Internal Rotation in Symmetric Tops

Abstract

Abstract Internal rotation in C2H6 and CH3SiH3 and similar symmetric tops has been investigated from 50 kHz to 1000 cm−1 using high resolution rotational, torsional, vibrational, and molecular beam spectroscopy. Electric dipole transitions that are forbidden in first order have been measured at high sensitivity with a Fourier transform microwave cavity spectrometer and infrared interferometers. The molecular beam avoided crossing technique has been developed to detect transitions that break the standard rotation–torsion–nuclear spin selection rules. The torsional mode has been studied from the harmonic limit to the case of nearly free internal rotation. Intensity and Stark shift measurements have been carried out in an investigation of the torsional dependence of the electric dipole operator. Fragmentation of the lowest lying parallel and perpendicular vibrational bands has been observed due to resonant and non-resonant interactions with dark states that differ from the bright states by as many as five torsional quanta. Giant torsional splittings are observed reflecting the fact that the frequency of tunneling through the hindering potential has been increased by more than a factor of ten. The results have serious implications for methyl enhancement of intramolecular vibrational relaxation. An effective Hamiltonian has been developed showing that the fragmentation is caused by Coriolis and anharmonic interactions mediated by the large amplitude motion. For each molecule studied, a multi-band fit to a large global data set has been made to within experimental error. The isotopic dependence of the torsional parameters for C2H6, CH3CD3, and C2D6 is discussed. This work is reviewed and possible future directions suggested.