Microwave Spectroscopy of Torsion Vibrations in Molecules: Approximation of Rigid Constraint on Structural Torsional Parameters ρ and F in Rho Axis Method

Abstract

In this paper, on example of the molecules with methyl top hindered internal rotation, we consider an approximation of rigid constraint on structural torsional parameters ρ and F in Rho axis method and its applicability to analysis of the observed molecular spectra. In rigid molecular structure approximation the existing functional relation between F and ρ parameters allows one to replace variation of these two parameters by variation of one structural parameter, namely moment of inertia of C_3v group that undergoes torsional vibration. The approximation has been tested via fitting spectra measurement data for a number of molecules for which the results of analysis employing standard Rho axis method are available in the literature. It is shown that structural distortions in the molecules with hindered internal rotation of C_3v group, which in the standard model of Rho axis method are associated with separate variation of the F and ρ parameters, are not significant in the ground torsional state and may be taken into account via fourth order Hamiltonian terms when the spectra of excited torsional states are fitted.