MQ-/-HETCOR

Fig. 20 2D HETCOR spectra of A1P04-14 (a) 27A1 31P R-INEPT at 9.4 T, (b) 27A1 31P HMQC at 9.4 T, (c) 27A1 31P R-INEPT at 18.8 T, (d) 27A1 31P HMQC at 18.8 T, (e) 27A1 31P SPAM-MQ-/-HETCOR with R-INEPT at 18.8 T, and (f) 27AI 3IP SPAM-MQ-D-HETCOR obtained using cross-polarization at 18.8 T. The total experimental times were 5 h (a), 5 h (b), 2 h (c), 2 h (d), 10 h (e) and 7 h (f). (Reproduced with permission from [235])...

Fig. 29 Pulse sequence used to record the MQ-/-HETCOR P Al spectrum of AIPO4-4O shown in Fig. 26b...

Recently, the combination of /-based correlation methods (INEPT) with MQMAS (MQ-/-HETCOR experiment) was accomplished (Figs. 26b and 29) [105]. Unlike MQ-HETCOR, which is based on the spatial proximity of nuclei probed by the dipolar interaction, MQ-/-HETCOR maps the chemical bonding between the quadrupolar nuclei and the coupled spin under high-resolution conditions. This technique does not require the spin-locking of quadrupolar nuclei as is the case with CP-based HETCOR experiments. The short T2 in solids may prevent the observation of relatively small /-couplings. 

Rocha et al. have reviewed recent advances in NMR spectroscopy of quadrupolar nuclei with half-integer spins. In particular, B, O, Na, A1, Ga, Nb MQ MAS applications have been considered. A range of related techniques, such as satellite transition (ST) MAS, inverse-STMAS NMR, fast amplitude modulation, and techniques based on the dipolar interactions between quadrupolar and spin-1/2 nuclei (CP MQ MAS and MQ HETCOR) and the recently introduced /-coupling based experiments (/-HMQC), have also been reviewed. 

P heteronuclear correlation spectra with isotropic resolution. The proposed experiment, MQ-/-HETCOR, uses MQ MAS NMR for elimination of the second-order quadrupolar broadening and INEPT, INEPTR, INEPT+ and DEPT sequences for the polarisation transfer. The experimental conditions leading to best sensitivity and resolution are detailed using AIPO4-I4 as a test sample. 

The details of this analysis, which is a generalization of a previous formalism for n = 1 [236], are beyond the scope of this review. Overall, the efficiency of R-INEPT and HMQC experiments has been found to be comparable, at least in applications involving 31P and 27Al. A major advantage of R-INEPT is that it can be combined with MQMAS or STMAS into a simple 2D heteronuclear correlation experiment, MQ/ST-J-HETCOR (Sect. 7.3). 

Figure 3.25. Na- P 2D HETCOR spectra of Na3P309 A. Obtained with the normal HETCOR pulse sequence and B. with MQ-MAS incorporated in the pulse sequence to obtain higher resolution from Wang, De Paul and Bull (1997) with permission of the copyright owner.

HETCOR), MQ REDOR (Rotational Echo Double Resonance), TRAPDOR (TRAnsfer of Populations in DOuble Resonance) and REAPDOR (Rotational Echo Adiabatic Passage Double Resonance), and the recently introduced /-cou-phng based experiments. A few other excellent reviews on MQMAS NMR have been recently pubhshed and complement the present review [1,2]. 

Solid state NMR techniques, such as CP, RAMP CP, CP TOSS, MQ DEPT, 2D HETCOR, which are relevant to the characterisation of natural organic matter have been reviewed by Cook. The pros and cons of many of the techniques are compared in an effort to provide guidance to the most beneficial utilisation of the NMR techniques for researchers interested in natural organic matter. 

The generation of indirect homonuclear correlation spectra was described by Hu et al. [92] for a Al—SPAM MQ-J-HETCOR experiment, which was transformed into a J-coupled Al—Al correlation spectrum or the equivalent to a 2D HOMCOR H-HSQC. The sample studied consisted of aluminium phosphate in polyhedral mode, AIPO4-I4. Due to the symmetric correlation representation, homonuclear cormectivities were r arded easier to interpret. 

The local structures of framework boron atoms in borosilicate zeolites B-p, B-SSZ-33 and B-SSZ-42 have been studied in the course of hydration/dehy-dration by employing solid-state NMR methods. In particular, characterisation of trigonal boron sites has been studied in great detail. B MAS NMR spectra showed that boron trigonally coordinated to the framework (B(OSi)3, denoted as B[3]) can be readily transformed to a defective trigonal boron site (B(OSi)2(OH), denoted as B[3]- I) as a result of hydration. The presence of B[3]-I sites was proven by utilising a number of different NMR methods including B MAS NMR at two different fields (11.7 and 19.6 T), B MQ MAS, B CP MAS, and B 2D HETCOR experiments. ... 

Figure 16 A source SPAM-MQ-J-HETCOR spectrum of AIPO4-I4, from which...

Figure 17 AI- AI J-coupled indirect covariance spectra obtained from the SPAM-MQ-J-HETCOR spectrum shown in Fig. 16. Reprinted with permission from Ref [86], Copyright 2007 Elsevier.

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