NUCLEAR-SPIN SPIN COUPLING IN THE METHANE ISOTOPOMERS

The coupling constants 1J(C, H) and 2J(H, H) of the methane molecule have been calculated as functions of bond-length extension and compression in the vicinity of equilibrium geometry. This has facilitated the prediction of the temperature dependences of these couplings. The calculations were carried out using various polarization propagator methods. There is a very large contribution from electron correlation to both couplings. The bond-length dependence is dominated by the Fermi-contact part of the coupling. 1J(C, H) is calculated to increase by 0.054 Hz upon increasing the temperature of 13CH4 from 200 to 400 K. This result is less than the observed value of 0.083 Hz due to the neglect of higher-order terms, including those involving the angle dependence of the coupling. 2J(H, D) in 12CH3D is calculated to be virtually temperature independent. The calculated total carbon-proton coupling at 300 K is 126.31 Hz, which is only 1 Hz greater than that experimentally observed. The calculated total proton-proton coupling at 300 K is -14.24 Hz, which is numerically greater by about 2 Hz than that calculated from a recent measurement on 12CH3D. © 1991.