Reversed phase liquid chromatographic retention behavior of polystyrene homopolymers

Summary Isocratic and gradient elution high performance liquid chromatographic measurements of the retention behavior of polystyrene homopolymers with molecular weights ranging from 2 kD to 390 kD were performed using mixed methylene chloride-methanol mobile phases of varying composition and a C-18 chemically bonded stationary phase supported on either 100 Å or 300 Å mean pore diameter silica. Isocratic measurements of the capacity factor, k, for different molecular weight homopolymers as a function of the volume fraction of methylene chloride, , permit determination of the critical composition, _c, which renders k=1 and the local slope, S=–lnk/_c of the lnk- isotherm at =_c, and also the dependence of _c and S on the degree of polymerization, M. Linear gradient elution measurements of _c and S were also performed and compared to the corresponding isocratic measurements. The general retention behavior and the dependence of _c and S on M compare favorably to the predictions of the theory of homopolymer retention and fractionation developed by Boehm, Martire, Armstrong, and Bui (BMAB). Comparison is also made between the present work and the experimental observations of other workers on related chromatographic systems involving hompolymer retention.     

Photoinduced Acylotropic Rearrangement of Crown-Containing 2-[N-Acetyl-N-(4-aminobenzo-15-crown-5)methylene]-2,3-dihydrobenzo[b]thiophen-3-one

2-N-(4-Aminobenzo-15-crown-5)methylene-2,3-dihydrobenzo[b]thiophen-3-one and its acetyl derivative were synthesized. Their treatment with lithium, sodium, or potassium iodide in acetonitrile solution leads to formation of the corresponding crown-metal complexes. Irradiation of the acylated compounds at 436 nm induces NO migration of the acetyl group to give O-acyl isomer which is characterized by a considerable blue shift of the long-wave absorption band ( = 70 nm). The quantum yields of the photo- induced acylotropic reaction for the free ligand and its complexes with alkali metal cations change in the series: _L > K⁺ > Na⁺ > Li⁺.     

Effect of adsorption,ionic strength and pH on the potential of the diffuse electric layer

Summary The potential, ₀, of the diffuse electric layer was determined by the equilibrium liquid film method (1). The method is based on calculating the ₀-potential by means of the DLVO theory of the electrostatic disjoining pressure at known concentration,C _el of the solution forming the microscopic liquid film and at known thickness,h _r of the film. The thickness of the film was measured by the microinterferometric method (7, 8). The calculation of the potential was carrried out by means of the more general eqs. [2] and [3], valid for a wide range ofC _el and ₀. The dependence of the potential of the diffuse electric layer on the concentration of surface activ agent (surfactant) stabilizing the liquid film was found. It was established that ionogenic SAS show a higher potential than the non-ionogenic ones. The potential of SAS-free solutions was determined — ca. 30 mV.The dependence of the potential of the diffuse electric layer on concentration and kind of electrolyte was also found.A strong effect of p_H on ₀-potential was observed and the isoelectric points were found. An analysis is made of the ₀-potential values obtained by the method of equilibrium liquid film with respect to their similarity to those on the boundary of the bulk liquid.

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Zusammenfassung Das Potential ₀ der diffusen elektrischen Doppelschicht wurde nach der Methode des flüssigen Gleichgewichtfilms in Schäumen (1) bestimmt. Die Dicke der Filme wurde interferometrisch (7, 8) gemessen.Es wurde die Abhängigkeit des ₀-Potentials von der KonzentrationC _S des grenzflächenaktiven Stoffes (Schaumfilmstabilisator) untersucht. Die ₀ (C _S)-Kurve hat am Anfang ein kleines Plateau, danach folgt ein Anstieg bis zu einem Sättigungswert. Dieser Sättigungswert ₀ ist für die verschiedenen Tenside unterschiedlich. Nichtionogene Tenside besitzen ein geringes ₀, die ionogenen Tenside geben höhere Werte.Es wurde weiterhin der Einfluß der Elektrolytkonzentration auf das ₀-Potential untersucht. Eine spezifische Wirkung der Elektrolyte wurde nicht gefunden. Dagegen wurde ein starker Einfluß des p_H-Wertes auf das ₀-Potential festgestellt. Bei einem bestimmten p_H-Wert fällt ₀ bis zu Null ab, d. h. ein isoelektrischer Punkt tritt auf. Die an Schaumfilmen gemessenen ₀-Werte wurden mit den ₀-Werten an der Oberfläche derselben Lösung verglichen.

