Isocratic separation of ginsenosides by high-performance liquid chromatography on a diol column at subambient temperatures

An improved high-performance liquid chromatographic method for separation of a number of ginsenosides has been developed. The influence of temperature (from 0 to 25°C) on the retention and separation of the ginsenosides was studied by applying a binary mobile phase (acetonitrile/water, 82:18 v/v) and a diol column (LiChrospher 100 Diol). The column temperature is one of the more important parameters for the retention and separation of the components investigated. Selected thermodynamic parameters, including changes of enthalpy (ΔH°) and entropy (ΔS°), were estimated from linear van’t Hoff plots, and possible retention mechanisms were discussed. Moreover, the best separation conditions were selected based on optimization criteria including maximum retention time (t _{R max}), minimum resolution (R _{s min}), and relative resolution product (r). Temperature regions close to 14°C offered the highest selectivity and almost equal distribution of the ginsenosides peaks across the chromatogram. Under such isocratic conditions, excellent separation of chromatographic standards and selected ginseng samples was achieved in less than 16 min.     

Solubility and Solution Thermodynamic Properties of 4-(4-Hydroxyphenyl)-2-butanone (Raspberry Ketone) in Different Pure Solvents

The solubility of 4-(4-hydroxyphenyl)-2-butanone (raspberry ketone) in six pure solvents was experimentally determined at temperatures ranging from 283.15 to 313.15 K under the pressure 0.10 MPa by employing a gravimetrical method. The experimental results indicate that the solubility of raspberry ketone in all studied solvents is temperature dependent, a rise in temperature brings about an increase in solubility. The experimental solubility data of raspberry ketone in six pure solvents (acetone, ethanol, ethyl acetate, n-propyl alcohol, n-butyl alcohol, and distilled water) was correlated by using several commonly used thermodynamic models, including the Apelblat, van’t Hoff and λh equations. The results of the error analysis indicate that the van’t Hoff equation was able to give more accurate and reliable predictions of solubility with root-mean-square deviation less than 0.56%. Furthermore, the changes of dissolution enthalpies (Δ_diss H°), dissolution entropies (Δ_diss S°) and dissolution Gibbs energies (Δ_diss G°) of raspberry ketone in the solvents studied were estimated by the van’t Hoff equation. The positive value of Δ_diss H°, Δ_diss S°, and Δ_diss G° indicated that these dissolution processes of raspberry ketone in the solvents studied were all endothermic and enthalpy-driven.     

Compatibility and decomposition kinetics studies of prednicarbate alone and associated with glyceryl stearate

In this work, TG/DTG and DSC techniques were used to the determination of thermal behavior of prednicarbate alone and associated with glyceryl stearate excipient (1:1 physical mixture). TG/DTG curves obtained for the binary mixture showed a reduction of approximately 37 °C to the thermal stability of drug ( T_{\textdm/\textdt = 0 \textDTG}^\textMax T_{{{\text{d}}m/{\text{d}}t = 0\,{\text{DTG}}}}^{\text{Max}} ). The disappearance of stretching band at 1280 cm⁻¹ (ν_as C–O, carbonate group) and the presence of streching band with less intensity at 1750 cm⁻¹ (ν_s C–O, ester group) in IR spectrum obtained to the binary mixture submitted at 220 °C, when compared with IR spectrum of drug submitted to the same temperature, confirmed the chemical interaction between these substances due to heating. Kinetics parameters of decomposition reaction of prednicarbate were obtained using isothermal (Arrhenius equation) and non-isothermal (Ozawa) methods. The reduction of approximately 45% of activation energy value (E _a) to the first step of thermal decomposition reaction of drug in the 1:1 (mass/mass) physical mixture was observed by both kinetics methods.     

