Calculation of reduced partition function ratios of isotopically substituted molecules on a quantum chemical base

By means of the semiempirical quantum chemical MINDO/3- and MNDO-MO-methods it is possible to perform calculations for use in evaluation or interpretation of isotope effects to such an extent that would not be rationally fossible by corresponding experiments. But only the calculated reduced partition function ratios of isotopically substituted molecules can be applied with sufficient reliability for discussions. The temperature dependence of the reduced partition function ratios of over 100 molecules, ions, and radicals regarding the H/D, 12C/13C-, 14N/15N-, 16O/18O-, 28Si/30Si-, 32S/34S-, and 35Cl/37Cl-substitution has been calculated. From these results general conclusions concerning the dependence of isotope effects from the chemical structure of the corresponding molecules have been drawn. In particular, a relationship between the reduced partition function ratio and the electronic charge of the substituted atom has been found. In addition, examples are given for the application of the calculation algorithm used above in connection with combined isotopic substitutions, radical cations, and transition states of chemical reactions. (author)