A Computational Study of the Effect of Bending on Secondary Kinetic Isotope Effects in S_N2 Transition States

摘要

Using conventional transition state theory,the secondary deuterium kinetic isotope effect (KIE) in the inversion S_N2 reaction of CH_3F and F~- is calculated to be small,0.98 (T=298 K).This is shown to be the result of a balance among opposing entropy and enthalpy terms.By contrast,KIE in the retention S_N2 mechanism is calculated to be large (1.5).Accordingly,KIE is a potential observable for discriminating between the two mechanisms.Large KIE's are also found for the inversion and retention mechanisms of the ion pair reactions between CH_3F and LiF.All of the transition structures leading to large KIE's have a bent FCF angle and an imaginary frequency that is sensitive to deuterium labeling.