Theoretical study of the potential energy surface governing the stereochemistry in chloroethyl radical reactions

Large-scale multireference configuration interaction calculations in a double-{xi}-type AO basis including polarization functions are carried out for the potential surface of the ClC{sub 2}H{sub 4} system. The charge distribution for various extreme points of the surface is discussed. The absolute minimum is found for an asymmetric ClC{sub 2}H{sub 4} structure. The symmetrical bridged nuclear conformation is also found to be stable with respect to dissociation into Cl + C{sub 2}H{sub 4}. The activation energy for rotation about the C-C axis is calculated to be around 18 kJ/mol, which is comparable to that for the 1,2 migration (around 26 kJ mol). The stereochemistry is governed by the fact that addition of Cl to C{sub 2}H{sub 4} (or dissociation) is a two-step reaction proceeding through a symmetrical intermediate. The direct addition pathway possesses a small barrier of about 8 kJ/mol.