Hybrid Density Functional Studies of Phenoxyl Free Radicals Modeling α-Tocopheroxyl

Abstract

Hybrid density functional calculations are used to study the geometry, spin density distribution and hyperfine coupling constants for a number of phenoxyl-type free radicals modeling the α-tocopheroxyl free radical. Stereoelectronic and inductive effects on the spin density distribution are investigated. Delocalization of the free radical spin density onto the para oxygen atom of the heterocyclic ring is allowed when the participating oxygen 2p z orbital is held closely perpendicular to the main ring system as occurs for the α-tocopheroxyl radical. The resultant stabilization of the free radical enhances antioxidant (radical scavenging) activity. The effect of electron donating and electron withdrawing substituents on the heterocyclic ring is also studied with the carboxylate electron donating substituent, shown to increase delocalization of free radical unpaired spin. Calculated 'H electron-nuclear hyperfine couplings are in good agreement with experimental determinations and calculated 1 3 C and 1 7 O anisotropic hyperfine coupling values are shown to provide an excellent direct guide to the spin density distribution of the phenoxyl free radicals studied.