Phase transitions in rubidium hydrogen sulfate: crystal structures at 293 and 200 K

The crystal structure of the paraelectric phase of rubidium hydrogen sulfate has been redetermined at room temperature to be monoclinic with a = 14.3503(14), b = 4.6187(4), c = 14.3933(14)?Å, β = 118.03(1)° (space group P2₁/n). Both the sulfate groups are found to be ordered, unlike in previous reports. The crystal structure of the ferroelectric phase at 200?K belongs to the noncentrosymmetric space group Pn with a = 14.2667(12), b = 4.5878(4), c = 14.2924(12)?Å, β = 118.01(1)°, with distorted sulfate groups. The change in the Rb coordination is discussed in terms of bond-valence-sum calculations. Variable-temperature powder X-ray diffraction patterns at temperatures above 393?K indicate a possible reduction in symmetry, suggesting a phase transition.     

Water-compatible molecularly imprinted polymers for selective extraction of ciprofloxacin from human urine

Water-compatible molecularly imprinted polymers (MIPs) were prepared in water-methanol systems for selective extraction and separation of ciprofloxacin from human urine samples. Molecular recognition properties, binding capability, and chromatographic applications of the MIPs were evaluated and the results revealed the obtained MIPs have high affinity for ciprofloxacin in aqueous environment and the selectivity can be easily controlled by adjusting the pH of mobile phase. After centrifugation, the urine samples were directly injected into the MIPs column and ciprofloxacin could be selectively retained on the column while other biological matrixes were quickly washed out. Good linearity was obtained from 0.1 to 100 mg L(-1) (r=0.999) with the relative standard deviations less than 3.6%. The limit of detection of the method was 0.03 microg/mL and the recoveries were more than 94.5% at three different concentrations. Moreover, by increasing the injection volume, the detection limits of the method could be improved more than 100-folds.     

Rapid chiral separation and impurity determination of levofloxacin by ligand-exchange chromatography

A sensitive, simple, and accurate method for determination of levofloxacin and its (R)-enantiomer was developed to determine the chiral impurity of levofloxacin in Cravit Tablets material by ligand-exchange high performance liquid chromatography. The effects of different kinds of ligands, concentration of ligands in mobile phase, organic modifier, pH of mobile phase, and temperature on enantioseparation were investigated and evaluated. Chiral separation was performed on a conventional C₁₈ column, where the mobile phase consisted of a methanol-water solution (containing10 mmol L⁻¹l-leucine and 5 mmol L⁻¹ copper sulfate) (88:12, v/v) and its flow-rate was set at 1.0 mL min⁻¹. The conventional C₁₈ column offers baseline separation of two enantiomers with a resolution of 2.4 in less than 20 min. Thermodynamic data (ΔΔH and ΔΔS) obtained by Van’t Hoff plots revealed the chiral separation is an enthalpy-controlled process. The standard curves showed excellent linearity over the concentration range from 0.5 to 400 mg L⁻¹ for levofloxacin and its (R)-enantiomer. The linear correlation equations are: y = 1.33 × 10⁵x + 6297 (r = 0.9991) and y = 1.34 × 10⁵x + 3565 (r = 0.9997), respectively. The relative standard deviation (RSD) of the method was below 2.3% (n = 3).     

On the Effect of Tether Composition on cis/trans Selectivity in Intramolecular Diels–Alder Reactions

