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51.
A. M. Amorim da Costa A. M. Amado M. Becucci Carola Kryschi 《Journal of Molecular Structure》1997,416(1-3):69-73
Phase transition in crystalline pure p-terphenyl and p-terphenyl: tetracene doped crystals was studied with Raman spectroscopy, for temperatures from 295 to 10 K. In particular, the torsional Raman vibrational mode with a “hard-core frequency” of 230.8 cm−1 was investigated in its frequency and bandwidth dependence upon temperature. The results were analyzed based on an order-disorder model allowing the determination of the activation energies and orientational correlation times of the molecular diffusive process in the monoclinic (above 193 K) and triclinic (below 193 K) phases of the crystals. The activation energy is observed to decrease from the monoclinic to the triclinic phase, whereas the orientational correlation times increase, both in the undoped and the doped crystals. The doping of p-terphenyl with tetracene appears to affect the activation energy and the orientational correlation times in a different way in each phase. 相似文献
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Carola Tortora Christina Mai Francesca Cascella Dr. Michael Mauksch Prof. Dr. Andreas Seidel-Morgenstern Prof. Dr. Heike Lorenz Prof. Dr. Svetlana B. Tsogoeva 《Chemphyschem》2020,21(16):1775-1787
Viedma deracemization is based on solution phase racemization, dissolution of racemic or scalemic conglomerates and crystal growth through autocatalytic cluster formation. With rate limiting racemization, its acceleration by appropriate catalysts may result in speeding up deracemization. A conglomerate-forming chiral compound may principally racemize directly, or via reverse of its formation reaction. For a hydrazine derivative, we investigated available racemization pathways in presence of pyrrolidine or thiourea amine as base catalysts: via Mannich or aza-Michael reaction steps and their reverse, or by enolization. Racemization by enolization was computationally found to dominate, both under water-free conditions and in presence of water, involving a multitude of different pathways. Faster racemization in presence of water resulted indeed in more rapid deracemization, when the base was pyrrolidine. Under water-free conditions, the role of water as enolization catalyst is assumed by chiral hydrazine itself – in autocatalytic racemization and in which both reactant and product are catalysts. 相似文献
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Werner Complexes with ω‐Dimethylaminoalkyl Substituted Ethylenediamine Ligands: Bifunctional Hydrogen‐Bond‐Donor Catalysts for Highly Enantioselective Michael Additions
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M. Sc. Subrata K. Ghosh Dr. Carola Ganzmann Dr. Nattamai Bhuvanesh Prof. Dr. John A. Gladysz 《Angewandte Chemie (International ed. in English)》2016,55(13):4356-4360
The racemic carbonate complex [Co(en)2O2CO]+ Cl? (en=1,2‐ethylenediamine) and (S)‐[H3NCH((CH2)nNHMe2)CH2NH3]3+ 3 Cl? (n=1–4) react (water, charcoal, 100 °C) to give [Co(en)2((S)‐H2NCH((CH2)nNHMe2)CH2NH2)]4+ 4 Cl? ( 3 a – d H4+ 4 Cl?) as a mixture of Λ/Δ diastereomers that separate on chiral‐phase Sephadex columns. These are treated with NaOH/Na+ BArf? (BArf=B(3,5‐C6H3(CF3)2)4) to give lipophilic Λ‐ and Δ‐ 3 a–d 3+ 3 BArf?, which are screened as catalysts (10 mol %) for additions of dialkyl malonates to nitroalkenes. Optimal results are obtained with Λ‐ 3 c 3+ 3 BArf? (CH2Cl2, ?35 °C; 98–82 % yields and 99–93 % ee for six β‐arylnitroethenes). The monofunctional catalysts Λ‐ and Δ‐[Co(en)3]3+ 3 BArf? give enantioselectivities of <10 % ee with equal loadings of Et3N. The crystal structure of Δ‐ 3 a H4+ 4 Cl? provides a starting point for speculation regarding transition‐state assemblies. 相似文献
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A method for the enantioselective separation of all-E-astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione), an important colorant in the feed industry, was developed. Different chiral stationary phases (CSPs) such as Pirkle phases (R,R Ulmo and l-leucine), modified polysaccharides and a beta-cyclodextrin have been investigated on their separation performance of astaxanthin enantiomers. Direct resolution was only achieved employing the Chiralcel OD-RH (cellulose-tris-3,5-dimethylphenyl-carbamate) under reversed phase conditions. The chiral separation of the enantiomeric forms of astaxanthin produced in microalgae and yeasts was reported. The yeast Xanthophyllomyces sp. produces astaxanthin predominantly in the R,R configuration, whereas in the green microalgae Scenedesmus sp. astaxanthin is built primarily in the S,S form. The separation method for the identification of astaxanthin enantiomers is of great interest since astaxanthin is used as functional food additive in human nutrition. Moreover the method may be used as a food chain indicator in farmed salmon. 相似文献