2H exchange experiments

Figure 18.12 Prototype scheme for quenched-flow 1H/2H-exchange experiments.

Multidimensional NMR experiments also provide additional information that is unavailable from onedimensional (ID) spectra even in the limit of high-resolution. For example, 2D 2H exchange experiments, which identify a particular change in molecular orientation between two evolution periods, have already been mentioned above.9 For dipolar-coupled nuclei, the combination of fast MAS with 2D multiple-quantum (MQ) spectroscopy achieves high resolution, while allowing the structural and dynamic information inherent to the dipolar couplings to be accessed. Specifically, using the homonuclear H- H double-... 

Silver porphyrins are interesting because of the existence of disilver(I), paramagnetic silver(II), and diamagnetic silver(III) complexes which has been known for a long time [7, 8]. The latter are favorably prepared by electrochemical [299] or peroxodisulfate [300-302] oxidation of silver(II) porphyrins. The Ag(III) porphyrins were used for and 2H resonance experiments [299] on one hand and for the determination of monomer-dimer equilibria [301] or self-exchange rate constants [301] at water-soluble silver porphyrins. 

Fig. 21. The new five-pulse sequence for recording static 2H exchange spectra.51 The experiment differs from the simple three pulse sequence in Fig. 19 in the addition of r-90°-r (or A — 90° - A) echo sequences before the t and h periods to avoid spectral distortions caused by receiver deadtime and finite pulse width problems. The broader pulses are 90° pulses the narrower ones 54.7° pulses.

Studies of dynamics in wet and dry carbohydrates have also been performed. Slow motions of polysaccharide chains in native starch and retrograding starch gels were monitored with two-dimensional 2H exchange and stimulated echo experiments.117 The effects of hydration on the dynamics of /j-cyclodextrin polymers have been examined with 13C T and H T p measurements,118 and WISE experiments were used to probe the effects of hydration on polysaccharide chains in maize starch forms A and B.119... 

In summary, for this area to move forward, methods need to be introduced to provide information additional to 2H lineshapes when studying more complex systems. Calculations such as those just described are one real possibility experimental possibilities include use of magic-angle spinning 2H NMR, and using the full anisotropy of 2H 7j measurements and 13C NMR. A very nice illustration of the use of several different techniques in a motional study examined the slow alkane motions in a urea/alkane inclusion compound via 2H relaxation measurements, selective inversion experiments and two-dimensional 2H exchange.125... 

These experiments with aconitase and isocitrate dehydrogenase established a selectivity for aconitase. When citrate is incubated with this enzyme in 2H20, one of the methylene hydrogens of citrate is replaced by 2H ( exchanged ). The other... 

One way by means of which the structural and dynamic information inherent to the anisotropic interactions can be probed in a site-selective fashion involves the investigation of samples where specific isotopic labels, e.g., 2H, 13C, 15N, are introduced. 2H is a spin 1= 1 nucleus, and the quadrupolar interaction dominates 2H NMR. By an analysis of onedimensional line shapes, fast dynamic processes (10-4 to 10-7 s) can be quantitatively probed, while two-dimensional exchange experiments allow the inves-... 

The atom % 2H values of the solvent-fractionated products are also shown in Table I. In previous hydrogenation experiments conducted without the use of a donor solvent (1,18), deuterium incorporation increased from the most soluble oil fraction to the insoluble residue. In E10 and El9, contact of the coal with Tetralin resulted in a uniform incorporation of deuterium in almost all of the four product fractions. In El9, the BMI fraction s high value of 61 atom % 2H may be due to direct gas-phase exchange and deuteration. 

It was found that CO exchange in (diphosphine)Rh(CO)2H complexes proceeds via the dissociative pathway [60], The decay of the carbonyl resonances of the (diphosphine)Rh(13CO)2H complexes indeed followed simple first-order kinetics. The experiments with ligand 20 at different 12CO partial pressure show that the rate of CO displacement is independent of the CO pressure. Furthermore, the rate is also independent of the (diphosphine)Rh(13CO)2H complex concentration, as demonstrated by the experiments with ligand 18. It can therefore be concluded that CO dissociation for these complexes obeys a first-order rate-law and proceeds by a purely dissociative mechanism. 

The second substrate glyoxylate approaches from the other side of the molecule and condenses as is shown. Since any one of the three protons in either R or S chiral acetyl-CoA might have been abstracted by base B, several possible combinations of isotopes are possible in the L-malate formed. One of the results of the experiment using chiral (R) acetyl-CoA is illustrated in Eq. 13-43. The reader can easily tabulate the results of removal of the 2H or 3H. However, notice that if the base -B removes 2H (D) or 3H (T) the reaction will be much slower because of the kinetic isotope effects which are expected to be Hk/"k 7 and "k/ k = 16. A second important fact is that the pro-R hydrogen at C-3 in malate is specifically exchanged out into water by the action of fumarate hydratase. From the distribution of tritium in the malate and fumarate formed using the two chiral acetates, the inversion by malate synthase was established. See Kyte231 for a detailed discussion. 

SLR time results provide novel information on the temperature dependence and distribution of local RFs and their dynamics. A direct evidence for incoherent phonon-assisted deuteron tunnelling in H-bonded systems has been observed from NMR experiments. The use of quadrupole-perturbed ID 2H NMR spectroscopy and in particular 2D 2H NMR exchange spectroscopy has found to be very promising as regards elucidating the details of the hydrogen dynamics. 

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