Calculation of the rotational constants A and C for isotopic species of slightly perturbed CH3CN symmetric top molecules

Abstract

A technique which was employed earlier to calculate the rotational constants of CHaCCH has been extended to the ground and two vibrational levels in the ~' 8 vibration of CH3CN for several isotopic species. The moments of inertia and a computer iteration technique over experimental data for each isotopic species were employed to evaluate the constant A,, in an excited vibrational state for a symmetric top molecule. This method gave good estimates for A,, for each isotope. The angle of bending and the orientation of each molecular system in reference frames, one fixed on the carbon atom at the --C----N site and the other at the center of mass, were explored. These results are discussed in this paper. The method, which was applied by Tam and Roberts to the mqo, n --1, 2, 3, 4, vibrations of CH3CCH earlier and which was extended to the ~t0 = 1 vibration of CH3CCH with 13C isotopic species, has been applied to 13C isotopic species of CH3CN and seems to be a useful tool to extract the value of A o. Each of these molecules shows reasonable dependency of A,. over vibrational levels, values of A,, calculated from the geometrical model are in good agreement with those obtained by fitting the terms in the frequency equations, which contained A,,, to the experimental data through an iteration technique in which the value of A was allowed to vary.