Raman spectrum of supercritical C(18)O2 and re-evaluation of the Fermi resonance

We report the first Raman spectra of fully (18)O-labeled supercritical CO(2) (scCO(2)) and various isotopic mixtures. The experimental results, coupled with ab initio molecular dynamics calculations, demonstrate that the frequencies assigned to the Fermi dyad of the CO(2) molecule transpose upon isotopic labeling of both oxygen atoms. Although the transposition of the Fermi dyad of CO(2) gas due to isotopic substitution has been discussed before, this is the first confirmation of the effect in the Raman spectrum of the supercritical fluid and provides necessary groundwork for future Raman spectroscopy studies of reactions in this important medium. More importantly, the work yields a quantitative assessment of the mixing of states upon labeling that provides the needed clarification concerning the pedigree of the assignments for the dyad of CO(2) under supercritical conditions.     

High-pressure speed of sound and compressibilities in heavy normal hydrocarbons:n-C23H48andn-C24H50

The speed of sound in the normal hydrocarbonsn- C₂₃H₄₈andn- C₂₄H₅₀has been measured up to 150 MPa in the temperature range (293.15 to 373.15) K using a pulse technique operating at 3 MHz. The data have been used to evaluate various thermophysical properties such as density, and isentropic and isothermal compressibilities up to 150 MPa with the help of additional density measurements performed at atmospheric pressure.     

Study on the reaction of CH2 with H2 at high temperature

Thermal decomposition of CH(2)I(2) [sequential C-I bond fission processes, CH(2)I(2) + Ar → CH(2)I + I + Ar (1a) and CH(2)I + Ar → (3)CH(2) + I + Ar (1b)], and the reactions of (3)CH(2) + H(2) → CH(3) + H (2) and (1)CH(2) + H(2) → CH(3) + H (3) have been studied by using atomic resonance absorption spectrometry (ARAS) of I and H atoms behind reflected shock waves. Highly diluted CH(2)I(2) (0.1-0.4 ppm) with/without excess H(2) (300 ppm) in Ar has been used so that the effect of the secondary reactions can be minimized. From the quantitative measurement of I atoms in the 0.1 ppm CH(2)I(2) + Ar mixture over 1550-2010 K, it is confirmed that two-step sequential C-I bond fission processes of CH(2)I(2), (1a) and (1b), dominate over other product channels. The decomposition step (1b) is confirmed to be the rate determining process to produce (3)CH(2) and the least-squares analysis of the measured rate gives, ln(k(1b)/cm(3) molecule(-1) s(-1)) = -(17.28 ± 0.79) - (30.17 ± 1.40) × 10(3)/T. By utilizing this result, we examine reactions 2 and 3 by monitoring evolution of H atoms in the 0.2-0.4 ppm CH(2)I(2) + 300 ppm H(2) mixtures over 1850-2040 K. By using a theoretical result on k(2) (Lu, K. W.; Matsui, H.; Huang, C.-L.; Raghunath, P.; Wang, N.-S.; Lin, M. C. J. Phys. Chem. A 2010, 114, 5493), we determine the rate for (3) as k(3)/cm(3) molecule(-1) s(-1) = (1.27 ± 0.36) × 10(-10). The upper limit of k(3) (k(3max)) is also evaluated by assuming k(2) = 0, i.e., k(3max)/cm(3) molecule(-1) s(-1) = (2.26 ± 0.59) × 10(-10). The present experimental results on k(3) and k(3max) is found to agree very well with the previous frequency modulation spectroscopy study (Friedrichs, G.; Wagner, H. G. Z. Phys. Chem. 2001, 215, 1601); i.e., the importance of the contribution of (1)CH(2) in the reaction of CH(2) with H(2) at elevated temperature range is reconfirmed.     

Reversible adsorption deformation of layered adsorbents at high pressures and temperatures

Isotherms of excessive adsorption of N₂ and Ar on three montmorillonite samples (natural sodium (NaMt) and ion-exchange pyridinium (PyMt) and polyhydroxyaluminum (PGAMt)) were measured in the pressure interval from 0.1 to 60 MPa and at temperatures 303, 343, 373, and 400 K. The results of measurements showed the inverse temperature dependence of the measured isotherms of excessive adsorption at elevated pressures and temperatures: for the isotherms at higher temperatures the amounts adsorbed exceed the values measured at lower temperatures. The deformation of montmorillonites with the temperature increase results in an increase in the adsorption volume of the adsorbent.     

