Resolution in 13C NMR of organic solids using high-power proton decoupling and magic-angle sample spinning

Abstract

The 13C NMR linewidths observed in organic solids by means of high-power proton decoupling and magic-angle sample spinning are roughly 10 to 100 times broader than resonances in the liquid phase. This paper investigates important 13C line-broadening mechanisms in organic solids and their dependences on experimental parameters, notably static and rf magnetic field strength. The discussion is limited to glassy (disordered, partially mobile) and crystalline (ordered, rigid) organics at natural isotopic abundance. Excluded are elastomers and systems with a third dipolar coupled nuclear species. Experimental data, primarily at 1.4 T, for glassy and semi-crystalline polymers as well as crystalline materials, illustrate and confirm the linebroadening mechanisms identified. For some specimens, 13C linewidths are compared at 1.4 and 4.7 T. It is found that a substantial linebroadening (0.5 to 6 ppm) corresponding to a dispersion of isotropic chemical shifts can arise from distributions of anisotropic sources of magnetic susceptibility, bond angles, or frozen molecular conformations; in other cases, the resonance lines may be split into many distinct lines by magnetic inequivalences present in the solid but not the liquid phase. For crystalline materials, methods for reducing the broadening from anisotropic bulk susceptibility are discussed. Other broadening mechanisms considered are: insufficient proton-decoupling fields, off-resonance decoupling, imperfections in magic-angle sample spinning, and motional modulation of both the carbon-proton dipolar coupling and the carbon chemical shift anisotropy. On consideration of these mechanisms, it is anticipated (and shown experimentally in limited cases) that no significant gain in resolution will be enjoyed at high magnetic fields, especially when variable-temperature operation is available. In some instances, degradation of resolution may occur at high field if large rf field strengths or high spinning rates cannot be achieved.