Double rotation technique

The double-rotation technique was introduced by Samoson et al. in 1988. This was the first real high-resolution method for half-integer spin quadrupolar nuclei. DOR is performed as a ID experiment in which the sample is spun simultaneously by an inner rotor that is inclined along an axis making an angle of 30.56° with respect to an outer rotor axis, and an outer rotor at an angle of 54.7° with respect to Bq. Unlike Eq. (35), here, the Hamiltonian contains instead of a product term of the form (O2)... 

The double-rotation technique may also be applied to a textured sample regarded as a pseudocrystal characterized by new pseudoprincipal susceptibility components whose magnitudes are functions of the monociystalline x p and Xj. and the degree of texturization. 

Commercially available equipment is used for solid-state static, MAS, OFF-MAS and double rotation techniques, and precautions have to be taken to discriminate between resonance signals and rotation side-bands by measuring the spectra at different spinning frequencies. The information that can be extracted from spectra under the static versus nonstatic conditions (where quadrupole perturbations, dipole interaction and chemical shift anisotropy... 

The inherent sensitivity of double resonance methods has led to studies of transition metal and rare earth compounds which would be difficult with more conventional methods. In chapter 10 we described the study of rotational transitions of the FeO molecule by Endo, Saito and Hirota [46] using a conventional ffee-space absorption cell, and by Allen, Ziurys and Brown [47] who used a high-temperature oven to study the reaction between iron vapour and nitrous oxide. We also mentioned, however, that a comprehensive study had been made by Krockertskothen, Knockel and Tiemann [48] using a double resonance technique which we now describe in detail. Allen, Ziurys and Brown [47] combined the data from different experiments to produce the best analysis. 

Si, 27ai, tH and I29xe are the nuclei studied to obtain information about the nature of the coke, pore blocking, location of the coke, diffusion of reactants, loss of crystallinity, or appearance of defects after coking. Sophisticated techniques such as CP/MAS, DOR (double rotation) or NMR imaging can enhance resonance lines or directly visualize the coke profile in a deactivated coked sample. 

In order to remove the anisotropic effects of quadrupolar interactions which can obscure MAS spectra other techniques such as Double Rotation (DOR) [18, 19], Dynamic Angle Spinning (DAS) [18] or Multiple Quantum Magic Angle Spinning (MQMAS) have to be employed. Of these the most simple to apply practically is MQMAS, which can be conducted on a standard MAS-equipped NMR spectrometer. A number of texts and review papers discuss MQMAS in significant technical detail [20, 21]. 

Dalby and Bennett " which has given accurate probabilities for a series of transitions. The technique is described briefly onpp. 291-2. Accurate determination of concentrations may still be hindered by self-absorption of the radiation, particularly in the case of the hydroxyl radical. Penner and co-workers have overcome the difficulty by the use of a double path technique, and are able to determine the rotational temperature and concentration of hydroxyl radicals in both flame and shock-tube studies. The single and double path emissivities are compared simultaneously, the double path beam being chopped to give modulation at about 5 sec intervals. The method of correction for line widths and Doppler broadening is discussed . 

Another polymorph of AlaSiOs, kyanite, contains four crystallographically inequivalent AlOfi sites with widely differing quadrupolar interactions but spread over a relatively small chemical shift range. A battery of Al NMR techniques, including MAS at different fields, satellite transition spectroscopy (Smith et al. 1994), MQ MAS NMR (Alemany et al. 1999, 1999a) and double rotation (Xu and Sherriff 1993, Smith et al. 1994) has been used to resolve the four sites and allow the simulation of the eomplete spectrum (Figure 5.5B). 

The synthesis of a range of silicon boron carbonitride ceramics can conveniently be carried out by thermal treatment of suitable amorphous precursor molecules in which the desired atomic linkages are already in place. A series of new amorphous precursor molecules has been investigated by "B MAS NMR and double resonance techniques such as Si- B) Rotational Echo Adiabatic Passage DOuble Resonance (REAP-DOR) and B-j Si) Rotational Echo DOuble Resonance (REDOR) (van Wiillen and Jansen 2001). The results indicate the presence of hexagonal BN and (3-Si3N4 in some... 

In this review, we shall introduce some basic concepts about sohd-state NMR of half-integer quadrupolar nuclei and discuss the most useful and promising methods presently available to study them. These include older methods, such as Double Rotation (DOR) and Dynamic Angle Spinning (DAS), and novel techniques including MQMAS, Quadrupolar Phase Adjusted Spinning Sidebands QPASS, SateUite Transition (ST) MAS and Inverse-STMAS NMR, and Fast Amplitude Modulation (FAM). We also discuss several techniques based on dipolar interactions between quadrupolar and spin-1/2 nuclei, such as Cross-Polarization (CP) MQMAS, MQ Heteronuclear Correlation Spectroscopy... 

Finally, we mention double-resonance techniques tailored to study the de-focusing process on rotational echoes generated by MAS rather than on static spin echoes. / otational Spin-Echo Double /fesonance (REDOR) [115], Notary Resonance decoupling (RRR) [116], and Transferred-Echo Double desonance... 

A major recent advance in the study of quadrupolar nuclei by solid-state NMR is the development of double-rotation (DOR) and dynamic-angle spinning (DAS) [ 158,159,163). These techniques are capable of removing not only the first-order effects such as chemical shift anisotropy, but also the second-order quadrupolar interaction, which, as we have seen above, affects all quadrupolar nuclei. The second-order interaction can be expanded as a function of Wigner rotation matrices D " , so that the shift of the central transition is... 

The most powerful tool to study the intermediate range order in amorphous Si/B/N/(C) networks is MAS-NMR spectroscopy including advanced double resonance techniques such as B- Si rotational echo double resonance (REDOR), Si- B] rotational echo adiabatic passage double resonance (REAPDOR), and C- B] REAPDOR. These methods utilize the heteronu-clear coupling between the observed nucleus and a nucleus in the second coordination sphere to get information about the internuclear distance of the two nuclei. 

The complete suppression of the influence of second-order quadrupole broadening requires the application of double rotation (DOR) [34-36] or dynamic angle spinning (DAS) [37-39] techniques. 

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