Structural measurements by solid-state NMR

Some solid-state NMR techniques developed recently by the authors to obtain direct information of molecular geometry are reviewed. C-N distances were very accurately determined by observing the Pake-doublet patterns in static samples using highly compensated inversion-pulse sequences. Deuteron two-dimensional exchange NMR based on spin diffusion in static powder samples was developed to determine interbond angles precisely, in which adiabatic cross polarization was used to overcome very long deuteron Ti in rigid solids and cross relaxation between two deuterons was caused by sample-turning during the mixing time. Rotational resonance was extended from the conventional rotating frame to the double tilted rotating frame by applying an rf field. Even when the chemical shift difference is small, a particular dipolar interaction can be recoupled in the double tilted relating frame, keeping the spinning speed fast enough to remove the spinning sidebands and to decouple the other dipolar interactions even in a fully labelled molecule. Furthermore, the present method allows us to switch the recoupling condition to another instantaneously. This feature gives many attractive possibilities for structural measurements in solids. Relayed anisotropy correlation NMR was proposed as a means to determine dihedral angles. In this method, a magnetization is developed under an anisotropic interaction and transferred to another nuclei, and the signal of the magnetization developed under another anisotropic interaction is observed. When an amplitude modulated rf-field is applied to a non-observed spin and the modulation frequency is matched to the MAS frequency, then the dipolar coupling with the observed spin can be recovered. Since the recoupling condition does not include the intensity of the rf field, this recoupling method is insensitive to inhomogeneity of the rf field. Distances obtained by solid-slate NMR and diffraction studies are known to be usually different from each other. The reason is that molecular vibrations influence measured distances, depending on the measurement method. The influence of stretching, deformation and lattice vibrations on measured NMR distances and also interbond angles are discussed. (C) 1998 Elsevier Science B.V.