Carbon deuterium

Two physically reasonable but quite different models have been used to describe the internal motions of lipid molecules observed by neutron scattering. In the first the protons are assumed to undergo diffusion in a sphere [63]. The radius of the sphere is allowed to be different for different protons. Although the results do not seem to be sensitive to the details of the variation in the sphere radii, it is necessary to have a range of sphere volumes, with the largest volume for methylene groups near the ends of the hydrocarbon chains in the middle of the bilayer and the smallest for the methylenes at the tops of the chains, closest to the bilayer surface. This is consistent with the behavior of the carbon-deuterium order parameters,. S cd, measured by deuterium NMR ... 

This means that the ionization and rearrangement need not be concerted and that symmetrical protonated ethylene can not be a major intermediate in the reaction. A similar experiment with isobutylamine and nitrous acid in heavy water gave products that contained no carbon-deuterium bonds. Since it is known that the -complex formed from isobutylene and acid is in rapid equilibrium with protons from the solvent, none of this can be formed in the nitrous acid induced deamination. This in turn makes it probable that the transition state for the hydrogen migration is of the sigma rather than the -bonded type.261... 

A primary isotope effect results when the breaking of a carbon-hydrogen versus a carbon-deuterium bond is the rate-limiting step in the reaction. It is expressed simply as the ratio of rate constants, i wlky,. The full expression of k /kn measures the intrinsic primary deuterium isotope for the reaction under consideration, and its magnitude is a measure of the symmetry of the transition state, e.g., -C- H- 0-Fe+3 the more symmetrical the transition state, the larger the primary isotope effect. The theoretical maximum for a primary deuterium isotope effect at 37°C is 9. The less symmetrical the transition state, the more product-like or the more substrate-like the smaller the intrinsic isotope effect will be. 

Mitulovic et al. [Ill] presented a procedure for the preparation of a-carbon deuterium-labelled a-amino acids from native amino acids via a Schiff-base racem-ization protocol in deuterated acetic acid involving a preparative chromatography step of the obtained Z-protected deuterium-labeled amino acid derivatives on the tBuCQN-CSP [ill]. The analytical control of the enantiomeric products after DNP, Z, or DNZ derivatization showed a high enantiomeric excess (97-98%) and also a high isotopic purity (99%) by MS. 

Carbon-proton and carbon-deuterium coupling constants are related to each other by eq. (3.15a), which follows not only from eq. (3.15) but also from Ramsay s theory [5, 10]. 

Figure 3.35 The deuterium effect in non-radiative transitions. The spacing of the vibrational levels of CD (carbon-deuterium) is much smaller than that of CH, so that for the same energy gap a much more distorted geometry of RD must be reached...

Carbon-deuterium bonds normally are broken more slowly than carbon-hydrogen bonds. This so-called kinetic isotope effect provides a general method for determining whether particular carbon-hydrogen bonds are broken in slow reaction steps. 

Synthesis of 8- and 12-carbon deuterium-labelled aldehydic esters... 

D. Carbon hydrogen bond is stronger than the corresponding carbon deuterium bond. 

B is correct. The question stem presents a mechanism for an elimination reaction (the product gains a double bond) that relies on a rapid C—H bond dissociation as the rate-limiting step. When the heavier deuterium (D) is used instead of a pure hydrogen atom, the reaction rate decreases because of a stronger carbon—deuterium bond. 

Because deuterium is heavier than hydrogen, the carbon-deuterium bond has a lower vibrational frequency than the carbon-hydrogen bond. This difference in frequency makes the carbon-deuterium bond slower to react. 

AC — D (carbon-deuterium) bond is electronically much like a C — H bond, and it has a similar stiffness, measured by the spring constant, k. The deuterium atom has twice the mass (m) of a hydrogen atom, however. 

Holzwarth G, Hsu EC, Mosher HS et al (1974) Infrared circular dichroism of carbon-hydrogen and carbon-deuterium stretching modes. J Am Chem Soc 96 251-252... 

Fig. 10.9. Depth profile changes in carbon, deuterium, oxygen and hydrogen in a-C D films due to air exposure at 650 K (a) before annealing, (b) after annealing for 2h, (c) after annealing for 4h, and (d) after annealing for 8h. The original film thickness was 730 nm. For each temperature step a fresh sample was used [62]...

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