One Collision—Two Substituents: Gas-Phase Preparation of Xylenes under Single-Collision Conditions

The fundamental reaction pathways to the simplest dialkylsubstituted aromatics—xylenes (C₆H₄(CH₃)₂)—in high-temperature combustion flames and in low-temperature extraterrestrial environments are still unknown, but critical to understand the chemistry and molecular mass growth processes in these extreme environments. Exploiting crossed molecular beam experiments augmented by state-of-the-art electronic structure and statistical calculations, this study uncovers a previously elusive, facile gas-phase synthesis of xylenes through an isomer-selective reaction of 1-propynyl (methylethynyl, CH₃CC) with 2-methyl-1,3-butadiene (isoprene, C₅H₈). The reaction dynamics are driven by a barrierless addition of the radical to the diene moiety of 2-methyl-1,3-butadiene followed by extensive isomerization (hydrogen shifts, cyclization) prior to unimolecular decomposition accompanied by aromatization via atomic hydrogen loss. This overall exoergic reaction affords a preparation of xylenes not only in high-temperature environments such as in combustion flames and around circumstellar envelopes of carbon-rich Asymptotic Giant Branch (AGB) stars, but also in low-temperature cold molecular clouds (10 K) and in hydrocarbon-rich atmospheres of planets and their moons such as Triton and Titan. Our study established a hitherto unknown gas-phase route to xylenes and potentially more complex, disubstituted benzenes via a single collision event highlighting the significance of an alkyl-substituted ethynyl-mediated preparation of aromatic molecules in our Universe.     

The catalytic activity of bimetallic Pt—Pd polymer nanocomposites in formic acid oxidation

The platinum-palladium/Nafion metal—polymer nanocomposites were synthesized by chemical reduction of metal ions in water—organic reverse microemulsion solutions. The catalytic activity of the synthesized polymer composites with bimetallic Pt—Pd nanoparticles was estimated in the oxidation of hydrogen and formic acid.     

Mechanism of formic acid decomposition on P?Mo heteropolyacid

Hydrated (undecomposed) form of heteropolyacid H₃PMo₁₂O₄₀/SiO₂ exhibits a higher activity in the formic acid decomposition than the corresponding dehydrated sample. The formic acid decomposition takes place on strong Br?nsted acid sites of the heteropolyacid.Ab initio SCF MO LCAO method was used for the calculation of the electronic state of two surface complexes of HCOOH molecule (S1 and S2) coordinated to a proton H⁺. The S1 complex is formed by proton addition to the carbonyl oxygen, whereas the S2 complex is formed proton addition to the oxygen atom of the C−O−H fragment of HCOOH. The selective weakening of the C−O bond and localization of the positive charge on the (O=C−H) fragment in the protonated complex S2 are favorable for the decomposition of formic acid to CO and H₂O.     

Decomposition of aqueous fluorene by γ-rays and product analysis

Fluorene degradation was investigated in aqueous solution, saturated with air or N₂O as a function of the absorbed radiation dose. The observed initial degradation yields (Gi values) are 2.41 and 1.04, respectively. In addition to these series, also studies were performed in media containing: 90% air and 10% N₂O (Gi=1.1), 50% air and 50% N₂O (Gi=0.9) and 10% air and 90% N₂O (Gi=0.5). In all cases, as major degradation products 9-fluorenone and 9-fluorene carboxylic acid were found in addition to a mixture of aldehydes and carboxylic acids. Their yield depends on the starting pollutant concentration, absorbed radiation dose and oxygen content in the solution. For explanation of the results, probable reaction mechanisms are given.     

Raman study of the molecular motions of pivalic acid: the liquid—plastic phase transition

Raman spectra of pivalic acid in the plastic and liquid phase have been measured. The reorientational correlation times have been evaluated from the ν_asCH, νCO and νCC bands as a function of temperature. The reorientational correlation time corresponding to ν_as CH and νCC bands is τ < 10⁻¹¹ s whilst for the νCO band τ = 4ps (T = 20°C). The calculated activation energy is 26 KJ mol⁻¹. The reorientation of the carboxylic groups which may be assisted by the proton transfer along the hydrogen bonds in dimers is discussed.     

