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 fluorescen ce excitation spectra of the mono- and disolvated water complexes of 2 -pyridone (2PY) have been obtained. Structures of 2PY-H2O in its S0 an d S1 electronic states have been determined from inertial parameters d erived 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 c arbonyl oxygen of 2PY, in both states. In the ground state, the struct ure of 2PY is found to change significantly with complexation. These c hanges are consistent with an increase in the zwitterionic character o f 2PY on solvation by water, which decreases on S1 double line arrow p oint left So excitation. 2PY-(H2O)2 also is planar in both states with respect to the heavy atoms. It exhibits three hydrogen bonds; a linea r bond between a water molecule and the amine hydrogen, a linear bond between a second water and the carbonyl oxygen, and a nonlinear bond b etween 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 proper ties of these complex structures are discussed.