A Novel Reaction of 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene (DBU) or 1,5‐Diazabicyclo[4.3.0]non‐5‐ene (DBN) with Benzyl Halides in the Presence of Water

1,8‐Diazabicyclo[5.4.0]undec‐7‐ene (DBU) reacted with benzyl halides in CH₂Cl₂/H₂O 1 : 1 (v/v) to afford a mixture of eleven‐membered cyclic amide 1 and seven‐membered cyclic amide 2 . When the reaction was carried out in EtOH/H₂O 1 : 1 (v/v), product 2 was obtained as the major product. 1,5‐Diazabicyclo[4.3.0]non‐5‐ene (DBN) gave the five‐membered cyclic amide 3 as the sole product under the same reaction conditions.     

Ultrasonic Investigation of the Gelation Process of Poly(Acrylamide) Gels

The time evolution of ultrasonic velocity and attenuation has been investigated in-situ during the gelation process of polyacrylamide (PAAm) hydrogels. Longitudinal ultrasonic pulses were transmitted through the gel samples and continuously recorded to obtain the magnitude and phase of the waves as a function of time and frequency, enabling the attenuation coefficient, α, and phase velocity, v_p, of PAAm gels to be determined. The reaction was characterized by (1) an initial rapid increase in α and v_p, and (2) a subsequent reduction after both quantities passed through a peak associated with the exothermic reaction for the PAAm gelation. The square of v_p is proportional to the longitudinal modulus of the sample and inversely proportional to the density, and the values of v_p for the aged gels were smaller than those before the gelation. The cross-linker concentration dependence was further examined in order to investigate the gelation process accompanied by phase separation.     

Energy resonance and inverse population of the product vibrational states for the reaction F + H2(v = 0) → HF(v′) + H,LCAC‐SW theoretical quantum scattering study

LCAC‐SW (linear combination of arrangement channel‐scattering wavefunction) method was used to calculate collinear state‐to‐state reaction probabilities for the reaction F + H₂(v = 0) → HF(v′) + H on the 6SEC potential energy surface. The results show that reaction probabilities P₀₂ and P₀₃ [i. e., v′ = 2,3 for reaction F + H₂ (v = 0) + HF(v′) + H] are primary, the population of product vibrational states is inverse and the reaction probabilities are oscillatory with collision energies, i.e., there is energy resonance in this reaction, which agrees with a new experiment.     

Direct approximation of the long- and short-range components of the exchange-correlation Kohn-Sham potential

An approximation scheme was developed for the Kohn-Sham exchange-correlation potential v_xcσ, making use of a partitioning of v_xcσ into a long-range screening v_scrσ and a short-range response v_resp component. For the response part, a model v^mod_respσ was used, which represents v_resp as weighted orbital density contributions, the weights being determined by the orbital energies. v^mod_respσ possesses the proper short-range behavior and the atomic-shell stepped structure characteristic for v_resp. For the screening part, two model potentials v^mod_scrσ were used, one with the accurate Slater potential; the other one with the generalized gradient approximation (GGA) for the exchange part. Both use the GGA for the Coulomb correlation contribution to v_scrσ. The scheme provides an adequate approximation to v_xcσ in the outer-valence region with both the proper asymptotics and a rather accurate estimate of the ionization potential from the highest one-electron energy and a reasonable estimate of atomic E_xc and total energies E_tot. © 1997 John Wiley & Sons, Inc.     

Kinetic Study of the Degradation of PAC-1 and Identification of a Degradation Product in Alkaline Condition

A selective and validated stability-indicating LC method was developed for the kinetic study of the degradation of PAC-1, which was carried out in aqueous solutions at 37, 60, 80 and 100 °C with pH 1.5–9.0. Separation was performed on a Kromasil C₁₈ column with acetonitrile–water–fomic acid (30:70:0.1, v/v/v) as mobile phase with a flow rate of 1.0 mL min⁻¹ at 281 nm. The degradation rate obtained indicated a first-order reaction law and the activation energy (E _a) was calculated. The results showed that temperature and pH values were significant factors affecting the degradation of PAC-1. An unknown degradation product in alkaline condition was isolated using a reverse-phase semi-preparative LC system. The structure of the degradation product is identified as 2-hydroxy-3-(2-propenyl)-[[2-hydroxy-3-(2-propenyl)phenyl]methylene]hydrazone utilizing the ¹H NMR, ¹³C NMR, IR and Q-TOF-MS techniques.

