SOLVENT EFFECTS ON MOLECULAR AND IONIC SPECTRA .6. HYDROGEN-BONDING AND THE DELOCALIZED NATURE OF THE FIRST (1)(N,PI-ASTERISK) EXCITED-STATE OF PYRAZINE

Our method (Parts 1-5(1-6)) for estimating solvent shifts of species w hich have strong specific interactions (e.g., hydrogen bonding) with t he solvent is applied to calculate the absorption and fluorescence sol vatochromic (solvent) shifts of dilute pyrazine in water. On the basis of interpretation of solvent shift data, pyrazine in its S-1 (1)(n,pi ) excited state has been thought to display reduced nuclear symmetry, with the excitation localized on just one of the two nitrogen atoms; this view has also been supported by electronic structure calculations . Such localization could occur, despite the presence of significant t hrough-bond interactions between the nitrogen lone pairs, if the reorg anization energy associated with symmetry breaking were sufficiently l arge. Here, the alternate description is developed for the electronic structure of this excited state of pyrazine based on studies of the fr ee molecule, of pyrazine-water clusters, and of pyrazine in dilute aqu eous solution. For the free molecule, extensive ab initio Davidson-cor rected CASSCF with MRCI calculations strongly suggest a high-symmetry geometry, and verify that this is the correct interpretation of the av ailable experiment data. For pyrazine-water clusters, only a high-symm etry model is shown capable of describing the observed high-resolution spectra, and for pyrazine in solution, only a high-symmetry model is shown to be capable of interpreting the observed fluorescence solvent shift.