Molecular isotope exchange

To excite Hi vibrationally by photochemical means, the Raman effect must be used. Although the stimulated Raman process has been used for this purpose in studies of vibrational energy transfer from Hi (and Di) to a variety of collision partners, ° and for an investigation of the effect of vibrational excitation on the molecular isotope exchange reaction, ... 

Isotopic Exchange Reactions. Exchange reactions between the isotopes of hydrogen are well known and well substantiated. The equihbrium constants for exchange between the various hydrogen molecular species have been documented (18). Kinetics of the radiation-induced exchange reactions of hydrogen, deuterium, and tritium have been critically and authoritatively reviewed (31). The reaction T2 + H2 — 2HT equiUbrates at room temperature even without a catalyst (30). 

This enzyme is of wide occurrence in bacteria where it is concerned with the reduction of nitrate and CO2 as well as sulphur. Methods for its estimation depend on measuring some activity of hydrogenase by (a) dye reduction (benzyl viologen or methylene blue), (b) isotopic exchange and (c) evolution of molecular hydrogen. Interpretation of quantitative results is difficult due to the complex relationship between the enzyme cell structure and the particular method selected. ... 

The concept of H2 chemisorbing on Rh as a molecular species may appear at first to be contradictory to other studies e.g.. Isotope exchange and TPD but It Is not, given the existence of klnetlcally distinct sites. An adsorption/desorption process can occur on a specific site via ... 

In earlier sections of this chapter we learned that the calculation of isotope effects on equilibrium constants of isotope exchange reactions as well as isotope effects on rate constants using transition state theory, TST, requires the evaluation of reduced isotopic partition function ratios, RPFR s, for ordinary molecular species, and for transition states. Since the procedure for transition states is basically the same as that for normal molecular species, it is the former which will be discussed first. 

The BO approximation, which assumes the potential surface on which molecular systems rotate and vibrate is independent of isotopic substitution, was discussed in Chapter 2. In the adiabatic regime, this approximation is the cornerstone of most of isotope chemistry. While there are BO corrections to the values of isotopic exchange equilibria to be expected from the adiabatic correction (Section 2.4), these effects are expected to be quite small except for hydrogen isotope effects. 

Conversions between equilibrium constants and fractionation factors are more complicated, as it is often necessary to accoimt for molecular stoichiometry and symmetry. For a generic balanced isotopic exchange reaction,... 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

The Catalysis of Isotopic Exchange in Molecular Oxygen G. K. Boreskov... 

The change in concentration of a molecular entity (being transformed) per unit time and usually symbolized by (f). This change in concentration (/.c., dc/dt) occurs in one direction only and applies to the progress of a reaction step (or sequence of steps) in a complex scheme that may even involve a set of parallel reactions. The term chemical flux can also refer to the progress of a chemical reaction(s) in one direction while that system is at equilibrium (Le., via isotope exchange at equilibrium). 

In previous sections we have shown clearly that intramolecular dihydrogen bonds X-H- H-Y, with X and Y representing various chemical elements, can exist in both the solid state and in solution. In addition, the bonds can be a critical factor in the control of molecular conformational states or effects on rapid and reversible hydride-proton exchanges related to the process shown in Scheme 5.1, or the well-known H-D isotope exchanges in similar subsystems [23]. Such bonds could also play an important role in the stabilization of transition states, appearing as a reaction coordinate in many transformations. This is particularly... 

Figure 1. Initial rate of isotopic exchange of the oxygen of oxides with molecular oxygen at 370°C, Curve 1 and their relative activity in hydrogen oxidation at 340°C, Curve 2. The activity of lanthanum oxide is taken to be unity (1).

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