Laser-induced kinetics: Arrhenius parameters for t-C4H9 + XI → i-C4H9X + I (X = H,D) and the Heat of Formation of the t-butyl radical

The metathesis reaction of DI with t-C₄H₉ generated by 351-nm photolysis of 2,2′-azoisopropane was studied in a low-pressure reactor (VLP? Knudsen cell) in the temperature range of 302–411 K. The data obeyed the following Arrhenius relation when combined with recent data by Rossi and Golden gathered by the same technique (t-C₄H₉ by thermal decomposition of 2,2′-azoisobutane): log k₂^D(M^?1s^?1) = 9.60 – 1.90/θ, where θ = 2.303RT kcal/mol for 302 K < T > 722 K. The metathesis reaction of HI with t-C₄H₉ was studied at 301 K and resulted in k₂^H(M^?1·s^?1) = (3.20 ± 0.62) × 10⁸. An analogous Arrhenius relation was calculated for the protiated system if the small primary isotope effect k₂^H/k₂^D was assumed to be √2 at 700 K. It was of the following form: log k₂^H(M^?1·s^?1) = 9.73 – 1.68/θ. Preliminary data of Bracey and Walsh indicate that earlier Arrhenius parameters determined for the reverse reaction are somewhat in error. Their value of log k₁(M^?1·s^?1) = 11.5 – 23.8/θ yields 7delta;H_f,300⁰(t-butyl) = 9.2 kcal/mol and S₃₀₀⁰(t-butyl) = 74.2 cal/mol7°K when taken in conjuction with this study.     

Direct observation and stability of active species in cationic polymerization: A reexamination of the polymerization of styrene initiated by triflic acid

Polymerization of styrene initiated by triflic acid in CH₂Cl₂ solution was reexamined, using a new stopped-flow device working in high purity conditions over a wide temperature range. Monomer and styryl cation were followed simultaneously through their respective absorbances at 290 and 340 nm. Initiation is very rapid, and cations concentration reaches a plateau the duration of which is depending on temperature. In our conditions (I₀ = 0.5 − 9.10⁻³M, M₀/I₀ = 1 to 20), cations concentration is so low at room temperature that it is almost unmeasurable. At −65°C, it is 100 times higher, remains constant for several seconds and complete termination takes place within a minute or more. Such a profile of cation evolution agrees with an equilibrium situation between initiation and a much more temperature-dependent backward deprotonation. Apparent initial rate of initiation is first order with respect to monomer, but the order with respect to initiator was found very high and variable with temperature (from 4.5 at −65°C to 3 at −20°C). This supports the presence, even if they are in low concentration, of acid high agregates, the reactivity of which increases with size. A first order monomer consumption is observed during the plateau, which leads to k_p values ranging from 10³ at −65°C to 9.10⁴ M⁻¹.s⁻¹ at −10°C (Ep^# = 43 kJ.mol⁻¹). The disappearance of cations, which follows the plateau, slows down and becomes unimolecular when monomer consumption is complete, and k_t values range from 6.10⁻²s⁻¹ at −65°C to 1.2s⁻¹ at −23°C (E_t^# = 33 kJ.mol⁻¹).     

Polymerization of t-butyl thiirane. III. Temperature effect on the stereoelective polymerization

The effect of temperature on stereoelective polymerization of t-butyl thiirane was studied. Although the overall kinetic scheme was not modified by a change of temperature, a strong effect on the stereoelectivity ratio was observed. Very high optical yields were obtained for polymerizations run below 0°C. On the other hand, at temperatures higher than 120°C, the stereoelection is inversed; i.e., the opposite enantiomer is chosen. The stereoselectivity ratio varies with temperature according to an Arrhenius relationship.     

Unusual Temperature-Retention Dependences Observed for Several Benzodiazepines in RP-HPLC Using Different Mobile Phase Compositions

Non-linear van’t Hoff plots were observed for several benzodiazepines over the usual temperature interval used in RP-LC (10–60 °C), when acetonitrile was added in the mobile phase. Such deviation from linearity was not observed when methanol or isopropanol was added to the mobile phase. Polynomial regressions were applied to fit the experimental data when acetonitrile was used as organic additive in mobile phase. The second-order polynoms may allow finding out the maximum retention depending on temperature, which can be within or out of the normal temperature range used in RP mechanism. A thermodynamic model deriving from the partition of two or more tautomers of the same analyte was proposed to explain this deviation from linearity of van’t Hoff plots.

