Homonuclear dipolar interactions

Homonuclear dipolar interactions described by the term HD are present in the spin system of abundant nuclei, such as H and 19F ... 

In the diluted spin system such as 13C and 29Si, homonuclear dipolar interactions are negligible because of the low natural abundance of nuclei under observation (S) in this case term HD describes dipolar interactions between unlike spins (I and S), which for 13C and 29Si are most frequently the neighboring protons ... 

Although the assumption that all apatite platelets are of equal thickness may not be well justified, it provides a useful approximation to the real situation. In fact, such assumption is inevitable for an ensemble technique like NMR. It is anticipated that HARDSHIP may be well suited to determine the relative thickness of apatite crystallites in different bone or dentin samples. We note in passing that C-REDOR, which is a hetero-nuclear recoupling technique with active suppression of homonuclear dipolar interaction,42 43 can be used to probe for the size of nanoparticles embedded in polymer matrix,44 provided that the nanoparticles do not... 

To determine orientation of an adsorbed molecule relative to the mineral surface, one would need to employ dipolar recoupling techniques to extract the distance constraints between the selected spin species of the molecule and of the surface. For polypeptide-HAp systems, 13C 31P REDOR has been carried out for uniformly 13C labeled molecules, where numerical simulations show that the effect of 13C dipole-dipole interaction is relatively minor.125 For a study of bone sample, o-phospho-L-serine was taken as the model compound for the setup of the 13C 31P REDOR experiments, where the data can be well analyzed by a l3C-3lP spin-pair model with the intemuclear distance equal to 2.7 A.126 Concerning the effect of 31P homonuclear dipolar interaction on the spin dynamics, Drobny and co-workers have carried out a detailed REDOR NMR study of polycrystalline diammonium hydrogen phosphate ((NH4)2F1P04).127,128 The results show that the 15N 31P REDOR data can... 

For abundant nuclei with spin V2, the spectrum is often dominated by heteronuclear or homonuclear dipolar interactions, i.e., the interactions between the magnetic moments of two neighbouring spins. In this case there is no isotropic contribution and q is zero, so that Equation 14.1 simplifies correspondingly. For a two-spin system one obtains a spin Hamiltonian of the form ... 

Different models of CP kinetics have been developed. The simplest model of CP dynamics (I-S) was derived for homogeneous solids where the I-S heteronu-clear interactions are relatively weak and the /-/ homonuclear dipolar interactions are sufficiently strong to provide efficient spin diffusion. For the abundant-dilute spin system, the CP dynamics can be described by equation (6) ... 

Since CP is governed by the heteronuclear dipolar interactions, the Tis time constant is related to the intemuclear distances and molecular mobility. The relaxation time describes the decay of intensity for longer contact times. Relaxation is mostly ensured by the Ft- lT homonuclear dipolar interactions. In contrast to the Tis time characteristic of the chemical group under study, relaxation time is a volume property averaged over the distance of ca. 

Whereas the I-S model is simple to understand and is widely applied, it is not sufficient to describe the CP kinetics for solids with heterogeneous populations of the source spins. The I-I -S model takes into account the efficiency of spin diffusion, which relies on homonuclear dipolar interactions and proceeds through ffip-ffop spin transitions. The I-I -S model relies on the existence of different proton populations, denoted I for the protons directly bound to an 5 spin under study and I for the rest of the proton network. The CP proceeds in two steps. A fast rise of the intensity is observed initially due to the transfer of the magnetization to a dilute spin (I -S) by the abundant spins in close proximity followed by a slow rise of the intensity or damped oscillation. Several equations have been proposed to describe the CP kinetics the simplest... 

Homonuclear dipolar decoupling can be achieved by using so-called multiple pulse sequences (MPS). These are carefully tailored short and intense pulse cycles averaging the homonuclear dipolar interactions by reorientadon of the nuclear spins. 

Rhim et al. have shown that application of a suitable sequence of strong RF pulses can reverse the time evolution of the coherent spin state caused by the homonuclear dipolar interaction [28]. Consequently, the influence of the dipo-... 

If the I-S heteronuclear dipolar interactions are weak, e.g. reduced by molecular mobility or spacial remoteness, while the I-I homonuclear dipolar interactions are moderate or strong, then the CP kinetics is likely to follow the classical I-S model. In most cases, it is described by Eq. (2) ... 

The presented technique, finite pulse r.f. driven recoupling (fpRFDR), restores homonuclear dipolar interactions based on constructive usage of finite pulse-width effects in a phase- and symmetry-cycled 7c-pulse train in which a rotor-synchronous n pulse is applied every rotation period. The restored effective dipolar interaction has the form of a ZQ dipolar Hamiltonian for static solids, whose symmetry in spin space is different from that obtained by conventional r.f. driven recoupling (RFDR) techniques. It has been demonstrated that the efficiency of recoupling by fpRFDR is not strongly dependent on chemical shift differences or resonance offsets in contrast to previous recoupling... 

A new heteronuclear dipolar interaction recoupling scheme suitable for the characterization of heteronuclear dipolar couplings for multiple-spin systems under very fast MAS condition has been proposed The new technique has been shown to be superior to the rotational echo double resonance method and other recoupling schemes as far as the interference of homonuclear dipolar interaction is concerned. The potential of the technique is illustrated for fluorapatite sample at a spinning frequency of 25 kHz. 

In terms of instrumentation the less demanding approach is multi-pulse excitation for averaging of the homonuclear dipolar interaction by trains of repetitive magic echoes during space encoding and acquisition of the spectroscopic response. Two such magic-echo... 

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