Infrared and Millimeter-Wave Study of the Four Lowest Torsional States of CH3CF3

An investigation of the torsion–rotation Hamiltonian of CH3CF3 in the ground vibrational state has been carried out using infrared and mm-wave spectroscopy. With infrared Fourier transform spectroscopy, the weak, torsional overtone (v6 = 2 ← 0) has been studied leading to the measurement of 382 frequencies between 405 and 440 cm−1 at a resolution of 0.005 cm−1. Torsional splittings on the order of 0.03 cm−1 were observed. With mm-wave methods, a total of 669 rotational transitions between 50 and 360 GHz have been measured at Doppler-limited resolution in the four lowest torsional states v6 = 0, 1, 2, 3. The experimental uncertainty attained for an isolated line was better than 10 kHz below 150 GHz, and somewhat larger at higher frequencies. For v6 = 3, torsional splittings as large as 8.7 MHz were observed. The global data set consisted of the current frequency determinations and the 443 measurements with molecular beam, microwave, and mm-wave methods analyzed by I. Ozier, J. Schroderus, S.-X. Wang, G. A. McRae, M. C. L. Gerry, B. Vogelsanger, and A. Bauder [J. Mol. Spectrosc. 190, 324–340 (1998)]. The observation of mm-wave R-branch transitions for v6 = 1 led to a change in the J-assignment of the forbidden (Δk = ±3) transitions reported earlier for this torsional state. A good fit was obtained by varying 24 parameters in a Hamiltonian that represented both the torsional effects and the sextic splittings. In the earlier work, the large reduced barrier height led to high correlations among several of the torsional distortion constants. With the current measurements, many of these correlations are substantially reduced. Improved effective values were determined for the height V3 of the hindering barrier and the first-order correction V6 in the Fourier expansion of the potential function. The dipole function which characterizes the transition moment of the torsional overtone (v6 = 2 ← 0) can be written as the product of a single effective dipole constant μT0,eff and the appropriate off-diagonal matrix element of (1 − cos 3α)/2, where α is the torsional angle. From an intensity analysis of the infrared spectrum, it has been determined that |μT0,eff| = 85.3(62) mD. A novel approach based on a simple regrouping of angular momentum operators is introduced for decoupling the torsional and rotational degrees of freedom.