The rotational partition-function of the symmetrical top and the effect of K-doubling thereon

Abstract

McDowell's earlier derivation of the partition function for a nonrigid symmetric top has been extended with a fourth-order correction for centrifugal distortion and correction terms for K doubling. Comparison with direct numerical summation (including K-doubling effects) indicates that the inclusion of the fourth-order term is necessary to ensure high accuracy in the computed enthalpy function and heat capacity above 2000 K, as well as that the effect of K doubling is very small at practical temperatures. The only effect of any practical importance is that of d2 for molecules with fourfold symmetry: the present approximate expressions represent this effect quite well.