The calculation and analysis of isotope effects on the nuclear spinspin coupling constants of methane at various temperatures

Abstract

New ab initio symmetry-coordinate carbon–proton and proton–proton spin–spin coupling surfaces for the methane molecule have been computed. Calculations have been performed at the SOPPA(CCSD) level using a large-basis set and a grid of 55 geometries on the two surfaces. The nuclear motion in the isotopomers CH4, CH3D, CH2D2, CHD3 and CD4 has been averaged over these surfaces to give values of J(13C, H) and J(13C, D) for the 13C isotopomers and J(H, D) for the 12C isotopomers at selected temperatures. Calculated isotope effects on J(C, H) and J(C, D) are very close to the values observed experimentally. This validates analysis into the various nuclear motion contributions to show that bond stretching at first order dominates and that angle bending at second order is also significant. However, the bond-stretching terms stem from the ‘other’ three bonds and there is surprisingly very little contribution from the bond containing the coupled nuclei. For the proton–deuteron coupling it is second-order bending which plays the major part but here all contributions are very small which explains why no change in J(H, D) has ever been observed along the series CH3D, CH2D2 and CHD3. The source of non-additivity in the isotope effects on J(C, H), J(C, D) and J(H, D) is located. It is believed that the present results are the most accurate ab initio results for the spin–spin coupling in any polyatomic molecule, possibly the most accurate for the magnetic property of any polyatomic molecule and among the most accurate for a molecular property of a pentatomic molecule.