Deuterium equilibrium

In a search fora pure y-deuterium equilibrium isotope effect, Kates (1978) studied [67] using C-nmr spectroscopy. The results showed equilibrating carbocation structures, and from the isotopic splitting of shifts of methyl groups equilibrium constants were determined. 

C2H6D. At —78° the presence of ethylene did not affect the hydrogen-deuterium equilibrium reaction rate. 

Saunders and coworkers have applied the isotopic perturbation technique to a number of persistent carbocations and have demonstrated that C—H hyperconjugation is a principal cause of P-secondary deuterium equilibrium isotope effects in stable carbocations. The results fully support the interpretation of secondary kinetic isotope effects in solvolytic substitution reactions. 

The magnitude of -deuterium equilibrium isotope effects The magnitude of the p-secondary deuterium kinetic isotope effects in solvolyses which proceed via a cationic transition state have been shown to be conformationally dependent (Sunko and Hehre, 1983). Shiner furnished two important representative examples relating the dihedral angle between an adjacent C—H or C—D bond and the vacant p-orbital of the cationic transition state with the magnitude of the kinetic isotope effect. In the... 

These results have been interpreted as a direct proof of the hyperconjuga-tional origin of the dependence of the P-deuterium equilibrium isotope effect on dihedral angle in a persistent carbocation. In [153] maximum overlap between the vacant p-orbital and one of the methyl C—D or C—H bonds is always possible leading to large equilibrium isotope splittings. In [154] the dihedral angle of 90° permits no overlap of the vacant p-orbital with the... 

Incorporation of deuterium into ammonia within the ammonia, hydrogen and deuterium equilibrium mixture... 

J. W. Pyper and C. K. Briggs, The Ortho-Para Forms of Hydrogen, Deuterium and Tritium Radiation and Self-induced Conpersion Kinetics and Equilibrium, Eawrence Eipermore Eaboratoy Report UCRE-52278, 1977. 

Penicillin sulfoxides can be epimerized by heat to afford thermal equilibrium mixtures of a- and /3-sulfoxides, the position of the equilibrium depending on the C(6) side chain (Scheme 5). Deuterium incorporation studies support a sulfenic acid, e.g. (18), as the intermediate in these transformations. This mechanism is also supported by the finding that when an a-sulfoxide epimerizes to a /3-sulfoxide there is a simultaneous epimerization at C(2) (71JCS(C)3540). With irradiation by UV light it is possible to convert a more thermodynamically stable /3-sulfoxide to the a-sulfoxide (69JA1530). 

Below — 140°C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above — 140°C, however, the spectmm of a second species was seen, and above — 100°C, this was the only spectmm observed. This second spectmm could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to — 140°C. The conclusion is that the ring opening of the cyclopropyl radical is a very facile process and that the lifetime of the cyclopropyl radical above — 100°C is very short. Even though the equilibrium favors the 3-butenyl radical, the reversible ring closure can be detected by isotopic labeling experiments, which reveal the occurrence of deuterium migration ... 

Data refer to H2 of normal isotopic composition (i.e. containing 0.0156 atom % of deuterium, predominantly as HD). All data refer to the mixture of ortho- and puru-forms that are in equilibrium at room temperature. 

Similar principles apply to ortho- and para-deuterium except that, as the nuclear spin quantum number of the deuteron is 1 rather than as for the proton, the system is described by Bose-Einstein statistics rather than the more familiar Eermi-Dirac statistics. Eor this reason, the stable low-temperature form is orriio-deuterium and at high temperatures the statistical weights are 6 ortho 3 para leading to an upper equilibrium concentration of 33.3% para-deuterium above about 190K as shown in Eig. 3.1. Tritium (spin 5) resembles H2 rather than D2. 

They found that deuterium labeled 2-phenyl-5-methoxy-4-[(methoxy-d3)-carbonyl]oxazole (7) scrambled on heating to give a 1 1 equilibrium mixture of 7 and the corresponding rearranged ester 9 ... 

Tlie amino form 84a exists in equilibrium with the imino structure 84b and not with 85 as concluded from the structure of products of deuterium exchange (87MRC757 91H329). 

The critical hydrogen content for the ductility loss increased with increasing hydrogen solubility in the alloy. The fracture surfaces were not characteristic of those found under conditions of SCC. In terms of hydrogen and deuterium solubility in a similar series of bcc alloys, the equilibrium constants were determined at infinite dilution as a function of temperature The free energy function was expressed in terms of the bound-proton model. 

The deuterium isotope effect is thought to arise from the effect on the equilibrium position of this A-nitrosation. This is also the case for the diazotization of aniline, but the isotope effect is larger, because two deprotonations are involved in the kinetics. 

On the basis of these correlations, Gold and Satchell463 argued that the A-l mechanism must apply (see p. 4). However, a difficulty arises for the hydrogen exchange reaction because of the symmetrical reaction path which would mean that the slow step of the forward reaction [equilibrium (2) with E and X = H] would have to be a fast step [equivalent to equilibrium (1) with E and X = H] for the reverse reaction, and hence an impossible contradiction. Consequently, additional steps in the mechanism were proposed such that the initial fast equilibrium formed a 7t-complex, and that the hydrogen and deuterium atoms exchange positions in this jr-complex in two slow steps via the formation of a a-complex finally, in another fast equilibrium the deuterium atom is lost, viz. 

Secondly, it has been found that the benzidine rearrangement is subject to a solvent isotope effect d2o/ h2o > 1- If a proton is transferred from the solvent to the substrate in a rate-determining step the substitution of protium by deuterium will lead to a retardation in the rate of reaction (primary isotope effect) whereas if a proton is transferred in a fast equilibrium step preceeding the rate-determining step as in... 

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