Theoretical studies on the mechanisms of proton transfer in Schiff bases

Abstract

Proton transfer reactions of salicylideneaniline (SA), N-tetrachlorosalicylideneaniline (Cl 4SA) and N-tetrachlorosalicylideneaniline-1-pyrenylamine (Cl 4SPY) have been investigated by using a semiempirical SCF MO method with an energy gradient technique. Thermochromic reactions involve the proton transfer from the hydroxyl oxygen to the imine nitrogen via six-membered ring transition states. From the calculated potential barriers, 81.27 and 87.27 kJ mol −1 for Cl 4SA and Cl 4SPY, respectively, it can be seen that the thermochromic reactions proceed easily at room temperature. The analyses of molecular orbitals and electronic distributions show that the thermochromism is also due to a change in the π-electron configuration induced by the proton transfer. Intermolecular interaction makes the thermochromic reaction of Cl 4SPY easier to proceed. Photochromic reaction of SA is initiated by excitation of the enol form to its first excited singlet state S 1. The population in the S 1 state is transferred either to the second excited singlet state of the keto form (S′ 2) along an adiabatic pathway or to the lowest excited triplet state of the keto form (T′ 1) via intersystem crossing, and finally relaxing to the trans-keto form (S′' 0) or cis-keto form (S′ 0).