SOLVENT EFFECTS ON MOLECULAR AND IONIC SPECTRA .8. THE (1)(N,PI-ASTERISK) 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 s olvent is applied to calculate the absorption and fluorescence solvato chromic (solvent) shifts of dilute pyridazine in water. The interpreta tion of the spectroscopy of pyridazine is complicated by the possibili ty that the S-1 state is nearly degenerate with one or more other elec tronic states. We evaluate solvent shifts for all possible low-lying s tates and conclude that if near degeneracies do indeed occur, then con siderable, nonobserved changes in the absorption band shape would be e xpected with solvent variation. This lends strong support to the argum ents suggesting that S-2 is somewhat removed from S-1. Only orthodox l inear hydrogen bonding to the ground state is found to be consistent w ith 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 ve rified from a molecular point of view. This completes our detailed mol ecular analysis of the solvent shifts of the diazines in dilute soluti on.