A direct ab initio molecular dynamics study of the finite temperature effects on the hyperfine coupling constant of methyl radical-water complexes

Temperature and solvation effects on the hyperfine coupling constants (HFCCs) of methyl radical have been investigated by means of direct ab initio molecular dynamics (MD) method. The complexes composed of methyl radical and H2O molecules, CH3(H2O)(n) (n = 0-2), were chosen as models of the solvation systems. The geometry optimizations of CH3(H2O) showed that the hydrogen of H2O molecule orients toward the carbon of CH3, and it is weakly bound to the carbon atom of CH3. The binding energies for n = 1 and n = 2 were calculated to be 1.50 and 2.82 kcal/mol at the MP2/6-311 + + G(d,p) level, respectively. The direct ab initio MD calculations indicated large temperature dependence of HFCCs: hydrogen-HFCC of CH3 decreases with increasing temperature. This large change is due to the fact that the structure of the complex is flexible and is significantly varied by thermal activation. Mechanism of the temperature dependence of HFCCs was discussed on the basis of theoretical results. (C) 2002 Elsevier Science B.V. All rights reserved.