- Ab-initio calculations on vibronic coupling in the lower triplet states of pyrimidine
- Journal of the American Chemical Society
- Pages (from-to)
- Document type
- Faculty of Science (FNWI)
- Van 't Hoff Institute for Molecular Sciences (HIMS)
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.
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