ABINITIO 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 B-3(1)(npi*), 3A2(npi*), and 3A1(pipi*) 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 npi* states, conformations corresponding to minimum energy are found that deviate significantly from planar. For the 3A1(pipi*) state, vibronic coupling similar to that between the B-3(1u) and 3E1u states of benzene has been found. From the calculations, we deduce a dynamic out-of-plane distortion of the molecule in the B-3(1u)(npi*) state, which provides for a coherent description of the results of optical and magnetic-resonance data available for the lowest triplet state of pyrimidine. The out-of-plane distortions occurring in the B-3(1)(npi*) and 3A2(npi*) states conform to the increased antibonding character of the pi-electron system upon npi* excitation.