A CASPT2//CASSCF study of vertical and adiabatic electron transitions of acrolein in water solution

The 1(n --> pi*) excited-state of acrolein in liquid water was studied theoretically by using the averaged solvent electrostatic potential from molecular dynamics method (ASEP/MD). The model combines a multireference perturbational CASPT2//CASSCF treatment in the description of the solute molecule with NVT molecular dynamics simulations in the description of the solvent. In this paper, we present two alternative methods for calculating solvent shift on adiabatic transitions and their performance is analyzed. In the first method, the free energy change during an adiabatic transition is calculated classically by using the free energy perturbation method. In the second method, it is calculated from the quantum values of the vertical absorption and emission electron transition energies. The 1(n --> pi*) excitation is accompanied by a charge flux from the oxygen to the carbon skeleton, this charge flux decreases the dipole moment of the excited-state with respect to the ground state value and, consequently, the solute-solvent interaction energy. This effect destabilizes the excited-state with respect to the ground state and produces a blue shift in the absorption and emission bands. For the emission process, there also exists an additional destabilization due to a partial desolvation of the excited state. The effect of the solvent electron polarization, the inclusion of the solute electron correlation, and the use of relaxed geometries in solution on the calculated solvent shift of the absorption and emission spectra are also analyzed.