Examination of approximations to hindered internal rotation in the calculation of energy level sums

Abstract

An examination has been made of the errors involved in various commonly used physical approximations for symmetric hindered rotors that are employed in the calculation of molecular energy level densities and sums. Some illustrative energy level sums obtained with use of these approximations are compared with true hindered rotor sums for a number of molecular systems. Several practical illustrations are also given of experimental systems reported in the literature. A facile higher energy approximation is offered for the accurate calculation of energy level densities or sums of states for molecules containing true hindered rotors wherein an energy-dependent correction is made to the total active internal energy of the corresponding molecule having free internal rotors. At lower energies where a librator approximation works well, a simple rule is offered for selection of an optimum equivalent librator frequency.