     

Effect of titanium based complex catalyst and carbon nanotubes on hydrogen storage performance of magnesium

Mg (MgH₂)-based composites, using carbon nanotubes (CNTs) and pre-synthesized titanium based complex (TCat) as the catalysts, were prepared by high energy ball milling technique. The use of both catalysts demonstrated markedly improved the hydrogen storage performance, e.g. a significant increase of hydrogen release rate and decrease of desorption temperature. The synthesized composites can absorb almost 6 wt% of hydrogen within 3 min at 200 °C and desorb 6 wt% hydrogen in 10 min at 310 °C. The influence of CNTs and TCat on desorption temperature was also investigated by using temperature programmed desorption (TPD). The TPD results reveal that the peak desorption temperature and the onset temperature can be lowered by 109 °C and 155 °C, respectively, compared to the non-catalyzed MgH₂. The reaction enthalpy and entropy of hydrogen desorption for the synthesized MgH₂-based composites are calculated by the van’t Hoff analysis to be 73.1 kJ/mol H₂ and 130.2 J/mol H₂ K, respectively.     

3‐[4′‐(Diethylboryl)phenyl]pyridine: Exclusive Crystallization of the Cyclic Tetramer

The crystallization of 3‐[4′‐(diethylboryl)phenyl]pyridine ( 1 ), which formed a mixture of oligomers in solution with the cyclic trimer as a major component, in acetone at 0 °C afforded a cyclic tetramer that co‐crystallized with solvent molecules. Similarly, solutions of compound 1 in toluene at 10 °C and in benzene at 8 °C furnished the cyclic tetramer with the incorporation of toluene and benzene molecules, respectively, thus suggesting that the cyclic tetramer was the minor component. ¹³C CP/MAS NMR spectroscopy of precipitates of compound 1 suggested that precipitation from acetone and toluene each afforded mixtures of the cyclic trimer and the cyclic tetramer, whereas precipitation from benzene exclusively furnished the cyclic tetramer. Therefore, it appeared that crystallization readily shifted the equilibrium towards the cyclic tetramer in benzene. The thermodynamic parameters for the equilibrium between these two oligomers in [D₆]benzene, as determined from a van′t Hoff plot, were ΔH°=?8.8 kcal mol^?1 and ΔS°=?23.7 cal mol^?1 K^?1, which were coincident with previously reported calculations and observations.     

Melt spinning of syndiotactic 1, 2-polybutadiene for preparation of carbon fibers

The thermal crosslinking and loss of vinyl unsaturation of syndiotactic 1, 2-polybutadiene(_s-PB) at 180–230°C were prevented by stabilizers with 3, 5-di-t-butyl-4-hydroxybenzyloxy group. The _s-PB samples (mp 140–198°C and MW 20,000–70,000) that contained the stabilizers could be melt-spun at a temperature below 220°C into 1-denier fibers to be used for the preparation of carbon fibers. The _s-PB fibers with higher mp and/or higher MW could be obtained by the addition of a high boiling solvent such as tetralin. The relationship between the molecular structures of _s-PB and the properties of resulting _s-PB fibers, including the degree of molecular orientation measured by birefringence and x-ray diffraction, is presented. Spun fibers showed small swellings here and there along the fiber axis, which would have resulted from the inhomogeneity of the melt of _s-PB spun at a temperature slightly above the melting point. The gelation was unlikely to occur.     

The ionic polymerizations of methyl glyoxylate

Methyl glyoxylate has been polymerized in CH₂Cl₂ solution either cationically or anionically to give the corresponding polyacetal. Thermodynamic and kinetic results of the polymerizations initiated by CF₃SO₃H, BF₃OEt₂, and NEt₃ are reported here. Monomer conversion was followed by UV and active centers concentrations were determined through phosphorus end-capping. A ceiling temperature of 26°C was observed for M_eq=l mol.H (T_c = 109°C for bulk) with ΔH°=−29.5 kJ.mo⁻¹ and ΔS°=−99 J.mo⁻¹.K⁻¹. Both initiations were found instantaneous and quantitative, and no termination was observable within the time scale of the reaction (second to hour). Cationic propagation appears to take place essentially on free ions (k_p+ ≈ 60 l.moH.s⁻¹ at −20°C) in equilibrium with a larger amount (K_d ≈ 6.7.10⁻⁶ mol.⁻¹) of much less reactive ion-pairs (k_p± ≈ 0.16 ± 0.03 1.mo⁻¹.s⁻¹). A stopped-flow device was used to follow the much more rapid anionic polymerization (k_p,app ≈ 8 ± 2.10³ 1.mo⁻¹.s⁻¹ at −20°C).     

