Proton-deuteron exchange

Electrophilic Attack. A variety of boranes, heteroboranes, and metaHaboranes undergo electrophilic substitution. SusceptibiUty of boranes to electrophilic attack is often detected by deuteron—proton exchange experiments. Eor example, electrophilic hydrogen—deuterium exchange of occurs at the l-,2-,3-, and 4-positions when exposed to DCl in the presence of AlCl (81). The trend to increasing positive sites in is... 

Proton ENDOR and DOUBLE ENDOR data on VO(acac)2 in frozen solutions have been reported by van Willigen2895. Since the CH-protons exchange with deuterons in CD3OD they may easily be discriminated from the methyl protons. From the single crystal-like ENDOR spectrum for B0 along g and the two-dimensional powder spectrum at gi, the hf coupling constants Ap 5 = 0.7 MHz and AfHj = 0.7 MHz, A 3 = 1.5 MHz, respectively, have been found. The relative signs of the principal... 

Labile protons exchange rapidly with each other and also with protons in water or with the deuterons in D2O. 

The protonation of a thiolate donor, formation of a nonclassical r 2-H2 complex, release of H2 and addition of D2, and the heterolytic cleavage of this D2 by the concerted attack of the Lewis acidic metal center and the Brpnsted basic thiolate donor are essential steps. The acidic thiol deuteron can exchange with EtOH protons. The resulting free protons and the deuteride complex yield HD and the coordinatively unsaturated species that is the actual catalyst. The detailed mechanism comprises a considerably larger number of steps (and equilibria) (143). For example, the occurrence of r 2-D2 and [M(D)(SD)] intermediates that exchange with H+ should give rise to [M(D)(SH)]... 

Solvent protons (or deuterons) may exchange with reactant protons, leading to labeled reactant. The isotopic fractionation factor, denoted (f>, indicates the equilibrium distribution of the isotopic label between solute (R-H) and solvent (S-D). For the reaction... 

All proton and deuteron NMR relaxation times are strongly affected by water/ biopolymer proton exchange and/or dipolar interactions. 

One method of investigating molecular motion in polymer physics is the observation of the temperature dependence of the line width of broad-line NMR spectra. However, since UPEC is composed of polyethylene and urea molecules, the protons in urea molecules must be replaced by deuterons in order to observe the behavior of the polyethylene chain by proton magnetic resonance. For this purpose, deuterated urea molecules were used in the preparation of UPEC (d-UPEC). In the preparation of d-UPEC, deuterated methanol has been used as a solvent in order to prevent proton exchange. In order to compare the new data with the data of bulk polymers, solution-grown polyethylene and extended-chain crystals of polyethylene were also used in the NMR study. 

Interestingly enough, both protons at C-11 are exchanged quite readily in 12-keto steroids. In these compounds C-11 is the only possible enolization site where the axial (/3) proton is probably expelled first. During ketonization, the deuteron attack is more likely to occur from the less hindered a-side. By this sequence the proton which was originally at the lla-equato-rial position becomes axial and readily available for expulsion in the next enolization step. Thus, isomerization of the C-11 hydrogens may be an important reason for the facile exchange at this position. (For a more detailed discussion of the mechanism of enolization and ketonization reactions, see ref 114.)... 

Fig. 16 Influence of the exchange of protons (H20) by deuterons (D20) on the efficiency of the hole transfer, measured by the product ratios Pqgg/Pg...

In the spectra of alkyl cobinamides two peaks have been observed at 3.89 and 4.42 which were assigned to the protons of a water molecule coordinated at the lower axial site (130). To confirm this assignment, it was found that addition of cyanide to methyl cobinamide, which displaces coordinated water, caused the peaks to disappear. Likewise, addition of excess D2O caused disappearance of the peaks through either ligand exchange or proton-deuteron exchange. 

This situation applies with weak hydrogen bonds at one extreme and very strong hydrogen bonds at the other with H and D confined to the same potential well. However, when the potential energy barrier has fallen sufficiently to allow the proton to escape the confines of its parent well, but leaves the deuteron trapped, then different values of the isotopic ratio can be observed (Fig. 7). The effect of isotopic exchange is now much more than merely one of doubling the reduced mass of the vibrating bond. When the proton is above the barrier, the force constant of the A—H bond, k A.—H),... 

The stable ruthenium alkyhdenes, used for catalysis of ring opening metathesis polymerizations, were found to exchange the alkylidene proton for a deuteron in D2O or in CD3OD (Scheme 9.4) [13],... 

We thought it preferable not to include in the discussion the electrophilic aromatic photosubstitution since well-studied examples 3ie still scarce (proton-deuteron and proton-triton exchange in acidic media protodeboronation). From our experience we have the feeling that many electrophiles are very efficient quenchers and that moreover it is not easy to choose systems where the concentrations of the potentially reacting electrophiles can be made high enough to react efficiently with the short-lived excited species. 

Labile protons can always be positively identified by in situ exchange with D2O. In practice, a normal H NMR spectmm is recorded then deuterium exchange of labile protons is achieved by simply adding a drop of deuterated water (D2O) to the NMR sample. Labile protons in -OH, -COOH, -NH2 and -SH groups exchange rapidly for deuterons in D2O and the H NMR is recorded again. Since deuterium is invisible in the iH NMR spectmm, labile protons disappear from the NMR spectmm and can be readily identified by comparison of the spectra before and after D2O is addition. 

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