Spectral shift of the n → π* transition for acetone and formic acid with an explicit solvent model

In our recent work, a new form of the electrostatic solvation energy for the nonequilibrium polarization has been derived by introducing the method of constrained equilibrium state in the framework of continuous medium theory. Up until now, the idea of the constrained equilibrium state method has not been introduced into the explicit solvent model by others; therefore this nonequilibrium energy form was further equivalently extended to the explicit solvent model in this work based on the discrete representation of the solvent permanent charges and induced dipoles. Making use of this expression in explicit solvent model, we modified the nonequilibrium module in the averaged solvent electrostatic potential/molecular dynamics program to implement numerical calculations. Subsequently, the new codes were applied to study the solvatochromic shifts of the n → π* absorption spectra for acetone and trans-formic acid in aqueous solution. The calculation results show a good agreement with the experimental observations. When our results of spectral shift are compared with those achieved directly from the continuum model, it can be seen that both the explicit solvent model and continuum model derived based on the constrained equilibrium approach can give reasonable predictions. The hydrogen bond effect was also discussed and deemed to be a dominant contribution to the spectral shift by calculating the n → π* absorption spectra of acetone-water complexes.     

The cis–trans transformation of cyclohexane formic acid ester compounds

Trans-alkyl cyclohexane formic acid phenol ester compounds are synthesised by reaction of trans-cyclohexane formic acid and phenols. This process also produces many harmful cis-alkyl cyclohexane formic acid phenol ester compounds that are harmful to the environment. Using p-toluenesulphonic acid as the catalyst in this experiment, these cis compounds were transformed to their trans form. The yields of trans-alkyl cyclohexane formic acid phenol ester compounds were more than 70%, which was significantly higher when compared to the yields of 40–60% obtained using the control method. The effects of catalyst, reaction time and reaction temperature on cis–trans transformation were investigated. The possible mechanism of cis–trans transformation has been discussed in this report.     

Electrocatalytic performance of PdCo–C catalyst for formic acid oxidation

PdCo–C catalyst was synthesized through a simple simultaneous reduction reaction with sodium borohydride in aqueous solution. Inductive coupled plasma emission spectrometer (ICP) and X-ray diffraction (XRD) technologies had been used to characterize the PdCo–C catalyst and proved that the amount of Co was 1.5 wt%, PdCo alloy was formed and possessed face-centered cubic (fcc) crystalline structure, and the average particle size was about 5.1 nm. The electrochemical tests (cyclic voltammetry (CV) and chronoamperometry (CA)) showed that the addition of Co could significantly improve the electrocatalytic activity and stability. The enhancement of electrocatalytic activity and stability was mainly ascribed to the interaction between Pd and additive Co, which facilitated the oxidation reaction of formic acid in direct pathway.     

Vapor—liquid equilibrium of hydrochloric acid solutions with the PRSV equation of state

Stryjek, R. and Vera, J.H., 1986. Vapor—liquid equilibrium of hydrochloric acid solutions with the PRSV equation of state. Fluid Phase Equilibria, 25: 279–290.The PRSV cubic equation of state and a new Margules-type composition-dependent mixing rule previously proposed have been used to correlate vapor—liquid equilibrium data for hydrochloric acid solutions. In total, four adjustable parameters are used for pure hydrogen chloride, pure water and their mixtures. The two binary parameters are determined from azeotropic point data. Results obtained for this highly nonideal system, presenting negative deviations from ideality, are better than those obtained with the modified cubic equation of Gibbons and Laughton.     

Mechanism of 1,2-shift through π-complex transition state

A systematic calculation of the potential curves or surfaces for 1,2-shift has been realized by using MNDO or other models in MOPAC programs. By referring to the previous authors' viewpoints, the 1,2-shift can be divided into two categories. 1,2-electron-deficient shift is that the electronic configuration of the atom which accepts the migrating group is a cation or an electron-deficient atom, and 1,2-anion shift is the one that the accepted atom of the migration group is a negative ion. In terms of the experimental facts and the calculation of the potential surfaces, in electron-deficient shift such as Beckmann or Baeyer-Villiger rearrangement, the migration occurs through a transition complex formed between the 7i-bond and the cation or electron-deficient migrating group, but in anion shift such as Wittig or Stevens rearrangement, the electron pair in it-orbit excites at first to π* orbit, and then the migration occurs through the new formed complex between the anion migration group and the vacant rc     

Two-parameter stochastic resonance in a model of electrochemical oxidation of formic acid on Pt

Stochastic resonance (SR) is shown in a two-parameter system, a model of electrochemical oxidation of formic acid on Pt. The driving current and the saturation coverage for carbon monoxide are two control parameters in this model. Modulation of an excitable focal stable state close to a Hopf bifurcation by a weak periodic signal in one parameter and noise in the other parameter is found to give rise to SR. The results indicate that the noise can enlarge a weak periodic signal and lead the system to be ordered. The scenario and novel aspects of SR in this system are discussed.     

