Ground and n–π* excited state structures of the hydrogen bonded complexes pyrazine⋅H2O and pyrazine⋅ (H2O)2

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Abstract
Vibronically resolved 1Ag→1B3u(n–π*) excitation and 3B3u(n–π*) →1Ag emission spectra are reported for isolated pyrazine, and the complexes pyraxine⋅Kr, pyrazine⋅H2O, and pyrazine⋅ (H2O)2 in solid neon host at 4.2°K. Distinct zero phonon lines appear in the complex spectra, enabling a separation of vertical and adiabatic excitation with respect to the low frequency intermolecular normal modes. In the monohydrate and bihydrate complexes, the hydrogen bond is only slightly weaker in the n–π* excited states. An upper limit for excited state hydrogen bond lengthening is obtained. The monohydrate 3n–π* complex decays purely radiatively, but a competitive intersystem crossing appears to occur in the bihydrate. The hydrate spectra and lifetimes are unchanged for D2O. Zero phonon lines are just resolved in the spectra of pyrazine isolated in solid ice at 10°K. These data enable a distinction to be drawn between specific solvation due to hydrogen bonding and general solvation due to a polar water environment.