1,1-Dimethylnaphthalenon-dimers as photocleavable linkers with improved two-photon-absorption efficiency and hydrolytic stability

Coumarins are well known for reversible dimer formation with wavelengths greater than 300 nm and dimer cleavage below 300 nm. In a photochemical [2+2]-cycloaddition a cyclobutane ring is formed. Formation as well as cleavage of the cyclobutane ring may be accomplished by a single-photon-absorption as well as by a two-photon-absorption triggered reaction. The coumarin system is of interest for various kinds of applications, ranging from drug delivery for ophthalmic implants to optical data storage. However, the two-photon-absorption coefficient of coumarin dimers is rather low falling in the range of 1 GM in the visible range. We present here a substitute for the coumarin dimer system which not only has an about one order of magnitude higher two-photon-absorption coefficient, but also overcomes several other problems of the coumarin dimer system. Coumarines and in particular coumarine dimers have a very limited solubility in common solvents and are susceptible to hydrolysis of the lactone ring, which leads to an undesired complexity in the photochemical cleavage reaction. The 1,1-dimethylnaphtalenone dimers introduced here show excellent stability, lead only to a single cleavage product, and have a two-photon-absorption coefficient of about 10 GM at 532 nm. These properties make the 1,1-dimethylnaphtalenone dimers a superior substitute over the well-known coumarin dimers in particular in applications where two-photon-absorption induced photocleavage is required.     

Catenane gel: synthesis of high molecular weight poly[2]catenanes by Sonogashira coupling polymerization

A first catenane polymer gel was obtained by Sonogashira coupling reaction of diiodo[2]catenane and 4,4′-diethynylbiphenyl. Because of the linearity and rigidity of the ethynylene group formed in the coupling reaction, the formation of undesired cyclic dimer was inhibited, and a very high molecular weight (M_n > 2.3 × 10⁵) poly[2]catenane was synthesized. The polymer was gelated without any chemical cross-linkage and swelled in DMF.     

High pressure effects on benzoic acid dimers: Vibrational spectroscopy

To understand pressure effects on dimer structure stability, Raman and FTIR spectroscopies were used to examine changes in H-bonded dimers of benzoic acid (BA). Experiments were performed on single crystals compressed to 33 GPa in a diamond anvil cell (DAC). Several changes in Raman spectra were observed in the range 6–8 GPa indicating modification in the dimer structure suggesting the lowering of molecular symmetry. Pressure increase above 15 GPa induced strong luminescence and a gradual change of the crystal color from white to yellow/brownish. FTIR measurements on the sample released from 33 GPa indicated formation of a new compound. It is proposed that molecules of this compound are composed of the hydroxyl group associated with alcohol, carbonyl group associated with ketone, and the sp³ hydrocarbon groups. This study demonstrates that sufficient high pressure compression and subsequent decompression can lead to significant changes in the H-bonded dimer structure, including the breaking of bonds and formation of new chemical compound.     

Renewable platform-chemicals and materials: Thermochemical study of levulinic acid

The standard molar enthalpy of formation in the gaseous state (?613.5 ± 2.2) kJ · mol^?1 of levulinic acid has been obtained from combustion calorimetry and results from the temperature dependence of the vapour pressure measured by the transpiration method. In order to verify the experimental data, first-principles calculations have been performed. Enthalpies of formation derived from G4 and G3MP2 methods are in an excellent agreement with the experimental results. Thermodynamic analysis of the hydrolysis of 5-hydroxymethylfurfural to the levulinic and formic acids has revealed very high feasibility of these reactions with equilibrium constants completely shifted to the desired reaction products even at T = 298.15 K.     

About the co-product of the OH radical in the reaction of acetyl with O2 below atmospheric pressure

Recent studies have shown that the reaction between acetyl radicals and O₂ at low pressures leads to the direct fast formation of OH radical, but the nature of co-products is controversial. Laser photolysis coupled to TDLAS (10–200 Torr, 298 K) has been employed to directly monitor two possible co-products of this reaction, CO and formaldehyde. Only CH₂O has been detected in yield of 0.29 ± 0.13, but with time constants much slow than the OH formation under these conditions; the observed CH₂O-time profiles are compatible with the known mechanism of peroxyacetyl secondary reactions.     

