MC/MO study of the solvent effect on the excitation energies of the (CH3)2NO radical in hydrogen-bonding and non-hydrogen-bonding solvents

A combination of Monte Carlo (MC) simulation and ab initio molecular orbital (MO) calculation was applied to dimethyl nitroxide (DMNO) in H2O, CH3OH, CH3CN, and (CH3)(2)CO solutions, and the solvent effect on the electronic structure and n-pi* and pi-pi* excitation energies was analyzed. The solution structures were,generated by MC simulations, and the ROHF-SCI calculation with the MIDI-4 basis set was carried out for each solution structure. The electronic structure and excitation energies in the four solutions were obtained by averaging the 100 solution structure's for each solution. Solvent effect was calculated by the point charge model and supermolecule model. In the point charge model, all solvent molecules were approximated by point charges at atomic nuclei, while in the supermolecule model the solute molecule and some of the solvent molecules were treated as a supermolecule surrounded by other solvent molecules approximated by point charges. The calculated n-pi* excitation energy increased (blue shift) in the four solvents as compared to that in the gas phase. The magnitude of the solvent effect reflects the dielectric constant of the solvent. The calculated En-pi* value in CH3OH was larger than that in CH3CN, whose dielectric constant is larger than that of CH3OH. This is due to the hydrogen-bonding ability of CH3OH and agrees well with experiment. The excitation energy was predicted to decrease in the four solvents, although the red shift was overestimated. The solvent effect was well elucidated by using the Mulliken charges of DMNO in the ground state and the excited states and the electrostatic potential generated by the solvent molecules.