Deuterium cycle

It is evident that fusion reactions become possible only at very high temperatures, and they are therefore called thermonuclear reactions. It is assumed that the deuterium cycle (a) prevails in the sun and in relatively cold stars, whereas the carbon cycle (b) dominates in hot stars. In the centre of stars densities of the order of lO g/cm and temperatures of the order of 10 K may exist, and under these conditions other thermonuclear reactions become possible ... 

It has been calculated that in the sun at a core temperature of 1.6 10 K about half of the energy is produced by the deuterium cycle, about half by the reaction sequence... 

AVT Barg BD BDHR BF BOF BOOM BOP BS W BSI BTA Btu/lb BW BWR BX CA CANDUR CDI CFH CFR CHA CHF CHZ Cl CIP CMC CMC CMC COC All-Volatile treatment bar (pressure), gravity blowdown blowdown and heat recovery system blast furnace basic oxygen furnace boiler build, own, operate, maintain balance of plant basic sediment and water British Standards Institution benzotriazole British thermal unit(s) per pound boiler water boiling water reactor base-exchange water softener cellulose acetate Canadian deuterium reactor continuous deionization critical heat flux Code of Federal Regulations cyclohexylamine critical heat-flux carbohydrazide cast iron boiler clean-in-place carboxymethylcellulose (sodium) carboxy-methylcellulose critical miscelle concentration cycle of concentration... 

Keeping in mind the entire set of components in the climate system as depicted in Figs 17-2,4-13, and 17-3, we can now re-examine Fig. 1-2 to emphasize that biogeochemical cycles are coupled with the climate system. The temperature (as inferred from the record of the deuterium to hydrogen ratio in Antarctic ice) covaries with CO2, CH4 and other species that derive from biological processes. Two simple, if extreme, possibilities can be drawn ... 

The oxidation state of the metal is of the utmost importance since no conversion is observed with complexes in a different oxidation state. CatalyticaUy active species are electron-rich d or d complexes. A general catalytic cycle has been proposed on the basis of deuterium-labeling experiments (Scheme 4-14) [280]. It is beUeved to occur for aU the catalysts used. 

Scheme 14 Top Plausible catalytic cycle as supported by deuterium labeling. Bottom ESI mass spectrum of a reaction mixture aliquot diluted 5000-fold in methanol from the hydrogen-mediated coupling of gaseous acetylene to an a-ketoester (Ar = p-N02Ph)...

Resolvable modulation is detected on a three-pulse echo decay spectrum of predeuterated 3-carotene radical (Gao et al. 2005) as a function of delay time, T. The resulting modulation is known as ESEEM. Resolvable modulation will not be detected for nondeuterated P-carotene radical since the proton frequency is six times larger. The modulation signal intensity is proportional to the square root of phase sensitive detection and interfering two-pulse echoes and suppressed by phase-cycling technique (Gao et al. 2005). Analysis of the ESEEM spectrum yields the distance from the radical to the D nucleus, a the deuterium coupling constant, and the number of equivalent interacting nuclei (D). The details related to the analysis of the ESEEM spectrum are presented in Gao et al. 2005. 

Deuterium is the baryometer of choice. During its post-BBN evolution, as gas is cycled through stars, D is only destroyed (setting aside rare astronomical events... 

Although a cobalt-catalyzed intermolecular reductive aldol reaction (generation of cobalt enolates by hydrometal-lation of acrylic acid derivatives and subsequent reactions with carbonyl compounds) was first described in 1989, low diastereoselectivity has been problematic.3 6 However, the intramolecular version of this process was found to show high diastereoselectivity (Equation (37)).377,377a 378 A Co(i)-Co(m) catalytic cycle is suggested on the basis of deuterium-labeling studies and the chemistry of Co(ll) complexes (Scheme 81). Cobalt(m) hydride 182, which is... 

To probe the reaction mechanism of the silane-mediated reaction, EtjSiD was substituted for PMHS in the cyclization of 1,6-enyne 34a.5 The mono-deuterated reductive cyclization product 34b was obtained as a single diastereomer. This result is consistent with entry of palladium into the catalytic cycle as the hydride derived from its reaction with acetic acid. Alkyne hydrometallation provides intermediate A-7, which upon cw-carbopalladation gives rise to cyclic intermediate B-6. Delivery of deuterium to the palladium center provides C-2, which upon reductive elimination provides the mono-deuterated product 34b, along with palladium(O) to close the catalytic cycle. The relative stereochemistry of 34b was not determined but was inferred on the basis of the aforementioned mechanism (Scheme 24). 

Insertion of the alkyne into the Pd-H bond is the first step in the proposed catalytic cycle (Scheme 8), followed by insertion of the alkene and /3-hydride elimination to yield either the 1,4-diene (Alder-ene) or 1,3-diene product. The results of a deuterium-labeling experiment performed by Trost et al.46 support this mechanism. 1H NMR studies revealed 13% deuterium incorporation in the place of Ha, presumably due to exchange of the acetylenic proton, and 32% deuterium incorporation in the place of Hb (Scheme 9). An alternative Pd(n)-Pd(iv) mechanism involving palladocycle 47 (Scheme 10) has been suggested for Alder-ene processes not involving a hydridopalladium species.47 While the palladium acetate and hydridopalladium acetate systems both lead to comparable products, support for the existence of a unique mechanism for each catalyst is derived from the observation that in some cases the efficacies of the catalysts differ dramatically.46... 

Asymmetric cyclization was also successful in the rhodium-catalyzed hydrosilylation of silyl ethers 81 derived from allyl alcohols. High enantioselectivity (up to 97% ee) was observed in the reaction of silyl ethers containing a bulky group on the silicon atom in the presence of a rhodium-BINAP catalyst (Scheme 23).78 The cyclization products 82 were readily converted into 1,3-diols 83 by the oxidation. During studies on this asymmetric hydrosilylation, silylrhodation pathway in the catalytic cycle was demonstrated by a deuterium-labeling experiment.79... 

Diene)Cr(CO)4 complexes serve as catalysts for the addition of hydrogen to 1,3-dienes to give 2Z-alkenes (equation 21)78a. Alternatively, Cr(CO)3(MeCN)3 may also be used as a catalyst for this reduction156. Use of deuterium instead of hydrogen affords the 1,4-dideuterio-2Z-alkene. The rate of reduction for uncomplexed acyclic dienes decreases in the order E, E— > E, Z— > Z, Z-dienes. This order parallels the ease of formation of the corresponding (diene)Cr(CO)4 complexes. These results implicate the formation of a 16 valence electron [VE] (diene)Cr(CO)3 intermediate as part of the catalytic cycle. 

Kinetic analyses and deuterium-labeling experiments have demonstrated that, remarkably, the reductive elimination of TEA and the formation of intermediate C is the rate-determining step in the (de)hydrogenation cycle. Accordingly, hydrogenation of the acceptor appears to be slower than dehydrogenation of the alkane substrate. This contrasts with the fact that catalytic olefin hydrogenation is well-established in transition-metal-mediated chemistry [10]. 

Godfrey JD (1962) The deuterium content of hydrous minerals from the East Central Sierra Nevada and Yosemite National Park. Geochim Cosmochim Acta 26 1215-1245 Goericke R, Fry B (1994) Variations of marine plankton 6 C with latitude, temperature and dissolved CO2 in the world ocean. Global Geochem Cycles 8 85-90 Goldhaber MB, Kaplan IR (1974) The sedimentary sulfur cycle. In Goldberg EB (ed) The sea, vol. 4. WUey, New York... 

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