Impact of configuration and fluorination on the solubility of octyl ester benzoate dimers in CO2

Data are reported on the phase behavior of hydrocarbon and semifluoroinated octyl ester benzoate dimers in CO₂ to temperatures of 100 °C and pressures of 1 600 bar. The experimental data at 75 °C demonstrate that the non-fluorinated head-to-head (H–H) dimer dissolves in CO₂ at ∼400 bar lower pressures than the non-fluorinated tail-to-tail (T–T) dimer. Even though partially fluorinating the octyl tails of the H–H and T–T dimers renders them soluble in CO₂ at pressures near 200 bar, it still takes ∼40 bar more pressure to dissolve the fluorinated T–T dimer as compared to the H–H dimer. The difference in pressures needed to dissolve these dimers is attributed to steric constraints on the coplanarity of the benzene rings imposed by the H–H regiochemistry that do not exist with T–T dimers. Semi-empirical quantum mechanics calculations suggest that the H–H dimer has a twisted, non-coplanar conformation due to the steric effect of the octyl ester groups while the T–T dimer has a less twisted conformation. Steric hindrance in the H–H dimer reduces considerably resonance or conjugation between the π electrons of the aromatic groups which also reduces the dipole moments of the H–H dimers compared to those of the T–T dimers.     

Thermodynamic study of solvent-free reaction between 17-methyltestosterone and o-aminophenol

In this work we investigated the possibilities of the solvent free synthesis of Schiff base (azomethyne) from 17-methyltestosterone and o-aminophenol. The study of the binary mixtures of 17-methyltestosterone and o-aminophenol was achieved by DSC, TG–DSC, and FTIR. The isolated compounds and reaction product were studied by adiabatic bomb calorimetry in order to determine the heat of reaction. The DSC data reveal a simple eutectic followed by a chemical reaction in liquid phases. From the DSC data we calculated the enthalpy of decomposition of reaction product as (44.65 ± 0.83) kJ · mol^?1. Schiff base formation by condensation reaction was highlighted by TG–DSC method and the structure of the solid product was confirmed by FTIR spectroscopy. The standard enthalpy of reaction was calculated from the standard molar enthalpy of formation of reactants and products.     

New,simple and validated kinetics spectrophotometric method for determination of moxifloxacine in its pharmaceutical formulations

The objective of this research was to develop a kinetic spectrophotometric method for determination of moxifloxacine (MOXF) in pure form and pharmaceutical formulations. The method was based on the formation of a colored N-vinyl chlorobenzoquinone derivative of MOXF by its reaction with 2,3,5,6-tetrachloro-1,4-benzoquinone in presence of acetaldehyde.The formation of the colored product was monitored spectrophotometrically by measuring the absorbance at 652 nm. Factors affecting the reaction were studied and optimized. The stoichiometry of the reaction was determined, and the reaction pathway was postulated. The activation energy of the reaction was calculated and found to be 6.65 kJ mol^?1. Under the optimized conditions, the initial rate and fixed time (at 5 min) methods were utilized for constructing the calibration graphs. The graphs were linear in concentration ranges 5–100 and 15–150 μg ml^?1 with limit of detection of 2.0 and 5.0 μg ml^?1 for the initial rate and fixed time methods, respectively. The analytical performance for both methods was fully validated, and the results were satisfactory. No interference was observed from the excipients that are commonly present in the pharmaceutical formulations. The proposed method was successfully applied to the determination of MOXF in its pharmaceutical formulations. The label claim percentages were 101.25 ± 0.73% and 100.92 ± 0.65% for the initial rate and fixed time method, respectively. Statistical comparison of the results with those obtained by a reference spectrophotometric method showed excellent agreement between the accuracy and precision of the two methods. The proposed method has great value in its application to the analysis of MOXF in quality control laboratories.     