Temperature dependent control of the solubility of gallium nitride in supercritical ammonia using mixed mineralizer

Using a mass-loss method, we investigated the solubility change of gallium nitride (GaN) in supercritical ammonia with mixed mineralizers [ammonium chloride (NH₄Cl)?+?ammonium bromide (NH₄Br) and NH₄Cl?+?ammonium iodide (NH₄I)]. The solubilities were measured over the temperature range 450–550 °C, at 100 MPa. The solubility increased with NH₄Cl mole fraction at 450 °C and 100 MPa. The temperature dependence of the solubility curve was then measured at an equal mole ratio of the two mineralizers. The slope of the solubility–temperature relationship in the mixed mineralizer was between those of the individual mineralizers. These results show that the temperature dependence of the solubility of GaN can be controlled by the mineralizer mixture ratio. The results of the van’t Hoff plot suggest that the solubility species were unchanged over the investigated temperature range. Our approach might pave the way to realizing large, high-quality GaN crystals for future gallium-nitride electronic devices, which are increasingly on demand in the information-based age.     

HPLC study of the host–guest complexation between fluorescent glutathione derivatives and β-cyclodextrin

RP-HPLC and the van’t Hoff law were used to study the association in which β-cyclodextrin forms inclusion complexes with aminothiol–phthaldialdehyde derivatives prepared from either glutathione (GSH) or γ-glutamylcysteine (γ-glucys) and either naphthalene-2,3-dicarboxaldehyde (NDA) or o-phthaldialdehyde (OPA). Elution was carried out at pH 8.5, the derivatization pH which gave the highest fluorescence signal during batch experiments. The variation of the retention factor (k) was monitored as a function of column temperature (10–35 °C) and β-cyclodextrin concentration (0–5 mM) in the mobile phase. Apparent binding constants, enthalpy and entropy were calculated from van’t Hoff plots for the complexation reaction. These data lay the groundwork for the improvement of high throughput GSH quantification methods using fluorimetry in biological and vegetal samples.     

Thermal decomposition of perfluoro-di-t-butyl peroxide

The thermal decomposition of perfluoro-di-t-butyl peroxide has been studied for the first time. The reaction was carried out in the gas phase between 5 and 600 torr in the 108-149°C temperature region. The products consisted solely of C₂F₆ and CF₃COCF₃. The decomposition was found to be first order and homogeneous. The rate constant is given by log k_decomp(sec^?1) = (16.2 ± 1.2) - (148.7 4.4)/2.3RT where R is 0.008314kJ/mol · °K. These Arrhenius parameters are consistent with those determined for the decomposition of di-t-butyl peroxide.     

Thermally induced polymerization of isobutylene in the presence of SnCl4: Kinetic study of the polymerization and NMR structural investigation of low molecular weight products

The polymerization reactivity of isobutylene/SnCl₄ mixtures in the absence of polar solvent, was investigated in a temperature interval from −78 to 60 °C. The mixture is nonreactive below −20 °C but slow polymerization proceeds from −20 to 20 °C with the initial rate r₀ of the order 10⁻⁵ mol · l⁻¹ · s⁻¹. The rate of the process increases with increasing temperature up to ∼10⁻² mol · l⁻¹ · s⁻¹ at 60 °C. Logarithmic plots of r₀ and M̄_n versus 1/T exhibit a break in the range from 20 to 35 °C. Activation energy is positive with values E = 21.7 ± 4.2 kJ/mol in the temperature interval from −20 to 35 °C and E = 159.5 ± 4.2 kJ/mol in the interval from 35 to 60 °C. The values of activation enthalpy difference of molecular weights in these temperature intervals are ΔH_Mn = −12.7 ± 4.2 kJ/mol and −38.3 ± 4.2 kJ/mol, respectively. The polymerization proceeds quantitatively, the molecular weights of products are relatively high, M̄_n = 1500–2500 at 35 °C and about 600 at 60 °C. It is assumed that initiation proceeds via [isobutylene · SnCl₄] charge transfer complex which is thermally excited and gives isobutylene radical‐cations. Oxygen inhibits the polymerization from −20 to 20 °C. Possible role of traces of water at temperatures above 20 °C is discussed. It was verified by NMR analysis that only low molecular weight polyisobutylenes are formed with high contents of exo‐ terminal unsaturated structures. In addition to standard unsaturated groups, new structures were detected in the products. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1568–1579, 2000     

