Time-dependent density functional theory study on the electronic excited-state hydrogen bonding dynamics of BHC-nicotinamide in MeOH solution

The density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods were performed to investigate the electronic excited-state hydrogen bonding dynamics of the hydrogenbonded complex formed by BHC-nicotinamide (BHCN) and methanol (MeOH). The ground-state geometry optimizations, electronic transition energies, corresponding oscillation strengths of the low-lying electronically excited states, and the optimized S1 excited-state geometry for the isolated BHCN and MeOH monomers, the hydrogen-bonded BHCN–MeOH dimers, and BHCN–2MeOH trimer complexes have been calculated by using the DFT and TDDFT methods, respectively. We have demonstrated that the intermolecular hydrogen bond C10=O14···H40−O39−Me is weaker than C16=O17···H46−O45−Me in the hydrogen-bonded dimers and trimer no matter whether in the ground state or the excited state. In addition, our results are consistent with the relationship between the electronic spectral shifts and excited-state hydrogen bonding dynamics: hydrogen bond strengthening can induce the relative electronic spectra redshift, whereas a blueshift will be induced. In addition, we focused our attention on the frontier molecular orbital and the results could reasonably explain the hydrogen bond strengthening or weakening mechanism.