The effectiveness of 1H decoupling in the 13C MAS NMR of paramagnetic solids:: An experimental case study incorporating copper(II) amino acid complexes

The use of continuous-wave (CW) H-1 decoupling has generally provided little improvement in the C-13 MAS NMR spectroscopy of paramagnetic organic solids. Recent solid-state C-13 NMR studies have demonstrated that at rapid magic-angle spinning rates CW decoupiing can result in reductions in signal-to-noise and that H-1 decoupling should be omitted when acquiring C-13 MAS NMR spectra of paramagnetic solids. However, studies of the effectiveness of modern H-1 decoupling sequences are lacking, and the performance of such sequences over a variety of experimental conditions must be investigated before H-1 decoupling is discounted altogether. We have studied the performance of several commonly used advanced decoupling pulse sequences, namely the TPPM, SPINAL-64, XiX, and eDROOPY sequences, in C-13 MAS NMR experiments performed under four combinations of the magnetic field strength (7.05 or 11.75 T), rotor frequency (15 or 30 kHz), and H-1 rf-field strength (71, 100, or 140 kHz). The effectiveness of these sequences has been evaluated by comparing the C-13 signal intensity, linewidth at half-height, LWHH, and coherence lifetimes, T'(2), of the methine carbon of copper(II) bis(DL-2 alanine) monohydrate, Cu(ala)(2)center dot H2O, and methylene carbon of copper(II) bis(DL-2-aminobutyrate), Cu(ambut)(2), obtained with the advanced sequences to those obtained without H-1 decoupling, with CW decoupling, and for fully deuterium labelled samples. The latter have been used as model compounds with perfect H-1 decoupling and provide a measure of the efficiency of the H-1 decoupling sequence. Overall, the effectiveness of H-1 decoupling depends strongly on the decoupling sequence utilized, the experimental conditions and the sample studied. Of the decoupling sequences studied, the XiX sequence consistently yielded the best results, although any of the advanced decoupling sequences strongly outperformed the CW sequence and provided improvements over no H-1 decoupling. Experiments performed at 7.05 T demonstrate that the XiX decoupling sequence is the least sensitive to changes in the H-1 transmitter frequency and may explain the superior performance of this decoupling sequence. Overall, the most important factor in the effectiveness of H-1 decoupiing was the carbon type studied, with the methylene carbon of Cu(ambut)(2) being substantially more sensitive to H-1 decoupling than the methine carbon of Cu(ala)(2)center dot H2O. An analysis of the various broadening mechanisms contributing to C-13 linewidths has been performed in order to rationalize the different sensitivities of the two carbon sites under the four experimental conditions. (C) 2007 Elsevier Inc. All rights reserved.