Search

The thermal decomposition of tert-butyl-1,3-cyclopentadiene is investigated in single-pulse shock-tube studies at various pressures and temperatures. The mechanism and rate constants are consistent and have provided support for the H-atom-assisted routes to the conversion of fulvene to benzene, which is proposed on the basis of theoretical investigations.

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]

Knudsen cell reactor measurements of heterogeneous reactions on the surfaces of sulfuric acid reveal that the rapid heterogeneous transformation of N2O5 to HNO3 influences the nitrogen species partitioning in the global stratosphere, resulting in ozone depletion. The low solubility of HNO3 indicates that HNO3 enters gas phase and does not cause denitrification in the global stratosphere. HCl is present in low levels in the sulfuric acid and is not involved in the depletion process.

We have used the single‐pulse shock tube technique with postshock GC/MS product analysis to investigate the mechanism and kinetics of the unimolecular decomposition of isopropanol, a potential biofuel, and of its reaction with H atoms at 918‐1212 K and 183‐484 kPa. Experiments employed dilute mixtures in argon of isopropanol, a radical scavenger, and, for H‐atom studies, two different thermal precursors of H. Without an added H source, isopropanol decomposes in our studies predominantly by molecular dehydration. Added H atoms significantly augment decomposition, mainly by abstraction of the tertiary and primary hydrogens, reactions that, respectively, lead to acetone and propene as stable organic products. Traces of acetaldehyde were observed in some experiments above ≈ 1100 K and establish branching limits for minor decomposition pathways. To quantitatively account for secondary chemistry and optimize rate constants of interest, we employed the method of uncertainty minimization using polynomial chaos expansions (MUM‐PCE) to carry out a unified analysis of all datasets using a chemical model–based originally on JetSurF 2.0. We find: k(isopropanol → propene + H2O) = 10(13.87 ± 0.69) exp(−(33 099 ± 979) K/ T) s−1 at 979‐1212 K and 286‐484 kPa, with a factor of two uncertainty (2σ), including systematic errors. For H atom reactions, optimization yields: k(H + isopropanol → H2 + p‐C3H6OH) = 10(6.25 ± 0.42)T2.54 exp(−(3993 ± 1028) K /T) cm3 mol−1 s−1 and k(H + isopropanol → H2 + t‐C3H6OH) = 10(5.83 ± 0.37)T2.40 exp(−(1507 ± 957) K /T) cm3 mol−1 s−1 at 918‐1142 K and 183‐323 kPa. We compare our measured rate constants with estimates used in current combustion models and discuss how hydrocarbon functionalization with an OH group affects H abstraction rates. [ABSTRACT FROM AUTHOR]

A single pulse shock tube with product analysis is employed to study the kinetics of the isomerization and decomposition reactions taking place in the pyrolysis of branched hydrocarbons, 4-methyl-1-pentyl radicals. The reaction rates are shown to be highly affected by the presence of the branched methyl moiety.

The relative rates of C-C and C-H β-scission reactions of isobutyl radicals (2-methylprop-1-yl, C4H9) were investigated with shock tube experiments at temperatures of (950 to 1250) K and pressures of (200 to 400) kPa. We produced isobutyl radicals from the decomposition of dilute mixtures of isopentylbenzene and observed the stable decomposition products, propene and isobutene. These alkenes are characteristic of C-C and C-H bond scission, respectively. Propene was the main product, approximately 30 times more abundant than isobutene, indicating that C-C β-scission is the primary pathway. Uncertainty in the ratio of [isobutene]/[propene] from isobutyl decomposition is mainly due to a small amount of side chemistry, which we account for using a kinetics model based on JetSurF 2.0. Our data are well-described after adding chemistry specific to our system and adjusting some rate constants. We compare our data to other commonly used kinetics models: JetSurF 2.0, AramcoMech 2.0, and multiple models from Lawrence Livermore National Laboratory (LLNL). With the kinetics model, we have determined an upper limit of 3.0% on the branching fraction for C-H β-scission in the isobutyl radical for the temperatures and pressures of our experiments. While this agrees with previous high quality experimental results, many combustion kinetics models assume C-H branching values above this upper limit, possibly leading to large systematic inaccuracies in model predictions. Some kinetics models additionally assume contributions from 1,2-H shift reactions-which for isobutyl would produce the same products as C-H β-scission-and our upper limit includes possible involvement of such reactions. We suggest kinetics models should be updated to better reflect current experimental measurements [ABSTRACT FROM AUTHOR]