     

Molecular Structure and Internal Rotation Potential of Dimethylphenylphosphine, According to Gas-Phase Electron Diffraction Data and Quantum-Chemical Calculations

Geometric parameters of dimethylphenylphosphine molecule were determined by gas-phase electron diffraction using a dynamic model in which the rotation of the PMe₂ group is treated as large-amplitude motion. Refinement of the structural parameters and parameters of the potential function was performed taking into account the geometry relaxation on the basis of HF/6-311++G** calculations. The internal rotation potential has a single minimum at 0° ( is the angle between the bisector of the MePMe angle and the phenyl ring plane) and may be described by the function of the form V() = 0.5V ₂(1 - cos2), where V ₂ = 0.38±0.36 kcal mol^{- 1}. The data obtained are compared with those for related molecules. Steric effects affect the geometry of the phenylphosphine molecule more significantly than does p- interaction.     

Apparent molar heat capacities and volumes of electrolytes and ions int-butanol-water mixtures

The apparent molar volumes V and heat capacities C _p, of NaCl, LiCl, NaF, KI, NaBPh₄ and Ph₄PCl have been determined in solutions of H₂O containing up to 40 mass% t-butyl alcohol (TBA) by flow densitometry and flow microcalorimetry. Combination of these results with literature data allows calculation of V and C _p, for 16 ions in these mixtures using the assumption that _tX(Ph₄P⁺) = _tX(BPh ₄ ^– ) where X=V or C _p and _tX is the change in X for a species on transfer from H₂O to TBA-H₂O mixtures. These are the first reported single ion values for C _p, in a mixed solvent. While whole electrolyte volumes and heat capacities show relatively smooth changes with solvent composition, _tX(ion) exhibit two well-developed extrema at around 10 and 25 mass% TBA. The shape of the _tX(ion) curves shows considerable uniformity among the alkali metal cations and the halide ions but the extrema become more pronounced with increasing size among the tetraalkylammonium ions. These extrema are analogous to those observed in aqueous organic mixtures of surfactants and are probably indicative of microphase transitions in these strongly interacting solvent mixtures.     

Apparent molar heat capacity and relative enthalpy of aqueous NaOH between 323 and 523K

The apparent molar heat capacity, C _p,, of aqueous NaOH has been measured at temperatures between 50 and 250°C and molalities from 0.05 to 1.5 mol-kg^–1. Enthalpies of dilution L were also determined at 99°C and apparent molar relative enthalpies L were calculated up to 1.9 m. Measurements were performed by means of a flow calorimetric apparatus constructed in our laboratory and standardized for C _p, and L with aqueous Na₂SO₄ and with the formation of water from its ions, respectively. Characteristics and performance of this calorimeter are described in detail. Pitzer's semiempirical equations are used for the representation of the results and a general fitting of C _p, data is reported using also recent literature values measured between 4 and 55°C. The fitted parameters are finally utilized, through an integration procedure, to derive a general equation to calculate L at any temperature between 4 and 250°C.     

Corrected values of osmotic and activity coefficients of aqueous NaTcO4 and HTcO4 at 25°C

Published isopiestic molalities for aqueous NaTcO₄ and HTcO₄ solutions at 25°C have been reexamined. Our calculations indicate that published smoothed values of the osmotic coefficients and mean molal activity coefficients _± of NaTcO₄ are substantially in error, by up to 100% for and 300% for _±. Published smoothed values of and _± for HTcO₄ solutions are in somewhat better agreement with the input data, but the reported are sometimes higher and sometimes lower than experimental values, by up to 6%. Consequently, we have reanalyzed isopiestic data for these two systems, and find that errors arose in representing the data mathematically. We report parameters for an extended form of the specific ion interaction equations and for Pitzer's equations, and corrected values of and _± are tabulated for aqueous NaTcO₄ and HTcO₄.     