The standard enthalpies of formation of l-asparagine and l-α-glutamine

The energies of combustion of l-asparagine and l-α-glutamine were measured in a static bomb adiabatic calorimeter. Corrections were made for the heats due to the ignition of sample and for the nitric acid formation. The derived enthalpies of formation in solid state of asparagine monohydrate, nonhydrated asparagine and glutamine are respectively ?1084.1 ± 3.0, ?788.1 ± 4.7 and ?834.3 ± 4.6 kJ mol^?1. The data of enthalpy of formation are compared with the literature values and with estimated values by means of group additivity, using parameters recommended by Domalski and Hearing. The discrepancies between experimental and calculated values are explained by considering the number and strength of intermolecular hydrogen bonds. The dehydration of asparagine monohydrate and the possible melting of the three amino acids were investigated by means of DSC. Glutamine melts without considerable decomposition at about 182 °C, while asparagines decompose during the fusion process (208 °C).     

Nonempirical analysis of the catalytic activity of the molecular environment – optimal static and dynamic catalytic fields for double proton transfer in formamide–formamidine complex

The double proton transfer reaction in model formamide–formamidine dimer mimicking adenine–thymine base pair was investigated. The physical nature of the catalytic activity of the first hydration shell has been analyzed using hybrid variation–perturbation decomposition of intermolecular interaction energy. The differential transition state stabilization energy is dominated by long range electrostatic multipole term, whereas electrostatic penetration, exchange and delocalization terms are negligible or partly cancel each other. This allowed to derive (within electrostatic approximation) the generalized static and dynamic properties of the molecular environment exerting optimal catalytic activity towards canonic → rare and rare → canonic tautomeric rearrangement.     

Selective kinetic spectrophotometric method for determination of gatifloxacin based on formation of its N-vinyl chlorobenzoquinone derivative

A selective and simple kinetic spectrophotometric has been developed, for the first time, for the determination of gatifloxacin (GAT) in its dosage forms. The method was based on the formation of a colored N-vinyl chlorobenzoquinone derivative of GAT by its reaction with 2,3,5,6-tetrachloro-1,4-benzoquinone in presence of acetaldehyde. The formation of the colored product was monitored spectrophotometrically by measuring the absorbances at 655 nm. The factors affecting the reaction were studied and optimized. The stoichiometry of the reaction was determined, and the reaction pathway was postulated. Under the optimized conditions, the initial rate and fixed time (at 5 min) methods were utilized for constructing the calibration graphs. The graphs were linear in the concentration ranges of 2–100 and 10–140 μg ml^?1 with limits of detection of 0.84 and 3.5 μg ml^?1 for the initial rate and fixed time methods, respectively. The analytical performance of both methods was fully validated, and the results were satisfactory. The proposed methods were successfully applied to the determination of GAT in its commercial dosage forms. The label claim percentages were 99.7–100.5 and 98.2–99.5% for the initial rate and fixed time methods, respectively. Statistical comparison of the results with those of the reference method showed excellent agreement and proved that there was no significant difference in the accuracy and precision between the reference and the proposed methods. The proposed methods are superior to all the previously reported spectrophotometric methods in terms of the procedure simplicity and assay selectivity.     

Kinetic study of low-temperature conversion of plastic mixtures to value added products

Batch-mode pyrolysis of 200.0 g samples of polymers was studied at low temperature. The cracking reaction was carried out in a stainless-steel autoclave with reaction temperatures of 360, 380, 400 and 420 °C, initial pressure of 6.325 kPa (absolute pressure) and reaction times of 0–240 min. Based on the experimental results, a four-lump kinetic model has been developed to describe the production distribution of the light fractions, middle distillates and heavy fraction. This model reasonably fitted the results in each reaction of operation conditions. It was also found that the pyrolysis kinetics of separated plastic, mixed plastic and mixed plastic containing additives can be described by the same kinetic model. The plastic additives have not had a great influence on the product distribution and kinetics of the mixed plastic pyrolysis. Finally, the optimum conditions of low-temperature conversion of plastic mixtures to value-added products were established. The formation of heavy fractions from HDPE was as high as 70 wt% at 380 °C at a reaction time of 250 min. During the thermal degradation of plastic mixtures, the heavy fractions yielded up 50 wt% for 30 min reaction time at 400 °C. The total activation energies for the conversion of HDPE and the plastic mixtures were estimated to be 217.66 kJ mol⁻¹ and 178.49 kJ mol⁻¹, respectively.     

