# HYDROGEN BONDING IN WATER CLUSTERS. STRUCTURES OF 2-PYRIDONE-$H_{2}O$ AND 2-PYRIDONE-$(H_{2}O)_{2}$ IN THEIR $S_{0}$ AND $S_{1}$ ELECTRONIC $STATES,^{1}$

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 Title: HYDROGEN BONDING IN WATER CLUSTERS. STRUCTURES OF 2-PYRIDONE-$H_{2}O$ AND 2-PYRIDONE-$(H_{2}O)_{2}$ IN THEIR $S_{0}$ AND $S_{1}$ ELECTRONIC $STATES,^{1}$ Creators: Held, A.; Pratt, D. W. Issue Date: 1993 Publisher: Ohio State University Abstract: Recently, we have obtained the rotationally resolved fluorescence excitation spectrum of the $2-pyridone(2PY)-^{18}$ O-labeled water complex. These results complement earlier work, in which we reported the spectrum of the protonated and seven deuterated species of $2PY-H_{2}O$. Having the complete substitution structure of the water molecule attached to 2PY provides important information about the geometry of the complex. Thus, the water molecule forms two planar, non-linear hydrogen bonds with the amine hydrogen and carbonyl oxygen of 2PY, in both states. Both hydrogen bond lengths increase on electronic excitation, suggesting a decrease in hydrogen bond strengths in the $S_{1}$ state relative to the $S_{0}$ state. The structure of the $H_{2}O$ molecule changes very little with complexation. But the 2PY structure is modified significantly. These changes are consistent with an increase in the zwitterionic character of 2PY on solvation by water. Spectra of the dihydrated complex of 2PY also were obtained. $2PY-(H_{2}O)_{2}$ also is planar with respect to the heavy atoms, in both states. It exhibits three hydrogen bonds; one between a water molecule and the amine hydrogen, one between a second water and the carbonyl oxygen, and one between the two water molecules themselves. The heavy atom separations between the water molecules and 2PY and the increase in these separations on excitation are identical to those in $2PY-H_{2}O$. But the $2PY-(H_{2}O)_{2} S_{1} \leftarrow S_{0}$ spectrum is blue shifted relative to that of $2PY-H_{2}O$, which suggests that, in fact, the hydrogen bonds in $2PY-(H_{2}O)2$ are stronger than those in $2PY-H_{2}O$, perhaps because of their more linear character. These and other structural and dynamical properties of these clusters will be discussed. Description: 1Supported by NSF (CHE-9224398). Author Institution: Department of Chemistry, University of Pittsburgh URI: http://hdl.handle.net/1811/18554 Other Identifiers: 1993-RF-6