9 research outputs found

    Ab Initio Computations and Active Thermochemical Tables Hand in Hand: Heats of Formation of Core Combustion Species

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    The fidelity of combustion simulations is strongly dependent on the accuracy of the underlying thermochemical properties for the core combustion species that arise as intermediates and products in the chemical conversion of most fuels. High level theoretical evaluations are coupled with a wide-ranging implementation of the Active Thermochemical Tables (ATcT) approach to obtain well-validated high fidelity predictions for the 0 K heat of formation for a large set of core combustion species. In particular, high level ab initio electronic structure based predictions are obtained for a set of 348 C, N, O, and H containing species, which corresponds to essentially all core combustion species with 34 or fewer electrons. The theoretical analyses incorporate various high level corrections to base CCSD­(T)/cc-pVnZ analyses (n = T or Q) using H<sub>2</sub>, CH<sub>4</sub>, H<sub>2</sub>O, and NH<sub>3</sub> as references. Corrections for the complete-basis-set limit, higher-order excitations, anharmonic zero-point energy, core–valence, relativistic, and diagonal Born–Oppenheimer effects are ordered in decreasing importance. Independent ATcT values are presented for a subset of 150 species. The accuracy of the theoretical predictions is explored through (i) examination of the magnitude of the various corrections, (ii) comparisons with other high level calculations, and (iii) through comparison with the ATcT values. The estimated 2σ uncertainties of the three methods devised here, ANL0, ANL0-F12, and ANL1, are in the range of ±1.0–1.5 kJ/mol for single-reference and moderately multireference species, for which the calculated higher order excitations are 5 kJ/mol or less. In addition to providing valuable references for combustion simulations, the subsequent inclusion of the current theoretical results into the ATcT thermochemical network is expected to significantly improve the thermochemical knowledge base for less-well studied species

    Resolving Some Paradoxes in the Thermal Decomposition Mechanism of Acetaldehyde

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    The mechanism for the thermal decomposition of acetaldehyde has been revisited with an analysis of literature kinetics experiments using theoretical kinetics. The present modeling study was motivated by recent observations, with very sensitive diagnostics, of some unexpected products in high temperature microtubular reactor experiments on the thermal decomposition of CH<sub>3</sub>CHO and its deuterated analogs, CH<sub>3</sub>CDO, CD<sub>3</sub>CHO, and CD<sub>3</sub>CDO. The observations of these products prompted the authors of these studies to suggest that the enol tautomer, CH<sub>2</sub>CHOH (vinyl alcohol), is a primary intermediate in the thermal decomposition of acetaldehyde. The present modeling efforts on acetaldehyde decomposition incorporate a master equation reanalysis of the CH<sub>3</sub>CHO potential energy surface (PES). The lowest-energy process on this PES is an isomerization of CH<sub>3</sub>CHO to CH<sub>2</sub>CHOH. However, the subsequent product channels for CH<sub>2</sub>CHOH are substantially higher in energy, and the only unimolecular process that can be thermally accessed is a reisomerization to CH<sub>3</sub>CHO. The incorporation of these new theoretical kinetics predictions into models for selected literature experiments on CH<sub>3</sub>CHO thermal decomposition confirms our earlier experiment and theory-based conclusions that the dominant decomposition process in CH<sub>3</sub>CHO at high temperatures is C–C bond fission with a minor contribution (∼10–20%) from the roaming mechanism to form CH<sub>4</sub> and CO. The present modeling efforts also incorporate a master-equation analysis of the H + CH<sub>2</sub>CHOH potential energy surface. This bimolecular reaction is the primary mechanism for removal of CH<sub>2</sub>CHOH, which can accumulate to minor amounts at high temperatures, <i>T</i> > 1000 K, in most lab-scale experiments that use large initial concentrations of CH<sub>3</sub>CHO. Our modeling efforts indicate that the observation of ketene, water, and acetylene in the recent microtubular experiments are primarily due to bimolecular reactions of CH<sub>3</sub>CHO and CH<sub>2</sub>CHOH with H-atoms and have no bearing on the unimolecular decomposition mechanism of CH<sub>3</sub>CHO. The present simulations also indicate that experiments using these microtubular reactors when interpreted with the aid of high-level theoretical calculations and kinetics modeling can offer insights into the chemistry of elusive intermediates in the high-temperature pyrolysis of organic molecules

