Theoretical spin density in nitroxides

Abstract

The evolution of the hyperfine tensors in the nitroxide series with increasing alkyl substitution on the NO group has been studied theoretically for radicals from H2NO to C5H10NO. A projection technique has been applied to the UHF wave-functions in order to correct spin densities for quartet contamination. The magnitude of the isotropic and anisotropic couplings reflects the substitution effect already observed on the spin distribution maps, that is a spin transfer from oxygen to nitrogen when alkyl groups are substituted to hydrogens in H2NO. The alternation of the signs of the couplings along the chain as well as the cos2 γ law for the coupling constants of atoms (C or H) in β position are verified in the series. The orientation of the anisotropic tensors with respect to the chemical bonds depends on the position of the atoms in the molecule (radical site, α, β positions); it is not affected by further substitution, except for strongly asymmetric configurations.