A deuteron NMR study of neopentane adsorbed on graphite. An example for the dominance of the magnetic shielding over the electric quadrupole coupling

The deuteron NMR spectra and spin-lattice relaxation times of neopentane adsorbed on the graphitized carbon black Graphon at surface coverages between 0.1 and 1.2 were measured at two different Zeeman fields (2.1 and 8.1 T) in the temperature range 80–290 K. For comparison purposes the ²H NMR spectra were measured for neopentane on boron nitride, on the nongraphitized carbon black Spheron, and on the graphitized carbon black Sterling MT. The spectra of neopentane on the graphitized carbon blacks in the Zeeman field B₀ = 8.1 T are practically completely determined by an extramolecular magnetic shielding-type coupling arising from the anisotropic polarization of the graphite support. The spin-lattice relaxation times are dominated at temperatures of 80 to 150 K by the quadrupole coupling, whereas above 150 K the anisotropic shielding coupling dominates at high Zeeman field. The analysis of the spin-lattice relaxation times allows one to evaluate the reorientation times of the molecules about their center of gravity as well as the translational surface diffusion coefficients. The isotropic molecular reorientation rates were found to be of the same order of magnitude as those in the bulk liquid and plastic phases. The surface diffusion coefficients (about 5 × 10⁻⁸ m² s⁻¹ at the bulk melting temperature 256.6 K) are about one order of magnitude larger than those in the bulk liquid.