Applications of high-resolution 13C and 15N n.m.r. of solids

Abstract

The combination of cross polarization, dipolar decoupling and magic angle spinning results in liquid-like high-resolution ¹³ C and ¹⁵ N n.m.r. spectra of a wide variety of solid materials. Structural determinations based on such ¹³ C n.m.r. spectra include the measurement of the extent to which pyrolysed polyacrylonitrile fibres (Orion) retain aliphatic character during the first step of the production of a carbon fibre, the determination of the chemical identity of the cross links formed from an acetyleneterminated polyimide resin, and the characterization of the metabolic products of a bacterial fermentation of wood lignin. All of these non-destructive analyses are performed on intact heterogeneous samples. The high resolution of the carbon experiment can also be exploited by obtaining proton spin-lattice relaxation parameters for chemically different protons in solids. Because of spin diffusion, these parameters are dependent on spatial proximity and so are helpful in measuring the homogeneity of solid blends of polymers such as poly(phenylene oxide) and polystyrene. High-resolution ¹³ C n.m.r. spectra of polymers can also be used for measuring microscopic chain dynamics. ¹³ C rotating-frame relaxation parameters observed for polycarbonate and poly (ethylene terephthalate) are related to the effects on motion of annealing, additives and structural substitutions. Individual relaxation rates are observed for individual carbons, so the behaviour of side groups is cleanly separated from that of the main chain. All of the line-narrowing and sensitivity-enhancing techniques applied to ¹³ C n.m.r. of solids work equally well for ¹⁵ N n.m.r. Use of ¹⁵ N rotating-frame and cross-polarization parameters leads to the assessment of the relative concentrations of ¹³ C - ¹⁵ N and ¹² C - ¹⁵ N pair concentrations in the main chains of multiply labelled proteins. Such measurements can be used to characterize the rate of protein turnover in fully expanded soybean leaves, as well as the details of protein synthesis in cultured soybean cotyledons.