Theoretical study of the electronic structure of HXY/XYH radicals (X[Double Bond]C,Si;Y[Double Bond]O,S).

The electronic structures of the HXY/XYH compounds (X[Double Bond]C,Si;Y[Double Bond]O,S) on the (2)A(') electronic ground state were investigated by applying the natural bond orbital (NBO) method to the computed B3LYP6-311G(**) wave functions. Different localized structures are proposed for the HXY and XYH isomers and the central XY unit is described as intermediate between a double and a triple bond in HCO, HCS, HSiO, and HSiS, similar to a double bond in COH, CSH, and SiSH, and clearly a single bond in SiOH. Through the comparison between the NBO results for the diatomic and hydrogenated compounds, the energy preferences on each pair of isomers and the computed geometrical parameters are explained. According to the structures proposed, the HXY compounds are sigma radicals with the spin density distributed along the molecular framework, while the XYH compounds are pi radicals with most of the unpaired spin located on an almost pure p orbital of the X atom. Finally, the amounts of spin density on natural atomic orbitals provided by the NBO method are used to explain the computed values of the isotropic and anisotropic hyperfine coupling constants.