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Transition state theory (TST) based on activation parameters computed using quantum mechanics calculations combined with the polarizable continuum model (QM/PCM) is a fundamental tool for investigating reaction rates in the liquid phase. In conventional QM/PCM methods, thermodynamic data and partition functions for a solute are often derived from a quasi-ideal gas treatment (IGT) widely implemented in commercially available computation packages. This approach tends to overestimate entropy because calculations of thermodynamic parameters in the liquid phase ignore hindered translational and rotational modes in real solutions. The present work formulated partition functions for more realistic solutes hindered by surrounding solvent molecules in conjunction with the basic QM/PCM concept. In addition, a configuration partition function for solute molecules at a standard concentration of 1 mol dm−3 was incorporated using a simple lattice model. The canonical partition function and thermodynamic functions were derived based on statistical thermodynamics for localized systems. Expressions for rate coefficients within TST were also derived with a consistent formalism based on the standard state selected in partition function calculations. The performance of the proposed method was assessed by predicting rate coefficients for three different Diels–Alder reactions and comparing these with experimental results. QM/PCM calculations at the G4//ωB97X-D/6-311++G(d,p)/IEF-PCM level of theory with corrections for the dispersion and repulsion energies were performed to obtain the electronic structures of stationary points on potential energy surfaces as a means of finding activation enthalpy, entropy and Gibbs energy values based on revised partition functions as well as predicting rate coefficients. The activation Gibbs energies obtained from our proposed method were lower than those obtained from the IGT method due to reasonable entropy computations. The proposed method overestimated the rate coefficients by one to two orders of magnitude compared to the experimental values, whereas the IGT method underestimated them by the same amount. This discrepancy arises because the proposed method calculates the partition function from the viewpoint of a localized system, whereas the IGT method calculates it from the viewpoint of a non-localized system. Given that actual liquids exist in a state between non-localized and localized systems, it is essential to formulate the partition function in a way that more accurately represents the liquid state. [ABSTRACT FROM AUTHOR]

The present study discusses computational fluid dynamics (CFD) modeling for combustion flows under high-pressure conditions, including supercritical pressure conditions. The real-fluid effects are considered in terms of thermodynamic properties, transport properties, and chemical kinetics. In the present model, the real-fluid effect on chemical kinetics is introduced via a modified equilibrium constant derived using the Gibbs free energy variation of chemical reactions and the fugacity. The results obtained with the present model are compared with available experimental data of high-pressure premixed H 2 / O 2 propagating flames diluted by Ar or He. We demonstrate in the H 2 / O 2 /Ar flame case that the real-fluid model provides a better prediction accuracy for the negative pressure dependence of the mass burning rate compared to the ideal-gas model. The improved prediction accuracy is primarily attributed to the proper estimation of thermodynamic properties such as unburnt-gas enthalpy via an appropriate equation of state and a departure function. The negative pressure dependence of unburnt-gas enthalpy of the H 2 / O 2 /Ar mixture with the real-fluid model significantly affects the flame speed prediction under high-pressure conditions. On the other hand, although the H 2 / O 2 /He mixture shows a positive pressure dependence of enthalpy, differences in the mass burning rate between the real-fluid and ideal-gas models are not significant for the H 2 / O 2 /He flame case. In the He-diluted case, the real-fluid effect is undermined owing to the low density of the H 2 / O 2 /He mixture. Thus, the real-fluid effect appears differently in the prediction of propagating flames depending on the species composition and thermodynamic conditions. The present study suggests that the positive or negative pressure dependence of enthalpy (i.e., the isothermal Joule–Thomson coefficient) is a metric to identify the real-fluid effects that appear. [ABSTRACT FROM AUTHOR]

The ammonium dinitramide‐based ionic liquid propellant (ADN‐based ILP), which is a mixture of ADN, monomethylamine nitrate (MMAN), and urea, is a low toxic monopropellant with a higher performance than that of hydrazine. To clarify the combustion wave structure of ADN‐based ILP, which has low volatility, we focused on the relationship between the phase state and temperature in ADN‐based ILP combustion and on clarifying the gas‐liquid phase reaction. The combustion still image and temperature distribution of ADN‐based ILP were obtained by strand burning tests with a high‐speed camera. As a result, two stages of the stable temperature region were found in the gas‐liquid phase. The pressure dependences of temperature in the stable temperature region were compared with the vapor pressure curves of some chemical substances and with the decomposition temperatures of ADN, MMAN, and urea. Then, it was inferred that the thermal decompositions of ADN, MMAN, and urea, as well as the evaporation of urea had occurred at the first stage of the stable temperature region. Also, it was found that the liquid ammonium nitrate had been dissociated at the second stage of the stable temperature region. For a report on the existence of dissociation products of MMAN and urea vapor on the burning surface at 1.2 MPa, the dissociation of MMAN and evaporation of urea would occur at the first stage of the stable temperature region at 1.2 MPa. As stated above, the combustion wave structure of ADN‐based ILP was developed at 1.2 MPa. [ABSTRACT FROM AUTHOR]