Volumes and compressibilities of pentanol in aqueous alkyltrimethylammonium bromide solutions at different temperatures

Speed of sound and density properties of ternary water-tetradecyltrimethylammonium bromide-pentanol system at 15, 25 and 35°C and of water-hexadecyltrimethylammonium bromide-pentanol system at 25, 35 and 45°C were measured at fixed alcohol concentration as a function of surfactant concentration. The apparent molar volumes V_,R and isentropic compressibilities K _,R ^S of pentanol in micellar solutions as a function of the surfactant concentration show irregular behavior which depends on the alkyl chain length of the surfactant and tends to disappear with increasing temperature. These anomalies are ascribed to micellar transitions. For both surfactants at high concentrations, V_,R decrease and the magnitude of the change seems to depend on the type of densimeter used. This observation is tentatively explained in terms of a correlation between the micellar structure and features of the densimeter. From this work and literature data, the apparent molar isothermal compressibilities K _,R ^T of the alcohol in micellar solutions were calculated at 25°C. V_,R , K _,R ^S and K _,R ^T are interpreted in terms of the distribution constant of the alcohol between the aqueous and the micellar phases and of the apparent molar property of the alcohol in the micellar and the aqueous phases. For a given surfactant increasing the temperature increases V_,R and K _,R ^S in the micellar phase while the distribution constant is weakly dependent. At a given temperature, an increase in the alkyl chain length of the surfactant increases the apparent molar volume and slightly changes the apparent molar compressibility of the alcohol in the micellar phase.     

Conformation analysis of diphosphine

Extended basis set ab initio computations are performed on HF, PNO-CI and CEPA level to determine the structure of P₂H₄ and the potential curve E() for rotation around the P-P axis. The structure parameters are optimized for dihedral angles of 0 ° (cis), 50 °, 80 ° (gauche or semi-eclipsed), 130 °, and 180 ° (trans). It turns out that P₂H₄ has a gauche equilibrium structure, a local minimum for trans which is 2.5 kJ/mol above gauche, a rather large cis barrier of 20 kJ/mol and a gauche trans barrier of 3.5 kJ/mol. The potential E() is extremely flat in the region 50 ° < < 310 °, where E() varies by less than 5 kJ/mol. Electron correlation tends to reduce the barriers but has no drastic effect on E().     

Role of rhenium in bimetallic reforming catalysts

The influence of rhenium on the behavior of a Pt/Al₂O₃ catalyst was studied in n-hexane reaction by means of dilution of the catalyst bed with Re/Al₂O₃. Parallel to activity, selectivity and ageing data obtained in slug pulse and continuous flow reactors, the amount of the surface carbon formed during the reaction was determined and its reactivity in hydrogen atmosphere was also studied. It is suggested that in the presence of rhenium due to the increase in the amount of hydrogen available for hydrogen consuming reactions including hydrodepolymerization of the carbonaceous surface overlayer, the routes of ageing processes are changed, resulting in lower carbon content on the catalyst and less pronounced catalyst deactivation.

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Pt/Al₂O₃ -, Re/Al₂O₃. , , , , . , , , , , , , .

     

Host-guest interactions. I. Volumes of the sucrose +β-cyclodextrin+water ternary systems at 25°C

Densities of sucrose+-cyclodextrin+water (SUC+CYC+W) ternary systems were measured at 25°C and mean apparent molar volumes of SUC+CYC (V_,m) in water in each composite mixture have been evaluated. Apparent molar volumes of SUC (V_,suc) have also been computed from the volume data of each ternary composite mixture. Variation of V_,m and V_,suc with respect to the total molality m_t of SUC+CYC and to that of SUC m_suc show the formation of inclusion complexes between SUC and CYC molecules with 12 and 11 stochiometries in composite mixtures of mole ratios 0.588 and 1.12 respectively.     

