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 s pectroscopy methods in an electric-resonance optothermal molecular-beam spe ctrometer. The eigenstate-resolved infrared spectrum shows the effects of i ntramolecular vibrational energy redistribution (IVR) through the fragmenta tion of each rotational level of the vibrationally excited state into a set of transitions. From the spectrum we determine the IVR lifetime of the asy mmetric -CH2(F) stretch to be 275 ps. The measured vibrational state densit y at 2980 cm(-1) is 44 states/cm(-1), and matches the value for the total s tate density obtained from a direct count. This agreement suggests that vib rational states of both the Gg' and Tt conformers are coupled by the intram olecular dynamics. From measurements of the c-type pure rotational transiti ons of the Gg' conformer we determine that the tunneling splitting for the Gg' ground state is less than 35 kHz. The infrared spectrum is characterize d 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 an alysis we estimate that the isomerization lifetime for the Gg' conformer of 2-fluoroethanol is 2 ns. (C) 1999 American Institute of Physics. [S0021-96 06(99)00104-X].