Pyrolysis of 5-chloropentan-2-one and 4-chloro-1-phenylbutan-1-one. Participation of the carbonyl group

The rates of elimination of 5-chloropentan-2-one and 4-chloro-1-phenylbutan-1-one in the gas phase have been determined in a static system, seasoned with allyl bromide, and in the presence of the chain inhibitor propene. The reactions are unimolecular and follow a first-order rate law. The working temperature and pressure ranges were 339.4–401.1°C and 46–117 torr, respectively. The rate coefficients for the homogeneous reactions are given by the following Arrhenius equations: for 5-chloropentan-2-one, log k₁(s^?1) = (13.12 ± 0.88) - (207.8 ± 11.0)kJ/mol/2.303RT; and for 4-chloro-1-phenylbutan-1-one, log k₁(s^?1) = (12.28 ± 1.09) - (185.2 ± 12.0)kJ/mol/2.303RT. The carbonyl group at the γ position of the C? Cl bond of haloketones apparently participates in the rate of pyrolysis. The five-membered conformation appears to be a favorable structure for anchimeric assistance of the C?O group in the gas-phase elimination of chloroketones.     

Steric influence in the direction of elimination of gas-phase pyrolyses of several highly branched secondary acetates

The rate coefficients for the gas-phase pyrolyses of a series of structurally related secondary acetates have been measured in a static system over the temperature range of 289.1–359.5°C and the pressure range 50.0–203.0 torr. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 3-hexyl acetate, log k₁ (s^?) = (12.12 ± 0.33) ? (176.1 ± 3.9)kJ/mol/2.203RT; for 5-methyl-3-hexyl acetate, log k₁ (s^?) = (13.17 ± 0.20) ? (186.2 ± 2.3)kJ/mol/2.303RT; and for 5,5-dimethyl-3-hexyl acetate, log k₁ (s^?) = (12.70 ± 0.19) ? (177.4 ± 2.2)kJ/mol/2.303RT. The direction of elimination of these esters has shown from the invariability of olefin distributions at different temperatures and percentages of decomposition that steric hindrance is a determining factor in the eclipsed cis conformation. Moreover, a more detailed analysis indicates that the greater the alkyl–alkyl interaction, the less favored the elimination tends to be. Otherwise, an increase of alkyl–hydrogen interaction caused steric acceleration to be the determining factor.     

The pyrolysis kinetics of ethyl esters in the gas phase. The alkyl substituent effect at the acyl carbon

The gas-phase elimination of ethyl 3-methylbutanoate and ethyl 3,3-dimethylbutanoate has been studied, in a static system, over the temperature range of 360–420°C and in the pressure range of 71–286 torr. The reactions are homogeneous, unimolecular, and follow a first-order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for ethyl 3-methylbutanoate, log k₁ (s^?1) = (12.70 ± 0.36) – (202.5 ± 4.4) kJ/mol/2.303RT, and for ethyl 3,3-dimethylbutanoate, log k₁ (s^?1) = (13.04 ± 0.08) – (207.1 ± 1.0) kJ/mol/2.303RT. Alkyl substituents at the acyl carbon of ethyl esters yield very close values in rates. Consequently it is rather difficult to offer some conclusion concerning the effect of these substituents.     

