Cis-trans separation

Ha281 also used a standard geometry for both cis and trans forms and with a near DZ basis he obtained a value of only 13 kJ for the cis-trans separation. This result is probably because the choice of geometry was far from the optimum. Pople and co-workers282 obtained a lower energy with a 4-31G basis set which should be less than DZ quality. 

A more extensive set of geometry optimization calculations was reported by Dykstra and Schaefer.285 A contracted DZ basis gave a lower SCF energy than in previous work and the results for the cis-trans separation are compared with the earlier work in Table 9. All five structural parameters were optimized and shown to give improvements in the results. The nature of the molecular orbitals was analysed in detail. Table 9 summarizes the results for the ground state. 

Table 9 Glyoxal ground-state energies, cis-trans separation, and rotational barrier...

Reference trans Energy cis-trans Separation Barrier ... 

Table 3 fists cycloaliphatic diamines. Specific registry numbers are assigned to the optical isomers of /n t-l,2-cyclohexanediamine the cis isomer is achiral at ambient temperatures because of rapid interconversion of ring conformers. Commercial products ate most often marketed as geometric isomer mixtures, though large differences in symmetry may lead to such wide variations in physical properties that separations by classical unit operations are practicable, as in Du Font s fractional crystallisation of /n t-l,4-cyclohexanediamine (mp 72°C) from the low melting (5°C) cis—trans mixture. 

The polyamide copolymer of dodecanoic acid with methylenedi(cyclohexylamine) (MDCHA, PACM) was sold as continuous filament yam fiber under the tradename QIANA. As late as 1981, over 145,000 t was produced using high percentages, typically 80%, of trans, trans MDCHA isomer. The low melting raffinate coproduct left after t,t isomer separation by fractional crystallisation was phosgenated to produce a Hquid aUphatic diisocyanate marketed by Du Pont as Hylene W. Upon terrnination of their QIANA commitment, Du Pont sold the urethane intermediate product rights to Mobay, who now markets the 20% trans, trans—50% cis, trans—30% cis, cis diisocyanate isomer mixture as Desmodur W. In addition to its use in polyamides and as an isocyanate precursor, methylenedi (cyclohexyl amine) is used directiy as an epoxy curative. The Hquid diamine mixture identified historically as PACM-20 is marketed as AMICURE PACM by Anchor Chemical for performance epoxies. 

The isomeric mixture is a colodess, mobile Hquid with a sweet, slightly irritating odor resembling that of chloroform. It decomposes slowly on exposure to light, air, and moisture. The mixture is soluble ia most hydrocarbons and only slightly soluble ia water. The cis—trans proportions ia a cmde mixture depend on the production conditions. The isomers have distinct physical and chemical properties and can be separated by fractional distillation. 

Dicbloroetbylene [cis + trans 540-59-0] M 96.9, b 60" (cir), d 1.284, b 48" (trans), d 1.257. Shaken successively with cone H2SO4, water, aqueous NaHC03 and water. Dried with MgS04 and distn separated the cis- and trans-isomers. 

The PES found by Smedarchina et al. [1989] has two cis-form local minima, separated by four saddle-points from the main trans-form minima. The step-wise transfer (trans-cis, cis-trans) - because of endoergicity of the first stage - displays Arrhenius behavior even at T < T. . The concerted transfer of two hydrogen atoms was supposed to become prevalent at sufficiently low temperatures. However, because of too high a barrier for the concerted trans-trans transition, this... 

To a solution of m-ethyl cinnamate (44, 352 mg, 85% pure, 1.70 mmol) and 4-phenylpyridine-A-oxide (85.5 mg, 29 mol%) in 1,2-dichloromethane (4.0 mL) was added catalyst 12 (38.0 mg, 3.5 mol%). The resulting brown solution was cooled to 4°C and then combined with 4.0 mL (8.9 mmol) of pre-cooled bleach solution. The two-phase mixture was stirred for 12 h at 4°C. The reaction mixture was diluted with methyl-t-butyl ether (40 mL) and the organic phase separated, washed with water (2 x 40 mL), brine (40 mL), and then dried over Na2S04. The drying agent was removed by filtration the mother liquors concentrated under reduce pressure. The resulting residue was purified by flash chromatography (silica gel, pet ether/ether = 87 13 v/v) to afford a fraction enriched in cis-epoxide (45, cis/trans . 96 4, 215 mg) and a fraction enriched in trans-epoxide cis/trans 13 87, 54 mg). The combined yield of pure epoxides was 83%. ee of the cis-epoxide was determined to be 92% and the trans-epoxide to be 65%. 

Shakhtman and coworkers67 carried out gas-solid chromatographic separation of cis/trans pairs, including l-phenylsulphonyl-2-(phenylthio)ethene and ethyl-w-styryl sulphone they used a graphitized C-black column at 230 °C, nitrogen carrier gas and FID. 

