Strategies for extracting NMR parameters from 23Na MAS, DOR and MQMAS spectra.: A case study for Na4P2O7

The Na-23 magic-angle spinning (MAS), double rotation (DOR) and multiple-quantum magic-angle spinning (MQMAS) NMR spectra of anhydrous sodium pyrophosphate, Na4P2O7, measured at five different Larmor frequencies (nu(L)) ranging from 105.8 MHz (corresponding to 400 MHz H-1 frequency) to 211.6 MHz (800 MHz) are analysed and the complete set of NMR parameters (C-qcc, eta(Q) and delta(iso)) of the four crystallographically inequivalent sodium sites were determined with high accuracy. Different approaches of spectra evaluation are discussed and their results are compared. The most reliable results are obtained from a combined evaluation of five DOR and three MQMAS spectra but also from two DOR and one MAS spectra or even from a single MQMAS spectrum all data can be derived. It is shown that Na4P2O7 may serve as a useful reference material for experimental set-up and reliability tests of the various NMR experiments. (C) 1999 Elsevier Science B.V. All rights reserved.