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

The Na-23 magic-angle spinning (MAS), double rotation (DOR) and multiple-qu antum magic-angle spinning (MQMAS) NMR spectra of anhydrous sodium pyrophos phate, Na4P2O7, measured at five different Larmor frequencies (nu(L)) rangi ng from 105.8 MHz (corresponding to 400 MHz H-1 frequency) to 211.6 MHz (80 0 MHz) are analysed and the complete set of NMR parameters (C-qcc, eta(Q) a nd delta(iso)) of the four crystallographically inequivalent sodium sites w ere determined with high accuracy. Different approaches of spectra evaluati on 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 spec trum all data can be derived. It is shown that Na4P2O7 may serve as a usefu l reference material for experimental set-up and reliability tests of the v arious NMR experiments. (C) 1999 Elsevier Science B.V. All rights reserved.