Intermolecular Potentials from NMR Data: I. CH4–N2 and CH4–CO2

Abstract

Proton spin–lattice relaxation times have been measured in the gaseous mixtures CH4–N2 and CH4–CO2 as a function of density and composition in the temperature region 300–600 K. The values of T1/ρ extrapolated to 100% N2 and CO2 respectively were found to be proportional to T−n where n = 0.87 ± 0.14 for CH4–N2 and n = 0.91 ± 0.10 for CH4–CO2. It is not possible to fit these data using a hard sphere potential for the isotropic part. Using the Bloom–Oppenheim theory and assuming that the correlation time of the spin–rotation interaction can be approximated by the average lifetime of a molecule in the given J state, it is shown that the above temperature dependence can be fitted by a 12–6 Lennard–Jones potential combined with an appropriate anisotropic potential. We have obtained the strengths of the repulsive and attractive terms in the anisotropic potential by a least squares fit of the data. Using the known values of the quadrupole moments of N2 and CO2, the octopole moment of CH4 has been obtained.