RADICAL-ADDITION TO ALKENES - AN ASSESSMENT OF THEORETICAL PROCEDURES

Ab initio calculations at a variety of levels of theory have been carried out for a number of prototypical radical addition reactions with the aim of identifying levels of theory suitable for general application. The calculated geometries have a relatively small dependence on basis set but a somewhat greater dependence on correlation procedure. The energetic quantities (barriers and reaction enthalpies) are much more sensitive to lever of theory, particularly to the correlation procedure used. Absolute barriers require methods such as UCCSD(T) or UQCISD(T) for an accurate description. Relative barriers for a series of related reactions are also best treated with methods such as UQCISD or UQCISD(T), but the simpler RMP2, PMP2, and UB-LYP procedures can fare reasonably well under specific circumstances. UMP2 performs poorly for both absolute barriers and relative barriers, a result that may be attributed to significant and variable spin contamination in the individual systems involved in radical addition reactions.