15N chemical shift referencing in solid state NMR

Solid-state NMR spectroscopy has much advanced during the last decade and provides a multitude of data that can be used for high-resolution structure determination of biomolecules, polymers, inorganic compounds or macromolecules. In some cases the chemical shift referencing has become a limiting factor to the precision of the structure calculations and we have therefore evaluated a number of methods used in proton-decoupled N-15 solid-state NMR spectroscopy. For C-13 solid-state NMR spectroscopy adamantane is generally accepted as an external standard, but to calibrate the 15N chemical shift scale several standards are in use. As a consequence the published chemical shift values exhibit considerable differences (up to 22 ppm). In this paper we report the N-15 chemical shift of several commonly used references compounds in order to allow for comparison and recalibration of published data and future work. We show that (NH4Cl)-N-15 in its powdered form (at 39.3 ppm with respect to liquid NH3) is a suitable external reference as it produces narrow lines when compared to other reference compounds and at the same time allows for the set-up of cross-polarization NMR experiments. The compound is suitable to calibrate magic angle spinning and static NMR experiments. Finally the temperature variation of (NH4Cl)-N-15 chemical shift is reported. (C)2014 Elsevier Inc. All rights reserved.