microwave-infrared double-resonance spectroscopy investigation of the asymmetric -CH2(F) stretch near 2980 cm-1

The asymmetric -CH2(F) stretch spectrum of 2-fluoroethanol near 2980 cm(-1) has been rotationally assigned using microwave-infrared double-resonance spectroscopy methods in an electric-resonance optothermal molecular-beam spectrometer. The eigenstate-resolved infrared spectrum shows the effects of intramolecular vibrational energy redistribution (IVR) through the fragmentation of each rotational level of the vibrationally excited state into a set of transitions. From the spectrum we determine the IVR lifetime of the asymmetric -CH2(F) stretch to be 275 ps. The measured vibrational state density at 2980 cm(-1) is 44 states/cm(-1), and matches the value for the total state density obtained from a direct count. This agreement suggests that vibrational states of both the Gg' and Tt conformers are coupled by the intramolecular dynamics. From measurements of the c-type pure rotational transitions of the Gg' conformer we determine that the tunneling splitting for the Gg' ground state is less than 35 kHz. The infrared spectrum is characterized by a large number of closely spaced infrared transitions. The clustering of vibrational energy levels is attributed to the weak interaction between the degenerate Gg' and nondegenerate Tt vibrational states. This lifting of the effective Gg' parity degeneracy is quantitatively investigated through the shape of the nearest-neighbor level spacing distribution. From this analysis we estimate that the isomerization lifetime for the Gg' conformer of 2-fluoroethanol is 2 ns. (C) 1999 American Institute of Physics. [S0021-9606(99)00104-X].