     

Relaxation of shear and normal stresses in step-shear deformation of a polystyrene solution. Comparison with the predictions of the Doi–Edwards theory

The shear stress σ, two components of birefringence, and extinction angle were measured for a concentrated polystyrene solution in step-shear deformation of magnitude of shear 0.3 ≤ γ ≤ 4.0. The stress-optical coefficient did not depend on either γ or time. The first and the second normal-stress differences v₁ and v₂ were evaluated with the use of the stress-optical law. Over a certain range of long times, σ could be factored as σ = γh(γ)G(t) and the quantity h(γ) agreed with the prediction of the Doi–Edwards theory based on the de Gennes tube model of entangled polymer chains. At short times the effect of γ on σ/γ was smaller than at long times. The relaxation spectrum became approximately independent of γ at the short-time end of the rubbery plateau region. The ratios v₁/σ and v₂/v₁ were independent of time and were in quantitative agreement with those predicted by the Doi–Edwards theory: v₁/σ was equal to γ, v₂/v₁ was negative, and |v₂/v₁| decreased with increasing γ.     

Cavity ring‐down study of BrO radicals: Kinetics of the Br + O3 reaction and rate of relaxation of vibrationally excited BrO by collisions with N2 and O2

Cavity ring‐down (CRD) techniques were used to study the kinetics of the reaction of Br atoms with ozone in 1–205 Torr of either N₂ or O₂, diluent at 298 K. By monitoring the rate of formation of BrO radicals, a value of k(Br + O₃) = (1.2 ± 0.1) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ was established that was independent of the nature and pressure of diluent gas. The rate of relaxation of vibrationally excited BrO radicals by collisions with N₂ and O₂ was measured; k(BrO(v) + O₂ → BrO(v − 1) + O₂) = (5.7 ± 0.3) × 10⁻¹³ and k(BrO(v) + N₂ → BrO(v − 1) + N₂) = (1.5 ± 0.2) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹. The increased efficiency of O₂ compared with N₂ as a relaxing agent for vibrationally excited BrO radicals is ascribed to the formation of a transient BrO–O₂ complex. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 125–130, 2000     

Vibrational energy disposal in reactions of fluorine atoms with hydrides of groups III,IV and V

The HF infrared chemiluminescence from the reactions of F atoms with B₂H₆, CH₄, CH₃F, CH₂F₂, CH₂Cl₂, CH₃ONO. CH₃NO₂, NH₃ (and ND₃). PH₃ and HNCO has been observed from a 300 K flowing-afterglow reactor. Experiments were done for a range of CH₄ and F atom concentrations to identify conditions which were free of vibrational relaxation and secondary reactions, and these conditions were used to assign initial HF(v) vibrational distributions for each reaction. The emission intensity from each reaction also was compared to that from CH₄ in order to obtain the relative HF formation rate constants at 300 K. Since the absolute rate constant for F + CH₄ is well established, the combination of all of these data provides absolute rate constants for HF(v) formation at 300 K. The ND₃ reaction was studied to obtain information on more vibrational levels in order to better estimate the HF(v = 0) and DF(v = 0) components of the ammonia distributions. With NH₃ and ND₃ there is no significant isotope effect on the energy disposal. Except for NHCO, for which an addition-elimination channel is possible, the HF(v) distributions are inverted and <f_v > = 0.60. Differences between the HF(v) distributions reported here and some other reports in the literature are noted: the present data are discussed as representative of direct H atom abstraction for 300 K Boltzmann conditions. The HCl infrared chemiluminescence from the F + CHCl₂ secondary reaction also was observed; the HCl(v) distribution was v₁: v₂: v₃: v₄: v₅ - 0.47: 0.23: 0.18: 0.08: 0.04.     

Optimization of Sample Preparation of Carrot-Fruit Juice for Determination of Antimony,Arsenic, and Selenium by Hydride Generation-Inductively Coupled Plasma Optical Emission Spectrometry

