Indirect determination of chemical shift by coupling evolution during adiabatic pulses
Peter W.A. Howe
The use of adiabatic 180° X-pulses within INEPT refocusing periods results in chemical shift-dependent evolution of J-couplings. This has been viewed as a disadvantage and several methods of overcoming it have been suggested. This article shows that there is the potential to use this chemical shift dependence to determine heteronuclear chemical shift without a heteronuclear evolution time. In this way, it possible to estimate heteronuclear chemical shift indirectly from a single one-dimensional proton-observe spectrum and determine it with high accuracy from a extensively-folded two-dimensional proton-observe spectrum.
Sensitivity enhancement of the central-transition signal of half-integer spin quadrupolar nuclei in solid-state NMR: Features of multiple fast amplitude-modulated pulse transfer
Mithun Goswami and P.K. Madhu
Sensitivity enhancement of solid-state NMR spectrum of half-integer spin quadrupolar nuclei under both magic-angle spinning (MAS) and static cases has been demonstrated by transferring polarisation associated with satellite transitions to the central m = −1/2 → 1/2 transition with suitably modulated radio-frequency pulse schemes. It has been shown that after the application of such enhancement schemes, there still remains polarisation in the satellite transitions that can be transferred to the central transition. This polarisation is available without having to wait for the spin system to return to thermal equilibrium. We demonstrate here the additional sensitivity enhancement obtained by making use of this remaining polarisation with fast amplitude-modulated (FAM) pulse schemes under both MAS and static conditions on a spin-3/2 and a spin-5/2 system. Considerable signal enhancement is obtained with the application of the multiple FAM sequence, denoted as m-FAM. We also report here some of the salient features of these multiple FAM sequences with respect to the nutation frequency of the pulses and the spinning frequency.
MAS NMR spectra of quadrupolar nuclei in disordered solids: The Czjzek model
Jean-Baptiste d’Espinose de Lacaillerie Christian Fretigny and Dominique Massiot
Structural disorder at the scale of two to three atomic positions around the probe nucleus results in variations of the EFG and thus in a distribution of the quadrupolar interaction. This distribution is at the origin of the lineshape tailing toward high fields which is often observed in the MAS NMR spectra of quadrupolar nuclei in disordered solids. The Czjzek model provides an analytical expression for the joint distribution of the NMR quadrupolar parameters υQ and η from which a lineshape can be predicted. This model is derived from the Central Limit Theorem and the statistical isotropy inherent to disorder. It is thus applicable to a wide range of materials as we have illustrated for 27Al spectra on selected examples of glasses (slag), spinels (alumina), and hydrates (cement aluminum hydrates). In particular, when relevant, the use of the Czjzek model allows a quantitative decomposition of the spectra and an accurate extraction of the second moment of the quadrupolar product. In this respect, it is important to realize that only rotational invariants such as the quadrupolar product can make sense to describe the quadrupolar interaction in disordered solids.