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

Abstract

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 Cs symmetry for the complex). This is in reasonable agreement with experiment and also with previous DFT and MP2 (second-order Møller−Plesset perturbation theory) calculations with large basis sets. Similar results are obtained for hydrogen bonding to the lowest (π,π*) excited state, S2 (1B2). However, for this complex in its first (n-π*) state, S1 (1B1), pyridine is found to adopt a boat configuration of only Cs 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-π*) 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,π*) excited state, the hydrogen bonding is to the electron-enhanced π 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.