Chemical exchange showing

Figure B2.4.1 shows the lineshape for intennediate chemical exchange between two equally populated sites without scalar coupling. For more complicated spin systems, the lineshapes are more complicated as well, since a spin may retain its coupling infonnation even though its chemical shift changes in the exchange.

Figure 10-165. Typical bayonet type heat exchanger, showing the key sparger arrangement internally as a part of each tube. (Used by permission Corsi, R. Chemical Engineering Progress, V. 88, No. 7, 1992. American Institute of Chemical Engineers. All rights reserved.)...

The conformation of the mixed p agonist/5 antagonist H-Tyr-c[-D-Orn-2-Nal-D-Pro-Gly-] in comparison to that of H-Tyr-c[-D-Orn-Phe-D-Pro-Gly-] was studied in DMSO-d6 by NMR spectroscopy and by molecular mechanics calculations [62,64]. Neither peptide showed nuclear Overhauser effects between C H protons or chemical exchange cross peaks in spectra obtained by total correlation and rotating frame Overhauser enhance-... 

Hydrolysis. NMR results show that TBT carboxylates undergo fast chemical exchange. Even the interfacial reaction between TBT carboxylates and chloride is shown to be extremely fast. The hydrolysis is thus not likely to be a rate determining step. Since the diffusivity of water in the matrix is expected to be much greater than that of TBTO, a hydrolytic equilibrium between the tributyltin carboxylate polymer and TBTO will always exist. As the mobile species produced diffuses out, the hydrolysis proceeds at a concentration-dependent rate. Godbee and Joy have developed a model to describe a similar situation in predicting the leacha-bility of radionuclides from cementitious grouts (15). Based on their equation, the rate of release of tin from the surface is ... 

Tributyltin carboxylates undergo rapid chemical exchange, as evidenced by NMR. As a consequence, even the interfacial reaction between tributyltin carboxylate and chloride is fast. IR, mass spectra, gas chromatographic retention time and chloride assay show that the product of the reaction is tributyltin chloride. 

The importance of the magnetic coupling is easily seen in Fig. 17 which shows two water proton MRD profiles for serum albumin solutions at the same composition (89). The approximately Lorentzian dispersion is obtained for the solution, and reports the effective rotational correlation time for the protein. The magnetic coupling between the protein and the water protons carries the information on the slow reorientation of the protein to the water spins by chemical exchange of the water molecules and protons between the protein and the bulk solution. When the protein is cross-linked with itself at the same total concentration of protein, the rotational motion of the protein... 

An even more useful property of supercritical fluids involves the near temperature-independence of the solvent viscosity and, consequently, of the line-widths of quadrupolar nuclei. In conventional solvents the line-widths of e. g. Co decrease with increasing temperature, due to the strong temperature-dependence of the viscosity of the liquid. These line-width variations often obscure chemical exchange processes. In supercritical fluids, chemical exchange processes are easily identified and measured [249]. As an example. Figure 1.45 shows Co line-widths of Co2(CO)g in SCCO2 for different temperatures. Above 160 °C, the line-broadening due to the dissociation of Co2(CO)g to Co(CO)4 can be easily discerned [249]. 

In many of these descriptions of lineshapes, chemical exchange line-shapes are treated as a unique phenomenon, rather than simply another example of relaxation effects on lineshapes. This is especially true for line-shapes in the intermediate time scale, where severe broadening or overlapping of lines may occur. The complete picture of exchange lineshapes can be somewhat simplified, following Reeves and Shaw [13], who showed that for two sites, the lineshape at coalescence can always be described by two NMR lines. This fact can be exploited to produce a clarified picture of exchange effects on lineshapes and to formulate a new method for the calculation of exchange lineshapes [16, 23]. This method makes use of the fact that lineshapes, even near coalescence, retain Lorentzian characteristics [13] (fig. 3). These lines, or coherences, are each defined by an intensity, phase, position, and linewidth, and for each line in the spectrum, the contribution of that particular line to the overall free induction decay (FID) or spectrum can be calculated. 

Water, as we have discovered, is a compound of two substances - hydrogen and oxygen. (The chemical formula for water, H2O, shows that it contains twice as much hydrogen as oxygen.) Many chemical reactions produce water. Copper oxide, for instance, reacts with hydrogen to form pure copper and water. More familiarly, bases and acids react together to form water as one of the products of chemical exchange. 

Fig. 18. 75.4-MHz 13C Bloch decay MAS spectra showing the dynamics of the toluenium ion. The cation was synthesized by reacting bromomethane-13C with benzene-13Q on AlBr3 at 233 K. The spectrum at 213 K shows all the peaks for the toluenium ion at 32 (methyl), 50 (C-4), 178 (C-3), 139 (C-2), and 201 ppm (C-l). The peak at 129 ppm was the unreacted benzene-13C6. At 243 K, the peaks were much sharper, and the 138 and 50 ppm peaks were NMR invisible. At 273 K, the spectrum shows two extra peaks at 128 and 73 ppm. All these spectral features are rationalized by the chemical exchange between the para and ortho isomers.

Fig. 8.4. The first EXSY experiment on a paramagnetic system [1] the 300 MHz spectrum, taken with mixing time of 50 ms, shows species in chemical exchange belonging to two different redox states of a cytochrome d, a protein containing four low spin hemes. The signals marked M1-M7 represent various heme methyl groups. EXSY cross peaks are observed between M, of two species containing two (II) or three (III) oxidized hemes, respectively.

In this chapter, we will introduce a new level of theoretical tools—the density matrix— and show by a bit of matrix algebra what the product operators actually represent. The qualitative picture of population changes in the NOE will be made more exact, the precise basis of cross-relaxation will be revealed, and a new phenomenon of cross-relaxation— chemical exchange—will be introduced. With these expanded tools, it will be possible to understand the 2D NOESY (nuclear Overhauser and exchange spectroscopy) and DQF-COSY experiments in detail. 

THE H-NMR spectrum of an unknown compound, C]HgO. This compound shows an OH in its IR spectrum. Its DU equals 0. Examination of its NMR spectrum indicates only alkyl-type H s.The integral provides the actual number of H s in this case, since they total eight.The broad singlet due to one H at 3.8 S is probably due to the hydroxy H, which is not coupled due to rapid chemical exchange.The two H s at 3.5 S appear as a triplet and must be coupled to two H s—the two H s at 1.5 S. 

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