Ab initio calculations of NMR and ESR coupling constants.

In Part I infinitesimal self-consistent field Hartree-Fock perturbation theory is applied to the calculation of the Fermi contact, electron orbital and spin-dipolar contributions to indirect nuclear spin-spin coupling constants in some small closed-shell molecules from ab initio LCAO-MO SCF wavefunctions. The dependence of the calculated values upon the size of Gaussian basis set employed and upon variations in molecular geometry is examined. The circumstances under which the method may be expected to yield results sufficiently reliable for most chemical purposes are indicated. In Part II the ab initio Unrestricted Hartree-Fock method with single spin annihilation is used in the LCAO-MO approximation with minimal Gaussian basis sets of atomic SCF orbitals to evaluate the isotropic and anisotropic electron-nuclear hyperfine coupling constants in some small doublet state free radicals of current experimental interest. This approach is applied to the identification of new radical species and to the elucidation of their molecular structures. In addition, the method is used to check the experimental assignments of hyperfine coupling constants deduced from ESR spectra and to investigate the reliability of Coulson's equation for the estimation of bond angles in triatomic ABA radicals.