Thermodynamic properties of water-β-cyclodextrin-dodecylsurfactant ternary systems

Densities, heat capacities and conductivities of water-surfactant--cyclodextrin (-CD) ternary systems were determined at 25°C. The surfactants studied were sodium dodecylsulfate (NaDS) and dodecyltrimethylammonium bromide (DTAB). From conductivity data, apparent critical micelle concentrations (cmc^*) and degree of ionization of micelles were obtained at a fixed -CD concentration (m_CD). From the cmc^* value and that in water (cmc) the stoichiometry of the surfactant--CD complex was calculated. At a given m_CD, the apparent molar volume V_,CD and heat capacity C_,CD of -CD in the two surfactants were calculated as functions of surfactant concentration m_S. For both NaDS and DTAB, V_,CD increases with m_S up to about the cmc beyond which it decreases to a constant value at high m_S, the opposite is observed for C_,CD. With NaDS, a jump in the C_,CD vs, m_S trend was detected and ascribed to a structural NaDS micellar transition. The apparent molar volume V_S and heat capacity C_S of NaDS and DTAB in the water--CD mixture 0.017 m were also obtained. From these properties and those in pure water, the volume V_S and heat capacity C_S of transfer of the surfactant from water to water+-CD mixture as functions of m_S were calculated. For both surfactants, the V_S vs. m_S trends increase to the cmc and then decrease in a monotonic manner, whereas C_S increases regularly with m_S in the pre-micellar region and is essentially constant in the post-micellar region. The V_S vs. m_S trends were qualitatively explained in terms of dispersed, complexed and micellized surfactant contributions.     

ESR spin-trapping study of the radicals produced in the photochemical reaction of Cr, Mo, W hexacarbonyls with carbon tetrachloride

The effect of UV and visible light on the catalytic reaction has been investigated in the systems containing Cr, Mo and W hexacarbonyls, nitrone and nitroso type of spin-traps in carbon tetrachloride. Under these reaction conditions not only radical CCl ₃ ^. but also dialkyl-, diphenyl- and acyl-phenylnitroxides have been trapped.

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, Cr, Mo W, . CCl ₃ ^. , , .

     

Torsional potential and strain energy distribution in an extended chain under stress

Torsional potentialV() for the single bond transformation in an extended hexadecane, subjected to elongation, has been determined by molecular mechanics calculations. The stored elastic energy significantly modifies the potentialV(), the conformational energies and the barriers of transition. Apart from the soft torsional coordinate, elastic energy is also dissipated considerably by bond stretching and angle bending. Maximal variations of the valence coordinates occur in the vicinity of the torsional defect and dampen along the chain. At higher elongation, the gauche minimum on the potentialV() disappears and the calculations predict the abrupt gauche to trans transition. The energetics of torsion of a deformed chain are compared with the experimental data on the hydrodynamic extension of polymers in dilute solution by elongational flow. The calculations also provide details of a single bond transformation mechanism at conformational interconversions in a long chain, proposed by Helfand.     

Quantitative study of the photostability of spiropyrans

A quantitative method of determining the quantum yield of irreversible photodegradation (_C) of photochromic spiropyrans (SPP) was proposed for the first time. The _C of a large number of SPP of different types (with a different heterocyclic part) were determined. It was shown that for all of the SPP studied (except for dithiolane SPP), _C is almost independent of the heterocyclic part and is determined by the structure of the chromene part of the SPP. In addition to photodegradation, photooxidation takes place as a result of the reaction of the triplet state of the closed form of the SPP with O₂ for nitrosubstituted SPP of all types. The value of _C is determined by the efficiency of the reaction of isomer X of the colored form with the solvent or impurities, i.e., the lifetime of this isomer.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 290–298, February, 1990.     

