Proton/deuterium exchange

The mobility of the proton in position 2 of a quaternized molecule and the kinetics of exchange with deuterium has been studied extensively (18-20) it is increased in a basic medium (21-23). The rate of exchange is close to that obtained with the base itself, and the protonated form is supposed to be the active intermediate (236, 664). The remarkable lability of 2-H has been ascribed to a number of factors, including a possible stabilizing resonance effect with contributions of both carbene and ylid structure. This latter may result from the interaction of a d orbital at the sulfur atom with the cr orbital out of the ring at C-2 (21). 

Substituted tetrazoles readily exchange the 5-hydrogen for deuterium in aqueous solution. A major rate-enhancing effect is observed with copper(II) or zinc ions due to complexation with the heterocycle. The rate of base-induced proton-deuterium exchange of 1-methyltetrazole is 10 times faster than 2-methyltetrazole (77AHC(2l)323). 

Hydrogen atoms in azolium ions can be removed easily as protons (e.g. 230—>232) exchange with deuterium occurs in heavy water. The intermediate zwitterion (e.g. 231) can also be written as a carbene, and in some cases this carbenoid form can be trapped or isolated as a dimer. 

In the H NMR spectrum five protons can be exchanged by deuterium. Here the molecular formula permits only OH groups. The shift values above 7.9) identify four phenolic OH groups and one less acidic alcoholic OH function = 4.0, overlapping). 

Charton has recently examined substituent effects in the ortho position in benzene derivatives and in the a-position in pyridines, quinolines, and isoquinolines. He concludes that, in benzene derivatives, the effects in the ortho position are proportional to the effects in the para position op). However, he finds that effects of a-sub-stituents on reactions involving the sp lone pair of the nitrogen atoms in pyridine, quinoline, and isoquinoline are approximately proportional to CT -values, or possibly to inductive effects (Taft s a ). He also notes that the effects of substituents on proton-deuterium exchange in the ortho position of substituted benzenes are comparable to the effects of the same substituents in the a-position of the heterocycles. 

Two possible reaction schemes for the photosensitized formation of semibullvalene are as follows, where the large dot indicates a proton label (all other protons were exchanged for deuterium) ... 

The best insight into the relaxation behavior of star polymers in dilute solution can be expected if, in addition to the whole star system, different parts of the star are considered separately. This can be achieved easily by neutron scattering techniques on systems where not only the entity of arms, but also single arms, the core or shell parts are labelled by proton deuterium exchange. With respect to the core-shell labelling it is convenient to build up the arms as diblock copolymers of A-B type with protonated or deuterated but otherwise chemically identical A and B blocks. 

The observation of a primary solvent deuterium isotope effect (kH/fa>) = 2-4 on the specific acid-catalyzed hydrolysis of vinyl ethers provides evidence for reaction by rate-determining protonation of the alkene.69 Values of kHikD 1 are expected if alkene hydration proceeds by rate-determining addition of solvent to an oxocarbenium ion intermediate, since there is no motion of a solvent hydron at the transition state for this step. However, in the latter case, determination of the solvent isotope effect on the reaction of the fully protonated substrate is complicated by the competing exchange of deuterium from solvent into substrate (see above). 

Significantly reduced rates for proton-deuterium exchange prove that the corresponding amide proton is either involved in stable hydrogen bonds, shielded from solvent access or both. Because of the ambiguity in interpretation, additional information about the persistence of hydrogen bonds stemming from structure calculations or from relaxation data should be available. 

Figure 23.4, illustrates the two dimensional COSY spectrum of a sugar 1-0-methyl a-D-glucopyranoside (1) that has been recorded on a 400 MHz NMR-spectrometer the sample was dissolved in D20 so that the OH protons get duly exchanged with Deuterium and are, therefore, not seen at all. Besides, the -NMR-spectrum has also been shown alongside both axis of the two dimensional spectrum in Figure 23.4. 

FIGURE 32. H NMR spectrum of filipin III, 3 mM in DMSO-dg, recorded at 400 MHz and 25 °C. The expanded region contains nine hydroxylic proton resonances that fully exchange with deuterium oxide and correspond to the nine hydroxyl groups of filipin III. No apodization functions were applied prior to the Fourier transformation. Reproduced by permission of John Wiley Sons from Reference 50... 

Silica gel and silica-alumina protons will exchange with deuterium in D2O at 150° or Da gas at 350°, as is evidenced by disappearance of the resonance signal. With partial exchange, the absorption line decreases in... 

Protons of 1,2,3-triazole rings can also be activated by A -oxides. The H(4) and H(5) protons in 3-substituted 1,2,3-triazole-1 -oxides (254) are more reactive but exchange with deuterium at a similar... 

The interaction of the OEC with magnetic nuclei (e.g. from substrate water) can also be studied by ENDOR and ESEEM techniques. Despite some initial controversy, there is now agreement that after H20/D20 exchange, proton resonances disappear and/or deuterium resonances can be detected for the S2 state.13-418,446 449,469 471 Thus, exchangeable protons exist at 3-6 A from the cluster. 

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