Solid state amorphization and glass transition of tri-O-methyl-β-cyclodextrin

Amorphous solid of tri-O-methyl--cyclodextrin was formed by grinding the crystalline sample with a vibrating mill at room temperature. The amorphising process was examined by X-ray powder diffraction technique and differential scanning calorimetry (DSC). The Bragg peaks disappeared and the enthalpy of crystallization became constant for the sample ground for 25 min, indicating the apparent completion of the amorphization. A glass transition of the ground amorphous solid was found by DSC. The glass transition temperature T_g moved from 58°C to 79°C with grinding. The saturated T_g of the ground sample was the same as that of the liquid-quenched glass. No significant difference between the ground and liquid-quenched amorphous solids was found in the X-ray diffraction patterns. Infrared spectra of both amorphous solids, however, showed a definite difference for the band at 1194 cm^–1 assigned to the rocking of the CH₃ groups which are located at the molecular periphery.

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Zusammenfassung Amorphes festes Tri-O-methyl--cyclodextrin wurde bei Raumtemperatur durch Mahlen einer kristallinen Probe in einer Vibrationsmühle hergestellt. Der Amorphisierungsprozeß wurde mittels Röntgenpulverdiffraktion und DSC verfolgt. Die Braggschen Peaks verschwanden und die Kristallisierungsenthalpie nahm nach einem Mahldauer von 25 min einen konstanten Wert an, was auf die augenscheinliche Beendigung des Amorphisierungsprozesses hinweist. Mittels DSC wurde ein Glasumwandlungspunkt des gemahlenen amorphen Feststoffes gefunden. Die Glasumwandlungstemperatur T_g stieg durch das Mahlen von 58°C auf 79°C. Der Sättigungswert T_g der gemahlenen Probe war der gleiche wie für flüssigkeitsabgeschrecktes Glas. In den Röntgendiffraktogrammen konnte zwischen den gemahlenen und den flüssigkeitsabgeschreckten amorphen Feststoffen keinen Unterschied feststellen. Die IR-Spektren von beiden amorphen Feststoffen zeigen einen signifikanten Unterschied für die Bande bei 1194 cm^–1, der Nickschwingung der CH₃-Gruppen am Molekülrand.

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Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60^th birthday

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Contribution No. 35 from the Microcalorimetry Research Centre.

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The authors thank the research group of Professor Yoshinobu Nakai, Faculty of Pharmaceutical Science, Chiba University, for their kind advice for the grinding-amorphization method. The authors are deeply indebted to Mr. Tetsuo Yamamoto for his X-ray diffraction experiments and Mr. Shin-ichi Ishikawa for his IR experiments.     

Quantum DFT and DF–DFT study of vibrational spectra of sulfuric acid,sulfuric acid monohydrate,formic acid and its cyclic dimer

Rigorous theoretical treatment of vibrational frequencies is critically important for the interpretation of unassigned experimental vibrational spectra and accurate determination of thermodynamic properties of molecular clusters. IR spectra of trans monomers of sulfuric and acetic acids, sulfuric acid monohydrate and cyclic dimer of the formic acid have been studied using DFT and DF–DFT methods using BLYP, B3LYP and PW91 with 12 different Pople and Dunning basis sets. New data for above-mentioned structures have been reported, scaling factors have been calculated and a comprehensive analysis of the performance of BLYP, B3LYP and PW91 methods has been performed. Comparison of the obtained results with experiments shows that results of pure PW91 and BLYP are better than predictions of well-established hybrid B3LYP method. Our analysis shows on the existence of the considerable difference in scaling factors weighted to high and low frequencies. In the case of formic acid dimer, the deviation the predicted low frequencies from the experimental data is considerable that leads to quite large (∼6–7 kcal mol⁻¹) uncertainties in the absolute values of the cluster Gibbs free energy. In order to determine an efficient computational strategy that comprises accuracy and reasonable computational costs, the effect of density fitting (DF) and basis set superposition error (BSSE) on the vibration frequencies has been investigated. We found that application of the DF that substantially (2.5–3.5 times) increases the performance of pure PW91 and BLYP methods gives excellent results, which are very close to those of conventional DFT. This suggests that DF–DFT is a viable low-cost alternative to conventional DFT in predicting vibrational spectra. It has been found that while vibrational spectra obtained using the counterpoise correction (CP) for the BSSE do not deviate much from uncorrected ones, the difference in absorption intensities between corrected and uncorrected spectra obtained using small and medium-sized basis sets is considerable. This suggests that application of DF–DFT uncorrected for the BSSE with large basis sets is a more efficient strategy of predicting vibrational spectra than the application of conventional DFT with small basis sets.     

