THEORETICAL-STUDY OF POTENTIAL WELLS AND BARRIERS FOR SN2 REARRANGEMENT IN THE SYSTEMS (XCH3X)- WITH X = F, CL, AND BR

Potential energy characteristics (well depths and barrier heights) for SN2 rearrangement of the three symmetrical systems (XCH3X)- with X = F, Cl, and Br have been calculated by using quantum chemical configuration interaction (CI) approaches with extended basis sets. The ion-dipole complex stabilities are found to be 11-13 kcal/mol for X = F and 8-9 kcal/mol for X = Cl and Br. The barrier heights (relative to the energy of the separate reactants) are obtained as 1-3 kcal/mol for X = F and Br but 7-8 kcal/mol for X = Cl. These results show some unexpected features; at present, they cannot be conclusively compared with experimental data. Therefore, our study should stimulate, on the one hand, further experimental investigations together with careful analysis of the measured quantities and, on the other hand, further refined quantum chemical treatments. © 1990, American Chemical Society. All rights reserved.