REAPDOR experiment

Fig. 5 (a) Structure directing agent (SDA) in the synthesis of zeolite ITQ-3 N quaternary nitrogen A-I carbon atoms of SDA, l,3,3,6,6-pentamethyl-6-azonium-bicyclo[3.2.1]octane some 13C line assignments were not unequivocally possible, so these are left open the orientation of the dipole moment was calculated with a semiempirical AM-1 simulation with the origin at the center of gravity of the molecular cation (b) 13C H 27A1 REAPDOR experiment on synthetic zeolite ITQ-3. Adapted from [204]... 

There have been many papers reporting the biological applications of solid-state 170 NMR.89-100 This section briefly describes several notable examples of 170 NMR for biological solids, including 13C-170 Rotational Echo Adiabatic Passage Double Resonance (REAPDOR) experiments,100 the ONIOM methods for NMR calculations27 and 170 NMR studies of carbohydrates98 and a peptide-lipid complex.92... 

The REDOR experiment has been proposed for spin-1/2 pairs. For quadmpolar nuclei-spin-1/2 pairs, the use of this experiment is limited. The main reason behind this is the fact that a significant quadmpolar broadening (in order of several MHz) makes the Sn pulses inefficient for the inversion of S populations. For such systems REDOR experiments can be applied if the n pulses are apphed to the spin-1/2 nucleus and only central transition is observed for a halfinteger spin. The transfer of population in double resonance (TRAPDOR) or rotational-echo adiabatic passage double resonance (REAPDOR) experiment offer a much better choice for the measurement of quadmpolar nuclei-spin-1/2 pair dipolar couplings. 

The development of the double resonance experiments described in Section 3.8.2 was targeted at spin- /i systems. TRAPDOR and Rotational-Echo Adiabatic Passage Double Resonance (REAPDOR) experiments are designed specifically for quadrupolar nuclei and do not work on systems containing only spin- /2 nuclei. These MAS experiments still rely on the modulation of the heteronuclear dipolar coupling (as in the REDOR experiment) to prevent an echo from refocusing, but the modulation is no longer by 180° pulses. 

The REAPDOR experiment (Gullion 1995, 1995a) can be considered as a variation of the TRAPDOR experiment, in which a train of rotor-synchronised 180° pulses is applied to the spin- /2 nucleus, spaced half a rotor period apart. The experiment uses an... 

It should be noted that in both the TRAPDOR and REAPDOR experiments, the rf field on the dephasing quadrupolar nucleus remains on for a considerable time, which can result in a Bloch-Siegert shift (Bloch and Siegert 1940) appearing as a misset in the zero-order phasing of the reduced echo compared to the full echo spectrum. 

At the qualitative level, NMR studies devoted to this issue exploit heteronuclear dipolar recoupling to correlate resolved MAS NMR signals of the nuclear species associated with different network formers. This information comes from the dephasing of the observe nuclei in REDOR, TRAPDOR, or REAPDOR experiments, or from one- or two-dimensional correlation spectroscopy involving coherence transfer via TEDOR (transferred echo double resonance) or... 

Here, we only briefly discuss the REAPDOR. REAPDOR experiment is specifically designed to measure the heteronuclear dipolar interaction involving, at least, one quadrupolar nucleus (irradiated spin). The pulse sequence is illustrated in Fig. 1.9 [101]. The initial transverse magnetization of J-spin (the observed spin) is usually generated by CP from proton. In the J-spin channel, two ti pulses are applied every rotor cycle except that the n pulse in the very middle of the pulse train is absent. An adiabatic pulse is... 

In this context, we must at least briefly mention the REAPDOR experiment [94] (rotational-echo adiabatic-passage double resonance), a technique that is very similar to the BJ3DOR pulse sequence, although it is more efficient, as it is designed especially for quadrupolar nuclei utilizing an adiabatic pulse of length for the polarization transfer (Fig. 25). In this... 

The apphcability of the Si Al REAPDOR experiment was tested for NaY zeohte [97] where the Si MAS NMR spectmm showed four well-separated resonances in the spectral region from —80 to —105 ppm [98]. In Fig. 26, a comparison is made between the reference and the measured Si Al REAPDOR dephased spectmm, as well as the different spectmm resulting from subtraction of the previous two. This clearly demonstrates that the resonance at 5( Si) = —100.0 ppm is unaffected by the reintroduction of the Si— Al dipolar coupling and can therefore be assigned to the Q (0A1) site. The remaining resonances exhibit PEAPDOR attenuations (Fig. 26) with a distinct rate described by the values o(h ai. These values, fe- ai = 0-16, 0.30, and 0.44, are determined from the initial part of the dephasing curves (4S/So<0.3) and differ considerably for each of the resonances at —94.8, —89.2, and —83.9 ppm, respectively. Because k is approximately constant... 

F A1 and A1 " F REDOR experiments and " F A1 TRAPDOR and REAPDOR experiments provided information on spatial proximities between F and A1 in many Al-based fluorides. We can cite aluminium fluoride phosphate glasses [40,41], aluminosilicate glasses [42] and zeolites dealuminated with NH4SiF6 [43,44]. Other examples are detailed in Section 5.3. 

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