Nonadiabatic Excited-State Dynamics with Hybrid ab Initio Quantum-Mechanical/Molecular-Mechanical Methods: Solvation of the Pentadieniminium Cation in Apolar Media

A new implementation of nonadiabatic excited-state dynamics using hybrid methods is presented. The current approach is aimed at the simulation of photoexcited molecules in solution. The chromophore is treated at the ab initio level, and its interaction with the solvent is approximated by point charges within the electrostatic embedding approach and by a Lennard-Jones potential for the nonbonded interactions. Multireference configuration interaction (MRCI) and multiconfiguration self-consistent field (MCSCF) methods can be used. The program implementation has been performed on the basis of the Columbus and Newton-X program systems. For example, the dynamics of penta-2,4-dien-1-iminium (PSB3) and 4-methyl-penta-2,4-dien-1-iminium cations (MePSB3) was investigated in gas phase and in n-hexane solution. The excited-state (S(1)) lifetime and temporal evolution of geometrical parameters were computed. In the case of PSB3 the n-hexane results resemble closely the gas phase data. MePSB3, however, shows a distinct extension of lifetime due to steric hindering of the torsion around the central bond because of solute-solvent interactions.