Polymerization by‐products with the formula (SiCl2)n or SinHxCl2n–x are accumulated at the reactor exhaust of SiC chemical vapor infiltration (CVI) from CH3SiCl3/H2, which occur as an explosive hazard. Thermodynamic constants of the chlorinated silane polymers (SiCl2)n and Cl(SiCl2)mCl (n ≤ 5, m ≤ 4) were obtained by quantum chemical calculations using the CBS‐QB3 method. Polymerization reaction pathways of SiCl2 to (SiCl2)n and Cl(SiCl2)mCl (n ≤ 3, m ≤ 3) were also determined using the CBS‐QB3 method, with higher accuracy than in previous reports. The thermodynamic and kinetic constants were incorporated into an existing elementary reaction model of CH3SiCl3 (methyltrichlorosilane; MTS) reacting with H2 to derive the UT2019 model. This enabled the decomposition of MTS into SiCl2 to be studied. Chemical equilibrium calculations and gas‐phase kinetic simulations were performed. The chemical equilibrium calculations revealed that the chlorinated silane polymers, which are explosive by‐products of CVI of SiC, are unstable in the gas phase at any temperature under reduced pressure conditions, suggesting that their formation could be inhibited by controlling the gas‐phase reactions. The gas‐phase kinetic simulations showed that by‐products with a polymerization degree of 3 are not produced above 400°C, but are rapidly produced in the gas phase below 400°C. These findings also apply to the chemical vapor deposition of Si from chlorinated silane precursors. [ABSTRACT FROM AUTHOR]

The kinetics and absolute rate constants of the gas-phase reaction of the hydroxymethyl radical (C[H.sub.2]OH) with molecular oxygen were studied using laser photolysis/near-IR absorption spectroscopy. The results showed slight negative temperature dependence in the measured temperature region.

The crystal structure of the high-pressure phase of hexahydro-1,3,5-trinitro-1,3,5-triazine ([gamma]-RDX) is investigated by using infrared spectroscopy and powder X-ray diffraction measurements followed by Rietveld refinements using a diamond anvil cell (DAC). The molecular structure of the [gamma] phase has displayed the same conformation as that of the alpha phase, but the torsion angles of N-N[O.sub.2] changed marginally.

A study used the flexible potentials and calculated the density of vibrational states and unharmonic coupling constants of crystalline explosives such as PETN(I), PETN(II), RDX, and NM(nitromethane). Results suggested that the energy transfer rate in secondary explosive crystals is a significant factor of their sensitivity and introduction of double bond, or hydrogen bonds, or caged structure into secondary explosives is expected to decrease the sensitivity.

Thermal decomposition of disilane was investigated using time-of-flight(TOF) mass spectrometry coupled with vacuum ultraviolet single-photon ionization (VUV-SPI) at a temperature range of 675-740 K and total pressure of 20-40 Torr. Comparison between the experimental results and the kinetic simulation results suggested that about 70% of consumed disilane was converted to trisilane, which was observed as one of the main reaction products under the present experimental conditions.

The absolute absorption spectrum of the benzyl radical in the visible region measured by ring-down spectroscopy in the visible region measured by cavity ring-down spectroscopy is presented. The absorption cross sections reported would facilitate the quantification of the benzyl radical in future kinetic studies.

Cavity ring-down spectroscopy (CRDS) was used to calculate the electronic absorption spectrum of the allyl radical (CH2CHCH2) between 370 and 420 nm at 297 K. In conclusion, the absorption cross-section of the allyl radical at 402.9 nm was determined to be (2.0 plusmn 0.04) x 10(super -19) cm(super 2) molecule(super -1) through study of time-dependent absorption traces.

A shock tube technique was used to examine the thermal decomposition of a series of primary, secondary and tertiary iodides. Specifically, the competition between the C-I bond fission and the four-center HI elimination in the thermal unimolecular decomposition of C3-C4 alkyl iodides has been analyzed. THe overall rate of constants was measured for primary iodides at temperatures of 950-1100 K. The change in the branching fractions for primary, secondary and tertiary iodides was mainly attributed to the change of the threshold energy for HI elimination processes.