Sample preparation procedures for the determination of As, Sb, and Se in carrot-fruit juice by hydride generation inductively coupled plasma optical emission spectrometry (HG-ICP OES) were examined. The applicability of a partial decomposition using aqua regia and simple dilution with a 2% (v/v) HNO₃ solution were tested and compared to a traditional treatment based on the wet digestion with a HNO₃/H₂O₂ mixture. The pre-reduction and hydride generation reaction conditions were evaluated. Under the optimal conditions, the hydrides were produced in the reaction of an acidified sample with NaBH₄ after pre-reduction with ascorbic acid [0.5% (m/v)] and KI [0.5% (m/v)] in 3 mol L^?1 HCl for total As and Sb, and boiling with HCl (6 mol L^?1) for total Se. The best results were obtained for the aqua regia procedure, resulting in limits of detection (LODs) between 1.2–2.4 ng g^?1 in the samples and recoveries from 90.9% to 109.1%. The method was successfully applied (without matrix effects) for the determination of As in dense mousse and pulp juice samples and for Sb in pulp juices. Standard solutions, processed in the same way as samples, were used for the calibration. Undecomposed matrix constituents strongly influenced Se; hence this element was determined using the method of standard addition. Concentrations of studied elements in analyzed products were at the trace level, that is, 6–32 ng g^?1, 4–10 ng g^?1, and 4–13 ng g^?1 for Se, As, and Sb, respectively.     

Theoretical analysis of sorption of a binary solvent in a polymer phase. I. Occurrence and character of inversion in preferential sorption

Osmotic and sorption equilibria in the system polymer–binary solvent can be represented with advantage in coordinates (u₁, v₃), where v₃ is the volume fraction of the polymer and u₁ gives the composition (volume fraction) of the binary solvent in the polymer phase. The coexistence lines and osmotic isobars are plotted; the former are used to read the preferential sorption ε of one of the solvent components in the polymer. The newly formulated equilibrium condition for the preferential sorption is applied to the Flory–Huggins theory extended by the ternary interaction parameter χ_T. This is used as a starting point for analyzing the conditions under which inversion of preferential sorption takes place, i.e., the sign of ε changes. The existence of inversion and the course of the inversion line in the v₃ versus u₁ plot are affected in a decisive manner by the extent to which the effect of the mutual interaction of solvent components prevails over the effect of the relative difference between their molar volumes and of the difference in strength of their interaction with the polymer. The effect of the ratio of molar volumes upon the preferential sorption increases with the concentration of the polymer, so that for v₃ not too far from unity the component having the smaller molecule is necessarily sorbed preferentially. If, therefore, both types of small molecules are not of the same size, the inversion vanishes for large v₃ even in systems where it actually occurs if v₃ is small. On the contrary, the same effect can in other cases have as its consequence an inversion at moderate values of v₃, even if it does not appear as v₃ approaches zero; a similar effect can also be produced by a nonzero value of the interaction parameter χ_T. The neighborhood of the inversion line can have a “divergent” or a “convergent” character, depending on whether the component being preferentially sorbed is that present in excess. The former case is observed with negative and the latter for the positive values of the binary solvent–solvent interaction parameter χ₁₂. The inversion with the divergent neighborhood has not yet been confirmed experimentally, owing to the small number of systems investigated.     

The complexes of vanadium(IV) oxydichloride with anisidines,toluidines, quinolines,tetrahydropyran, and tetrahydrothiophene

VOCl₂ was prepared by reducing VOCl₃ with sulphur under an atmosphere of dry nitrogen. The VOCl₂ with tetrahydrofuran formed the complex VOCl₂(C₄H₈O)₂. The complexes of VOCl₂ with anisidines, toluidines, quinolines, tetrahydropyran and tetrahydrothiophene were prepared by the reaction of VOCl₂(C₄H₈O)₂ and the respective ligand. Infrared spectra of the complexes were determined in nujol and hexachlorobutadiene mulls and assignments of vanadium-oxygen double bond, v(V?O), vanadium-nitrogen, v(V? N), vanadium-oxygen, v(V? O), vanadium-sulphur, v(V? S) and vanadiumchlorine, v(V? Cl), stretching vibrations made. The possible structural formations of the complexes are proposed.     

Visible and near-IR spectroscopy of endohedral Gd@C82(C 2v ) and Ho@C82(C 2v ) metallofullerenes and their monoanions

Solutions of endohedral Gd@C₈₂(C _2v ) and Ho@C₈₂(C _2v ) metallofullerenes are studied by means of visible and near-IR spectroscopy upon their conversion from neutral to the anionic form via a redox reaction with the electron donor potassium perchlorotriphenylmethide K(18-crown-6)[C(C₆Cl₅)₃]. The concentrations of the studied solutions of endohedral Gd@C₈₂(C _2v ) and Ho@C₈₂(C _2v ) metallofullerenes in o-dichlorobenzene were determined from the spectroscopic data, and their molar extinction coefficients are calculated.

