Hydrogen exchange mixed kinetics

A neglected topic is secondary deuterium IEs on the acidity of metal hydrides. These are related to IEs on CH acids only in that equlibration can be slow. There are many metal hydrides, including polyhydrides, whose acidities have been measured, but not with D.64 67 It is often possible to exchange deuterium into the hydrides, and the hydrides and mixed hydrides have unusual properties and kinetic IEs,68 69 as well as possibilities for hydrogen bonding that might show IEs.70... 

A number of research groups have used SIFT instruments for measurements directed toward IS chemistry. The Birmingham group of Adams and Smith, the inventors of the SIFT technique [16], was particularly active in this regard and a major focus of their SIFT measurements was the systematic study of reactions of hydrogenated ions, e.g. CH,, CjH,, NH , HnS+, HnCO+ etc., with numerous molecular species [18]. Further contributions by this group include detailed studies of isotope exchange in ion-neutral reactions, studies for which the SIFT is eminently suited, since the ion source gas and the reactant gas are not mixed. From these studies and detailed kinetic models of interstellar ionic reactions, it is now understood that the observed enhancement of the rare isotopes (e.g. D, 13C) in some IS molecules is due to the process of isotope fractionation in ion-neutral reactions [19]. 

Solvent isotope effects. Much mechanistic information can be obtained about reactions involving proton transfer from solvent kinetic isotope effects, particularly in solvents of mixed isotopic composition. For practical reasons work is essentially confined to H/D effects, especially those in water. Unlike ordinary primary hydrogen isotope effects, solvent isotope effects have to take into account a host of exchangeable sites, subject to equilibrium as well as kinetic isotope effects. A key concept is that of the fractionation factor, (p, which is the deuterium occupancy of a site in a 1 1 H2O/D2O mixture more formally it is defined by equation l.l ... 

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