Photoisomerization Dynamics in Solution

Abstract

Coulombic interaction is inversely proportional to the distance between two charged particles and converges to zero very slowly; a quite large cutoff-radius distance is required to treat molecular dynamics in solution. One of theoretical approaches to solve the problem is the Ewald summation. A further development, the particle-mesh Ewald (PME) summation, accelerates the original approach and is nowadays available from variety of MD program packages. In the present work, the PME summation was applied to the ONIOM scheme (PME-ONIOM) for validating Coulombic interaction in solution. The two different photochemical reactions were actually performed as nonadiabatic ab initio MD with the Zhu-Nakamura trajectory surface hopping, (i) the photoisomerization of a (Z)-penta-2,4-dieniminium cation (PSB3) in methanol solution, and (ii) the photochromic reaction between 1,3-Cyclohexadiene (CHD) and 1,3,5-cis-Hexatriene (HT) in both hexane and ethanol solvents. The two different S₁-state lifetimes of PSB3 were found to be 3.483 ps and 82 fs, which were consistent with the experimental trend, moreover the mystery of the product branching ratio could be solved. On the other hand, the high nonpolar-nonpolar affinity between CHD and the solvent was observed in hexane solvent, which definitely affected the S₁ lifetimes, the product branching ratio of CHD : HT, and solute (CHD) dynamics. In ethanol solvent, however, the CHD solute was isomerized in the solvent cage caused by the first solvation shell. The photochemical dynamics in ethanol solvent results in the similar property to the process appeared in vacuo (isolated CHD dynamics).