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-pro ton spin-spin coupling constant J(H-1, H-2) in methane with the variat ion of the H-1-C-H-2 interbond angle are reported. The other coupling constants, including J(C, H), were simultaneously calculated. The coup ling 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 de pendence is slightly larger at the SCF level of calculation than at th e 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, whe re alpha12 is the increase in the interbond angle with respect to equi librium (-10-degrees less-than-or-equal-to alpha12 less-than-or-equal- to 10-degrees).