Time-Dependent Density Functional Theory Study on Excited-State Hydrogen Bonding Dynamics of Acetic Acid Hydrates

The time-dependent density functional theory (TDDFT) method was performed to investigate the hydrogen-bonding dynamics of acetic acid (AA) hydrates in aqueous solution. For AA, it tends to be both active hydrogen acceptor and donor and denote double H-bonds as O-A center dot center dot center dot H-W and H-A center dot center dot center dot O-W, resulting in ring structure hydrates. The ground-state geometry optimizations and electronic transition energies and corresponding oscillation strengths of the low-lying electronically excited states for the isolated AA monomer and the hydrogen-bonded ring structure hydrates are calculated by the density functional theory and TDDFT methods, respectively. Different types of intermolecular hydrogen bonds are formed between one AA molecule and water molecules. According to Zhao's rule on the excited-state hydrogen bonding dynamics, it can be found that the intermolecular hydrogen bonds O-A center dot center dot center dot H-W and H-A center dot center dot center dot O-W are strengthened in electronically excited states of the hydrogen-bonded ring structure hydrates, with the excitation energy of a related excited state being lowered and electronic spectral redshifts being induced. Moreover, the hydrogen bond strengthening in the electronically excited state is crucial to the photophysics and photochemistry of hydrates with AA in aqueous solution.