Collect. Czech. Chem. Commun. 1982, 47, 3206-3220

Reduced moment of inertia for internal rotation and quantum-chemical description of thermodynamics of isomeric molecular complexes: Study of HF-ClF system

Zdeněk Slanina

The J. Heyrovský Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 121 38 Prague 2


Relations are studied between values of reduced moment of inertia Ired of a system consisting of a rigid frame and one asymmetric top evaluated within the exact approach and within simpler models of one symmetrical top and of two coaxial tops with particular respect to situation in the isomeric van der Waals molecules HF.ClF and ClF.HF. It has been shown that replacement of the exact approach by the model of two coaxial tops is generally more acceptable physically than that by the model of symmetrical top. For the both isomeric complexes, the error introduced by the coaxial tops approach is always lower than that introduced by transition to the second of the two basis sets (4-31G and 4-31G*) recently used for the study of structure of the complexes. Reevaluation of thermodynamics of this isomeric system using the exact approach for Ired has been carried out, and the problem of relative stabilities has been analyzed. According to the 4-31G* theoretical results, The HF.ClF isomer predominates completely in the equilibrium mixture of the two isomers up to the temperature of 40 K at least, their equimolecular ratio being not attained until at 90 K. Above this temperature, the ClF.HF isomer becomes relatively more abundant (even though not completely predominant) component of the equilibrium mixture. The paper gives proof of independence between Ired and the frame-top assignment. Inaccuracy in former Ired values and in thermodynamic characteristics of Cl2S2 are pointed out as a byproduct.