Deuterium isotope effects, and hydrogen

They both produce the same intermediate, and because of the deuterium isotope effect, more hydrogen is lost than deuterium. 

As we have discussed in depth elsewhere, despite the similarities in the structures of hypericin and hypocrellin, which are centered about the perylene quinone nucleus, their excited-state photophysics exhibit rich and varied behavior. The H-atom transfer is characterized by a wide range of time constants, which in certain cases exhibit deuterium isotope effects and solvent dependence. Of particular interest is that the shortest time constant we have observed for the H-atom transfer is 10 ps. This is exceptionally long for such a process, 100 fs being expected when the solute H atom does not hydrogen bond to the solvent [62]. That the transfer time is so long in the perylene quinones has been attributed to the identification of the reaction coordinate with skeletal motions of the molecule [48, 50]. 

Isotope Effect and Hydrogen Bond Type. Singh and Wood (146) have calculated using Stepanov s approximation the effect of isotopic substitution of hydrogen by deuterium on the R(A..B) distance of an A—H.. B hydrogen bond. They found that the R(A.. B) distance increases (positive isotope effect) on deuteration if the potential energy... 

Since these first reports, Iwahara and other investigators have studied the conductivities (both ionic and electronic), conduction mechanism, deuterium isotope effect, and thermodynamic stability of these materials. The motivation for most of this work derives from the desire to utilize these materials for high temperature, hydrogen-fiieled solid oxide fuel cells. In a reverse operation mode, if metal or metal oxide electrodes are deposited onto a dense pellet of this material and heated to temperature T, the application of an electric potential to the electrodes will cause a hydrogen partial pressure difference across the pellet according to the Nemst equation ... 

Section 5.17 A P C—D bond is broken more slowly in the E2 dehydrohalogenation of alkyl halides than a p C—H bond. The ratio of the rate constants / h/ d is a measure of the deuterium isotope effect and has a value in the range 3-8 when a carbon-hydrogen bond breaks in the rate-determining step of a reaction. 

Matsunaga I, Yamada A, Lee DS, Obayashi E, Fuji-wara N, Kobayashi K, Ogura H, Shiro Y (2002) Enzymatic reaction of hydrogen peroxide-dependent peroxygenase cytochrome P450s kinetic deuterium isotope effects and analyses by resonance Raman spectroscopy. Biochemistry 41 1886-1892... 

Nelson, P.F., Lukey, C.A. and Cant, N.W. (1989), Measurements of Kinetic Isotope Effects and Hydrogen/Deuterium Distributions over a Methane Oxidative Coupling Catalyst, J. Catal. 120, 216. 

The transformation of the intermediate, I, to the final product, N-allyl-2,4-dinitroanilme, involves the breaking of both a carbon-chlorine bond and a nitrogen-hydrogen bond. Whether this takes place in two consecutive steps or in a single, concerted process, a valid mechanism must be consistent with both the absence of a measurable deuterium isotope effect and the fact that the observed rates are affected by the process in which the nitrogen-hydrogen bond is broken. 

Limbach H H 1991 Dynamic NMR spectroscopy in the presence of kinetic hydrogen/deuterium isotope effects NMR Basic Principles and Progress vol 23, ed P Diehl, E Fluck, H Gunther, R Kosfeld and J Seelig (Berlin ... 

Kinetic isotope effects are an important factor in the biology of deuterium. Isotopic fractionation of hydrogen and deuterium in plants occurs in photosynthesis. The lighter isotope is preferentially incorporated from water into carbohydrates and tipids formed by photosynthesis. Hydrogen isotopic fractionation has thus become a valuable tool in the elucidation of plant biosynthetic pathways (42,43). 

When one of the ortho hydrogens is replaced by deuterium, the rate drops from 1.53 X 10 " s to 1.38 X lO s. What is the kinetic isotope effect The product from such a reaction contains 60% of the original deuterium. Give a mechanism for this reaction that is consistent with both the kinetic isotope effect and the deuterium retention data. 

A second piece of evidence in support of the E2 mechanism is provided by a phenomenon known as the deuterium isotope effect. For reasons that we won t go into, a carbon-hydrogen bond is weaker by about 5 kj/mol (1.2 kcal/mol) than the corresponding carbon-rfaiiferiwm bond. Thus, a C-H bond is more easily broken than an equivalent C-D bond, and the rate of C-H bond cleavage is faster. For instance, the base-induced elimination of HBv from l-bromo-2-phenylethane proceeds 7.11 times as fast as the corresponding... 

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