TDDFT and MS‐CASPT2 Study of the Excited States of Para ‐Methoxymethylcinnamate

The vertical and adiabatic excitation energies of the ππ*, ππ*′, and nπ* states of the trans and cis isomers of para ‐methoxymethylcinnamate ( p ‐MMC) were calculated using time‐dependent density functional theory and multistate complete active space second‐order perturbation theory (MS‐CASPT2) with reasonably sized basis sets. The adiabatic excitation energies of the ππ* state of trans ‐ p ‐MMC calculated by TD‐CAM‐B3LYP and MS‐CASPT2 are in excellent agreement with the experimental value. The nπ* state lies at a higher energy than the ππ* state in the adiabatic excitation energy calculations, indicating that the barrier to internal conversion from the ππ* state to the nπ* state will be considerable. This result is opposite to the previous theoretical result. The microsolvation effect of a water molecule on p ‐MMC is examined by calculating the excitation energies. The origin of the stabilization of the ππ* state and destabilization of the nπ* state by microsolvation has also been elucidated using natural population analysis.