ACCURATE AB-INITIO CARBON PROTON AND PROTON-PROTON SPIN-SPIN COUPLINGSURFACES 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 a t each of fifty-one distinct geometries. For each geometry the Fermi c ontact, spin-dipolar, orbital paramagnetic and orbital diamagnetic con tributions were individually calculated. The resulting surfaces are tr eated as functions of symmetry displacement coordinates. Sixteen indep endent coefficients are required to describe the carbon-proton couplin g surface and twenty-two independent coefficients to describe the prot on-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 s tretching for the carbon-proton coupling, the greater importance of an gle-bending for the proton-proton coupling and the almost total cancel lation of the orbital paramagnetic and orbital diamagnetic parts of th e proton-proton coupling at all geometries. There is unexpected eviden ce that the Fermi contact contribution to the proton-proton coupling b ecomes more positive as the distance between the coupled protons incre ases. The data obtained are sufficient for calculating the coupling co nstants 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 com plete to date for spin-spin coupling in a polyatomic molecule.