Intramolecular Diels–Alder (IMDA) transition structures (TSs) and energies have been computed at the B3LYP/6‐31+G(d) and CBS‐QB3 levels of theory for a series of 1,3,8‐nonatrienes, H₂C?CH? CH?CH? CH₂? X? Z? CH?CH₂ [? X? Z? =? CH₂? CH₂? ( 1 ); ? O? C(?O)? ( 2 ); ? CH₂? C(?O)? ( 3 ); ? O? CH₂? ( 4 ); ? NH? C(?O)? ( 5 ); ? S? C(?O)? ( 6 ); ? O? C(?S)? ( 7 ); ? NH? C(?S)? ( 8 ); ? S? C(?S)? ( 9 )]. For each system studied ( 1 – 9 ), cis‐ and trans‐TS isomers, corresponding, respectively, to endo‐ and exo‐positioning of the ? C? X? Z? tether with respect to the diene, have been located and their relative energies (E_rel^TS) employed to predict the cis/trans IMDA product ratio. Although the E_rel^TS values are modest (typically <3 kJ mol^?1), they follow a clear and systematic trend. Specifically, as the electronegativity of the tether group X is reduced (X?O→NH or S), the IMDA cis stereoselectivity diminishes. The predicted stereochemical reaction preferences are explained in terms of two opposing effects operating in the cis‐TS, namely (1) unfavorable torsional (eclipsing) strain about the C4? C5 bond, that is caused by the ? C? X? C(?Y)? group’s strong tendency to maintain local planarity; and (2) attractive electrostatic and secondary orbital interactions between the endo‐(thio)carbonyl group, C?Y, and the diene. The former interaction predominates when X is weakly electronegative (X?N, S), while the latter is dominant when X is more strongly electronegative (X?O), or a methylene group (X?CH₂) which increases tether flexibility. These predictions hold up to experimental scrutiny, with synthetic IMDA reactions of 1 , 2 , 3 , and 4 (published work) and 5 , 6 , and 8 (this work) delivering ratios close to those calculated. The reactions of thiolacrylate 5 and thioamide 8 represent the first examples of IMDA reactions with tethers of these types. Our results point to strategies for designing tethers, which lead to improved cis/trans‐selectivities in IMDAs that are normally only weakly selective. Experimental verification of the validity of this claim comes in the form of fumaramide 14 , which undergoes a more trans‐selective IMDA reaction than the corresponding ester tethered precursor 13 .     

Synthesis of aryl alkynyl carboxylic acids and aryl alkynes from propiolic acid and aryl halides by site selective coupling and decarboxylation

The coupling of propiolic acid with aryl iodides afforded the aryl alkynyl carboxylic acids and aryl alkynes in generally good yields. Aryl alkynyl carboxylic acids were obtained when the reaction was performed in the presence of Pd(PPh₃)₂Cl₂ (2.5 mol %), dppb (5.0 mol %) and DBU (5 equiv) at 50 °C. For the synthesis of the terminal aryl alkynes, the reaction was conducted in the presence of Pd(PPh₃)₂Cl₂ (2.5 mol %), dppb (5.0 mol %), DBU (5.0 equiv), and Cu(acac)₂ (10 mol %) at 25 °C for 5 h, and further reacted at 60 °C for 6 h.     

Crystal structure and ionic conductivity of a new bismuth tungstate, Bi3W2O10.5

The compound Bi₃W₂O_10.5 was synthesized by the solid-state technique from Bi₂O₃ and WO₃ in stoichiometric quantities. Single crystals were grown by the melt-cooling technique and the crystal structure was solved in the tetragonalI4/m space group witha = 3.839 (1) ?,c = 16.382 (5) ?,V = 241.4 (1) ?³,Z = 4 and was refined to anR inde_x of 0.0672. The structure represents a modification of the Aurivillius phase and consists of [Bi₂O₂]²⁺ units separated by WO₈ polyhedra. a.c. impedance studies indicate oxide ion conductivity of 2.91 10⁻⁵ Scm⁻¹ at 600°C. Dedicated to Prof J Gopalakrishnan on his 62nd birthday.     

Comparison of the elution curves of pure components and their mixture and prediction of elution curves by a theoretical plate model in preparative gas-liquid chromatography

Summary Two close-boiling materials, diethyl ether (DEE) and dichloromethane (DCM), were separated to investigate the effects of the pure components and the mixture on elution in preparative gas-liquid chromatography.Nitrogen was used as the carrier gas, and the chromatographic column (1 cm I.D. and 0.75m length) was packed with Chromosorb A with different quantities of dinonyl phthalate stationary phase and particle sizes. Below ca. 7% (by wt.) of feed concentration, the experimental elution curves of pure DEE and DCM were almost the same as those of the mixture, and the theoretical plate model can be used successfully to predict the elution curves.