    Unconventional Peroxy Chemistry in Alcohol Oxidation: The Water Elimination Pathway

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    Predictive simulation for designing efficient engines requires detailed modeling of combustion chemistry, for which the possibility of unknown pathways is a continual concern. Here, we characterize a low-lying water elimination pathway from key hydroperoxyalkyl (QOOH) radicals derived from alcohols. The corresponding saddle-point structure involves the interaction of radical and zwitterionic electronic states. This interaction presents extreme difficulties for electronic structure characterizations, but we demonstrate that these properties of this saddle point can be well captured by M06-2X and CCSD­(T) methods. Experimental evidence for the existence and relevance of this pathway is shown in recently reported data on the low-temperature oxidation of isopentanol and isobutanol. In these systems, water elimination is a major pathway, and is likely ubiquitous in low-temperature alcohol oxidation. These findings will substantially alter current alcohol oxidation mechanisms. Moreover, the methods described will be useful for the more general phenomenon of interacting radical and zwitterionic states

    Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a Room-Temperature Flow Reactor

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    Chirped-pulse Fourier transform millimeter-wave spectroscopy is a potentially powerful tool for studying chemical reaction dynamics and kinetics. Branching ratios of multiple reaction products and intermediates can be measured with unprecedented chemical specificity; molecular isomers, conformers, and vibrational states have distinct rotational spectra. Here we demonstrate chirped-pulse spectroscopy of vinyl cyanide photoproducts in a flow tube reactor at ambient temperature of 295 K and pressures of 1–10 μbar. This <i>in situ</i> and time-resolved experiment illustrates the utility of this novel approach to investigating chemical reaction dynamics and kinetics. Following 193 nm photodissociation of CH<sub>2</sub>CHCN, we observe rotational relaxation of energized HCN, HNC, and HCCCN photoproducts with 10 μs time resolution and sample the vibrational population distribution of HCCCN. The experimental branching ratio HCN/HCCCN is compared with a model based on RRKM theory using high-level ab initio calculations, which were in turn validated by comparisons to Active Thermochemical Tables enthalpies

    Temperature and Pressure-Dependent Rate Coefficients for the Reaction of Vinyl Radical with Molecular Oxygen

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    State-of-the-art calculations of the C<sub>2</sub>H<sub>3</sub>O<sub>2</sub> potential energy surface are presented. A new method is described for computing the interaction potential for R + O<sub>2</sub> reactions. The method, which combines accurate determination of the quartet potential along the doublet minimum energy path with multireference calculations of the doublet/quartet splitting, decreases the uncertainty in the doublet potential and thence the rate constants by more than a factor of 2. The temperature- and pressure-dependent rate coefficients are computed using variable reaction coordinate transition-state theory, variational transition-state theory, and conventional transition-state theory, as implemented in a new RRKM/ME code. The main bimolecular product channels are CH<sub>2</sub>O + HCO at lower temperatures and CH<sub>2</sub>CHO + O at higher temperatures. Above 10 atm, the collisional stabilization of CH<sub>2</sub>CHOO directly competes with these two product channels. CH<sub>2</sub>CHOO decomposes primarily to CH<sub>2</sub>O + HCO. The next two most significant bimolecular products are OCHCHO + H and <sup>3</sup>CHCHO + OH, and not C<sub>2</sub>H<sub>2</sub> + HO<sub>2</sub>. C<sub>2</sub>H<sub>3</sub> + O<sub>2</sub> will be predominantly chain branching above 1700 K. Uncertainty analysis is presented for the two most important transition states. The uncertainties in these two barrier heights result in a significant uncertainty in the temperature at which CH<sub>2</sub>CHO + O overtakes all other product channels