H-NMR spectroscopy can be used to determine alkenesulfonates in mixtures [115]. Under normal conditions, 1-alkenesulfonate shows a signal separated from the other positional isomers [122]. Moreover, the utilization of a lanthanide shift reagent makes possible even the separation of the signals of isomeric alkenesulfonic acids and hydroxyalkanesulfonic acids as their methyl esters [124]. 13C-NMR spectroscopy, which is not as quantitative, simply gives the cis/trans ratio of the main positional isomer. 

Consequently, due to preferred cis-cis orientation a dimeric structure is observed for the indium complex and an unprecedented cis-trans arrangement in the thallium structure leads to a polymeric aggregate. Further N-NMR spectroscopic studies show that the aluminum and gallium complexes are stable contact ion pairs even in solution whereas the indium and thallium compounds are solvent-separated ion pairs in THE solution. 

Chandler, L.A. and Schwartz, S.J., HPLC separation of cis-trans carotene isomers in fresh and processed fruits and vegetables, J. Food ScL, 52, 669, 1987. 

Baillet, A., Corbeau, L., Rafidson, R, and Ferrier, D., Separation of isomeric compounds by reversed-phase high-performance liquid chromatography using Ag+ complexation. Application to cis-trans fatty acid methyl esters and retinoic acid photoisomers, /. Chromatogr., 634, 251, 1993. 

The direct irradiation of cis- and frans-2-butenes in solution separately and in admixture gives stereospecific dimerization along with the more efficient cis-trans isomerization<78) ... 

Tsukida, K., Saiki, K., Takii, T., and Koyama, Y. 1982. Separation and determination of cis/trans-P-carotenes by high-performance liquid chromatography../. Chromatogr. 245 359-364. 

Chen, X. (1991). An electrochemical, EPR study of carotenoids and high performance liquid chromatography separation of cis-trans isomers of canthaxantin. MS thesis, The University of Alabama, Tuscaloosa, AL. 

Mancini et al. (2004) reported the use of achiral-chiral LC-LC to separate the various forms of allethrin. A silica gel column was used to separate the allethrin into cis, trans isomers. Then, the cis or trans peak was switched onto a Chiralcel OJ column to separate the stereoisomers. The other set of isomers was analyzed in a subsequent run on the achiral-chiral system. 

Heats of formations (A//f) of the possible two trans- and four cA-fused stereoisomers of the 9-hydroxyperhydropyr-imido[2,1 / 1,4]oxazine 287 derived from 3-(2-hydroxyethyl)-l,3-propanediamine and glyoxal were calculated by the AMI program. Experimentally, 287 was obtained as 3 1 cis—trans mixture the isomers were not separated < 1996H(43)1991 >. 

GC-MS operated in electron impact (El) mode was only sporadically used for the determination of some UV filters such as 4-MBC, EHMC, and OC. Separation was achieved on a 60 m x 0.25 mm i.d. DB-5 column, with 0.25-pm film thickness. For quantification of the compounds, data acquisition was performed in selected ion monitoring (SIM) mode recording three characteristic ions per compound. GC-MS allowed the differentiation between the two isomers (cis/trans) for 4-MBC and EHMC. 

A cis/trans mixture of 1,2-dimethylgermacyclopentanes 56 and 57 was prepared as outlined in Scheme 1632. The isomeric germanes are stable and can be separated by spinning band distillation. Stereochemical assignments were made by comparison of the chemical shifts of the methyl carbons in the 13C NMR spectra with those of the analogous dimethylcyclopentanes and silacyclopentanes. 

Clomiphene citrate is used as a mixture of E (58) and Z (60) isomers to treat infertility. Photolysis of a chloroform solution with a high-pressure mercury lamp gave the expected phenanthrenes (59) and (61), which were separated and identified by GC-MS. Study of the separate isomers (58) and (60) indicated that rapid cis-trans photoisomerization preceded ring closure so that each gave a mixture of phenanthrenes (60) and (61) (Scheme 2.2) [52],... 

A similar strategy was also applicable for the synthesis of six- and eight-membered siloxanol-ring systems. Hydrolysis of z-PrRSiCl2 (R = Ph, o-Tol) with ZnO and KOH provided the six-membered siloxane rings as a mixture of two constitutional isomers cis-trans-(i-PrRSiO)3 (539, R = Ph 540, R = o-Tol) and Wzr-(z-PrRSiO)3 (541, R = Ph 542, R = o-Tol) which were separated by preparative HPLC. Subsequent reaction with HC1/A1C13 and hydrolysis of the chloro intermediates yielded the same product for both isomers, namely Wzr-[z-Pr(OH)SiO]3 543, implying that isomerization occurs under these conditions (Scheme 75).484... 

The bis-benzo-15-crown-5 ferrocene compound [7] containing two vinylic linkages was formed in a mixture of three isomeric components, the cis-cis, cis-trans and trans-trans isomers, which proved inseparable. However, the precedent of insignificant differences found between the magnitudes of the metal cation-induced anodic shifts in the ferrocenyl redox potentials of the respective separated cis and trans isomers [2a] and [2b] led us to use the same isomeric mixture of [7] throughout the subsequent FABMS and electrochemical group 1 and 2 metal cation complexation experiments,... 

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