Dipolar suppression

Dipolar coupling constant for the H- H interaction Dipolar coupling constant for the H- P interaction Dipolar coupling constant for the P- P interaction Dipolar dephased (dipolar suppression)... 

Differential cross-polarization Dipolar suppression period Calcium hydroxyapatite Calcined HA (dehydrated)... 

Following the first procedure, mineral standards were synthesized and characterized by P MAS NMR [31,42]. BD, CP and DD spectra of mineral standards and bone samples were compared, giving particular attention to chemical shifts (Table 2),linewidths and rotational sideband patterns [35]. A typical set of spectra, acquired for mineral standards, is given in Fig. 7. Each BD spectrum comes from all P-sites, while its CP counterpart exposes a fraction of the P-sites capable of obtaining polarization from surrounding protons. The DD experiment is performed with a time interval without proton decoupling inserted just after CP. This is called a dipolar suppression period (DS), because... 

Solid-state C MAS NMR has been used only to study various synthetic car bonatoapatites and dental enamel [23] (Fig. 24). Chemical shifts for CO various apatite locations were determined and response of the peaks to dipolar suppression and heating was investigated. For CP, a long contact time of 10 ms was required. For BD, a recycle delay of 60 s was used. In Fig. 24, spectra A and B were recorded with and without CP. CHA-A gives a relatively sharp peak and does not cross-polarize, because COf" replaces OH , depleting the main proton source for the polarization. The spectra C and D were not adequately described in [23], so it is not sure whether BD or CP was applied. It is... 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

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). 

The estimated correlation times for the loop domains of the order of 10 4 s are obtained for the suppressed peaks in the [l-13C]amino-acid-labelled bR, including Gly, Ala, and Leu residues as shown in Figure 24C. The loop dynamics can be also examined by measurements of the 13C-1H dipolar couplings by DIPSHIFT experiment in which fluctuations of the Co,-Cp vector result in additional motional averaging as order parameters, in addition to the rotation of Ala methyl groups which scales the dipolar... 

Any heteronuclear dipolar recoupling techniques developed for systems containing multiple 3H spins must have the property to suppress actively the dipolar... 

Fig. 8 Schematic diagrams for the following pulse sequences (A) single pulse excitation/magic-angle spinning, (B) total suppression of sidebands, and (C) delayed decoupling, or dipolar dephasing.

Kemp et al., 1978). The rate is slowest in an aqueous solution and is enhanced in aprotic and/or dipolar solvents. The rate augmentation of 106—108 is attainable in dipolar aprotic solvents such as dimethyl sulfoxide and hexamethylphosphoramide (HMPA). Interestingly, the decarboxylation rate of 4-hydroxybenzisoxazole-3-carboxylate [53], a substance which contains its own protic environment, is very slow and hardly subject to a solvent effect (1.3 x 10-6 s-1 in water and 8.9 x 10-6 s-1 in dimethylformamide Kemp et al., 1975). The result is consistent with the fact that hydrogen-bonding with solvent molecules suppresses the decarboxylation. 

PNPA in dry aprotic media and the reaction was efficiently suppressed by minute amounts of water (much less than 1 M). In the reaction of tetraethylammonium N-methylmyristohydroxamate [63 R = C13H27] in acetonitrile, for instance, an increase in water concentration from 3.3 mM to 960 mM caused the rate to decrease from 845 M I s-1 to 8.5 M-1 s-1. Thus, the value of the rate constant is almost meaningless without accurate determination of water concentration. This finding contrasts with the previously held view that the reactivity of anions in dipolar aprotic solvents is... 

Fig. 7.11 J modulation in a constant time HSQC in order to measure dipolar couplings. A Watergate pulse is introduced in order to optimize water suppression. The intensity of cross peaks is given by /(7") = C cos(nJT)exp(-T/T2) The right...

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. 

Fig. 15. Comparison of a water suppressed muscle spectrum and a spectrum from yellow bone marrow containing almost pure fat (triglycerides). Measurement parameters STEAM sequence, TE=10 ms, TM=15 ms, TR = 2 s, 40 acq., VOI (11 X 11 X 20) mm. (a) Spectrum from TA muscle recorded after careful positioning of the VOI, avoiding inclusion of macroscopic fatty septa allows separation of extramyocellular (EMCL, broken lines) and intramyocellular lipid signals (IMCL, dotted lines) based on susceptibility differences. For this reason characteristic signals from fatty acids occur double. Signals of creatine (methyl, Crs, and methylene, Cr2) show triplet and doublet structure, respectively, due to dipolar coupling effects. Further signals of TMA (including carnitine and choline compartments), Taurine (Tau), esters, unsaturated fatty acids (-HC=CH-), and residual water are indicated, (b) Spectrum from yellow fatty bone marrow of the tibia with identical measuring parameters, but different amplitude scale.

With ions or dipolar substrates, radical ions undergo nucleophilic or electrophilic capture. Nucleophilic capture is a general reaction for many alkene and strained-ring radical cations and may completely suppress (unimolecular) rearrangements or dimer formation. The regio- and stereochemistry of these additions are of major interest. The experimental evidence supports several guiding principles. 

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