Hydrogen bonding in water complexes. Structures of 2-pyridone-H2O and 2-pyridone-(H2O)2 in their S0 and S1 electronic states

Rotationally resolved S 1 ←S 0 fluorescence excitation spectra of the mono- and disolvated water complexes of 2-pyridone (2PY) have been obtained. Structures of 2PY-H 2 O in its S 0 and S 1 electronic states have been determined from inertial parameters derived by fitting spectra of the protonated species, seven deuterated species, and the 18 O-labeled water species. The water molecule forms two planar, nonlinear hydrogen bonds with the amine hydrogen and the carbonyl oxygen of 2PY, in both states. In the ground state, the structure of 2PY is found to change significantly with complexation. These changes are consistent with an increase in the zwitterionic character of 2PY on solvation by water, which decreases on S 1 ←S 0 excitation