The Molecular Structure and the Puckering Potential Function of Octamethylcyclotetrasilane,Si4Me8, Determined by Gas Electron Diffraction and Relaxation Constraints from Ab Initio Calculations

Gas electron diffraction data are applied to determine the geometrical parameters of the octamethylcyclotetrasilane molecule using a dynamic model in which the ring puckering is treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined, taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree–Fock level of theory using a 6-311G** basis set. The potential function has been described as V() = V ₀[(/ _e )² – 1]² with V ₀ = 1.0 ± 0.5 kcal/mol and _e = 28.3 ± 1.9°, where is the puckering angle of the ring. The geometric parameters at the minimum of V() (r _a in Å, in degrees and errors given as three times the standard deviations including a scale error) are as follows: r(Si—C)_av = 1.894(3), r(Si—Si) = 2.363(3), r(C—H) = 1.104(3), CSiC = 109.5(6), SiSiSi = 88.2(2), SiCH = 111.7(6), C = 4.1, where the tilt C was estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds.     

Kinetics of photoreduction of 9,10-phenanthrenequinone in the presence of amines and polymethylbenzenes

A study of the kinetics of photoreduction of 9,10-phenanthrenequinone in the presence of hydrogen donors (para-substituted N,N-dimethylanilines and polymethylbenzenes) showed that plots of the quantum yield of photoreduction (_H) and apparent reaction rate constant (k _H) vs. oxidation potential of hydrogen donors are extreme. In the presence of amines, k _H and _H increase, as a whole, whereas they decrease in the presence of polymethylbenzenes. In coordinates _H-G _e (G _e is the change in the free energy of electron transfer) for pairs quinone-H donor, _H increases with G _e approaching to zero. For the amine series, this effect is mainly in the exothermic region of G _e (G _e < 0). For the series of polymethylbenzenes, this increase is observed in the endothermic region (G _e > 0).__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2381–2385, November, 2004.     

Marked differences between acetonitrile/water and methanol/water mobile phase systems on the thermodynamic behavior of benzodiazepines in reversed phase liquid chromatography

Summary Two different methods were used to determine the separation factor at different temperatures and the Gibbs-Helmholtz parameters ((H), (S)) of two adjacent benzodiazepines on a chromatogram were obtained from plots of ln versus 1/T. We first studied each factor (fraction of water in the ACN/water mixture and column temperatureT), which controls the retention mechanism, and then we examined the simultaneous variation of all these factors. The changes in (H) and (S) in relation to a volume fraction of water in an ACN/water mixture were examined. In the ACN/water system, (H) was fairly constant in the acetonitrile region of 0.52 and appears to be a roughly linear function of for 0.52. In this system (S) is approximately a parabolic function of with an optimum at 0.52. The retention mechanism of ten benzodiazepines was found to be significantly different in the methanol/water and ACN/water mixtures. The separation optimization of these ten benzodiazepines was then considered. A fraction of water of 0.43 in the ACN/water mixture and a column temperature of 44°C gave the most efficient separation conditions in the ACN/water mixture.     

Rearrangement ofα -trifluoromethyl-β-dialkylamino-β-fluoroacrylic acid N.N-dialkylamides

Conclusions When heated in the presence of a catalytic amount of BF₃ etherate,-trifluoromethyl--diethylamino--fluoroacrylic acid N, N-dimethylamide is reversibly isomerized to-trifluoromethyl--dimethylamino--fluoroacrylic acid N, N-diethylamide. The methyl esters of-trifluoromethyl--diethylammo--fluoroacrylic acid and-trifluoromethyl--phenoxy--fluoroacrylic acid N,N-dimethylamide are not isomerized under the same conditions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No.1, pp.137–141, January, 1976.     

Steric effect of terminal substituents in azomethins and thermal stability of the nematic phase

The paper studies the steric eflect of terminal substituents on the temperature T_c of the phase transition nematic liquid crystal-isotropic liquid in mesogenic azomethins with one or two noninteracting or strongly interacting conformation degrees of freedom. The linear dependence T_c(Q) has been confirmed, where Q=(cos² _N) is the conformation paramefer of the molecular ensemble; _N is the angle between the planes of the bridging group CH=N and the aniline ring in the benzylideneaniline fragment. Direct and indirect steric eflects of lateral substituents are shown to take place, which explain an anomalous variation of T_c in some compounds upon substitutions in them.L. V. Kirensky Institute of Physics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 89–97, July–August, 1993.Translated by L. Smolina