MC/MO study of the electronic structure and hyperfine coupling constant of the nitrogen 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 hyperfine coupling constant (hfcc) of nitrogen in DMNO was analyzed. The solution structures were picked up from the MC simulations, and a ROHF-SCI calculation with the MIDI-4 basis set was carried out for a supermolecule including one DMNO and a few solvent molecules surrounded by other solvent molecules approximated by point charges. The calculated hfcc of the N atom in DMNO in these solvents reflects the dielectric constant and the hydrogen-bonding ability of solvent and agrees with the experimental trend observed for di-tert-butyl nitroxide in solutions. In the H2O and CH3OH solutions, these are solvent molecules that are hydrogen-bonding and have a strong interaction with DMNO. For this interaction, the hfcc is larger in the CH3OH solution than in the CH3CN solution, although the dielectric constant of CH3CN is larger than that of CH3OH. Electron transfer between DMNO and the solvent molecules was acting in two directions: one from DMNO to solvent molecules around the N-O group and the other from solvent molecules to DMNO around the methyl groups. These electron transfers polarize the;rr-electron system of DMNO in the same direction as the electrostatic interaction does and increase the hfcc of the N atom.