Calculation of heavyhyphen;atom kinetic isotope effects: Barrier curvature and internal coordinate effects with threehyphen;element reaction coordinates

摘要

Effects of barrier curvature and of a restricted method for selection of values for offhyphen;diagonal force constants in the transition state were studied by computation for several threehyphen;element reaction coordinates assumed for decomposition of formic acid; the temperature independent (TIF) and temperature dependent factors (TDF) of the rate constant ratioksol;kprime;equals;k12csol;k13cwere calculated. As barrier curvature increases, the area in eigenvector space corresponding to valid transition states decreases in extent and alters in shape; these changes depend on the method of selection of certain secondary offhyphen;diagonal force constants. TIF is not affected. TDF is affected, particularly in the part of eigenvector space where theextentof the allowed region is being reduced by barrier curvature. While shapes and values associated with the TIF and TDF surfaces in eigenvector space can be related at least approximately to the inputFhyphen; andGhyphen;matrix properties in some cases, such accounting is generally not accurate in the first order high temperature approximation.