Solvent effects on molecular and ionic spectra .8. The (1)(n,pi*) excited states of pyridazine in water

Our method (parts I-VII) for estimating solvent shifts of species that have strong specific interactions (e.g., hydrogen bonding) with the solvent is applied to calculate the absorption and fluorescence solvatochromic (solvent) shifts of dilute pyridazine in water. The interpretation of the spectroscopy of pyridazine is complicated by the possibility that the S-1 state is nearly degenerate with one or more other electronic states. We evaluate solvent shifts for all possible low-lying states and conclude that if near degeneracies do indeed occur, then considerable, nonobserved changes in the absorption band shape would be expected with solvent variation. This lends strong support to the arguments suggesting that S-2 is somewhat removed from S-1. Only orthodox linear hydrogen bonding to the ground state is found to be consistent with observed solvent shifts; hydrogen bonding to the excited states of pyridazine is shown to be relatively weak, and the generally accepted interpretations of the solvent shifts of pyridazine are explicitly verified from a molecular point of view. This completes our detailed molecular analysis of the solvent shifts of the diazines in dilute solution.