AN ABINITIO STUDY OF SOME FREE-RADICAL HOMOLYTIC SUBSTITUTION-REACTIONS AT SELENIUM

Ab initio molecular orbital calculations at the HF/3-21G(*) and HF/HUZ-SV** levels of theory predict that homolytic substitution by alkyl and other radicals at the selenium atom in alkyl selenides proceeds via a T-shaped transition structure in which the attacking and leaving radicals are colinear, or nearly so. The calculated energy barriers suggest that the nature of the leaving group is important in these reactions, whereas the nature of the attacking alkyl radical is not. HF/HUZ-SV** calculations put the energy barrier for the attack on methaneselenol by the alkyl radicals in this study at 96-105 kJ mol-1. MP2 single-point correlation correction reduces most of these barriers to about 71-80 kJ mol-1, while approximately 68 kJ mol-1 is obtained using the MP4 correction. Formation of methyl radical from methaneselenol by reaction with silyl radical is predicted to be extremely favourable with an MP4/HUZ-SV**//HF/HUZ-SV** calculated barrier of only 12.6 kJ mol-1, reinforcing the importance of this type of reaction in generating alkyl radicals.