Nutation NMR

Nutation NMR spectroscopy was introduced by Samoson and Lippmaa for the study of half-integer quadrupolar nuclei. For quadrupolar nuclei in the rotating frame, the nutation frequencies and amplitudes of the excited Zeeman 

The multiple-quantum magic-angle spinning (MQMAS) technique is one of the most important NMR advances for half-integer quadrupolar nuclei that has been made in recent years. Through a correlation between the multiple and single-quantum coherences, the anisotropic second-order quadrupolar 

(P) is the amplitude of the detected coherence, which is a complicated function of RF pulse intensity and duration as well as spinning speed. After Fourier transformation, the second-order quadrupolar and isotropic chemical shifts of any observed signals can be readily inferred from their FI and F2 positions. This procedure is useful for signals with or without well-defined spectral features. For an exposition of this technique at the introductory level, readers are referred to an article by Chan.  

To date, only a few one-site Co(III) complexes have been studied by MQMAS, viz. K3[Co(CN)d, 0 Na3Co(N02)6 2 nd [Co(TPP)MeIm2]BF4.  

The corresponding TQMAS spectra are shown in Figs 11 and 12. These preliminary studies show that the presence of chemical shift anisotropy could severely deteriorate the intensity of the multiple-quantum coherence. Since most Co systems have substantial chemical shift anisotropies, the general utility of such experiment in this area remains doubtful at the present stage. Nevertheless, in view of the powerful simulation protocol proposed for 2D MQMAS spectra and the launching of commercial MAS probes capable of spinning speed in excess of 30 kHz, more cobalt complexes should soon become accessible by MQMAS at low Bq field. 

To calculate the nutation spectrum the density matrix formalism is used, in which the Hamiltonian is diagonalised either numerically (Kentgens et al. 1987) or analytically (Pandey et al. 1986, Janssen and Veeman 1988). 

In Section 2.2.6, the nutation frequency of a quadrupolar nucleus was seen to vary from (I + /2)v] when vi vq to V when V vq. The nutation spectra display distinct features in the region where vi vq. Simulation of nutation behaviour has shown that the useful range of rf field-strengths lies with vq/vi in the range 0.05-1. The distinct features of nutation spectra allow the quadrupole parameters to be obtained. This means that only quadrupole frequencies up to a certain limit are accessible with currently available rf-fields of 500 kHz in specially dedicated probeheads, and in fields of around 100 kHz in most commercially available systems. This constraint especially limits the investigation of spin I = 3/2 nuclei such as Na. 

In cases where more than one resonance could be observed, one would like to increase the resolution in the F2-dimension by applying MAS. However, as was discussed previously, rotationally-induced spin-locked magnetisation can appear when an 

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Nuclear electric quadrupole QCC (quadrupole coupling constant), (asymmetry parameter) Line-shape analysis, nutation NMR Coordination symmetry... 

NMR properties, 33 213, 274 in nutation-NMR spectroscopy, 33 333 in sheer silicate smdies of, 33 340-341 layer structure, 32 184-186 on metal surfaces, 32 194-197 model, oscillatory reactions, 39 97-98 number... 

Fig. 6. Na 2D nutation NMR spectra of 69LaNaY in hydrated (a) and rehydrated (b) states.

Of greater relevance catalytically is that the combined use of l3C enrichment and 13C nutation NMR spectroscopy can distinguish between proposed rival mechanisms for the Ziegler-Natta catalyzed polymerization of acetylene. In the four-center insertion mechanism the enriched acetylene (HC =C H) is incorporated as shown in Scheme 6. It is to be noted that the, 3C—13C bond label is here incorporated into a carbon-carbon double bond, the length of which is significantly smaller than that of a carbon-carbon single bond, which is how the enriched acetylene would be incorporated in the two-center mechanism shown in Scheme 7. The results of nutation experiments leave little doubt that the Ziegler-Natta polymerization of acetylene proceeds by a four-center mechanism. 

Quadrupole Nutation NMR Studies of Second Generation Faujasitic Catalysts... 

Quadrupole nutation NMR of nuclei with half-integer spin in powdered samples (1-8) can distinguish between nuclei subjected to different quadrupole interactions, the signals from which overlap in ordinary NMR spectra. The technique can be used for the determination of the local... 

During the course of hydrothermal dealumination (ultrastabilisation) of zeolite Y quadrupole nutation NMR detects four kinds of aluminium sites. In addition to signals from the framework (F), non-framework tetrahedral (NFT) and non-framework octahedral (NFO) aluminium there is a signal which we attribute to distorted framework (DFT) aluminium bonded to hydroxyl nests formed during dealumination. 

Quadrupole nutation NMR is a technique, introduced by Samoson and Lippmaa (27), where in a two-dimensional way the effect of the quadrupole interaction is separated from other line broadening interactions, simply by allowing the magnetization to evolve during an incremented periode tj, under the influence of a strong radio frequency field. 

As an example, quadrupole nutation NMR of nuclei with half-integer quadrupolar spin in zeolitic materials can distinguish between nuclei of the same chemical element subjected to different quadrupole interactions, the signals of which overlap in conventional spectra. The situation is favourable for half-integer quadrupolar spins since the m=l/2 <-> m= -1/2 transition for these nuclei is broadened by the quadrupole interaction only in second-order perturbation theory. The technique can be usefully applied for the determination of the local environment of A1 in zeolitic catalysts (28). It allows discrimination between species of similar chemical shift but different quadrupolar coupling constants (see Figure 5). The main difficulty in the interpretation is the complex spectmm that results from a nutation experiment since it can consist of many overlapping powder patterns (29). 

Fig. 5. 27Al quadrupole nutation NMR spectrum of 80% NH4-exchanged Y zeolite (Si/Al=2.5). Numbers in parentheses have units of kHz (for Fj) and ppm (for F2), respectively (Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam). 

The distinction between Al species having the same chemical shifts and strongly overlapping lines but different quadrupolar couplings can be made via the two-dimensional quadrupole nutation NMR technique (13) (see chapter 2.)... 

The Nb NMR spectra of a number of alkali and lead niobates have been acquired using MAS, DAS, MQMAS and two-dimensional nutation NMR to improve the resolution and determine values of the quadrupolar products Pq for these materials (Prasad et al. 2001). The 9.4 T Nb MAS NMR spectrum of LiNbOs spun at 25 kHz (Figure 10.16A) shows a lineshape dominated by the second-order quadrupolar interaction, with only marginal improvement in resolution at a field of 14.1 T. For most of the metal niobates, the second-order interaction is not removed by MAS alone, even at high magnetic fields and fast spinning speeds. The expected improvement in Nb resolution provided by DAS spectroscopy is offset by significant homonuclear... 

As illustrated by the model compound work summarized in Fig. 3, Na spin echo decay spectroscopy is suitable in principle for elucidating the spatial distribution of sodium ions in sohds [11,44-47]. Meaningful results can be obtained provided that the measurement is carried out by selective excitation of the central j 1/2> -1/2> transition, which needs to be ascertained by nutation NMR. Furthermore, sub-ambient temperatures may be required to suppress any potential influence of sodium dynamics. To date, promising applications have been pubhshed using Na spin echo decay spectroscopy (1=3/2) on sodium-containing glasses, to address the vital issue of cation clustering. Fig-... 

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