Synthesis and characterization of a novel (μ3-oxo)tetraruthenium cluster

A novel (μ₃-oxo)tetraruthenium cluster (2) was synthesized in an isolated yield of 73% by the reaction of a zerovalent ruthenium complex, Ru(η⁶-cot)(η²-dmfm)₂ (1) [cot = 1,3,5-cyclooctatriene, dmfm = dimethyl fumarate], with water under reflux in 1,4-dioxane for 6 h. The structure and the X-ray characterization of this novel complex 2 as well as a possible formation mechanism of 2 are disclosed.     

Estimation of (vapour + liquid) equilibrium of binary systems (tert-butanol + 2-ethyl-1-hexanol) and (n-butanol + 2-ethyl-1-hexanol) using an artificial neural network

(Vapour + liquid) equilibrium (VLE) data are important for designing and modelling of process equipment. Since it is not always possible to carry out experiments at all possible temperatures and pressures, generally thermodynamic models based on equations of state are used for estimation of VLE. In this paper, an alternate tool, i.e. the artificial neural network technique has been applied for estimation of VLE for the binary systems viz. (tert-butanol + 2-ethyl-1-hexanol) and (n-butanol + 2-ethyl-1-hexanol). The temperature range over which these models are valid is (353.2 to 458.2) K at atmospheric pressure. The average absolute deviation for the temperature output was in range 2% to 3.3%. The results were then compared with experimental data.     

Biomolecule-free,selective detection of clenbuterol based on disposable screen-printed carbon electrode

We report here the development of a selective clenbuterol sensor made of disposable screen-printed carbon electrode (SPCE) without the need of adding any biorecognition element. Good analytical performance was achieved through the proper function of both the oxygen functionalities and edge plane sites on the “preanodized” SPCE (SPCE*). It is the amino group of clenbuterol to effectively form hydrogen bond with the SPCE* to induce the adsorption of clenbuterol. The edge plane sites enhance the electron transfer process and further help the dimer formation of clenbuterol to generate electroactivity for analysis. Square wave voltammetry was applied to increase the detection sensitivity with a linear response in the range of 7–1000 ppb and a detection limit of 0.51 ppb (S/N = 3). In the real sample analysis, results observed were satisfactory with meat, human blood, and human urine. High reproducibility in sensor fabrication further favors the disposable purpose of applications.     

Kinetic and thermodynamic behaviour of CF4 clathrate hydrates

This study presents experimental kinetic and thermodynamic data for CF₄ clathrate hydrates. Experimental measurements were undertaken in a high pressure equilibrium cell with a 40 cm³ inner volume. The measurements of experimental hydrate dissociation conditions were performed in the temperature range of (273.8 to 278.3) K and pressures ranging from (4.55 to 11.57) MPa. A thermodynamic model based on van der Waals and Platteeuw (vdW–P) solid solution theory was used for prediction and comparison of hydrate dissociation conditions and the Langmuir constant parameters for CF₄ based on Parrish and Prausnitz equation are reported. For the kinetics, the effect of initial pressure and temperature on the induction time, CF₄ hydrate formation rate, the apparent rate constant of reaction, storage capacity, and water to hydrate conversion during the hydrate formation were studied. Kinetic experiments were performed at initial temperatures of (275.3, 276.1 and 276.6) K and initial pressures of (7.08, 7.92, 9.11, 11.47 and 11.83) MPa. Results show that increasing the initial pressure at constant temperature decreases the induction time, while CF₄ hydrate formation rate, the apparent rate constant of reaction, storage capacity, and water to hydrate conversion increase. The same trends are observed with a decrease in the initial temperature at constant pressure.     

