Theoretical Studies on the Hydrogen Atom transfer Reaction (￥±)

摘要

The hydrogen atom transfer reaction between substituted methane, CH3X, and its radical, CH2X(X = H, F, CH3, CN, OH and NH2) was studied by MINDO/3 method. The transition state(TS) structure and energy barriers were determined and variation of the transition state and of the reactivity due to the change of X were analyzed based on the potential energy surface characteristics. It was found that the greater the radical stabilization energy, the looser the TS becomes; the TS occurs at about 15% stretch of the C-H bond, which becomes longer as the radical stabilization energy of CH2X increases. The intrinsic barrier, ツE× X,X, of the reaction with X was found to increase in the order H < F < CH3 < CN < OH < NH2. The degree of bond stretch of the C-H bond at the TS also had the same order indicating that the homolytic bond cleavage of the C-H bond is rate-determining. Orbital interalictions at the TS between LUMO of the fragment C---H---C and the symmetry adapted pair of nonbonding, n【(n1【n2), or pi orbitals of the two X atoms were shown to be the dominant contribution in determining tightness or looseness of the TS. The Marcus equation was shown to apply to the MINDO/3 barriers and energy changes of the reaction.