Perturbation treatment of doublet splittings in rotational spectra of molecules with two equivalent conformations separated by nonequivalent energy barriers

A method is described for analyzing doublet splittings in rotational spectracaused by tunneling between two equivalent conformations, under the restriction that the tunneling motion be effectively aperiodic. The splittings can mainly be fitted with three parameters: the purely torsional doublet splitting ΔE and two Euler angles β and γ, which characterize the transformation between the principal-axis system and the Eckart-axis system. The method is numerically very fast when the splittings are small enough to allow the use of perturbation theory. Examples are given to show that a simple correlation between β and γ with the path of the tunneling process is possible

The microwave spectrum of fluoroacetic acid

The microwave spectrum, rotational constants and centrifugal distortion parameters for fluoroacetic acid are reported. The identification was done, without use of the Stark-effect, by direct comparison of observed and calculated frequencies

Internal rotation with a low threefold barrier ; comparison between different basis functions

For the treatment of the effects of a C3v internal rotor on the rotational spectrum of a molecule we have compared two different sets of basis functions, i.e. Mathieu functions and free rotor functions. In both cases the internal axes system was used. The behaviour of two model substances, based on the geometries of 1-chloro-2-butyne and meta-fluorotoluene, was studied for values of V3 up to 100 cm−1. It was found that for this range of barriers the use of basis functions from more than one torsional state is often necessary for a good description of the energy levels. The size of the matrix to be diagonalized depends on the barrier height and the molecular geometry, but is often larger for free rotor functions than for Mathieu functions. Thus for low barrier problems the use of the latter type of functions, which have not often been used in that region, should be more seriously considered