Structure and dynamics of ammonia adsorbed on graphitized carbon black. Part 4.—Nuclear magnetic resonance spectra

Abstract

Proton nuclear magnetic resonance experiments on ammonia adsorbed on Graphon are described. Proton chemical shifts, spin–lattice relaxation times and linewidths have been measured as a function of temperature and coverage.The variation of the signal from adsorbed fluid ammonia has been used to measure adsorption isotherms, the isosteric heat of adsorption and melting curves of solid ammonia in contact with Graphon. All these results agree well with gravimetric and neutron scattering experiments described in three earlier papers.The chemical shift of adsorbed ammonia is large (39 p.p.m. to high field) and can be attributed to the effects of anisotropy in the susceptibility of graphite on ammonia molecules adsorbed close to the graphite basal planes. A semi-quantitative model of the effect shows that, above ≈ 195 K (the melting point of bulk solid ammonia), there can only be limited multilayer formation. As the temperature increases, the chemical shift decreases because ammonia molecules are promoted to layers further from the surface. At low temperatures the chemical shift also decreases because ammonia starts to aggregate at heterogeneities on the surface where there is no susceptibility effect. This is correlated with neutron scattering results. The two experiments taken together show the gradual transition with decreasing temperature from anisotropic fluid, through aggregates of ammonia similar to the super-cooled liquid, to bulk solid, the whole process resembling heterogeneous nucleation.The linewidths are abnormally large at all temperatures and coverages. There are two sources of the broadening, diffusion through local magnetic field gradients in the sample, and the possible restriction of the motion of the adsorbate to two dimensions. Other mechanisms which have been considered by other authors are eliminated by considering the neutron scattering results.The spin–lattice relaxation time of adsorbed ammonia is abnormally small (≈ 20 ms) and shows some evidence of a minimum corresponding to a correlation time of 2 × 10–9 s. Partial deuteration shows that the mechanism is intramolecular dipolar relaxation. Consideration of the frequency spectrum of the diffusive motions of adsorbed ammonia, known from quasielastic neutron scattering, indicates that the slow correlation time may arise from a slow rate of jumping between the molecular layers of adsorbate.