An improved experiment for heteronuclear-correlation 2D NMR in Solids

Abstract

Techniques have been proposed in the past (I, 2) for the measurement of twodimensional heteronuclear-correlation spectra of solids. However, they have not been widely used in practice, and seem to be capable of producing useful results only for a few, carefully selected materials. In this Communication an improved heteronuclear chemical-shit&on-elation experiment is introduced which can be applied to the analysis of ‘H13C and other heteronuclear correlations in a wide variety of crystalline and amorphous materials of practical interest, such as polymers, pharmaceutical products, and foods. The dominant interaction between proton and 13C nuclei in most solids is the direct heteronuclear dipolar coupling. Because this interaction depends entirely on the distance between the nuclei, regardless of chemical bonding, heteronuclear correlation by two-dimensional NMR provides a method for studying the geometry of molecules in the solid state. This includes both the stereochemistry of individual molecules and the positioning of adjacent molecules relative to each other. Another advantage of heteronuclear correlation is that it separates the proton resonances over the much larger 13C chemical-shift range. Therefore, the technique can provide well-resolved proton chemical-shift information for samples where it is impossible to resolve the proton resonances with any standard, one-dimensional spectroscopic technique. The complete experiment is illustrated in Fig. 1. It has the classic four-part structure (3) of preparation, evolution, mixing, and detection which is common to virtually all 2D NMR. In this case, the spin system is prepared by a single pulse, following which the protons are allowed to evolve in order to “label” them according to their chemical shifts. After the evolution period, a special mixing pulse sequence is applied to transfer polarization selectively from the protons to the carbon spins via the heteronuclear dipolar interaction. Finally, the 13C FID is acquired during cw proton decoupling. During the entire experiment, the sample is rotated about the magic angle in order to suppress broadening due to chemical-shift anisotropy. Phase cycling may be added to the basic experiment to suppress potential artifacts. The phase-cycling scheme used to obtain all data presented here is shown in Table 1. The 180” phase alternation of the initial pulse P is an implementation of the wellknown “spin-temperature alternation” method for eliminating pulse dead-time effects.