THEORETICAL-STUDIES ON THE S(N)2 PROCESSES AT PRIMARY AND SECONDARY CARBON CENTERS

Ab initio MO calculations with the 6-31+G* basis sets are carried out on the identity exchanges X(-) + RX reversible arrow XR + X(-) with X = F and Cl, and geometries and the HF (Delta E(HF) (double dagger)) and MP2 activation barriers (Delta E(MP) (double dagger)) are reported. Relatively constant transition-state tightness, d(double dagger)(X...X), is found for R with primary (R = CH2Y) and secondary (R = (CHYY2)-Y-1) carbon centers, respectively. The difference in d(double dagger)(X...X) between primary and secondary carbon centers is ca. 0.10 Angstrom. For highly electronegative X and/or Y, the correlated energy barrier, Delta E(MP)(double dagger), is invariably lower than the Hartree-Fock energy barrier, Delta E(HF)(double dagger) . A satisfactory correlation with r = 0.90 for 11 R groups is found between the theoretical Delta E(MP)(double dagger) values for the identity exchanges and the experimental log k(2) values for the reactions of ROSO(2)C(6)H(5) with aniline in acetonitrile at 65.0 degrees C.