Hydride shift in substituted phenyl glyoxals: Interpretation of experimental rate data using electronic structure and variational transition state theory calculations

Abstract

Experimental rate data for hydride transfer in some p-substituted phenyl glyoxals in 80: 20 dioxan:water are presented including kinetic hydrogen isotope effects. The roles of different substituents are discussed using electronic structure calculations of the potential energy surfaces at the SM5.4/PM3 level together with calculations employing variational transition state theory and multidimensional tunneling. p-NO₂ substitution has the most pronounced effect on the rate parameters, which can be understood in terms of the relative destabilisation of the reactant and reduced quantum mechanical tunneling through the barrier. The use of different models for tunneling suggests that when coupled with the SM5.4/PM3 method, the higher level of theories tend to overestimate the degree of tunneling.