Conformational and phase transformations of chlorocyclohexane at high pressures by Raman spectroscopy

Pressure induced conformational and phase transformations of chlorocyclohexane (CCH) were investigated in a diamond anvil cell by Raman spectroscopy at room temperature. Pure CCH was compressed up to 20 GPa and then decompressed to ambient pressure. The conformational equilibrium was shifted by pressure from equatorial to axial conformers in the fluid phase below 0.7 GPa, consistent with previous observations. Upon further compression, several solid-to-solid phase transitions were identified by the observation of markedly different Raman patterns as well as different pressure dependences of characteristic Raman modes. The possible structures of these phases were analyzed in correlation with previously observed solid phases at low temperatures. Finally, CCH exhibits pressure hysteresis and partial reversibility upon decompression which result in the formation of the phases with different Raman patterns from those obtained upon compression. The difference can be interpreted as conformational contribution as well as the intrinsic plasticity of CCH crystals.     

Catalytic activity of Pt and Pd in gaseous reactions of H2 and CH4 oxidation at low pressures

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The diffusion of CO2, CH4, C2H4, and C3H8 in polyethylene at elevated pressures

Diffusion and solubility coefficients have been determined for the CO₂^?, CH₄^?, C₂H₄^?, and C₃H₈-polyethylene systems at temperatures of 5, 20, and 35°C and at gas pressures up to 40 atm. Diffusion coefficients were obtained from rates of gas absorption in polyethylene rods under isothermal-isobaric conditions by means of a new diffusivity apparatus. The concentration dependence of the diffusion coefficients was represented satisfactorily by Fujita's free-volume model, modified for semicrystalline polymers, while the solubility of all the penetrants in polyethylene was within the limit of Henry's law. Semiempirical correlations were found for the free-volume parameters in terms of physicochemical properties of the penetrant gases and the penetrant-polymer systems. These correlations, if confirmed, should permit the prediction of diffusion and permeability coefficients of other gases and of gas mixtures in polyethylene as functions of pressure and temperature.     

Fourier transform Raman spectroscopy at high pressures: Preliminary results of sulphur to 56 kbar

The feasibility of obtaining Fourier transform (FT) Raman spectra at high pressure in a diamond anvil cell has been investigated. By using a high pressure cell in conjunction with a micro-FT-Raman system, FT-Raman spectra with good signal-to-noise ratios have been measured for sulphur to 56 kbar. In general, the internal modes show significant increases with pressure. No phase transition was observed throughout the pressure range studied.     

Some peculiarities of interactions in the Hf2Fe−H2 system at low temperatures and high pressures

Hydride formation was studied in the Hf₂Fe−H₂ system at hydrogen pressure of up to 2000 atm in a temperature range from 195 to 295 K. Hydride phases of different compositions were studied by the X-ray diffraction method. The hydrogenation reaction in the system can take two pathways to form two stable hydride phases depending on the conditions of initial hydrogenation. Absorption of hydrogens at a pressure of about 2000 atm yields a hydride which contains two H atoms per metal atom. Models of the arrangement of hydrogen atoms in the crystal lattice of hydride phases were suggested. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 33–36, January, 1997.     

Clathrate formation in binary aqueous systems with CH2Cl2, CHCl3 and CCl4 at high pressures

P,T,X phase diagrams of the CH₂Cl₂-H₂O, the CHCl₃-H₂O and the CCl₄-H₂) systems have been studied by DTA in the pressure range 10^–3 to 5.0 kbar. Under pressure the cubic structure II (CS-II) hydrates forming in all the systems are replaced by hydrates with the composition M·7.3 H₂O whose stoichiometry and positive dT/dP values of melting lead us to believe that they are CS-I hydrates.In the CH₂Cl₂ and CHCl₃ systems the nonvariant point coordinates of the hydrate transformationQ ₂ ^h (l₁h17h7l₂, where l₁ and l₂ are liquid phases abundant in water and hydrate former, respectively, h₁₇ and h₇ are hydrates with hydrate numbers 17 and 7, respectively) areP = 0.6 kbar, T = –1.5°C andP =2.65 kbar,T = –10.5°C, respectively. In the CCl₄ system the 4-phaseQ ₃ ^h point (l₁h₁₇h₇s, where s is crystalline CCl₄) has coordinatesP = 0.75 kbar and T = 0.4°C.The main obstacle of the present study, the very slow achievement of equilibrium, has been eliminated by adding small amounts (0.25% by mass) of surfactants followed by ultrasonic mixing. We have shown that this accelerates the achievement of equilibrium without changing its position.     

Fermi resonance - the theory of isotropic and anisotropic Raman spectra in dilute solutions

Raman scattering from dilute solution was studied limiting considerations to one active molecule, in which the Fermi resonance occurs, interacting with the molecules of an inert solvent. For such a system, the isotropic and anisotropic correlation functions ⧸CFs⧸ for symmetric top molecule have been calculated and examples of Fermi resonance spectra ⧸FRS⧸ are presented.     

