High-resolution 14N NMR in polycrystalline solids

The N-14 NMR spectra of solids are usually very broad due to the presence of large quadrupole coupling constants. However, even partial excitation of the whole spectrum can give valuable information. With magic angle spinning (MAS), the spectrum consists of a number of peaks, but normally the centerband cannot be readily distinguished from the spinning sidebands. Multiple-pulse methods of sideband elimination, such as TOSS and PASS, cannot be used for N-14 because of its short spin-spin relaxation time. On the other hand, the sidebands can be eliminated by systematic data treatment. First, the signal-to-noise ratio (S/N) is enhanced by co-adding all the peaks in a MAS spectrum in a periodic way. Then, several spectra obtained at different spinning rates are added or multiplied together to identify the centerband. In the centerband region of the spectrum obtained from the addition method, the residual sidebands can be distinguished from the weak signals by the use of logical or digital filtering. Results obtained by using these methods to treat the spectra of two mixtures of KNO3, Pb(NO3)(2), and NH4Cl are shown. The experimental requirements are not very stringent, the S/N ratio is good, and peaks covering a large range of chemical shifts can be readily observed with high-resolution. (C) 1999 American Institute of Physics. [S0021-9606(99)00616-9].