Molecular Orbital Considerations on the Molecular Structures of 2-Pyridone and 1-Methyl-2-pyridone in the Lowest Excited Singlet State and Their Electronic Properties in the Corresponding State

Abstract

In order to determine the molecular geometries of 2-pyridone and 1-methyl-2-pyridone in the ground and lowest excited singlet states the ab initio molecular orbital calculations at the RHF and RCIS levels using the 6-31+G (d) basis set were carried out. And also, the molecular orbital calculations at the B3LYP level using the 6-31G (d) basis set were carried out to obtain the molecular structures of these compounds in the ground state. From calculated results the following conclusions were drawn: (1) The ring frame-works of 2-pyridone and 1-methyl-2-pyridone in the ground state are planar within the precision of the present molecular orbital calculations. (2) In the lowest excited singlet state (S1 state: abbrev. ) the ring frame-works of these compounds are nonplanar. The angle between the plane containing the bond angle of the ring C3-N1-C4 and the bond N1-H2 is 30.4° for 2-pyridone, and the angle between the plane containing the bond angle of the ring C3-N1-C4 and the bond N1-C2 is 22.6°for 1-methyl-2-pyridone. (3) In the S1 state the energy differences between the nonplanar and planar structures are 4.30 and 6.72 kJ mo1-1 for 2-pyridone and 1-methyl-2-pyridone, respectively. (4) The UV absorption band of the nonplanar model at the longest wavelength side shifts to the lon ger wavelength than that of the planar model, while the oscillator strength of the corresponding band of the former model is smaller than that of the latter one. (5) In the S1 state of 2-pyridone the planar conformation is better than the nonplanar conformation for the dimer formation and the complex formation with the carboxylic acid such as acetic acid. (6) The S1 states of these compounds are partially of n, π* character due to the non-planarity of their ring frame-works.Therefore, the S1 state may be mixed easily wit h the near-lying π, π * and n, π* excited states through the vibronic-coupling and spin-orbit coupling. These couplings lead to the deactivation of the S1 state. The reason of low fluorescence quantum yields of 2-pyridone and its relevants is due to the nonplanar structure in the S1 state.