Experimental measurements and kinetic modeling of CH4/O2 and CH4/C2H6/O2 conversion at high pressure

A detailed chemical kinetic model for homogeneous combustion of the light hydrocarbon fuels CH₄ and C₂H₆ in the intermediate temperature range roughly 500–1100 K, and pressures up to 100 bar has been developed and validated experimentally. Rate constants have been obtained from critical evaluation of data for individual elementary reactions reported in the literature with particular emphasis on the conditions relevant to the present work. The experiments, involving CH₄/O₂ and CH₄/C₂H₆/O₂ mixtures diluted in N₂, have been carried out in a high‐pressure flow reactor at 600–900 K, 50–100 bar, and reaction stoichiometries ranging from very lean to fuel‐rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Finally, the mechanism was extended with a number of reactions important at high temperature and tested against data from shock tubes, laminar flames, and flow reactors. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 778–807, 2008     

Ethene elimination from CH3CH2NH=CH 2 + : Reaction pathways at the boundary between stepwise and concerted processes

The elimination of ethene from CH₃CH₂NH=CH ₂ ⁺ is characterized by ab initio procedures. This reaction occurs through several asynchronous stages, but without passing through formal intermediates. A potential energy barrier to hydrogen migration from the β carbon to N is largely determined by the energy required to cleave the CN bond, but is lowered slightly by H transfer from the β to the α carbon and then to N. The complex [C₂H ₅ ⁺ NH=CH₂] is bypassed, even though that complex could exist at energies only slightly above that of the transition state for ethene elimination. Furthermore, conversion of a substantial reverse activation energy into energy of motion causes CH₂=NH ₂ ⁺ and CH₂=CH₂ to dissociate faster than they can form [CH₂=NH ₂ ⁺ CH₂=CH₂]. Comparison of results for CH₃CH₂NH=CH ₂ ⁺ to ab initio ones for methane from CH₃CH₂CH ₃ ⁺ and elimination of ethene from CH₃CH₂O=CH ₂ ⁺ and CH₃CH₂CH=OH⁺ reveals that these dissociations occur in a similar but, in each case, a distinct series of asynchronous steps or stages, and that there is no sharp demarcation between concerted and stepwise eliminations as presently defined. In dissociations of CH₃CH₂NH=CH ₂ ⁺ , loss of electron density at the C in the breaking N bond leads the transfer of electron density to that carbon by migration of a hydrogen from the adjacent C. We attribute this to a requirement for the moving H to be close to Cα before the moving H can start to develop covalent bonding to Cα. It is also concluded that elimination of ethene from CH₃CH₂NH=CH ₂ ⁺ avoids a Woodward-Hoffmann symmetry-imposed barrier by H migrating sufficiently from the β to the α carbon on the way to N, so that the dissociation is essentially a 1,1 rather than a 1,2 elimination.     

Raman spectroscopy under high pressures and DFT calculations of the amino acid l-glutamine

l-glutamine crystal was obtained by the slow evaporation method and its crystallographic structure was verified by X-ray diffraction experiments and the Rietveld method. The vibrational modes of l-glutamine were investigated through Raman spectroscopy and the normal modes were obtained using the Density Functional Theory with the B3LYP functional and set of bases 6-31G++(d, p). With such approach, it was possible to make a theoretical-experimental comparison of the results obtained and to furnish a more precise assignment of the normal modes. The crystal was submitted to high pressure conditions and the Raman spectra between 3055 and 40 cm⁻¹ were recorded for pressures up to 6.1 GPa in a diamond anvil cell. This study allowed us to understand that the crystal undergoes a reversible structural phase transition around 4.0 GPa, characterized mainly by spectral changes in the region of the external modes.     

Crystal structure and stability of Tl2CO3 at high pressures

The crystal structure of dithallium carbonate, Tl₂CO₃ (C2/m, Z = 4), was investigated at pressures of up to 7.4 GPa using single‐crystal X‐ray diffraction in a diamond anvil cell. It is stable to at least 5.82 GPa. All atoms except for one of the O atoms lie on crystallographic mirror planes. At higher pressures, the material undergoes a phase transition that destroys the single crystal.     

CH_3自由基与C_2H_5CN高温反应机理的理论研究

用G3(MP2)//B3PW91/6-311G(d,p)双级别方法研究了CH_3自由基与C_2H5_CN的反应机理和动力学性质.计算表明反应存在抽氢、加成-消除和取代3种机理7条反应通道.用CVT方法计算了所有反应通道在1 000K~3 000K温度范围内的速率常数,结果表明计算值与实验值符合得很好.