2-proton exchange with deuterium

Desensitizers, in photography. 78 Deuterium, exchange with proton-2 of thiazolium salts, 31 Deviation theory. 69 Dialkylacyiphosphonates. adducts with thiazolium salts. 35... 

The best insight into the relaxation behavior of star polymers in dilute solution can be expected if, in addition to the whole star system, different parts of the star are considered separately. This can be achieved easily by neutron scattering techniques on systems where not only the entity of arms, but also single arms, the core or shell parts are labelled by proton deuterium exchange. With respect to the core-shell labelling it is convenient to build up the arms as diblock copolymers of A-B type with protonated or deuterated but otherwise chemically identical A and B blocks. 

Remember that any proton which is acidic enough is prone to undergo deuterium exchange. Methylene protons alpha to a carbonyl for example, may exchange if left standing with D2O for any length of time, as they can exchange via the keto-enol route (i.e., Structure 7.1). 

Proton spectra obtained on I HJ formic add solutions differ from those discussed in the foregoing insofar as the add deuterium exchange with the imidazole imino protons causes collapse of the signal near 6 = 13 instead, a sharp singlet appears at 6 = 10.7. A second singlet near 6 = 8.3 has also been attributed to the exchanging add medium63). 

Some areas which are not covered are isotope effects on proton and deuterium exchange with solvent, for example, the water-hydronium ion system (Saunders et al., 1984), deuterium isotope effects on acid and base strength (Halevi et al., 1979), on amino acids (Petersen and Led, 1979) and on hydration of cobalt (II) (Saunders and Evilia, 1985). Solvent-dependent isotope effects on equilibria involving hydrogen bonds in carbohydrates and... 

Protons attached to the a-carbon atom in metal-carbene complexes undergo rapid base-catalyzed hydrogen-deuterium exchange with hydroxylic solvents (Kreiter, 1968). For example, the half-life for exchange of the a protons in VII in acetone-D20 with no added base is 23 min at 40° (Casey, 1970). This rapid exchange implies the existence of an intermediate carbanion. These... 

Deuterium exchange with the solvent does occur during the reaction, but at a rather slow rate. A symmetrical reaction intermediate must exist, since the rate of incorporation of tritium from the solvent into D-madelate as the substrate yields equimolar amounts of D-and L-product [50]. Thus the data are consistent with the formation of an a-carbanion intermediate with an enzymatic base group acting as the proton acceptor [50]. The proton transfer has to be rate-limiting, as indicated by the approximately 5-fold primary isotope effect for deuterium. In the enzyme-substrate complex, the epimerization occurs with a rate constant of the order of 10 s ... 

The mobility of the proton in position 2 of a quaternized molecule and the kinetics of exchange with deuterium has been studied extensively (18-20) it is increased in a basic medium (21-23). The rate of exchange is close to that obtained with the base itself, and the protonated form is supposed to be the active intermediate (236, 664). The remarkable lability of 2-H has been ascribed to a number of factors, including a possible stabilizing resonance effect with contributions of both carbene and ylid structure. This latter may result from the interaction of a d orbital at the sulfur atom with the cr orbital out of the ring at C-2 (21). 

Deuterium does not give a signal under the conditions of H NMR spectroscopy Thus replacement of a hydroxyl proton by deuterium leads to the disappearance of the OH peak of the alcohol Protons bonded to nitrogen and sulfur also undergo exchange with... 

Substituted tetrazoles readily exchange the 5-hydrogen for deuterium in aqueous solution. A major rate-enhancing effect is observed with copper(II) or zinc ions due to complexation with the heterocycle. The rate of base-induced proton-deuterium exchange of 1-methyltetrazole is 10 times faster than 2-methyltetrazole (77AHC(2l)323). 

Hydrogen atoms in azolium ions can be removed easily as protons (e.g. 230—>232) exchange with deuterium occurs in heavy water. The intermediate zwitterion (e.g. 231) can also be written as a carbene, and in some cases this carbenoid form can be trapped or isolated as a dimer. 

In theory two carbanions, (189) and (190), can be formed by deprotonation of 3,5-dimethylisoxazole with a strong base. On the basis of MINDO/2 calculations for these two carbanions, the heat of formation of (189) is calculated to be about 33 kJ moF smaller than that of (190), and the carbanion (189) is thermodynamically more stable than the carbanion (190). The calculation is supported by the deuterium exchange reaction of 3,5-dimethylisoxazole with sodium methoxide in deuterated methanol. The rate of deuterium exchange of the 5-methyl protons is about 280 times faster than that of the 3-methyl protons (AAF = 13.0 kJ moF at room temperature) and its activation energy is about 121 kJ moF These results indicate that the methyl groups of 3,5-dimethylisoxazole are much less reactive than the methyl group of 2-methylpyridine and 2-methylquinoline, whose activation energies under the same reaction conditions were reported to be 105 and 88 kJ moF respectively (79H(12)1343). 

Azaindolizine, 5-chloro-nucleophilic substitution, 4, 458 8-Azaindolizine, 7-chloro-nucleophilic substitution, 4, 458 Azaindolizines basicity, 4, 454 electronic spectra, 4, 445 electrophilic substitution, 4, 453 halogenation, 4, 457 hydrogen/deuterium exchange, 4, 458 NMR, 4, 447, 449 nucleophilic attack, 4, 458 protonation, 4, 453 reaction with isothiocyanates, 4, 513 reactions, 5, 267 reviews, 4, 444 UV spectra, 4, 446, 449 Azaindolizines, amino-tautomerism, 4, 452... 

Protons which are bonded to heteroatoms (XH protons, X = O, N, S) can be identified in the H NMR spectrum by using deuterium exchange (treatment of the sample with a small amount of D2O or CD3OD). After the deuterium exchange ... 

The stereochemistry of hydrogen-deuterium exchange at the chiral carbon in 2-phenylbutane shows a similar trend. When potassium t-butoxide is used as the base, the exchange occurs with retention of configuration in r-butanol, but racemization occurs in DMSO. The retention of configuration is visualized as occurring through an ion pair in which a solvent molecule coordinated to the metal ion acts as the proton donor... 

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