The first singlet (n,π*) and (π,π*) excited states of the hydrogen-bonded complex between water and pyridine

The excited-state hydrogen bonding between a pyridine molecule and a water molecule has been investigated by a series of theoretical methods including direct and time-dependent density functional theory (DFT and TD-DFT), complete-active-space self-consistent-field (CASSCF) with second-order perturbation-theory correction (CASPT2), and equation-of-motion coupled-cluster (EOM-CCSD). All calculations indicate that the water:pyridine complex on the ground state has strong hydrogen-bonding with binding enthalpies ranging from 4.5 to 5.9 kcal mol(-1) after basis set superposition error, zero-point, and thermal correction, with the water molecule lying perpendicularly to the pyridyl plane (total C-s symmetry for the complex). This is in reasonable agreement with experiment and also with previous DFT and MP2 (second-order Moller-Plesset perturbation theory) calculations with large basis sets. Similar results are obtained for hydrogen bonding to the lowest (pi,pi*) excited state, S-2 (B-1(2)). However, for this complex in its first (n-pi*) state, S-1 (B-1(1)), pyridine is found to adopt a boat configuration of only C-s symmetry with the water above the pyridyl plane. Both the EOM-CCSD and CASPT2 calculations indicate that reasonably strong hydrogen-bonding occurs to pyridine in the (n-pi*) state, with the calculated bond enthalpies ranging from 4.0 to 4.5 kcal mol(-1). Hence, we find that excited-state hydrogen bonding to azines remains important, but that it has a different motif from the usual linear hydrogen bonding found in ground-state systems. For the (n,pi*) excited state, the hydrogen bonding is to the electron-enhanced pi cloud of the aromatic ring. A new, much more complex picture is presented for hydrogen bonding in azines which is qualitatively consistent with observed spectroscopic data.