Interaction of Radicals with σ-Holes

The noncovalent interactions of the CH₃^•, Cl^•, CF₃^•, and C(CH₃)₃^• radicals with CF₃X molecules (X = Cl, Br, I) have been investigated using CCSD(T)/aug-cc-pVTZ energy calculations on MP2/aug-cc-pVTZ-optimized geometries. The electrophilic chlorine atom prefers to complex with the equatorial belt of the heavier halogen X, whereas the nucleophilic radicals CH₃^• and C(CH₃)₃^• coordinate preferentially with the σ-hole of X to give C_{3 v} complexes. Complexation energies are generally small, ranging up to 5 kcal mol^-1 for CF₃I with either the chlorine atom or the ^tBu radical. Within the continuum from pure one-electron (SOMO-LUMO) to pure three-electron (SOMO-HOMO) bonds, the results can be interpreted in terms of predominantly one-electron bonds for nucleophilic radicals but closer to three-electron bonds for the chlorine atom. The trifluoromethyl radical binds very weakly via interhalogen interactions; the carbon radical center plays no role in the bonding.