Ab initio studies of structural features not easily amenable to experiment. 42. Molecular geometries and conformational analysis of methylbutanoate

Conformational energy profiles were calculated for τ₁, the C? C? C?O torsion, and τ₂, the C? C? C? C torsion, of methyl butanoate, using Pulay's ab initio gradient procedure at the 4-21G level with geometry optimization at each point. In addition, the structures of seven conformations were fully relaxed, including the energy minima (τ₁, τ₂) = (0, ?60), (0, 180), (120, 180), (120, ?60), and the maxima (0, 0), (180, 180), and (60, ?60). The calculated geometries confirm the previously formulated rule that, in saturated hydrocarbons, a C? H bond trans to a C? C bond (C? H_s) is consistently shorter than a C? H bond (C? H_a) trans to another C? H bond. Specifically, for X? C(α) (? O)? C(β)? C(γ)? C(δ) systems, the following rules can be formulated, incorporating results from previous studies of butanal, butanoic acid, and 2-pentanone: (1) C(δ)? H_s < C(δ)? H_a in all the conformers in which the δ-methyl group is remote from the ester group; whereas, in all the conformers in which nonbonded interactions are possible between the C(δ)-methyl and the ester groups, the bonding pattern is affected by a C? H ?O?C interaction. (2) In the most stable conformers, (0, 60), C(β)? H_a < C(β)? H_s, and C(γ)? H_a < C(γ)? H_s, regardless of X. (3) The average C? C bonds in the τ₂ = 180° conformers are consistently shorter than those with τ₂ = 60° (compared at τ₁ constant). In the most stable conformations (τ₁ = 0°, τ₂ = 60° or 180°), the bonding sequence is consistently C(α)? C(β) < C(β)? C(γ) < C(γ)? C(δ); whereas, when τ₁ = 120°, C(α)? C(β) < C(β)? C(γ) > C(γ)? C(δ).     

Kinetics of methoxy-NNO-Azoxymethane hydrolysis in strong acids

The kinetics of methoxy-NNO-azoxymethane (I) hydrolysis in concentrated solutions of strong acids (HBr, HCl, HClO₄, and H₂SO₄) has been investigated by a manometric method. The gas evolution rate is described by the equation corresponding to two consecutive first-order reactions, with the rate constant of the second reaction considerably exceeding the rate constant of the first reaction, i.e., k ₂ {ie17-1} k ₁. The temperature dependences of k ₁ (s⁻¹) in 47.59% HBr in the temperature range from 60 to 90°C and in 64.16% H₂SO₄ between 80 and 130°C are described by Arrhenius equations with IogA= 12.7 ± 1.5 and 13.6 ± 1.4 and E _a = 115 ± 10 and 137 ± 10 kJ/mol, respectively. The parameters of the Arrhenius equation for the rate constant k ₂ for the reaction in 64.16% H₂SO₄ between 80 and 130°C are IogA= 9.1 ± 2.5 and E _a = 91 ± 18 kJ/mol. An analysis of the UV spectra of compound I in concentrated H₂SO₄ shows that I is a weak base $ (pK_{BH^ + } \approx - 6) $ (pK_{BH^ + } \approx - 6) . The rate-determining step of the hydrolysis of I is the attack of the nucleophile on the carbon atom of the MeO group of the protonated molecule of I. The resulting methyldiazene dioxide decomposes via a complicated mechanism to evolve N₂, NO, and N₂O. The pseudo-first-order rate constant k ₁ of the reaction at 80°C depends strongly on the acid concentration and on the type of nucleophile (Br⁻, Cl⁻, or H₂O). The relationship between k ₁ and the rate constant k of the bimolecular nucleophilic substitution reaction (S_N2) is given by the linear equation log$ [k_1 /(C_H + C_{Nu} )] = m^ \ne m*X_0 + \log (k/K_{BH^ + } ) $ [k_1 /(C_H + C_{Nu} )] = m^ \ne m*X_0 + \log (k/K_{BH^ + } ) , where $ C_{H^ + } $ C_{H^ + } and C _Nu are the concentrations of H+ and nucleophile, respectively; X ₀ is the excess acidity; and m ^≠ and m* are coefficients. The Swain-Scott equation log$ (k_{Nu} /k_{H_2 O} ) = ns $ (k_{Nu} /k_{H_2 O} ) = ns , where n is the nucleophilicity factor and s is the substrate constant (s = 0.72), is applicable to the rate constants k of the S_N2 reactions of the protonated molecule of I with Br⁻, Cl⁻, and H₂O.