Ab-initio calculations on vibronic coupling in the lower triplet states of pyrimidine

Ab initio calculations at the UHF and CASSCF levels have been performed in which the geometry of pyrimidine in the 3B1(np*), 3A2(np*), and 3A1(pp*) states has been optimized. The calculations clearly demonstrate the influence of vibronic coupling on the molecular and electronic structure of the molecule in these states. For the np* states, conformations corresponding to min. energy are found that deviate significantly from planar. For the 3A1(pp*) state, vibronic coupling similar to that between the 3B1u and 3E1u states of benzene has been found. From the calculations, a dynamic out-of-plane distortion of the molecule in the 3B1(np*) state, which provides for a coherent description of the results of optical and magnetic-resonance data available for the lowest triplet state of pyrimidine, is deduced. The out-of-plane distortions occurring in the 3B1(np*) and 3A2(np*) states conform to the increased antibonding character of the p-electron system upon np* excitation.