MONTE-CARLO AND MOLECULAR-ORBITAL STUDY OF SOLVENT EFFECT ON THE ELECTRONIC-STRUCTURE AND HYPERFINE COUPLING-CONSTANTS OF THE (CH3)(2)NO RADICAL - THE EFFECT OF ELECTRON-TRANSFER BETWEEN THE SOLUTE AND SOLVENTMOLECULES

Ab initio UHF MIDI-4 calculations have been carried out for the dimeth yl nitric oxide (DMNO) radical in the H2O and CHCl3 solutions. The con figurations of the solvent molecules were selected from the snapshots in the Monte Carlo (MC) simulation for the solution, and the electroni c structure and the hyperfine coupling constant (hfcc) of DMNO were ob tained by averaging the results of 300 configurations. The point charg e approximation in which all solvent molecules were represented by poi nt charges indicated that the electrostatic interaction polarizes the N-O bond and increases the spin density on the N atom. When some of th e solvent molecules near DMNO were treated as a supermolecule together with DMNO and the remaining solvent molecules were approximated by po int charges, the electron transfer occurs between DMNO and H2O molecul es in two directions, from DMNO to H2O around the N-O group and from H 2O to DMNO around the CH3 groups. These electron transfers polarize th e N-O bond in the same direction as the electrostatic interaction and increase the hfcc of the N atom. The contribution of the electron tran sfer accounts for 40% of the solvent effect on hfcc of DMNO. (C) 1998 Elsevier Science B.V. All rights reserved.