     

Crazing and shear deformation in crosslinked polystyrene

Thin films of polystyrene (PS) are bonded to copper grids and crosslinked with electron irradiation. When the films are strained in tension regions of local plastic deformation, either crazed or plane stress deformation zones (DZs), nucleate and grow from dust particles. the nature of the local deformation, as well as the local extension ration λ, is determined by transmission electron microscopy. The behavior of the PS glass is consistent with its being a network of molecular strands of total density v = v_E + v_X, where v_E is the entangled strand density inferred from melt elasticity measurements of uncrosslinked PS and v_X is the density of crosslinked strands determined from the ratio of the applied electron dose to the electron dose for gelation. when v is less than 4 × 10²⁵ m^?3 (<1.3v_E), only crazes are observed whose microstructure is similar to those in uncrosslinked PS. As v increases from 4 × 10²⁵ to 8 × 10²⁵ m^?3 (from 1.3v_E to 2.5v_E) shear deformation begins to compete with crazing. As v increases above 8 × 10²⁵ m^?3, only shear DZs are observed, the strain in which becomes progressively more diffuse as v increases. The λ in the crazes and DZs correlate well with λ_max, the maximum extension ratio of a strand in a network of density v computed using the Porod×Kratky model. For crazes ln(λ) ? 0.9 ln(λ_max) and for DZs ln(λ) ? 0.55 ln(λ_max). The strain at which crack nucleation is first observed increases as v increases from <5% in uncrosslinked PS with v = 3.3 × 10²⁵ m^?3 to >20% in PS with v = 33 × 10²⁵ m^?3 (v = 10v_E); crosslinking to still higher crosslink densities, e.g., v = 14v_E, results in cracks which propagate in a catastrophic manner at low applied strains. An optimum v thus exists, one not too high to suppress local shear ductility but high enough to suppress crazes which can act as crack nucleation sites. these results are compared with previous results on a variety of linear homopolymers, copolymers, and polymer blends that are characterized by a wide range of v (v = v_E). The transitions from crazing to crazing plus shear and from crazing plus shear to shear only take place at almost identical values of v. In addition the correlation between λ in the crazes and DZs and λ_max for a single network strand is the same for both classes of polymers. This agreement implies that chain scission is the major mechanism by which strands in the entanglement network are removed in forming fibril surfaces. Craze suppression, by either increasing v in the crosslinked polymer or v_E in the uncrosslinked ones, is due to the extra energy required to break more main-chain bonds to form these surfaces.     

Purification of hydrophilic and hydrophobic peptide fragments on a single reversed phase high performance liquid chromatographic column

Hydrophilic peptides generated from enzymic fragmentation of proteins are difficult to purify because they are either weakly bound or unretained by the reversed phase C₁₈ columns favoured for liquid chromatographic separation of peptide mixtures. To overcome this difficulty, peptides that were not bound or only weakly bound by a C₁₈ RP column were reacted with phenyl isothiocyanate (PITC), as used in the initial step in Edman sequencing. The hydrophobic phenylthiocarbamyl (PTC) peptide derivatives produced by the reaction were rechromatographed on the same column. Peptides generated by tryptic digestion of equine cytochrome C were used as a model system to test whether a complete set of peptide fragments could be purified by this method using just one column and solvent system. All the expected hydrophobic tryptic peptides bound to the RP column and were resolved by elution with acetonitrile, but no hydrophilic peptides were recovered as pure fractions. The column breakthrough fraction was reacted with PITC and rechromatographed on the same column, producing a profile consisting of 19 bound peaks. Further rechromatography of some of the fractions at different column temperatures enabled all six of the expected hydrophilic peptides to be purified and identified. The technique has also been applied to the sequence determination of coat protein from peanut stripe potyvirus protein, eight hydrophilic tryptic peptides being recovered and identified as PTC derivatives.     

Determination of Chlorzoxazone in Rat Plasma by LC-ESI-MS/MS and Its Application to a Pharmacokinetic Study

A sensitive LC-ESI-MS/MS method for determination of chlorzoxazone in rat plasma has been developed. Chromatographic separation was achieved on a Zorbax SB-C₁₈ column, with 45:55 (v/v) acetonitrile–water as the mobile phase. A LC-ESI-MS/MS was performed in a multiple reactions monitoring (MRM) mode using target ions m/z 167.5→131.6 for chlorzoxazone and m/z 230.7→185.6 for phenobarbital (internal standard). The calibration plots were linear over the range of 10.0–2,000 ng/mL. Intra-day and inter-day precisions were better than 5.1% and 6.8%, respectively. The validated method was successfully used to analyze the drug in samples of rat plasma for pharmacokinetic study.     

