The calculation of isotopic partition functions ratios by a perturbation theory technique. 2. Dissection of the isotope effect

Statistical mechanical perturbation theory is applied to the evaluation of molecular partition functions and (reduced) isotopic partition function ratios ((s/sub 2//s/sub 1/)f) are calculated. The unperturbed vibrational problem corresponds to noninteracting stretches, bends, torsions, etc. It is demonstrated that an analysis of the unperturbed problem yields important insights into the magnitude of (s/sub 2//s/sub 1/)f values and into the isotope effects on chemical equilibria which can be calculated from these quantities. In such an analysis, one does not need to consider normal mode vibrational frequencies but one considers instead the frequencies corresponding to noninteracting individual coordinates. The contributions of the individual coordinates to (s/sub 2//s/sub 1/)f values and to isotope effects on equilibrium constants are presented.