Hydrogen-deuterium exchange rate constant

Hydrogen-deuterium exchange rates of H2 and in some 3-substituted pyrazine 1 -oxides in Na0D-D20 have been correlated with a constants, and the log of the H2 exchange rates have been shown to be rectilinearly related to the pK values of these compounds (745). A considerable exchange-rate enhancement was caused by replacement of the =C4-H function in a pyridine A-oxide by a =N4 with formation of a pyrazine A-oxide (745). 

Let kunfold he the rate constant for unfolding, kfdd the rate constant for folding, and be the hydrogen/deuterium exchange rate for an amide... 

The hydrogen-deuterium exchange H2 + D2 — 2HD is the simplest heterogeneous reaction taking place on the surface of semiconductors. This reaction has been thoroughly studied experimentally. It has been shown that under the influence of illumination (other conditions being constant) the rate of the reaction is considerably changed. 

While the photoenhancement of the hydrogen deuterium exchange at MgO has been found to be due solely to an increase in the rate constant, investigations of the photocatalyzed carbon monoxide oxidation with ZnO, NiO and Co304 as catalysts have shown that the photoenhancement in these cases is due to drastic changes in the apparent activation energy 82-88)... 

The relative mobilities of the axial and equatorial hydrogen atoms from C(2) and C(6) positions of rigid cyclohexanones have been determined by several authors from the relative rate constants of hydrogen-deuterium exchange. Indeed, according to the principle of microscopic reversibility, the elementary rate constants for axial and equatorial proton detachment correspond to those of proton addition on the two faces of the enol or enolate (26). 

Acid-catalysed hydrogen-deuterium exchange in norcamphor has also been investigated by Werstiuk and Banerjee (1977) (DOAc—D20—DC1 medium). It was observed that exo-deuteron addition to the enol is also preferred, but with a slightly smaller selectivity (x 190). This would mean that, if torsional factors cause preferential base-catalysed exo-exchange, they also occur for acid-catalysed keto-enol tautomerism. However, the absence of important torsional strain effects on the rate constants of acid-catalysed enolisation of cyclic and bicyclic ketones contradicts this assumption. 

Dipole moments of 3-aminopyrazine 1-oxide (3.43D, dioxane) (749) and 3,5-bisdimethylaminopyrazine 1-oxide (4.87, benzene) (663) have been measured. Second-order rate constants for hydrogen-deuterium exchange of H2 and in sodium deuteroxide-deuterium oxide solutions of 3-amino- and 3-dimethylaminopyrazine 1-oxides have been determined as 2.3 x 10 , 1.1 x 10 , and 3.3 x 10 and 4.5 x 10 l/mol min, respectively. These results (and those of other 3-substituted pyrazine 1-oxides) have been correlated with o constants, and the log of the H2 exchange rates has been shown to be linearly related to the p/fa values of these compounds (745). 

P cm 21.8 Show in detail how the enolization mechanism accounts for the following facts (a) the rate constants for acid-catalyzed hydrogen-deuterium exchange and bromination of acetone are identical (b) the rate constants for acid-catalyzed racemization and iodination of phenyl rec- tyl ketone are identical. 

Fig. 1. The temperature dependence of the hydrogen-deuterium exchange. T is the absolute temperature, and K a quantity proportional to KxK2IKx+K%, Kt and Kt are the rate constants of Eqs. 3 and 4. (From Boato et al, J, Chem. Phys. 24, 783, 1956. ...

For the parent pyrido[2.3-c/]pyndazine, the rate constants of the hydrogen-deuterium exchange of the ring protons by deuterium oxide in neutral, acidic or basic medium have been measured.47... 

NHC catalysis of Stetter and benzoin reactions by triazolidenes has been investigated via in situ observation of intermediate 3-(hydroxybenzyl)azolium salts of the benzaldehyde substrates. Equilibrium constants for their formation are reported, together with rate constants for hydrogen-deuterium exchange at the a-carbon. 

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