BARRIER WIDTHS AND TUNNELING IN THE 4-CENTERED SYN ELIMINATION OF H-XFROM ETHYL-X - THE ROLE OF TRANSITION-STATE ASYMMETRY

The syn elimination of HX from CH(3)CH(2)X 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 hav e been examined ab initio for X = H, BH2, CH3, NH2, NH3+, OH, OH2+, F, Cl, and Br, using 3-21G, 3-21G(d), 6-31G(d), and MP2/6-31G(d) optimiz ed structures and vibration frequencies. Four-centered transition stru ctures are found for all but X = H, BH2, and CH3; with BH2, no transit ion structure exists at MP2/6-31G(d); with H and CH3, the transition s tructures are three-centered. In addition, although the syn mechanism has lower energy for X = H than the four-centered Woodward-Hoffman all owed 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 tabl e. Concurrently, the X-H-C angle in the transition state increases. Th e increase in k(H)/k(D) as the X-H-C angle becomes more linear is the direction predicted by E.S. Lewis for hydride (deuteride) transfer, an d by More O'Ferrall for hydron transfer. One-dimensional corrections t o these isotope effects predict significant quantum mechanical tunneli ng only in the cases of F and OH, for which the Bell and Eckart barrie rs are narrower than those obtained from intrinsic reaction coordinate (IRC) calculations. In contrast, for NH2, where tunneling seems unimp ortant the Eckart barrier is wider than the IRC. A quantitative measur e 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.