Theoretical study of the electronic and hyperfine structures of the HSO and SOH radicals

The electronic structures, the spin density distributions, and the isotropic and anisotropic hyperfine coupling constants of the HSO and SOH radicals have been studied by applying the natural bond orbital (NBO) method to the B3LYP/6-311++G(3df,3pd) densities. Within the NBO framework, the SO unit is properly described as intermediate between a single and a double bond and the HSO isomer is more stable due to the favorable pπ(SO) overlap and the high strength of the HS bond. The computed spin densities indicate that both compounds are π-radicals, although the unpaired spin in HSO is shared by the sulfur and oxygen atoms and it is mostly located on the sulfur for SOH. The different spin density distributions and the values of the isotropic and anisotropic hyperfine coupling constants are properly explained on the basis of the proposed NBO structures.