First Singlet (n,π*) Excited State of Hydrogen-Bonded Complexes between Water and Pyrimidine

Hydrogen bonds from water to excited-state formaldehyde and from water to excited-state pyridine have
been shown to display novel motifs to traditional hydrogen bonds involving ground states, with, in particular
for 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 the
hydrogen bonding can dramatically alter the electronic structure of the excited state. Such effects are rare for
ground-state interactions for which hydrogen bonding usually acts to merely perturb the electronic structure
of the participating molecules. It arises as the (n,π*) excitation of isolated pyrimidine is delocalized over
both nitrogens but asymmetric hydrogen bonding causes it to localize on just the noninteracting atom. As a
result, the excited-state hydrogen bond in H2O:pyrimidine is suprisingly very similar to the ground-state
structure. 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, despite
failure of experiments to actually do so. They also provide a simple model for the intricate control over
primary charge separation in photosynthesis exerted by hydrogen bonding, and for solvent-induced electron
localization in symmetric mixed-valence complexes. All conclusions are based on strong parallels found between
the 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 verified
through detailed comparison of computed properties with experimental data for both the isolated molecules
and the ground-state hydrogen bond.