    Temperature and Pressure-Dependent Rate Coefficients for the Reaction of Vinyl Radical with Molecular Oxygen

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    State-of-the-art calculations of the C<sub>2</sub>H<sub>3</sub>O<sub>2</sub> potential energy surface are presented. A new method is described for computing the interaction potential for R + O<sub>2</sub> reactions. The method, which combines accurate determination of the quartet potential along the doublet minimum energy path with multireference calculations of the doublet/quartet splitting, decreases the uncertainty in the doublet potential and thence the rate constants by more than a factor of 2. The temperature- and pressure-dependent rate coefficients are computed using variable reaction coordinate transition-state theory, variational transition-state theory, and conventional transition-state theory, as implemented in a new RRKM/ME code. The main bimolecular product channels are CH<sub>2</sub>O + HCO at lower temperatures and CH<sub>2</sub>CHO + O at higher temperatures. Above 10 atm, the collisional stabilization of CH<sub>2</sub>CHOO directly competes with these two product channels. CH<sub>2</sub>CHOO decomposes primarily to CH<sub>2</sub>O + HCO. The next two most significant bimolecular products are OCHCHO + H and <sup>3</sup>CHCHO + OH, and not C<sub>2</sub>H<sub>2</sub> + HO<sub>2</sub>. C<sub>2</sub>H<sub>3</sub> + O<sub>2</sub> will be predominantly chain branching above 1700 K. Uncertainty analysis is presented for the two most important transition states. The uncertainties in these two barrier heights result in a significant uncertainty in the temperature at which CH<sub>2</sub>CHO + O overtakes all other product channels

    Rate Constant and Branching Fraction for the NH<sub>2</sub> + NO<sub>2</sub> Reaction

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    The NH<sub>2</sub> + NO<sub>2</sub> reaction has been studied experimentally and theoretically. On the basis of laser photolysis/LIF experiments, the total rate constant was determined over the temperature range 295–625 K as <i>k</i><sub>1,exp</sub>(<i>T</i>) = 9.5 × 10<sup>–7</sup>(<i>T</i>/K)<sup>−2.05</sup> exp­(−404 K/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. This value is in the upper range of data reported for this temperature range. The reactions on the NH<sub>2</sub> + NO<sub>2</sub> potential energy surface were studied using high level ab initio transition state theory (TST) based master equation methods, yielding a rate constant of <i>k</i><sub>1,theory</sub>(<i>T</i>) = 7.5 × 10<sup>–12</sup>(<i>T</i>/K)<sup>−0.172</sup> exp­(687 K/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, in good agreement with the experimental value in the overlapping temperature range. The two entrance channel adducts H<sub>2</sub>NNO<sub>2</sub> and H<sub>2</sub>NONO lead to formation of N<sub>2</sub>O + H<sub>2</sub>O (R1a) and H<sub>2</sub>NO + NO (R1b), respectively. The pathways through H<sub>2</sub>NNO<sub>2</sub> and H<sub>2</sub>NONO are essentially unconnected, even though roaming may facilitate a small flux between the adducts. High- and low-pressure limit rate coefficients for the various product channels of NH<sub>2</sub> + NO<sub>2</sub> are determined from the ab initio TST-based master equation calculations for the temperature range 300–2000 K. The theoretical predictions are in good agreement with the measured overall rate constant but tend to overestimate the branching ratio defined as β = <i>k</i><sub>1a</sub>/(<i>k</i><sub>1a</sub> + <i>k</i><sub>1b</sub>) at lower temperatures. Modest adjustments of the attractive potentials for the reaction yield values of <i>k</i><sub>1a</sub> = 4.3 × 10<sup>–6</sup>(<i>T</i>/K)<sup>−2.191</sup> exp­(−229 K/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> and <i>k</i><sub>1b</sub> = 1.5 × 10<sup>–12</sup>(<i>T</i>/K)<sup>0.032</sup> exp­(761 K/<i>T</i>) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, in good agreement with experiment, and we recommend these rate coefficients for use in modeling

