The Lowest Singlet (n,π*) and (π,π*) Excited States of the Hydrogen-Bonded Complex between Water and Pyrazine

The hydrogen bonding between water and pyrazine in its ground, lowest (n,π*), and lowest (π,π*) states is
investigated using density-functional theory (DFT), time-dependent density function theory (TD-DFT), coupled-cluster singles and doubles (CCSD) theory and equation-of-motion coupled cluster (EOM-CCSD) theory.
For all states, the minimum-energy configuration is found to be an orthodox linear hydrogen-bonded species,
with the bond strength increasing by 0.4 kcal mol-1 upon formation of the (π,π*) state and decreasing by 1.0
kcal mol-1 upon formation of the (n,π*) state. The calculated solvent shifts for the complexes match
experimental data and provide a basis for the understanding of the aqueous solvation of pyrazine, and the
excited-state complexes are predicted to be only short-lived, explaining the failure of molecular beam
experiments to observe them. Quite a different scenario for hydrogen bonding to the (n,π*) excited state is
found compared to those of H2O:pyridine and H2O:pyrimidine:  for pyridine linear hydrogen bonds are unstable
and hydrogen bonds to the electron-enriched π cloud are strong, whereas for pyrimidine the excitation localizes
on the nonbonded nitrogen leaving the hydrogen-bonding unaffected. For H2O:pyrazine, the (n,π*) excitation
remains largely delocalized, providing a distinct intermediary scenario.