Communication: Conformation state diagram of polypeptides: a chain length induced α-β transition

By using a generic coarse grained polypeptide model, we perform multicanonical molecular dynamics simulations for determining the equilibrium conformation state diagram of a single homopolypeptide chain as a function of the chain length and temperature. The state diagram highlights the thermal regimes of stability for various conformational patterns in polypeptides, including swollen, random and collapsed coils, globular structures, extended and bended α helices, and compact β bundles. Remarkably, at low temperatures we observe a sharp transition from extended α helix to compact β bundles as the chain length increases. This finding indicates that the chain length is one of the intrisic factors that can trigger α-β transformations in a broad class of polypeptides.     

First representative of phosphorylated formic acid hydrazides with three P—C bonds: synthesis and addition to the phosphorylated 4-methylenequinones

Russian Chemical Bulletin - Diethylphosphinylmethanehydrazide, the first representative of the phosphorylated formic acid hydrazides with three P—C bonds was obtained, which can exhibit...     

Polyether—phosphinite for One—phase Catalysis Coupled with Two—phase Separation

Polyether moiety was introduced to the phosphinite and the phosphinite modified rhodium complex formed in situ was highly active in the hydroformylation of higher olefins in organic monophase system. After reaction, on cooling to room temperature,the catalyst could precipitate out from organic phase and was easily separated by decantation and reused six times without obvious decreasing in activity.     

Electrocatalytic oxidation of formic acid on functional MWCNTs supported nanostructured Pd–Au catalyst

In this work, PdAu nanocatalysts with different weight ratio of Pd and Au supported on functional multi-walled carbon nanotubes (f-MWCNTs) were prepared, and their electrocatalytic activity for the oxidation of formic acid was also studied. The electrocatalysts were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrochemical results showed that the 4Pd1Au/f-MWCNTs (by weight) catalyst, exhibited distinctly higher activity and better stability in formic acid electrooxidation than the Pd/f-MWCNTs catalyst. The Nano-Au improves potentially the performance of Pd-based electrocatalysts for the direct formic acid fuel cells (DFAFCs).     

The dissociation quotients of formic acid in sodium chloride solutions to 200°C

The dissociation quotients of formic acid were measured potentiometrically from 25 to 200°C in NaCl solutions at ionic strengths of 0.1, 0.3 1.0, 3.0, and 5.0 mol-kg^–1. The experiments were carried out in a concentration cell with hydrogen electrodes. The resulting molal acid dissociation quotients for formic acid, as well as a set of infinite dilution literature values and a calorimetrically-determined enthalpy of reaction, were fitted by an empirical equation involving an extended Debye Hückel term and seven adjustable parameters involving functions of temperature and ionic strength. This regressional analysis yielded the following thermodynamic quantities for 25°C: logK=–3.755±0.002, H^o=–0.09±0.15 kJ-mol^–1, S^o=–72.2±0.5 J-K^–1-mol^–1, and C _p ^o =–147±4 J-K^–1-mol^–1. The isocoulombic form of the equilibrium constant is recommended for extrapolation to higher temperatures.     

Infrared characterization of the HCOOH···CO2 complexes in solid argon: stabilization of the higher-energy conformer of formic acid

The complexes of formic acid (HCOOH, FA) with carbon dioxide are studied by infrared spectroscopy in an argon matrix. Two trans-FA···CO(2) and one cis-FA···CO(2) complexes are experimentally identified while the calculations at the MP2(full)/6-311++G(2d,2p) level of theory predict one more minimum for the cis-FA···CO(2) complex. The complex of the higher-energy conformer cis-FA with CO(2) is prepared by vibrational excitation of the ground-state trans-FA conformer combined with thermal annealing. The lifetime of the cis-FA···CO(2) complex in an argon matrix at 10 K is 2 orders of magnitude longer than that of the cis-FA monomer. This big difference is explained by the computational results which show a higher stabilization barrier for the complex. The solvation effects in solid argon are theoretically estimated and their contribution to the stabilization barriers of the higher-energy species is discussed. The relative barrier transmissions for hydrogen tunneling in the cis-FA···CO(2) complex and cis-FA monomer are in good agreement with the experimental decay rates.