IV. Radical Conformation

Conformations of a molecule are usually viewed as arrangements of atoms that differ only by rotation about one or more single bonds. This view needs to be expanded where radicals are con­cerned because in some instances radicals with pyramidal configurations change from one con­for­mation to another by inversion of configuration at the radical center. Consider the case of the ethyl radical (Figure 4). This radical is reported to have a pyramidal configuration with a 6.2^o dis­tortion from planarity and an extremely low (0.15 kcal/mol) energy barrier between con­for­ma­tions.^20–22 For the ethyl radical bond rota­tion and inversion of configuration both can contribute to conversion of one conformation into another (Figure 4).²⁰

The intriguing complications associated with changes in conformation of alkyl radicals are not a major focus for conformational analysis of carbo­hy­drates. The primary objective where car­bo­hydrate radicals are con­cerned is to determine why a particular conformation is preferred and then to under­stand how con­for­mation influences reactivity. ESR spectroscopy is an essen­tial tool in achieving these objectives because it enables direct observation of radicals and, in so doing, pro­vides valuable information about their confor­mation.