Hybrid density functional studies on the EPR parameters of heterosubstituted vinyl radicals: substituent effect on the isotropic hyperfine couplings with H and C nuclei

Isotropic hyperfine parameters of a set of vinyl radicals are investigated using the B1LYP hybrid density functional. The systems studied are RH_βC_β=CH radicals, where R=H, BH₂, CH₃, NH₂, OH and F. Theoretical results indicate that electronegativity of the substituent strongly affects the magnitude of hyperfine coupling with hydrogen nuclei as well as with ¹³C_β. A_iso(¹³C_β) varies from −8.7 (4.9) to 17.4 G (−17.8 G) for Z (E) isomers of the radicals depending on the R group, BH₂ and F, respectively. In the same order, for Z (E) isomeric forms A_iso(¹H_β) diminishes from 40.1 (67.7) to 18.4 G (40.9 G) and A_iso(¹H) – from 25.6 (24.1) to 1.5 G (1.3 G). The effect of the substituents on the spin and electron density distribution is discussed in the framework of natural population analysis and theory of atoms in molecules.