Photoinduced hydrogen-transfer reactions in pyridine-water clusters: Insights from excited-state electronic-structure calculations

Recent experiments have shown that photoexcited pyridine can abstract a hydrogen atom from a water molecule in pyridine-water clusters containing at least four water molecules. To explain these findings, we explored the electron-driven proton-transfer reaction from water to pyridine in pyridine-(H₂O)_n, n = 1–4, complexes with ab initio methods. It is shown that the photoinduced electron/proton transfer reaction is energetically possible for all clusters. The calculations reveal that the hydrogen bond between pyridine and the adjacent water molecule is weakened (strengthened) in the ¹nπ^∗ (¹ππ^∗) excited state, which is unfavorable (favorable) for the H-atom transfer reaction. For pyridine-(H₂O)_n clusters with n = 1–3, the steepest descent path leads to a local minimum of ¹nπ^∗ character, while for the pyridine-(H₂O)₄ cluster, this path leads to a local minimum of ¹ππ^∗ character. The transition state calculations show the presence of this ¹ππ^∗ minimum substantially reduces the barrier height for the H-atom transfer reaction. These results provide a tentative explanation of the experimental observations.