Suppressing the dipolar interaction

In order to effectively suppress the dipolar interaction, it is necessary to apply a decoupling RF field satisfying the condition 1 (typically by a factor of 3), where is the H gyromagnetic ratio, and represents... 

The problem is to discuss the generalized polarizability ae(1.49) with the matrix a 1 not commuting with that of the dipolar interactions, 0. To show that the pure retarded interactions may be discarded in the dynamics of mixed crystals, we assume here that the coulombic interactions are suppressed in (ft. The interaction tensor is then reduced to its retarded term (1.74). Then the dispersion is given by (1.35) ... 

Several pulse methods were developed for estimation distances between two slowly-relaxing spins. In a pulse electron-electron double resonance (PELDOR) technique a spin echo is created by a two-pulse sequence at one microwave frequency. The timing of a pulse at a second microwave frequency is varied (Milov et al., 1998). This method is suitable for analysis of weak dipolar interactions. 3-pulse PELDOR with all three pulses at the same microwave frequency ( 2 + 1 sequence) was proposed by Raitsimling and his co-workers (2000). A specific feature of the 2 + 1 technique is suppression of dipolar interaction of randomly distributed spins, which allows the selection of a dipolar interaction between radicals. Using a 4- pulse experiments it was possible to eliminate an inherent dead experimental deadtime that limits the magnitude of the dipolar interaction in 2 + 1 sequence and in 3-pulse ELDOR experiments (Pannier et al., 2000). 

Fig. 15. Four multi-pulse sequences for suppressing homonuclear dipolar interaction (a) WAHUHA, (b) MREV-8, (c) BR-24 and (d) CORY-24. One cycle is drawn for each sequence, l e longer delays are double the length of the shorter ones.

In the preceding section, we discussed methods of suppressing dipolar interactions as a means of eliminating spin diffusion in relaxation measurements, but dipolar interactions are also the principal source of line-broadening in solid-state spectra, and therefore suppression of dipolar interactions also offers the possibility of obtaining high resolution spectra of solids in which chemical shift information is discernible. It is this, rather than the suppression of spin diffusion that has been the motivation in the development of these techniques, particularly the multiple pulse sequences. 

However, in the presence of motion, the efficacy of REDOR is severely limited the dynamic processes average the dipolar interaction, consequently leading to biased, overestimated interatomic distances. Thus, when studying amorphous polymer electrolytes or the amorphous parts of a heterogeneous system, REDOR NMR spectroscopy must be performed at temperatures below the T, where all the mobility is suppressed. ... 

High sorption capacities with respect to protein macromolecules are observed when highly permeable macro- and heteroreticular polyelectrolytes (biosorbents) are used. In buffer solutions a typical picture of interaction between ions with opposite charges fixed on CP and counterions in solution is observed. As shown in Fig. 13, in the acid range proteins are not bonded by carboxylic CP because the ionization of their ionogenic groups is suppressed. The amount of bound protein decreases at high pH values of the solution because dipolar ions proteins are transformed into polyanions and electrostatic repulsion is operative. The sorption maximum is either near the isoelectric point of the protein or depends on the ratio of the pi of the protein to the pKa=0 5 of the carboxylic polyelectrolyte [63]. It should be noted that this picture may be profoundly affected by the mechanism of interaction between CP and dipolar ions similar to that describedby Eq. (3.7). 

Efforts to understand the state of hydrogen in metals and metal hydrides have involved the use of NMR for many years. This study combines the conventional solid state NMR techniques with more recently developed high-resolution, solid state NMR techniques (5,6). Conventional NMR techniques furnish information on dipolar interactions and thus can furnish static geometrical information on hydrogen positions and information on proton motion within such solids. The newer multiple pulse techniques suppress proton-proton dipolar interaction and allow information on other, smaller interactions to be obtained. This chapter reports what the authors believe is the first observation of the powder pattern of the chemical shift tensor of a proton that is directly bonded to a heavy metal. 

Figure 6 is a plot of the proton NMR spectrum obtained from H2Os3(CO)io when using an eight-pulse cycle (5, 6,16,17,18) to suppress the effects of proton-proton dipolar interactions. The curve results from a computer fit that assumes the lineshape is caused by the chemical shift tensor. The center of the spectrum is near r = 19 ppm, and thus it agrees reasonably with that expected from the solution NMR results (r = 21.7 ppm (37)). The three principal values of the tensor, according to this fit, are at r values 5.6 ppm, 19.9 ppm, and 31.6 ppm. Since approximately one-third of the proton pairs interact with a near... 

---