Electronic structure of pyrazine: a study of the geometries in the lowest excited singlet π *← n and π*←π states |
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Authors: | SK Srivastava PC Mishra |
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Institution: | Department of Physics, Spectroscopy Laboratory, Banaras Hindu University, Varanasi- 221005 India |
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Abstract: | Electronic, excitation energies, charge distributions and geometries of pyrazine in the lowest excited singlet π*←π and π*←n states have been studied by the VE—PPP, CNDO/2 and CNDO/s-CI molecular orbital methods. Study of the change of geometry in the π*←n excitation requires localization of the density matrices in the ground and excited states, and with the help of these σ-bond orders are defined. Charge distributions and bond orders in the lowest excited singlet π*←π and π*←n states are compared. Whereas in the lowest singlet π*←π excitation the pyrazine ring expands uniformly, in the case of the π*←n excitation C-C bonds contract whereas C-N bonds elongate. The predictions of theory are in agreement with experimental results, showing that the method used can be employed to obtain reliably the trends of geometry changes following a π*←π excitation of a molecule before a more complete theoretical or experimental study is performed. |
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