The forbidden rotational Q branch of CH 3SiF 3: A study in internal rotation

Abstract

The pure rotational spectrum driven by the small dipole moment produced perpendicular to the symmetry axis by centrifugal distortion has been investigated for CH 3SiF 3 in the ground vibrational state using a Fourier transform waveguide spectrometer. Between 10.9 and 17.0 GHz, four ( k + 3 ← k) series in the Q branch have been measured in the lowest torsional state v 6 = 0 for k = 4, 5, 6, and 7 with 54 ⩽ J ⩽ 65. In each transition, the quantum number σ = 0, +1, −1 labelling the different torsional sub-levels is conserved. For given ( J, k), splittings from ∼10 to ∼45 MHz have been observed between lines with different values of σ. The global data set includes the anticrossing molecular beam energy differences of [W.L. Meerts, I. Ozier, Chem. Phys. 71 (1982) 401–415] as well as the mm-wave R branch frequencies and ( A 1 − A 2) splittings of [P. Dréan, J.-M. Colmont, J. Demaison, L. Dore, C. Degli Esposti, J. Mol. Spectrosc. 176 (1996) 23–27]). A good fit was obtained by varying 15 molecular parameters characterizing the torsion–rotation Hamiltonian H TR for the vibrational ground state. Because of the strong correlation between two of the quartic torsion–distortion parameters ( F 0,3 K and D 0, Km ) and a redundancy connecting the centrifugal distortion constants, four models were obtained yielding comparable fits. In each case, effective values were determined for the A-rotational constant and the height of the potential hindering the internal rotation. A high precision determination of the structural parameter ρ was made that is the same in all four models. For the off-diagonal quartic centrifugal distortion constant ε 0 and the sextic constants H 0, J , H 0, JK , H 0, KJ , and h 0,3, the differences in the values obtained in the two different reductions used have been explained in terms of the redundancy connecting these parameters. For σ = 0, +1, −1, the energy level pattern for (| k| = 3) is discussed for the case where the pure torsional energy splitting and the matrix elements off-diagonal in k are of comparable magnitude. A method is described of using an R branch study of the resulting σ-splittings for (| k| = 3) to probe the zeroth-order torsional Hamiltonian.