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1. IntroductionWe have compiled gas phase enthalpies of formation for nine hydrogen-oxygen species ([H.sub.x][O.sub.y]) and selected recommended values for H, O, OH, [H.sub.2]O, H[O.sub.2], [H.sub.2][O.sub.2], [O.sub.3], H[O.sub.3], and [H.sub.2][O.sub.3]. Compared [...]

In this work, we compiled and evaluated rate expressions for reactions relevant to the decomposition and combustion of CH2F2 (difluoromethane, refrigerant R‐32) in CH2F2/O2/N2 flames. The recommended values have been used in premixed flame calculations, reported elsewhere, to model experimentally derived burning velocities determined using a constant volume spherical flame method. In this work, we also provide a detailed description of the reaction pathways for decomposition and combustion of CH2F2. This work is part of a larger effort at NIST to characterize and predict the flammability of new refrigerant working fluids and their blends for consideration as replacements of current refrigerants with high global warming potentials. [ABSTRACT FROM AUTHOR]

Inthis article, the first-principles prediction of enthalpiesof formation is demonstrated for 669 polycyclic aromatic hydrocarbon(PAH) compounds and a number of related functionalized molecules.It is shown that by extrapolating density functional theory calculationsto a large basis set limit and then applying a group based correctionscheme that good results may be obtained. Specifically, a mean unsigneddeviation and root mean squared deviation from the experimental enthalpiesof formation data of 5.0 and 6.4 kJ/mol, respectively, are obtainedusing this scheme. This computational scheme is economical to computeand straightforward to apply, while yielding results of reasonablereliability. The results are also compared for a smaller set of moleculesto the predictions given by the G3B3 and G3MP2B3 variants of the Gaussian-3model chemistry with a mean unsigned deviation and root mean squareddeviation from the experimental enthalpies of formation of 4.5 and4.8 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

In this article, recent work on calculating high-quality enthalpies of formation for polycyclic aromatic hydrocarbons (PAHs) based on both density functional theory (DFT) and Gaussian-3 (G3) model chemistry methods is discussed. It is shown that through the use of an empirical correction model, the systematic errors in low-level DFT calculations can be controlled to produce reliable thermochemistry. It is further shown that the G3 results have a more regular systematic error and as a consequence a simpler model may be used to correct for systematic deviations. It is seen that the resulting enthalpy of formation values are in good agreement with the available experimental data, and that the predictions are sufficiently robust to point out certain errors in experimental determinations. This work was done as part of a larger effort to create a curated data set of property data for a large set of PAH molecules. In furtherance of this goal, UV/Vis spectra have been computed using time-dependent DFT. The collection of PAH data is being made publicly available through a web-based database that is briefly described. [ABSTRACT FROM AUTHOR]

The decomposition and intramolecular H-transfer isomerizationreactions of the 1-pentyl radical have been studied at temperaturesof 880 to 1055 K and pressures of 80 to 680 kPa using the single pulseshock tube technique and additionally investigated with quantum chemicalmethods. The 1-pentyl radical was generated by shock heating dilutemixtures of 1-iodopentane and the stable products of its decompositionhave been observed by postshock gas chromatographic analysis. Etheneand propene are the main olefin products and account for >97% ofthe carbon balance from 1-pentyl. Also produced are very small amountsof (E)-2-pentene, (Z)-2-pentene,and 1-butene. The ethene/propene product ratio is pressure dependentand varies from about 3 to 5 over the range of temperatures and pressuresstudied. Formation of ethene and propene can be related to the concentrationsof 1-pentyl and 2-pentyl radicals in the system and the relative ratesof five-center intramolecular H-transfer reactions and β C–Cbond scissions. The 3-pentyl radical, formed via a four-center intramolecularH transfer, leads to 1-butene and plays only a very minor role inthe system. The observed (E/Z)-2-pentenescan arise from a small amount of beta C–H bond scission inthe 2-pentyl radical. The current experimental and computational resultsare considered in conjunction with relevant literature data from lowertemperatures to develop a consistent kinetics model that reproducesthe observed branching ratios and pressure effects. The present experimentalresults provide the first available data on the pressure dependenceof the olefin product branching ratio for alkyl radical decompositionat high temperatures and require a value of ⟨ΔEdown(1000 K)⟩ = (675 ± 100) cm–1for the average energy transferred in deactivatingcollisions in an argon bath gas when an exponential-down model isemployed. High pressure rate expressions for the relevant H-transferreactions and β bond scissions are derived and a Rice RamsbergerKassel Marcus/Master Equation (RRKM/ME) analysis has been performedand used to extrapolate the data to temperatures between 700 and 1900K and pressures of 10 to 1 × 105kPa. [ABSTRACT FROM AUTHOR]