4,882 research outputs found
Rate constants of the fulvenallenyl recombination with propargyl and its role in PAH formation: a theoretical and kinetic modeling study
No full textThe temperature- and pressure-dependent rate constants of the reaction of fulvenallenyl (C7H5) and propargyl (C3H3) radicals have been explored using advanced electronic structure methods and kinetics theories. The results show that the head-C3H3 + tail-C7H5 addition is the fastest, followed by the tail + tail addition, and that at typical combustion conditions of 1500 K and 1 atm, the reaction mostly results in collisional stabilization of entrance channel adducts without further cyclization, whereas the well-skipping pathway to fulvalene - a precursor to naphthalene, prevails at high temperatures. The formation of the aromatic two-ring isomers, naphthalene, methylene-indenes, and azulene, can be enhanced only at high temperatures and much lower pressures, when the collisional stabilization of the intermediate wells is not efficient. The computed phenomenological rate constants were consequently simplified using master equation-based lumping and the pseudo steady state approximation to reduce the size of the system. This post-processing confirmed fulvalene as the main two-ring aromatic product and thus, its contribution should be considered in kinetic models. While naphthalene is only a trace product of the C7H5 + C3H3 reaction, it can be formed from fulvalene via H-assisted isomerization. The impact of the updated thermochemistry and rate constants in CRECK and ITV kinetic models on mole fractions of relevant cyclic aromatic species was also analyzed. The results suggest that the relevance of the C7H5 + C3H3 reaction has been likely overestimated in the current literature and needs to be carefully reevaluated by revising the pathways involving the formation and consumption of C7H6 and C7H5 and improving predictions of their precursors, such as unsaturated C4 species in flames
Elaine M. Alexander: 2023 Cook Prize Gold Medal Acceptance Speech
No full textAuthor Elaine M. Alexander gives an acceptance speech for Anglerfish: The Seadevil of the Deep, illustrated by Fiona Fogg (Candlewick)https://educate.bankstreet.edu/cook/1007/thumbnail.jp
p-p* vibronic spectrum of ethylene from ab initio calculations of the Franck-Condon factors
No full textA theoretical study of the structure, energetics and the n-p* electronic transition of the acetone + nH2O (n=1-3) comple_TTS.DATE
No full textKenneth M Alexander - Author and Artist
Get PDFI was born to Dennis and Kathleen Alexander in a single motor garage at 21 Limerick Road in Athlone. In those days, the midwife would do her rounds on a bicycle at the time when the stork was seen flying over the now-collapsed, missing going, gone forever Athlone Towers. Either that or she went to the foot of Table Mountain and placed a hollowed out pumpkin with a precision cut hole in one side. The monkey would come, stick his or her hand in the hole, grab some pips and in trying to pull its hand out in a fist, it gets stuck. The midwife then pounces on the helpless monkey, knocks it out with her case, and then stuffs "it" into that same black case and off she motors on her "dik" wheel bicycle to deliver the latest addition to an Athlone family. The monkey cries with relief when let out of the case. I have since moved on from that belief system. For some reason, the majority of the employers I worked for still believe that. In fact, far too many white people still do. To them we are monkeys and they pay us with peanuts
The Fragmentation of Melamine: A Study via Electron-Impact Ionization, Laser-Desorption Ionization, Collision-Induced Dissociation, and Density Functional Calculations of Potential Energy Surface
No full textMechanism and kinetics of the oxidation of propargyl radical by atomic oxygen
No full textAutomated reaction path discovery tools have been used to map out the C3H3O potential energy surface and determine the prevailing channels for oxidation of the propargyl radical with atomic oxygen. The energy of the stationary points was then evaluated at the CCSD(T)/CBS//ωB97X-D/6-311+G(d,p) level of electronic structure theory. The C3H3 + O total and individual product channel rate constants were evaluated with Rice-Ramsperger-Kassel-Marcus Master Equation calculations combined with variable reaction coordinate transition state theory assessment of barrierless entrance channels. The total reaction rate so determined is in quantitative agreement with experiments, highlighting the relevance of accounting for C3H3 + O recombination on the excited electronic state surface. C2HCHO (propynal) + H are predicted as the prevailing reaction products, followed by CH2CCO (propadienone) + H, with minor contributions from C2H2 + HCO and C2H3 + CO. The calculated rate constants have been utilized in kinetic simulations, which tested the impact of the C3H3 + O reaction on chemical reactivity in premixed laminar flames of acetylene, ethylene, allene, propyne, and benzene. The results displayed macroscopic changes of the model predictions at low pressures (< 0.1 atm) and rich conditions, a reduction in the consumption of propargyl via C3H3 + O, and a substantial decrease in the C2H2 + HCO reactivity to C2H3 + CO. The updated model showed an improved performance in predicting C2 species in the considered flames
Fulvenallenyl Radical (C7H5·)-Mediated Gas-Phase Synthesis of Bicyclic Aromatic C10H8 Isomers: Can Fulvenallenyl Efficiently React with Closed-Shell Hydrocarbons?
