Intermolecular interactions in nuclear magnetic resonance. XI. The 13C and proton medium shifts of CH4 in the gas phase and in solution

The 1H and 13C resonances of methane have been measured at 298 K, both as pure CH4 in the gas phase as function of the density as well as at low concentration in solution, using polar, non polar, isotropic, as well as magnetically anisotropic solvents. It is found that both the gas and the solution proton shift data can be readily understood in terms of the Van der Waals shielding σw and the mean-square field due to the dispersive interactions: [Formula: see text]. Either a macroscopic reaction field model or a binary collision statistical mechanical model for [Formula: see text] can be used with an approximately equal degree of success. Whereas the 13C gas phase shifts of CH4 are also proportional to the density, the 13C gas-to-solution shifts indicate that, apart from σw, there exists even in non-polar isotropic solvents another effect which is not proportional to [Formula: see text]. The present study also includes a new proposal regarding [Formula: see text], a discussion of anisotropy shielding σa in magnetically anisotropic solvents and a discussion of the effective B parameters (1H and 13C) of CH4 in the gas and the solution state.