Rotational Mode Specificity in the F- + CH3Y [Y = F and CI] SN2 Reactions

More than 12 million quasidassical trajectories are computed for the F- + CH3Y(v = 0, JK) [Y = F and CI] S(N)2 reactions using full-dimensional ab initio analytical potential energy surfaces. The initial (J, K = 0) and (J, K = [J = 0, 2, 4, 6, 8] rotational state specific cross sections are obtained at different collision energies (E-coll) in the 1-20 kcal mol(-1) range, and the scattering angle and initial attack angle distributions as well as the mechanism-specific opacity functions are reported at E-coll = 10 kcal mol(-1). The tumbling rotation (K = 0) inhibits the F- + CH3F reaction by a factor of 3 for J = 8 at E-coll = 10 kcal mol(-1). This tumbling rotational effect becomes smaller at low and high E-coll, and the tumbling motion affects the cross sections of F- + CH3Cl by only a few percent. The spinning rotation (K = J) hinders both reactions by factors in the 1.3-1.7 range for J = 8 at low E-coll, whereas slight promotion is found as the E-coll increases. The tumbling rotation may counteract the attractive ion-dipole forces, and the spinning motion hinders the complex formation, thereby decreasing the reactivity.