Infrared and millimeter-wave study of the four lowest torsional states of CH3CF3

An investigation of the torsion-rotation Hamiltonian of CH,CF, in the groun d vibrational state has been carried out using infrared and mm-wave spectro scopy. With infrared Fourier transform spectroscopy, the weak, torsional ov ertone (v(6) = 2 <-- 0) has been studied leading to the measurement of 382 frequencies between 405 and 430 cm(-1) at a resolution of 0.005 cm(-1) Tors ional splittings on the order of 0.03 cm(-1) were observed. With mm-wave me thods, a total of 669 rotational transitions between 50 and 360 GHz have be en measured at Doppler-limited resolution in the four lowest torsional stat es v(6) = 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 f requencies. For v(6) = 3, torsional splittings as large as 8.7 MHz were obs erved. The global data set consisted of the current frequency determination s and the 443 measurements with molecular beam, microwave, and mm-wave meth ods analyzed by I. Ozier, J. Schroderus, S.-X. Wang, G. A. McRae, M. C. L. Gerry B. Vogelsanger, and A. Bauder [J. Mel. Spectrosc. 190, 324-340 (1998) ]. The observation of mm-wave R-branch transitions for v(6) 1 led to a chan ge in the J-assignment of the forbidden (Deltak = +/-3) transitions reporte d earlier for this torsional state. A good fit was obtained by varying 24 p arameters in a Hamiltonian that represented both the torsional effects and the sextic splittings. In the earlier work, the large reduced barrier heigh t led to high correlations among several of the torsional distortion consta nts. With the current measurements, many of these correlations are substant ially reduced. Improved effective values were determined for the height V-3 of the hindering barrier and the first-order correction V-6 in the Fourier expansion of the potential function. The dipole function which characteriz es the transition moment of the torsional overtone (v(6) = 2 <-- 0) can be written as the product of a single effective dipole constant mu (o,eff)' an d the appropriate off-diagonal matrix element of (1 - cos 3 alpha)/2, where Phi is the torsional angle. From an intensity analysis of the infrared spe ctrum, it has been determined that \ mu0(eff)(T)\ = 85.3(62) mD. A novel ap proach based on a simple regrouping of angular momentum operators is: intro duced for decoupling the torsional and rotational degrees of freedom. (C) 2 001 Academic Press.