Theoretical isotope effects studies

Abstract

Two theoretical isotope effects studies were carried out. The purpose of the first study was to develop a method for determining force fields of molecules from experimental thermodynamic data such as isotope-exchange equilibrium constant measurements or vapor pressure isotope effect measurements instead of from spectroscopic data. The purpose of the second study was to examine how certain structural features of molecules affect the prevalence of temperature-dependence anomalies of isotope-exchange equilibrium. __The technique used in the first study utilized the orthogonal polynomial expansion of the logarithmic form of the isotopic partitions function ratio (ln --6.. f), which is an explicit though non-linear function of the individual force constants. A method using the Gauss-Newton non-linear least squares fitting technique was developed in this study to solve for the unknown force constants. __Molecules for which force fields were available in the s2 literature were examined. The [] f values for these molecules were calculated from the known force fields for a series of temperatures and were used as the "experimental" data. The molecules were then treated as if their force fields were unknown. The non-linear least squares analysis was applied to a set of "guessed" values of the force constants in an effort to regenerate the "original II force constants.