Purification of CIS-trimedlure isomers by high-performance liquid chromatography

Summary High-performance liquid chromatographic procedures were developed to make it possible to obtain the fourcis-trimedlure isomers (V, W, X, and Y) in pure form. Trimedlure-V and-Y were each readilt separated from the four-componentcis-trimedlure mixture through high-performance liquid chromatographic analysis on 5-μm and 3-μm silica, but trimedlure-W and-X were not adequately resolved. Chromatography of 5-μm silica of the mixtures obtained by epimerization of thetrans-trimedlure isomers, C and B₂, yielded the respective epimers, trimedlure-W and-X, in pure form.     

一种新的改性聚乙二醇毛细管气相色谱固定相

合成了一种新的聚乙二醇衍生物，将其用作毛细管气相色谱固定相，研究了其色谱性能，并成功地分离了二甲苯、氯甲苯、溴甲苯、二氯苯等二取代苯位置异构体；并对其手性分离能力进行了初步研究和讨论。     

Peculiar basis set dependence of the energetics of C2S2H2 isomers. In search of adequate and affordable basis set for routine calculations

Quantum mechanical calculations using the Hartree-Fock (HF), post-Hartree-Fock {MP2 and CCSD(T)}and gradient-corrected hybrid density functional variant, B3LYP methodology in conjugation with basis sets like 6-31G*, 6-311G** and cc-pVTZ, were employed to critically probe the right quality of basis set and the appropriate level of theory that can be applied in modeling the organo-sulfur compounds. The relative stability ordering of 1,2-dithiete and dithioglyoxal reveal the delicate basis set dependence especially on ‘S’ atom. Single point calculations at the B3LYP, MP2 and CCSD(T) levels using a series of basis set ranging from double ζ quality all the way up to the quintuple ζ quality were done to illustrate the effect of electron correlation and the basis set dependency for these compounds. Basis set requirements are much higher than affordable for medium sized molecules as very slow convergence is seen even when the calculations are carried out with basis set as high as cc-pV5Z at the B3LYP and MP2 level of theories.     

Photoinduced nitrosyl linkage isomers in complexes based on the photochromic cation [RuNO(NH3)5] with the paramagnetic anion [Cr(CN)6] and the diamagnetic anions [Co(CN)6] and [ZrF6]

Two metastable nitrosyl linkage isomers SI and SII are generated by light irradiation in the spectral range 370–500 nm in the two diamagnetic compounds [RuNO(NH₃)₅][Co(CN)₆] and [RuNO(NH₃)₅]₂[ZrF₆]₃ as well as in the paramagnetic compound [RuNO(NH₃)₅][Cr(CN)₆]. The frequencies of the ν(NO) stretching vibrations of SI and SII identify SI as the isonitrosyl Ru–O–N isomer and SII as the side-on η² isomer of NO. The population, i.e., the number of generated linkage isomers, is determined from the decrease of the area of the fundamental ν(NO) and of the higher harmonic 2 · ν(NO) of the ν(NO) stretching vibration of the ground state. Using differential scanning calorimetry (DSC) the heat release during the thermal decay of the metastable linkage isomers is determined. The activation energies, frequency factors, and the energetic position of the metastable linkage isomers are determined from the DSC and infrared spectroscopic experiments. It is found that the exchange of the counter ion significantly influences the energetic positions of the linkage isomers, while the activation energy and frequency factor are much less affected.     

硫原子团簇负离子的螺旋结构

在使用B3LYP密度泛函进行几何构型优化和振动频率计算得到的硫原子团簇负离子的结构中,分子的总能量最低的S9- 到 S13-的同分异构体呈螺旋状构型。另外也计算了螺旋状的Sn- (n = 14~20)的结构。大多数的的硫负离子是链状结构,这与相应中性硫原子团簇的环状构型完全不同。     

Nanotubes from isomeric dibenzoylmethane molecules

Organic nanotubes of various diameters were fabricated from the isomeric molecule dibenzoylmethane (DBM) by using an immersing technique with ordered porous alumina membrane as the template. The ratio of the enol isomers of DBM increased as the diameters of the nanotubes decreased. In addition, although almost no fluorescence could be detected for the DBM monomer, a striking enhancement in the fluorescence emission intensity of the nanotubes was observed as the diameters decreased. This is due to the increased ratio of the enol isomers.     

Access to lincomycin N-oxide isomers controlled by reaction conditions

Oxidation of lincomycin with H₂O₂ in alkaline media leads to N-oxides, besides the conversion of thiomethyl group into sulfoxides and sulfones. NH₄OH favors formation of the S-isomer; both R- and S-isomers of the N-oxide are formed in the presence of NaOH. Addition of acetonitrile markedly accelerates the reaction.     

C3H4: Density functional study of interconversion of isomers

The B3LYP density functional has been used to calculate the reaction paths for the interconversion of the three stable isomers of C₃H₄, namely, propyne, allene, and cyclopropene, and the results compared with experimental data. It is found that the trans nonplanar isomer of vinylmethylene plays a central role in the interconversions. Other thermochemical parameters, like the entropies of reaction and activation, have also been calculated. The rate constants of the various competing reactions have been calculated using the Rice–Ramsperger–Kassel–Marcus theory. Kinetic isotope effects for the reactions have also been investigated to gain an insight into their mechanisms. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Photochemical transformations of dinitrogen tetraoxide in a frozen glassy matrix

The presence of minima on the potential energy surface was shown by semiempirical calculations. These minima correspond to five isomers of dinitrogen tetraoxide: two symmetrical structures of the O₂N-NO₂ type, two nonsymmetrical structures of the O₂N-ONO type, and nitrosonium nitrate NO⁺NO₃ ^–. Quantum yields of photochemical reactions of dinitrogen tetraoxide in a matrix of glassy methylcyclohexane (at 77 K) were determined experimentally to be equal to l, 0.15, and 10^–5 for photoisomerization of symmetrical N₂O₄ into the nonsymmetrical isomer, reverse photoisome rization, and photodissoeiation to form the stabilized pair of NO₂ fragments, respectively. Measurements of the degree of orientation of the products of photochemical transformations performed by photolysis with polarized light show that the photoisomerization occurs via the intramolecular mechanism in a matrix cage, and the photodissoeiation is associated with considerable migration of the molecular fragments formed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 854–858, April, 1996.     

Convenient Synthesis of Indene Derivatives via Intramolecular Friedel-Crafts Cyclization of Tetraaryl Substituted 1,3-Butadienes

A convenient and efficient method for the highly regioselecfive synthesis of indene derivatives 4 and 5 from tetraaryl substituted 1,3-butadienes 3 was described. The method involves an intramolecular Friedel-Crafts cyclization and a corresponding double-bond positional shift of isomers 4 and 5 in the presence of different Lewis acids under mild conditions with higher than 90% yields.