Homonuclear dipolar coupling

The basic RFDR element consists of two 7i-pulses each centered in the middle of each rotor period in two consecutive rotor periods leading to a cyclic rf field. In the toggling frame of ideal (i.e., infinitely strong) 7i-pulses, the Hamiltonian of the chemical shift terms and the homonuclear dipolar coupling interaction is split up into three commuting parts ... 

As demonstrated by Griffin, Levitt, and coworkers in the late 1980s [21, 93], it is also possible to recouple homonuclear dipolar couplings through interference between isotropic chemical shifts and the rotor revolution. This phenomenon, called rotational resonance, occurs when the spinning frequency is adjusted to a submultiple of the isotropic chemical shift difference, i.e., ncor = ct> so o) °. To understand this experiment, the dipolar coupling Hamiltonian in (10) is transformed... 

Fig. 10.22. Diagram showing the cross-polarization from protons, H, to a heteronucleus, X, such as carbons. Heteronuclear dipolar coupling enables the transfer of magnetization from H to X, such as protons to carbons. Homonuclear dipolar coupling between the abundant protons enables the redistribution of proton spin energy through spin diffusion.

Fig. 11.2 Solid-state 13C NMR spectra of a powder sample of U-13C-15N-glycine illustrating the broad NMR resonances in the static sample (a) and the effects of magic-angle spinning at 5 kHz (b) and 10 kHz (c). Both the chemicalshielding tensors and the homonuclear dipolar coupling...

Radio-frequency driven recoupling (RFDR) [58] uses rotor-synchronized 180°-pulses to prevent the averaging of the homonuclear dipolar coupling by the MAS rotation. A single 180°-pulse is placed in the middle of each rotor period (Fig. 11.6a), often using an XY-8 phase cycle [66]. The efficiency of the recoupling depends on the isotropic chemical-shift difference of the two spins and the size and relative orientation of their CSA tensors. 

Solid-state analogs of the HMQC [103, 104] and HSQC [105] experiment, MAS-J-HMQC and MAS-J-HSQC [106, 107], have been proposed. They rely on the suppression of the large homonuclear dipolar couplings by FSLG irradiation of the protons. In contrast to the liquid-state implementations, both experiments use low-y detection in the solid state. 

One of the important advantages of NMR spectroscopy is that it makes possible calculation of the H- H distances in dihydrogen-bonded complexes formed in solution. As we have shown above, the formation of dihydrogen bonds is accompanied by the appearance of H- H contacts that are shorter than a sum of the van der Waals radii of H (< 2.4 A). Snch short H- -H contacts are connected directly with the spin-lattice (Ti) NMR relaxation of target nnclei [32]. The contacts between two nuclei cause strong homonuclear dipolar coupling, DCh-h. which is written as... 

The carbon-proton dipolar interaction and the chemical shift anisotropies broaden the lines in solid state 13C NMR spectra. The major effect arises from the dipolar coupling of the carbon nuclei with neighboring protons homonuclear dipolar couplings between two adjacent 13C nuclei are neglegible because of their low natural abundance. The large magnitude of dipolar 13C— H coupling (up to 40 kHz) results in broad and structureless proton-coupled 13C NMR absorptions. 

NMR imaging of solids is a technique that can be divided into two areas, depending upon the characteristics of the sample (7-6). For "soft" solids, the presence of local, rapid molecular motions reduces homonuclear dipolar coupling and averages anisotropic chemical shielding. Thus, the natural line width is narrow (long T2) and it is relatively easy to encode spatial information with a magnetic field... 

Restoring the interactions that are averaged out by (high-speed) MAS has been the most active research area in solid-state NMR in recent years. Novel pulse sequences for this purpose, including those can restore heteronuclear dipolar coupling, homonuclear dipolar coupling, chemical shift, etc., have been continually proposed. Some of these sequences were reviewed in the previous article,1 hence only new developments in this direction will be summarized in the following. 

It is known that, at the HORROR condition, homonuclear dipolar couplings between xHs are reintroduced, leading to partial removal of the heteronuclear coupling.28 29 It is worth noting that at 60 and 120 kHz the proton decoupling is insufficient due to the rotary resonance effect. It has been shown theoretically by Emsley and co-workers,19 that the first and second rotary resonances are partially quenched by the presence of homonuclear couplings. 

The simplest technique that enables phosphorus-phosphorus correlations for complex multi-component system and elucidation of structural constraints is the PDSD sequence.48 This experiment is relatively easy to set up however, the distances between spins cannot be established with satisfactory precision. One common way of determining distance type information using spin-7i NMR is based on the acquisition of the DQ filtered spectra as a function of the DQ mixing time.49 The recently introduced DQ homonuclear recoupling sequences based on C and/or R symmetry developed by Levitt and co-workers allows very precise analysis of intemuclear distances.50 The accuracy of distances obtained from these DQ experiments depends on the effective suppression of contributions to the spin Hamiltonian other than the required homonuclear dipolar coupling terms. 

At the center of the echo all resonance offsets from interactions linear in the spin quantum number are canceled as long as these interactions operate for the full duration ofTE. Linear spin interactions include chemical shifts, heteronu-clear dipolar couplings, field inhomogeneity, field gradients, and transmitter frequency offsets but do not include quadru-polar and homonuclear dipolar couplings. There will however be a net phase evolution induced by an interaction to the extent its duration or intensity is not balanced with respect to the two halves of TE (that is, the balance with respect to amount of phase evolution on either side of the 180° pulse). 

Elimination of homonuclear dipolar coupling is a significant challenge in solid-state NMR of nuclei with high natural abundance and large y (particularly H and... 

Most homonnclear correlation experiments rely on the homonuclear dipolar couplings to provide information... 

Double-Quantum. (DQ) Proton Spectroscopy. Multiple-quantum (MQ) coherences are routinely used in high-resolution solution-state NMR spectroscopy. The MQ coherences cannot be observed directly and the application of two-dimensional methodology is required. The Double-Quantum (DQ) methodology is particularly useful to derive the information on the spatial proximity of different spins on the basis of the homonuclear dipolar couplings. ... 

Both the heteronuclear and the homonuclear dipolar coupling depend on the orientation of the inter-nuclear vector and the inverse cube of the distance, and often give rise to broad lines in powders. However the characteristic lineshape has features, particularly singularities. For an isolated pair of spin- /2 nuclei a pair of strong singularities... 

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