Barrier Widths and Tunneling in the Four‐Centered Syn Elimination of H‐X from Ethyl‐X. The Role of Transition State Asymmetry

Abstract

AbstractThe syn elimination of HX from CH3CH2X produces a C2H4‐HX complex, which is stabilized relative to ethylene and HX. The primary kinetic H/D isotope effects in the complex‐forming steps of such reactions have been examined ab initio for X = H, BH2, CH2, NH2, NH3+, OH, OH2+, F, Cl, and Br, using 3–21G, 3–21G(d), 6–31G(d), and MP2/6–31G(d)optimized structures and vibration frequencies. Four‐centered transition structures are found for all but X = H, BH2, and CH3; with BH2, no transition structure exists at MP2/6–31G(d); with H and CH3, the transition structures are three‐centered. In addition, although the syn mechanism has lower energy for X=H than the four‐centered Woodward‐Hoffman allowed anti mechanism, C‐H, C‐C, and H‐H bond breaking would be favored over either pathway. The semiclassical primary kinetic isotope effect increases systematically as the central atom of a neutral X is varied from left to right along a row, or down a column, of the periodic table. Concurrently, the X‐H‐C angle in the transition state increases. The increase in kH/kD as the X‐H‐C angle becomes more linear is the direction predicted by E.S. Lewis for hydride (deuteride) transfer, and by More O'Ferrall for hydron transfer. One‐dimensional corrections to these isotope effects predict significant quantum mechanical tunneling only in the cases of F and OH, for which the Bell and Eckart barriers are narrower than those obtained from intrinsic reaction coordinate (IRC) calculations. In contrast, for NH2, where tunneling seems unimportant, the Eckart barrier is wider than the IRC. A quantitative measure of barrier width is the imaginary frequency of the transition state. Tunneling occurs when this is large, when the barrier is large, and, most importantly, when the transition structure is symmetrical.