The calculation of molecular parameters for a molecule with an internal rotor

A derivation for the formulas to calculate centrifugal distortion constants , based on a recent formulation [Duan and Takagi, Phys. Lett. A (1995)] of centrifugal distortion effects for a molecule containing a threefold symmet ric internal rotor, is presented. Some constants which are independent of t he barrier derivatives, especially the constants representing interactions between torsion and rotation, are given in terms of molecular structural pa rameters and force constants. These calculated constants are helpful in the reduction of the Hamiltonian and in the analysis of observed transitions. The derived formulas are applied to numerical calculations of the centrifug al distortion constants of methanol. It is shown that most of the calculate d constants are in good agreement with those obtained from the fitting to e xperimental data. (C) 1999 American Institute of Physics. [S0021-9606(99)01 530-5].