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

Experimental rate data for hydride transfer in some p-substituted phenyl gl yoxals in 80 : 20 dioxan : water are presented including kinetic hydrogen i sotope effects. The roles of different substituents are discussed using ele ctronic structure calculations of the potential energy surfaces at the SM5. 4/PM3 level together with calculations employing variational transition sta te theory and multidimensional tunneling. p-NO2 substitution has the most p ronounced effect on the rate parameters, which can be understood in terms o f the relative destabilisation of the reactant and reduced quantum mechanic al 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.