Deuterated solvents Deuterium isotope effect

Much attention has been paid to the chemistry and biological chemistry of singlet dioxygen and superoxide ion. The lifetime of the former species, known to be solvent-dependent, is much increased for deuterated acetone, acetonitrile, benzene and chloroform compared to the undeuterated solvent. This is a remarkable solvent deuterium isotope effect. Electron transfer to 02 from tetramethylphenylenediamine to give superoxide has been confirmed, the rate of reaction being close to the diffusion-controlled limit. Kinetic studies on the photochemical formation of superoxide from jU-superoxodecacyanodicobalt(III) ions and oxygenated ethanol solutions have been reported. 

The effect of solvent upon k2 has been reported , and it was concluded that the activated complex is not sufficiently polar to be called ionic . The oxidations of toluene and triphenylmethane exhibit primary kinetic deuterium isotope effects of 2.4 and ca. 4 respectively. No isotopic mixing occurred during formation of the Etard complex from a mixture of normal and deuterated o-nitrotoluene . The chromyl chloride oxidation of a series of substituted diphenylmethanes revealed that electron-withdrawing substituents slow reaction while electronreleasing groups have the opposite effect, the values ofp andp being —2.28 + 0.08 and —2.20 + 0.07 respectively . ... 

These experiments demonstrate the importance of proton transfer processes during hole transfer through DNA. S. Steenken has already remarked that a proton shift between the G C bases stabilizes the positive charge [23]. If such a proton shift is coupled with the hole shift, a deuterium isotope effect should arise. Actually, H/D isotope effects are described by V. Shafiro-vich, M.D. Sevilla as well as H.H. Thorp in their articles of this volume. Experiments with our assay [22] also demonstrate (Fig. 16) that hole transfer in protonated DNA (H20 as solvent) is three times more efficient than in deuterated DNA (D20 as solvent). If this reflects a primary isotope effect, it shows that the charge transfer is coupled with a proton transfer. 

Data for THE show otherwise [20[. Solvent deuteration increases the yield of RMgBr by 15%. decreases its enantiomeric excess, decreases the yield of RH(D), and increases its enantiomeric excess. These effects rcllect a deuterium isotope effect that diverts R- from s to r. The increase in (RMgBr) indicates that I5% of RBr gives R-that are diverted in this manner. The discrepancy between this value and the lower (RH), 5-6%. found in THF is probably due lo quantitative uncertainties in the analyses. 

Hartshorn and Ridd48 showed that there is a negligible solvent isotope effect on nitrating anilinium ions in sulphuric acid and deuterated sulphuric acid (cf. an earlier less accurate determination by Brickman and Ridd48). The absence of a solvent isotope effect also argues against reaction on the free base because the free base concentration would be lower by a factor of about four in the deuterium-containing medium. Consequently, the differences in the rate coefficients in Table 6... 

Probing for proton transfer in the rate-determining step can be accomplished using deuterated compounds or solvents [63,86,91,138,140], If the deuterated explosive is available, it can be examined neat and in solution. If no deuterium kinetic isotope effect (DKIE) is exhibited by the neat deuterated explosive, then hydrogen transfer can be ruled out in the... 

In the presence of picric acid, the reaction products were the same, but general acid catalysis could be observed. The solvent isotope effect was determined using a mixture of 82.5 % ethanol and 17.5 % water. If 38 % of the mobile (O-bonded) hydrogen of the solvent was replaced by deuterium the reaction rate was decreased by a factor of 1.45. Linear extrapolation to 100% deuteration led to an approximate result of kH/kD 2.2. Similar experiments referring to general catalysis and solvent isotope effects have been done for the ethanolysis of 9-diazo-fluorene, and similar results have been obtained [215]. 

Isotope discrimination studies were employed to deduce if the double proton transfer of proline racemase is concerted or stepwise [88]. Isotope discrimination is an alternative manifestation of the multiple kinetic isotope effect techniques previously discussed, wherein racemization is conducted in mixed isotopic solvents of H2O and D2O and the discrimination in the incorporation of solvent deuterium is measured. If the double proton transfer is stepwise, deuteration of the substrate... 

The main uses of deuterium are in tracer studies to follow reaction paths and in kinetic studies to determine isotope effects. " A good discussion with appropriate references is in Comprehensive Inorganic Chemistry, Vol. 1, pp. 99-116. The use of deuterated solvents is widespread in proton nmr studies to avoid interference from solvent hydrogen atoms, and deuteriated compounds are also valuable in structural studies involving neutron diffraction techniques. 

The last two reactions, which are extremely fast, account for the stoichiometric relationship 2 ArN2 + H2Q. The reaction is second order kinetically, however, because only one molecule of diazonium ion is consumed up to the rate-determining step of the reaction. The solvent isotope effect is attributed to the fact that the for deuterated hydroquinone, DO—CgH4—OD, is 4.16 times less than that of hydroquinone. The absence of deuterium in the nitrobenzene when the reaction is done in D2O indicates that the decomposition of the Ar—N2 to product does not involve hydroquinone and that hydroquinone is not the source of the hydrogen that is added to the benzene ring in the final step of the reaction. In fact, the partial deuteration of the nitrobenzene suggests that the decomposition of the aryldiazenyl radical occurs by two pathways (equation 56), where SH is the solvent for the reaction. 

Tidwell and coworkers have examined the addition of water to 1 -cyano-1 -ethoxyethene in aqueous sulphuric acid at 25 °C. The primary hydrogen-deuterium kinetic isotope effect found when the reaction is done in deuterated solvent increases from 3.77 to 5.44 as the percent sulphuric acid in the solvent increases from 6.6 M to 10.8 M. These isotope effects demonstrate that the slow step of the reaction is protonation of the vinyl substrate to form the a-cyanocarbocation and that the product is formed in subsequent, fast steps of the Ad-E2 mechanism (equation 83). 

The simplest model is when a single site determines the entire solvent isotope effect. Consider a mixture of HOH and DOD in which the mole fraction of DOD is n, so that DOD/HOH ratio is n/(l-n). For an exchangeable hydrogen in a reactant molecule AH, the deuterium/protium ratio will also be [AD]/[AH] = nJ (l- ) if the equilibrium constants for protons and deuterons are the same at that position. There will be normally some selection, so that the actual ratio is where is ihe fractionation factor for that exchangeable reactant position. Such exchanges of hydrogen can take place not only in the reactant molecule itself but also in the transition state of reaction in which it participates. In this transition state, there is a similar deuterium/protium ratio 4>rn/ii n), where is the fractionation factor for the same exchangeable position in the transition state. The total rate of reaction is now the sum of the rates for the protonated and deuterated species, and so the rate of reaction is proportional to... 

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