Theoretical study of the bridging in β-Halo Ethyl

Abstract

Large-acale multi-reference configuration interaction (MRD-CI) calculations in a quite flexible AO basis are employed to study the energy hypersurface for the reaction intermediates XC$_3$H$_4$ with X = Cl, Br and F. Particular emphasis is therby placed on determining the equilibrium conformations, the CH$_2$ rotation barrier and the energy surface for a possible bridging (shuttling motion (1a] of X between the two carbon centers). The absolute minimum in the potential energy surface is found in all three cases for the asymmetric ß-halo radical in general agreement with ESR data at an XCC angle of ca. 110°, a c-c separation somewhat shorter than a single bond and an approximate sp3 type hybridization ($\alpha _2 \approx$ 135-140°). In FC$_2$H$_4$ the energy difference between the minimum in the symmetric conformation and the absolute minimum is found to be more than 30 kcal so that shuttling seems impossible in agreement with experimental findings. In BrC$_2$H$_4$ the difference between these two potential minima is only between 1-2 kcal, i.e., smaller than the barrier to CH$_2$, rotation, so that· shuttling is favored, while ClC$_2$H$_4$ takes an intermediate position between these extremes. The use of correlated wavefunctions is found to be quite important for such a study; the results are related to various kinetic studies of these radicals