Stress relaxation of polybutadiene at large deformation. Measurements of stress and birefringence in shear and elongation

The rheological behavior of an uncrosslinked polybutadiene on sudden application of finite strain was examined. The shear stress σ, two components of birefringence, and the extinction angle were measured in shear (magnitude of shear γ ≤ 3.5) and tensile stress and the birefringence were measured in uniaxial elongation (elongation ratio λ ≤ 3.8). Measurements were performed at 30°C with a tensile tester equipped with appropriate sample holders. The stress-optical coefficient was 3.01 × 10^?9Pa^?1. The first and second normal-stress differences v₁ and v₂ were separately evaluated with the use of stress-optical law. The Lodge—Meissner relation v₁ = γσ held good. The ratio v₂/v₁ was independent of time and varied from about ?0.3 to ?0.2 with increasing γ in the range of measurements. Each of the stress components was factored into a function of strain and one of time, and the latter was common to all the stress components. Simple formulas were proposed to represent stress components in step deformations.     

Quantitative Determination of Four Water-Soluble Compounds in Herbal Drugs from Lamiaceae Using Different Chromatographic Techniques

HPTLC-densitometric and HPLC–UV techniques were used for qualitative and quantitative determination of luteolin-7-O-glucuronide, lithospermic acid, rosmarinic acid and caffeic acid in several herbal drugs from the Lamiaceae family: Thymi herba, Serpylli herba, Majoranae herba and Menthae piperitae folium. Unmodified silica gel (HPTLC Si60) and silica gel chemically modified with aminopropyl groups (HPTLC NH₂) were used during the investigation process. Among HPTLC methods the best resolution and selectivity was achieved with mobile phases: diisopropyl ether–acetone–formic acid–water (50:30:10:10, v/v/v/v) and acetone–formic acid (85:15, v/v), respectively. Plates were densitometrically evaluated. Contents of analyzed compounds in the studied aqueous extracts prepared from herbal drugs were established using both techniques. The results from the HPTLC-densitometric analysis have been compared with those from HPLC–UV on a C18 column with acetonitrile–water–formic acid as a mobile phase. The chromatographic methods were validated for linearity, LOD, LOQ, repeatability, intermediate precision and recovery. An analysis of variance showed that the HPTLC-densitometric and HPLC–UV methods are equivalent and sufficiently precise for the estimation of polyphenolic compounds mentioned above, in investigated herbal drugs. All of the suggested methods (HPTLC NH₂, HPTLC Si60 and HPLC RP18) give results with good agreement.     

Electrochemical behavior of catechol in the presence of 2-methyl-1,3-cyclopentanedione: application to electrosynthesis

Abstract  

Electro-oxidation of catechol in the presence of 2-methyl-1,3-cyclopentanedione as a nucleophile was investigated in water–acetonitrile (90:10 v/v) solution. The results indicate that the o-benzoquinone electrogenerated participates in a Michael addition reaction with this nucleophile. The electrosynthesis of 2-(3,4-dihydroxyphenyl)-2-methylcyclopentane-1,3-dione was carried out. The product was characterized by NMR, MS, FT-IR, and elemental analysis. An EC mechanism was deduced from voltammetric and spectroscopic data. Also, the Michael addition reaction rate constant (k _m) was estimated using digital simulation of voltammograms.     

Simple LC Method with Chemiluminescence Detection for Simultaneous Determination of Arbutin and l-Ascorbic Acid in Whitening Cosmetics

Simultaneous determination of arbutin (ART) and l-ascorbic acid (AA) by HPLC with chemiluminescence detection is proposed for the first time. This method is based on the CL reaction of acidic potassium permanganate with ART and AA in the presence of formaldehyde as enhancer. The separation was performed on a C₁₈ column with a 90:10 (v/v) mixture of 0.02 M phosphate buffer and methanol as mobile phase. The effects of several conditions on HPLC resolution and CL emission were studied systematically. The linear ranges were 0.5–50 and 1–200 μg mL⁻¹ for ART and AA, respectively. The detection limits were 0.2 and 0.3 μg mL⁻¹, respectively. The method was successfully applied to the determination of ART and AA in whitening cosmetics.