HYDROGEN-BONDING IN WATER COMPLEXES - STRUCTURES OF 2-PYRIDONE-H2O AND 2-PYRIDONE-(H2O)2 IN THEIR S0 AND S1 ELECTRONIC STATES

Rotationally resolved S1 double line arrow pointing left S0 fluorescence excitation spectra of the mono- and disolvated water complexes of 2-pyridone (2PY) have been obtained. Structures of 2PY-H2O in its S0 and S1 electronic states have been determined from inertial parameters derived by fitting spectra of the protonated species, seven deuterated species, and the O-18-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 S1 double line arrow point left So excitation. 2PY-(H2O)2 also is planar in both states with respect to the heavy atoms. It exhibits three hydrogen bonds; a linear bond between a water molecule and the amine hydrogen, a linear bond between a second water and the carbonyl oxygen, and a nonlinear bond between the two water molecules themselves. An apparent nonplanarity of the hydrogen bond ''bridging'' the two water molecules gives rise to separate conformers of 2PY-(H2O)2. The structural and dynamical properties of these complex structures are discussed.