No full textTo understand the reactivity of resonantly stabilized radicals, often found in relevant concentrations in gaseous environments, it is important to determine their main reaction pathways. Here, it is investigated whether the fulvenallenyl radical (C7H5·) reacts preferentially with closed-shell molecules or radicals. Electronic structure calculations on the C10H9 potential energy surface accessed by the reactions of C7H5· with methylacetylene (CH3CCH) and allene (H2CCCH2) were combined with RRKM-ME calculations of temperature- and pressure-dependent rate constants using the automated EStokTP software suite and kinetic modeling to assess the reactivity of C7H5· with closed-shell unsaturated hydrocarbons. Experimentally, the reactions were attempted in a chemical microreactor heated to 998 ± 10 K by preparing fulvenallenyl radicals via pyrolysis of trichloromethylbenzene (C7H5Cl3) and seeding the radicals in methylacetylene or allene carrier gas, with product identification by means of photoionization mass spectrometry. The measured photoionization efficiency curve of m/z = 128 was assigned to a linear combination of the reference curves of two C10H8 isomers, azulene (minor) and naphthalene (major), presumably resulting from the C7H5· plus C3H4 reactions. However, the calculations demonstrated that these reactions are too slow, and kinetic modeling of processes in the reactor allowed us to conclude that the observation of naphthalene and azulene is due to the C7H5· plus C3H3· reaction, where propargyl is produced by direct hydrogen atom abstraction by chlorine (Cl) atoms from allene or methylacetylene and Cl stem from the pyrolysis of C7H5Cl3. Modeling results under the copyrolysis conditions of toluene and methylacetylene in high-temperature shock tube experiments confirmed the prevalence of the fulvenallenyl reaction with propargyl over its reactions with C3H4 even when the concentrations of allene and methylacetylene largely exceed that of propargyl. Overall, the reactions of fulvenallenyl with both allene and methylacetylene were found to be noncompetitive in the formation of naphthalene and azulene thus attesting the inefficiency of the fulvenallenyl radical reactions with the prototype closed-shell hydrocarbon species. In the meantime, the new reaction pathways revealed, including H-assisted isomerizations between C10H8 isomers and decomposition reactions of various C10H9 isomers, emerge as relevant and are recommended for inclusion in combustion kinetic models for naphthalene formation
Alexander, the Crown Prince
No full textThe Life of Alexander by Plutarch and the Alexander Romance by Ps.-Callisthenes are the only two sources that deal with Alexander’s birth, childhood and youth. Both works deliver a large number of anectodes: but only in the case of Plutarch’s biography do these anecdotes maintain a firm connection with reality. In the Alexander Romance, the author offers the reader a story full of plot twists. Alexander’s youth ended abruptly in the autumn of 336, when his father Philip II was murdered in Aigai, during the ceremonies organized to celebrate the wedding between Cleopatra, Philip’s daughter and Alexander’s sister, and the king of Epirus, Alexander called Molossos: the crown prince became king in a sudden and most unexpected way
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