Nanospace geometry-sensitive molecular assembly

Interaction of a molecule with micropore walls strongly depends on the micropore width. Molecules confined in the micropore tend to form an intermolecular structure inherent to each molecule/pore system in order to lower the whole molecular energy. Supercritical NO is adsorbed in micropores of zolite or activated carbon fiber in the form of a dimer at 303 K. The NO dimerization varies with the micropore width. CCl₄ molecules only in pore of pore width =1.0 nm at 303 K form a plastic crystalline structure which is observed at 246–250 K in the bulk phase. H₂O molecules are associated with each other to form an ordered assembly in carbon micropores at 303 K; the smaller the pore width, the more ordered the assembly structure. The presence of preadsorbed H₂O noticeably enhances adsorption of supercritical CH₄ in carbon micropores at 303 K due to methane nanohydrate formation, which has an optimum pore width of 1 nm.     

Photocleavage of dimers of coumarin and 6-alkylcoumarins

The cleavage of four coumarin dimers, the syn-head-to-tail (ht) dimer of parent coumarin (syn-ht-CC1), the anti- and syn-hh dimers of 6-methylcoumarin (anti-hh-CC2 and syn-hh-CC2, respectively) and the anti-hh dimer of 6-dodecylcoumarin (anti-hh-CC3), was studied by UV–vis and IR spectroscopy and HPLC upon direct 254 nm irradiation as well as sensitized excitation. The quantum yield of dimer splitting is Φ_sp = 0.1–0.3 in various solvents and the effects of structure and solvent polarity are small. In certain solvents some of the dimers produced CO₂ along with the monomers in the splitting reaction. Electron transfer from dimers to the triplet state of sensitizers, such as benzophenone or 9,10-anthraquinone, was observed in acetonitrile.     

Quantitative NMR spectroscopy of binary liquid mixtures (aldehyde + alcohol). Part III: (1-decanal or 3-phenylpropanal or 2-chlorobenzaldehyde) + (methanol or ethanol or 1-propanol)

The thermodynamic properties of binary liquid mixtures of (aldehyde + alcohol) are strongly influenced by chemical reactions in particular around and below ambient temperature. In two previous publications the chemical reaction equilibrium was investigated by ¹³C – Fourier transform NMR-spectroscopy at temperatures between 255 K and 295 K for a series of aldehydes (acetaldehyde, 1-propanal, 1-butanal, 1-heptanal) with three alcohols (methanol, ethanol, 1-propanol). Here these investigations are extended to three more aldehydes (1-decanal, 3-phenylpropanal and 2-chlorobenzaldehyde, respectively). The results for the binary systems with decanal or 3-phenylpropanal as the aldehyde in the binary mixture (aldehyde + alcohol) confirm the expectations from the first parts of this series, i.e., that the majority of the constituents of the mixture is present as hemiacetal and the first two poly(oxymethylene) – hemiacetals. The numerical results for the chemical reaction equilibrium constants from the previous investigations can be used to predict quantitatively the speciation in binary systems of ((either 1-decanal or 3-phenylpropanal) + (either methanol or ethanol or 1-propanol)). However the experimental results with 2-chlorobenzaldehyde reveal a different behaviour. In all investigated systems (2-chlorobenzaldehyde + alcohol) the most important reaction product was the corresponding acetal whereas the amounts of hemiacetal were very small. While the amounts of hemiacteal could still be quantified, it was not possible to quantify the amount of any poly(oxymethylene) – hemiacetal.     

New experimental heat capacity and enthalpy of formation of lithium cobalt oxide

The heat capacity of LiCoO₂ (O3-phase), constituent material in cathodes for lithium-ion batteries, was measured using two differential scanning calorimeters over the temperature range from (160 to 953) K (continuous method). As an alternative, the discontinuous method was employed over the temperature range from (493 to 693) K using a third calorimeter. Based on the results obtained, the enthalpy increment of LiCoO₂ was derived from T = 298.15 K up to 974.15 K. Very good agreement was obtained between the derived enthalpy increment and our independent measurements of enthalpy increment using transposed temperature drop calorimetry at 974.15 K. In addition, values of the enthalpy of formation of LiCoO₂ from the constituent oxides and elements were assessed based on measurements of enthalpy of dissolution using high temperature oxide melt drop solution calorimetry. The high temperature values obtained by these measurements are key input data in safety analysis and optimisation of the battery management systems which accounts for possible thermal runaway events.