MOLECULAR-ORBITAL STUDIES ON THE ELECTRON IC-STRUCTURE OF SALICYLALDEHYDE AND ITS ISOMERIZATION REACTION IN THE EXCITED AND GROUND-STATES

In order to make clear the order of the n, pi and pi, pi* excited sin glet states of salicylaldehyde and the shapes of the potential energy curves of isomerization reaction from salicylaldehyde to its proton-tr ansferred structure in the excited and ground states, the molecular or bital calculations were carried out within the framework of the ab ini tio 6-31G level with the geometry-optimization using the Gaussian 90 s ource program on the FACOM VP-100E computer. From the present calculat ions the following 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, pi character and the s econd singlet state is of the pi, pi character. The intramolecular hy drogen bond becomes weaker in the n, pi singlet state than in the gro und state. The n, pi singlet state of salicylaldehyde with the intram olecular hydrogen bond is higher in energy than that of salicylaldehyd e without the intramolecular hydrogen bond. These results suggest that the isomerization from the structure with the hydrogen bond to that w ithout the hydrogen bond may proceed in the lowest n, pi singlet stat e. (3) The lowest pi, pi state of salicyladehydye is higher in energy than that of the proton-transferred structure. The calculated potenti al energy curve suggests that the isomerization reaction from salicyla ldehyde to the proton-transferred structure may occur through the intr amolecular hydrogen bond in the lowest pi, pi singlet state, while it may be difficult in the ground state.