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

Abstract

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 H 2O molecules, CH 3(H 2O) n ( n=0–2), were chosen as models of the solvation systems. The geometry optimizations of CH 3(H 2O) showed that the hydrogen of H 2O molecule orients toward the carbon of CH 3, and it is weakly bound to the carbon atom of CH 3. 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 CH 3 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.