Photochemical isomerization; quantum dynamics; conical intersection; potential energy surface
This work deals with quantum chemistry and quantum dynamics of cis-trans isomerization via conical intersections for three model systems of reduced dimensionality. The focus is on obtaining clear pictures of the quantum dynamics in the excited electronic state using fulvene, 4-(methyl-cyclohexylidene)-fluoromethane as model systems. The effect of solvent polarity on the conical intersection is also investigated using 4-cyclopentadienylidene-1,4-dihydropyridine as model system.
Ab initio potential energy surfaces are built for the ground and first excited electronic states of the molecules 4-(methyl-cyclohexylidene)-fluoromethane and 4-cyclopentadienylidene-1,4-dihydropyridine. For fulvene , model potential energy surfaces are generated based on analysis of the corresponding ab initio potential energy surfaces. To investigate the dynamics of the isomerization process, the nuclear wave packets evolving on one or two dimensional coupled potential energy surfaces are simulated for the fulvene and the 4-(methyl-cyclohexylidene)-fluoromethane. For this purpose, the original adiabatic potential energy surfaces with kinetic couplings are transformed to the diabatic ones with potential couplings. To investigate the effect of laser pulses on the dynamics of the isomerization process, peliminary simulations based on one dimensional ab inito potentials and dipole functions of the 4-(methyl-cyclohexylidene)-fluoromethane model system are carried out.
It is found that for fulvene, radiationless decay due to vibrations along the symmetric allylic stretch is faster than the radiationless decay along the torsional coordinate. For 4-(methyl-cyclohexylidene)-fluoromethane, it is found that torsional/rotational motions can be conserved in the excited as well as in the ground state. Furthermore, the model 4-cyclopentadienylidene-1,4-dihydropyridine, the conical intersection between the ground and first excited electronic shifts in non-polar solvents, whereas in polar solvents the degeneracy is lifted.
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