Hydrogen Bonding and Protonation Effects on the Electronic Spectra of Nitrogen Heterocycles

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Hydrogen bonding and protonation effects on the electronic spectra of nitrogen heterocycles, especially pyridine, have been studied theoretically, employing quantum mechanical calculation.<BR>Hartree-Fock SCFMO's have been computed assuming two extreme models : (1), There is no transfer of n-electron on nitrogen toward proton, but bare proton is located near the nitrogen atom of neutral molecule (Model I). (2), There is considerable charge transfer from nitrogen atom to proton, leading to covalent binding between them (Model II). In addition, calculations based on the perturbation theory using SCFMO's of pyridine as zeroth order wave function have been performed, and also classical point charge-point charge interaction energies between proton and π-electron distributions for ground and excited states have been computed. The results calculated with model I give very good agreement with experimental observation.Other calculations, however, lead to the results which give rather poor agreements with experimental observations. A careful examination of these results leads to the following conclusions : (a), the real circumstance of pyridinium ion in solution is intermediary of model and model II. (b), the distortion of σ-core caused by charge transfer from nitrogen atom toward proton, is considerable.
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