| Abstract
| - Hydrogen bonds from water to excited-state formaldehyde and from water to excited-state pyridine havebeen shown to display novel motifs to traditional hydrogen bonds involving ground states, with, in particularfor H2O:pyridine, strong interactions involving the electron-rich π cloud dominating the (n,π*) excited state.We investigate H2O:pyrimidine and various dihydrated species and reveal another motif, one in which thehydrogen bonding can dramatically alter the electronic structure of the excited state. Such effects are rare forground-state interactions for which hydrogen bonding usually acts to merely perturb the electronic structureof the participating molecules. It arises as the (n,π*) excitation of isolated pyrimidine is delocalized overboth nitrogens but asymmetric hydrogen bonding causes it to localize on just the noninteracting atom. As aresult, the excited-state hydrogen bond in H2O:pyrimidine is suprisingly very similar to the ground-statestructure. These results lead to an improved understanding of the spectroscopy of pyrimidine in liquid water,and to the prediction that stable excited-state hydrogen bonds in H2O:pyrimidine should be observable, despitefailure of experiments to actually do so. They also provide a simple model for the intricate control overprimary charge separation in photosynthesis exerted by hydrogen bonding, and for solvent-induced electronlocalization in symmetric mixed-valence complexes. All conclusions are based on strong parallels found betweenthe results of calculations performed using density-functional theory (DFT) and time-dependent DFT (TDDFT),complete-active-space self-consistent-field (CASSCF) with second-order perturbation-theory correction(CASPT2) theory, and equation-of-motion coupled cluster (EOM-CCSD) theory, calculations that are verifiedthrough detailed comparison of computed properties with experimental data for both the isolated moleculesand the ground-state hydrogen bond.
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