Intermolecular Potentials from NMR Data: II. CH4, CF4, and SiF4

Abstract

Proton and fluorine spin–lattice relaxation times per unit density, T1/ρ, in pure gases CH4, CF4, and SiF4 are experimentally found to be proportional to T−1.5 where T is the temperature. Using either a hard sphere or a 12–6 Lennard–Jones potential for the isotropic part of the intramolecular potential, it is shown here that these data can be analyzed to obtain information on the anisotropic part of the potential in these systems. The analysis is made within the framework of the Bloom–Oppenheim theory, 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. From the strength of the attractive term in the anisotropic potential, the octopole moments of all the three molecules are obtained. These values are comparable with the values reported earlier based on other techniques for CH4 and CF4 in case of the Lennard–Jones model. The octopole moments of CH4 and CF4 obtained from the hard sphere model are smaller by 25% than those from the Lennard–Jones model. For SiF4, we obtain values of 4.14 × 10−34 e.s.u. cm3 and 5.3 × 10−34 e.s.u. cm3, as the octopole moments from the Lennard–Jones and the hard sphere models respectively and no other value exists in the literature.