Intramolecular proton transfer reactions in internally hydrogen-bonded Schiff bases: ab initio and semiempirical study

Abstract

The barrier heights of the ground state proton transfer reactions in a series of substituted salicylaldehyde anils and two symmetric Schiff bases, N, N′-bis(salicylidene)- p-phenylenediamine and N, N′-bis(2-hydroxy-1-naphthylmethylene)- p-phenylenediamine, were calculated using ab initio (4-31G and 6-31G ∗∗ basis sets) and semiempirical (AM1 and PM3) quantum-chemical methods. The minimum energy paths at the AM1 and 4-31G levels were followed for the salicylaldehyde anils. On the basis of the analysis of the intramolecular proton transfer reactions in the symmetric Schiff bases which may occur through a one- or two-step mechanism, it is suggested that the latter mechanism is more probable.