Quadrupolar spins

During an rf-field irradiation, the Hamiltonian governing a quadrupolar spin system can be written in the rotating frame as... 

Finally, Deschamps et al. reported for the first time a homonuclear connectivity map between the quadrupolar nuclei based on J couplings [284], The experiment uses a relayed heteronuclear transfer via spin-1/2 nuclei to a second quadrupolar spin via four chemical bonds (in this case / -mediated Al-O- P-O- Al transfer), in a scheme equivalent to the heteronuclear SQ correlation (HSQC) experiment. 

Frydman, L and Harwood, ).S. (1995) Isotropic spectra of half-integer quadrupolar spins from bidimensional magic-angle spinning NMR./. Am. Chem. Soc., 17, 5367-5368. 

The nucleus is quadrupolar (spin 7/2, natural abundance 99.76%), and thus, the spectra can be affected by both the first- and second-order quadru-pole interaction, though the second-order broadening is generally not the largest source of line broadening in these materials. In general, three major anisotropic interactions influence the line shapes seen in the NMR spectra of solid samples (i) the qua-... 

D. luga, H. Schafer, R. Verhagen and A. P. M. Kentgens, Population and coherence transfer induced by double frequency sweeps in half-integer quadrupolar spin systems. /. Magn. Reson., 2000,147,192-209. 

A general treatment of spin coupled with quadrupolar spins was given using density matrix theory.27 This formulation enables one to calculate, on the same theoretical basis, the lineshapes of the systems with different ratios of the quadrupolar interaction to the Zeeman interaction. Additionally, it includes the spinning sidebands very naturally. 

It was shown that REDOR or its variants can be used when one coupling partner is a quadrupolar spin.113,114 However, there are some limitations, such as the fact that only a fraction of the spin-1, nuclei are involved in the dephasing.115 Accordingly, more specific dephasing methods have been proposed for this type of system. One is TRAPDOR (transfer of population in double resonance), which uses continuous irradiation.116 The other is REAPDOR (rotational-echo, adiabatic passage, double resonance),117,118 which is a more straightforward extension of REDOR. 

More recently Tarasov et al. provided unambiguous evidence for the superposition of scalar and quadrupolar spin-lattice relaxation mechanisms in the case of Al in the corresponding mixed tetrahalogenoaluminates AlCl Br4 . (244) From the temperature dependence of Ti( Al) in AICl4 and AlBr the scalar contributions to the Al relaxation rates are determined, providing the following estimates for the respective scalar coupling constants J( Al- Cl) 650 Hz and J( Al- Br) = 750 Hz. 

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). 

Because of the complex spin-dynamics of the CP transfer involving quadrupolar nuclei, CP-based methods may not be quantitative. An alternative approach to CP spectral editing, introduced by Fernandez et al. [70], combines MQMAS with (quadrupolar spin)-observe (spin l/2)-dephase Rotational Echo Double Resonance (REDOR) [90]. The method relies on reintroducing the het-... 

Theory of Quadrupolar Spins Line Narrowing Techniques... 

High-resolution spectra of half-integer quadrupolar spins... 

THEORY OF QUADRUPOLAR SPINS LINE NARROWING TECHNIQUES... 

High-resolution spectra of half-integer quadrupolar spins can be obtained if the observable elements of the density matrix I - 1/2X1/21 have accumulated the same... 

A single RF pulse is sufficient to create MQC in half-integer quadrupolar spin systems, as long as the strength of the RF pulse Pj < vq, which is often the case. Similarly, a single RF pulse can also convert the MQC to observable SQC. We denote such a basic sequence as Ph- h which is sketched in Fig. 5a, where Ph denotes a hard unmodulated rectangular pulse often called CW pulse. The first pulse is optimized to excite MQC and has a phase ]. The second pulse with a phase converts MQC to SQC which is detected by the receiver. The receiver phase ec is cycled to select the required coherence pathways and to eliminate the undesired ones. The value of the receiver phase equals — [, Ap,,] where Ap,- is the difference between the coherence order after and... 

In general, the best MQMAS sequence for the highest resolution in any quadrupolar spin system is obtained with the use of an MQNQ experiment." Here, for instance, in a spin- system, instead of correlating either the 5Q or 3Q with SQC, the second-order quadrupolar frequencies resulting from a 5QC evolution period are refocused with 3QC evolution and only then transferred and detected via SQC. The pulse sequence for this is shown in Fig. 8a where both SQC and 3QC are subjected to a fj evolution (split-fj) to refocus second-order quadrupolar broadening. This, though yielding a better resolution, is prone to sensitivity problems more than other MQMAS schemes. 

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