Temperature dependence of the carbon-13 proton spin-spin coupling constant in gaseous methane

The nuclear spin-spin coupling constant 1 J(13C, H) in 13C-enriched methane gas has been observed by 13C resonance to increase from 125·264 Hz at 180 K to 125·347 Hz at 369 K. The dependence on temperature is non-linear with a much steeper variation at higher temperatures. Very similar results are observed in 1H resonance. There appears to be a very slight dependence of 1 J(13C, H) on density at the lower temperatures. The theory of the temperature dependences of 1 J(13C, H) and 1 J(13C, D) in the low density, gaseous, isotopomers of methane 13CH x D4 - x is presented in terms of the coefficients of a single carbon-13 proton spin-spin coupling surface, the elements of the tensor of vibration-rotation theory and the average values of reduced normal coordinates and their squares. Results are applicable to the temperature dependences of all X, Y coupling constants and Y nuclear shielding constants in X Y 4 molecules of Td symmetry. Application of the theory to the results on 13CH4 gives an equilibrium value of 119·18(16) Hz which shows that the nuclear motion contributes more than 6 Hz to the observed coupling. The quantity (Jr + 3Js ) is found to be 368(10) Hz Å-1; Jr is the derivative of the coupling with respect to the length of the bond involved in the coupling and Js is the derivative with respect to the length of a bond not involved in the coupling.