Parallel Session: Internal rotation, Contributed Talk (15min)

Analysis of the low torsional barrier of a heavy C3v top: the case of trifluroacetic acid

L. Zou1, R. A. Motiyenko1, L. Margulès1
1Universite de Lille, Laboratoire PhLAM UMR 8523 CNRS

Trifluoroacetic acid (TFA, CF3COOH) is the final degradation product of many hydrochlorofluorocarbons, hydrofluorocarbons, and hydrofluoroolifines in the troposphere. The CF3 internal rotor is a heavy C3v top that strongly couples with the overall rotation of the molecule (ρ = 0.68). Despite the low torsional barrier of about 230 cm-1, the high mass of the top leads to low tunneling probability in the ground state. The A-E splitting is not observable in the ground state rotational spectrum even using  molecular beam Fourier transform microwave spectroscopy which typical resolution is few kHz[1]. In previous studies, the splitting was observed only for the vt=4 excited torsional state[2], whereas the rotational lines for the vt=1-3 states were not well measured. The missing lines from the vt=1-3 states lead to the question of what the exact torsional barrier is, and in which state the tunneling splitting becomes observable. We perform a new global analysis of the TFA vt=0-4 states with new spectral measurements from 50 to 330 GHz, and from 1.2 to 1.5 THz. In our data set, the tunneling splitting is observed for vt=4 lines, and a small portion of vt=3 lines. Our analysis was performed using the rho-axis method and RAM36 code. This joint analysis of five torsional states resulted in accurate determination of the V3 and V6 potential energy terms. The latest results will be presented.

[1] Antolinez, S., Alonso, J., Dreizler, H. and Sutter, D., Zeitschrift für Naturforschung A1999, 54, 524-538.

[2] Stolwijk, W. M. and van Eijck, B. P., Journal of Molecular Spectroscopy, 1985, 113, 196-207.