Molecular Orbital Studies on the Electronic Structure of Salicylaldehyde and its Isomerization Reaction in the Excited and Ground States

Abstract

In order to make clear the order of the n, π* and n, π* excited singlet states of salicylaldehyde and the shapes of the potential energy curves of isomerization reaction from salicylaldehyde to its proton-transferred structure in the excited and ground states, the molecular orbital calculations were carried out within the framework of the ab initio 6-31G level with the geometry-optimization using the Gaussian 90source program on the FACOM VP-100E computer.
From the present calculations the followin g conclusions were obtained: (1) In the ground state the intramolecular hydrogen bonded salicylaldehyde is most stable. (2) Its lowest excited singlet state is of the n, π* character and the second singlet state is of the n, π* character. The intramolecular hydrogen bond becomes weaker in the n, π* singlet state than in the ground state. The n, π* singlet state of salicylaldehyde with the intramolecular hydrogen bond is higher in energy than that of salicylaldehyde without the intramolecular hydrogen bond. These results suggest that the isomerization from the structure with the hydrogen bond to that without the hydrogen bond may proceed in the lowest n, π* singlet state. (3) The lowest n, π* state of salicylaldehdye is higher in energy than that of the proton-transferred structure. The calculated potential energy curve suggests that the isomerization reaction from salicylaldehyde to the proton-transferred structure may occur through the intramolecular hydrogen bond in the lowest n, π* singlet state, while it may be difficult in the ground state.