A natural bond orbital analysis of hydrocarbon radicals

The methodology of natural bond orbital (NBO) analysis has been applied to the study of alkyl and cycloalkyl radicals with the objective of finding relationships between donor/acceptor computed stabilization energies and isotropic Fermi contact couplings (HFCs). A linear correlation was found between computed β-C-H hyperfine coupling constants and 2nd order perturbation energies, i.e. hyperconjugation. A linear correlation was established between HFCs and the complete range of cos2 θ of dihedral angles defined by the orbital axes of 2pC-radicals with vicinal β-C-H* bonds. The dominant spin transmission mechanism is attributed to hyperconjugation with smaller contributions from spin polarization.