15min:

BROOKS H. PATE, DAVID GREEN, SARAH HAMMOND AND JOHN KESKE, Department of Chemistry, University of Virginia, Charlottesville, VA 22901.

We have measured the high resolution (5 MHz) infrared spectrum of the asymmetric -CH₂(F) stretch of 2-fluoroethanol (HO-CH₂-CH₂F) near 2980 cm^-1 using an electric-resonance optothermal molecular beam spectrometer. The infrared spectrum has been assigned using microwave-infrared double-resonance spectroscopy techniques. The infrared spectrum originates in rotational levels of the most stable conformer, denoted Gg', where both internal rotors (C-C and C-O) are in a gauche conformation. The barrier to conformational isomerization is approximately 1000 cm^-1. Above this energy two new conformations are accessible, Tt and Tg. Two features of the infrared spectrum may indicate conformational isomerization following excitation of the C-H stretch: 1) The degeneracy of the two Gg' enantiomers is lifted in the infrared spectrum. For 2-fluoroethanol, the degeneracy is lifted only when highly torsionally excited states, or states from the Tt conformation, are involved in the dynamics. 2) There is a strong correlation between rotational states assigned to separate levels of near-degenerate K_a-asymmetry doublets indicating energy flow between these states. These properties of the single-photon infrared spectrum are discussed in the context of subsequent infrared-microwave double-resonance spectroscopy measurements which unambiguously demonstrate conformational isomerization following C-H stretch excitation.