ANGULAR-DEPENDENCE OF GEMINAL SPIN-SPIN COUPLING-CONSTANTS IN A PROTOTYPE CH2 GROUP - J(H,H) VERSUS INTERBOND ANGLE IN METHANE

Correlated ab initio calculations of the dependences of the proton-proton spin-spin coupling constant J(H-1, H-2) in methane with the variation of the H-1-C-H-2 interbond angle are reported. The other coupling constants, including J(C, H), were simultaneously calculated. The coupling constants J(H-1, H-2) and J(C, H-1) increase nearly linearly with the opening of the interbond angle whereas J(H-2, H-3) and J(C, H-3) decrease. J(H-3, H-4) is only very slightly affected. The changes are due almost solely to changes in the Fermi contact term. The angular dependence is slightly larger at the SCF level of calculation than at the correlated level. The change in the geminal coupling constant J(H-1, H-2) with respect to equilibrium geometry is given to within 0.025 Hz by DELTAJ(H-1, H-2) (Hz) = 41.57 sin alpha12 - 8.90 sin2 alpha12, where alpha12 is the increase in the interbond angle with respect to equilibrium (-10-degrees less-than-or-equal-to alpha12 less-than-or-equal-to 10-degrees).