Avoided-crossing molecular-beam study of the torsion-rotation energy levels of CH 3CF 3

Abstract

The avoided-crossing molecular-beam electric resonance method has been used to determine the leading parameters in the torsion-rotation Hamiltonian of CH 3CF 3, which was selected as a prototype for the case of a symmetric rotor with small (≤ 500 kHz) internal rotor splittings. Stark anticrossings have been studied for ( K= ±2↔∓ 1) with J= 2−6; hyperfine anticrossings have been studied for ( K= ±2↔0),(±2↔±1), and (±)↔0) with J≤2. A detailed investigation of the hyperfine case has been carried out involving selection rules, relative intensity calculations, Zeeman studies, and combination differences. Several pure rotational transitions for the lowest two torsional states have been measured between 93 and 114 GHz with a mm-wave spectrometer. The ( J= 1 ← 0) transition in the ground torsional state has been measured with the molecular beam spectrometer. Stark and Zeeman studies have been carried out with conventional molecular beam techniques. It has been determined that the effective rotational constant A eff=5502.904(3) MHz, the effective height of the threefold barrier to internal rotation V eff=1093 (11) cm −1 and the moment of inertia of the methyl top I α=3.21(4) Å 2. It has been found that the electric dipole moment μ=2.34720(13) D and the distortion dipole moment constant μ D=3.220(11) μD. The magnitudes and signs of the molecular g-factors have been obtained: g∥=−0.0226(13) nm and g ⊥=−0.117(1) nm. In addition, values have been determined for the B-rotational constant and several distortion constants.