The forbidden rotational Q-branch of CH3CF3:: Torsional properties and (A1-A2) splittings

The pure rotational spectrum driven by the small distortion dipole moment perpendicular to the symmetry axis has been investigated between 8 and 18 GHz for CH3CF3 in the ground vibrational state using a pulsed Fourier transform waveguide spectrometer. This molecule has been selected as a prototype for the case of a symmetric top with small (similar to 500 kHz) torsional energy splittings in the ground torsional state (v(6) = 0). In this state, six (k +/- 3 <-- k) e-branch series have been measured for lower state K = \k\ between 3 and 8 with 27 less than or equal to J less than or equal to 75, For (v(6) = 1), three series with lower state K between 5 and 7 with 49 less than or equal to J less than or equal to 66 have been observed. In two of these series, the torsional fine structure extending over similar to 6.8 MHz has been fully resolved. The (A(1) - A(2)) splitting has been measured in the (v(6) = 0) series (K = 6 <-- 3) for 37 less than or equal to J less than or equal to 74. The global data set of 443 frequencies included avoided-crossing molecular-beam splittings of Meerts and Ozier (1991. Chem. Phys. 152, 241-259) and mm-wave R-branch measurements of Bocquet et nl. (1994. J, Mol. Spectrosc. 165, 494-499). In a weighted least-squares analysis, a good fit was obtained by varying 18 parameters in a Hamiltonian that represented both the torsional effects and the sextic splittings. Effective values have been determined for both rotational constants, eight torsional parameters including the barrier height, six diagonal centrifugal distortion constants, and two centrifugal distortion constants (epsilon and epsilon(J)) that characterize the (Delta k = +/-3) matrix elements. The difficulties are discussed that arise in defining a unique model for the torsional terms in the Hamiltonian when a high barrier symmetric top is investigated by distortion moment spectroscopy. The redundancies are investigated that exist in the quartic and sextic Hamiltonian for a near-spherical top such as CH3CF3. (C) 1998 Academic Press.