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 C3v 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