ACCURATE AB-INITIO CARBON PROTON AND PROTON-PROTON SPIN-SPIN COUPLING SURFACES FOR THE METHANE MOLECULE

Accurate ab initio values of the carbon-proton and proton-proton spin-spin coupling constants of the methane molecule have been calculated at each of fifty-one distinct geometries. For each geometry the Fermi contact, spin-dipolar, orbital paramagnetic and orbital diamagnetic contributions were individually calculated. The resulting surfaces are treated as functions of symmetry displacement coordinates. Sixteen independent coefficients are required to describe the carbon-proton coupling surface and twenty-two independent coefficients to describe the proton-proton coupling surface to second order in the symmetry coordinates. Values for these coefficients are presented. The principal features of the results are the dominance of the Fermi contact contributions to the two couplings at all geometries, the greater importance of bond stretching for the carbon-proton coupling, the greater importance of angle-bending for the proton-proton coupling and the almost total cancellation of the orbital paramagnetic and orbital diamagnetic parts of the proton-proton coupling at all geometries. There is unexpected evidence that the Fermi contact contribution to the proton-proton coupling becomes more positive as the distance between the coupled protons increases. The data obtained are sufficient for calculating the coupling constants J(C, H), J(C, D), J(C, T), J(H, D), J(H, T) and J(D, T) in the isotopomers of methane for any temperature of experimental interest. It is believed that the results obtained are the most accurate and complete to date for spin-spin coupling in a polyatomic molecule.