Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Relations are studied between values of reduced moment of inertia I_red 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 I_red 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 I_red and the frame-top assignment. Inaccuracy in former I_red values and in thermodynamic characteristics of Cl₂S₂ are pointed out as a byproduct.