Deuterium isotope effect, upper

Upper excited-state emission spectroscopy deuterium isotope effect, 227-229 diacid porphyrins, 110-112 diamagnetic... 

Since the lower pathway has proton removal as an integral part of the mechanism, you could use isotopic substitution in two ways. First make the deuterated compound D. The kinetic deuterium isotope effect should be primary for the lower path but very near 1 for the upper path. Moreover, if you looked at the product, there should be no deuterium left in the product if the lower path is followed, whereas deuterium should be retained in the product if the addition elimination path is followed. 

Hence, the first clearcut evidence for the involvement of enol radical cations in ketone oxidation reactions was provided by Henry [109] and Littler [110,112]. From kinetic results and product studies it was concluded that in the oxidation of cyclohexanone using the outer-sphere one-electron oxidants, tris-substituted 2,2 -bipyridyl or 1,10-phenanthroline complexes of iron(III) and ruthenium(III) or sodium hexachloroiridate(IV) (IrCI), the cyclohexenol radical cation (65" ) is formed, which rapidly deprotonates to the a-carbonyl radical 66. An upper limit for the deuterium isotope effect in the oxidation step (k /kjy < 2) suggests that electron transfer from the enol to the metal complex occurs prior to the loss of the proton [109]. In the reaction with the ruthenium(III) salt, four main products were formed 2-hydroxycyclohexanone (67), cyclohexenone, cyclopen tanecarboxylic acid and 1,2-cyclohexanedione, whereas oxidation with IrCl afforded 2-chlorocyclohexanone in almost quantitative yield. Similarly, enol radical cations can be invoked in the oxidation reactions of aliphatic ketones with the substitution inert dodecatungstocobaltate(III), CoW,20 o complex [169]. Unfortunately, these results have never been linked to the general concept of inversion of stability order of enol/ketone systems (Sect. 2) and thus have never received wide attention. 

Deuterium Isotope Effect. Ubbelohde and co-workers have done much work on the effect of deuterium substitution on the structures of H bonded crystals (1729, 2067, 1728, 1727, 2071, 522, 739, 2068). The results are reviewed in the summary paper, 2068. In most crystals the D bond is slightly longer than the H bond. Furthermore, in those crystals which are ferroelectric, the deuterated crystal has a higher ferroelectric transition temperature (upper Curie temperature). Some of the results summarized by Ubbelohde and Gallagher are... 

Figure 6.9 Neutron scattering data from a binary mixture, annealed at 160°C, of a hydrogenous polystyrene (molecular weight 870,000) and a deuterated polystyrene (molecular weight 1,150,000). The solid curve is a fitted theoretical curve with x equal to 1.9 x 10-4, and the upper broken curve is for x =0 expected in the absence of the deuterium isotope effect. (From Bates and Wignall.19)...

For secondary deuterium isotope effect at not too high temperatures the zero point energy difference between initial and final state for an isotope exchange equilibrium is directly related to the standard free energy change and has an exponential effect on the equilibrium constant. The principal possibilities of the temperature dependenee of equilibrium isotope effects are discussed in the literature (Melander and Saunders, 1980 Collins and Bowman, 1970). For most practical purposes to evaluate the temperature dependence of secondary isotope effects it has been found that approximate solutions are sufficient. The low temperature approximation (i.e. when the populations of upper vibrational levels are negligible) of the temperature dependence of the equilibrium isotope effect has the form (10) (Stern el al.. 

Evidence for a single energy minimum. Saunders and Kates (1980) have measured deuterium isotope effects on the nmr spectra of deuteriated 2-norbornyl cations. No additional isotope splitting or broadening was observed in the spectra of either 2-Di- or S.S-D -norbornyl cations. The upper limit for an isotope splitting thus cannot be more than 2-2.5 ppm. This result is in marked contrast to the large splittings observed in the model cations [104] and [105]. 

This mechanism would predict that a plot of l/d>r vs. 1/[BH2] should be linear with an intercept equal to l/Oisc. The experimental results 1 have been plotted in this way in Figure 3.1. The straight lines in Figure 3.1 are consistent with but do not prove the suggested mechanism. The slope for the lower line (BH2) is equal to 0.05, while that for the upper is 0.133. This difference in rate upon replacement of a hydrogen atom with deuterium indicates that the reaction is subject to an isotope effect. Therefore the abstraction... 

Radicals abstract deuterium from the molecule S D much less readily than they abstract hydrogen from the corresponding compound S H for example in the polymerization of styrene at 60°—70° C, the transfer constant for n C4H9- SH is about four times that for n C4H9- SD (30). Upper and lower limits for the isotope effect can be set by considering the type of bonding in the transition state. Experimental study of the effect can lead to information concerning the nature of the transition state. 

One simple test is to measure the level of radioactivity from the sample. Synthetic vanillin is not radioactive. However, natural vanilla, like all natural products, is. This is, of course, because atmospheric carbon dioxide contains some radioactive 14C formed by exposure to cosmic radiation in the upper atmosphere. Plants then incorporate this into their photosynthetic pathway and produce metabolites, which exhibit a low level of radioactivity. Synthetic vanillin is prepared from coal tar, which is not radioactive since the 14C has long since decayed. However, unscrupulous dealers know this and can synthesise radiolabelled or hot vanillin and dose it into synthetic material so that the level of radioactivity matches that of a natural sample. Another method of checking for naturalness must therefore be found. When plant enzymes synthesise molecules, they, like all catalysts, are susceptible to isotope effects. The vanilla plant is no exception and examination of the distribution of hydrogen and carbon isotopes in the vanillin molecule reveals that the heavier deuterium and 13C isotopes accumulate at certain specific sites. A suitable NMR spectrometer can determine the isotopic distribution in a sample and the cost of using 2H, 13C and 14C labelled synthetic materials to replicate the NMR spectra and radioactivity of natural vanillin in a synthetic sample would not be financially attractive. Furthermore, the 2H and 13C labelling patterns in the vanilla bean are different from those of other natural shikimate sources and so the NMR technique can also distinguish between vanillin from vanilla and vanillin produced by... 

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