    Theoretical Determination of the Rate Coefficient for the HO<sub>2 </sub>+ HO<sub>2</sub> → H<sub>2</sub>O<sub>2</sub><i>+</i>O<sub>2</sub> Reaction: Adiabatic Treatment of Anharmonic Torsional Effects

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    The HO<sub>2</sub> + HO<sub>2</sub> → H<sub>2</sub>O<sub>2</sub> + O<sub>2</sub> chemical reaction is studied using statistical rate theory in conjunction with high level ab initio electronic structure calculations. A new theoretical rate coefficient is generated that is appropriate for both high and low temperature regimes. The transition state region for the ground triplet potential energy surface is characterized using the CASPT2/CBS/aug-cc-pVTZ method with 14 active electrons and 10 active orbitals. The reaction is found to proceed through an intermediate complex bound by approximately 9.79 kcal/mol. There is no potential barrier in the entrance channel, although the free energy barrier was determined using a large Monte Carlo sampling of the HO<sub>2</sub> orientations. The inner (tight) transition state lies below the entrance threshold. It is found that this inner transition state exhibits two saddle points corresponding to torsional conformations of the complex. A unified treatment based on vibrational adiabatic theory is presented that permits the reaction to occur on an equal footing for any value of the torsional angle. The quantum tunneling is also reformulated based on this new approach. The rate coefficient obtained is in good agreement with low temperature experimental results but is significantly lower than the results of shock tube experiments for high temperatures

    Electronic States of the Quasilinear Molecule Propargylene (HCCCH) from Negative Ion Photoelectron Spectroscopy

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    We use gas-phase negative ion photoelectron spectroscopy to study the quasilinear carbene propargylene, HCCCH, and its isotopologue DCCCD. Photodetachment from HCCCH<sup>–</sup> affords the <i>X̃</i>(<sup>3</sup>B) ground state of HCCCH and its <i>ã</i>(<sup>1</sup>A), <i>b̃</i> (<sup>1</sup>B), <i>d̃</i>(<sup>1</sup>A<sub>2</sub>), and <i>B̃</i>(<sup>3</sup>A<sub>2</sub>) excited states. Extended, negatively anharmonic vibrational progressions in the <i>X̃</i>(<sup>3</sup>B) ground state and the open-shell singlet <i>b̃</i> (<sup>1</sup>B) state arise from the change in geometry between the anion and the neutral states and complicate the assignment of the origin peak. The geometry change arising from electron photodetachment results in excitation of the ν<sub>4</sub> symmetric CCH bending mode, with a measured fundamental frequency of 363 ± 57 cm<sup>–1</sup> in the <i>X̃</i>(<sup>3</sup>B) state. Our calculated harmonic frequency for this mode is 359 cm<sup>–1</sup>. The Franck–Condon envelope of this progression cannot be reproduced within the harmonic approximation. The spectra of the <i>ã</i>(<sup>1</sup>A), <i>d̃</i>(<sup>1</sup>A<sub>2</sub>), and <i>B̃</i>(<sup>3</sup>A<sub>2</sub>) states are each characterized by a short vibrational progression and a prominent origin peak, establishing that the geometries of the anion and these neutral states are similar. Through comparison of the HCCCH<sup>–</sup> and DCCCD<sup>–</sup> photoelectron spectra, we measure the electron affinity of HCCCH to be 1.156 ± <sub>0.095</sub><sup>0.010</sup> eV, with a singlet–triplet splitting between the <i>X̃</i>(<sup>3</sup>B) and the <i>ã</i>(<sup>1</sup>A) states of Δ<i>E</i><sub>ST</sub> = 0.500 ± <sub>0.01</sub><sup>0.10</sup> eV (11.5 ± <sub>0.2</sub><sup>2.3</sup> kcal/mol). Experimental term energies of the higher excited states are <i>T</i><sub>0</sub> [<i>b̃</i>(<sup>1</sup>B)] = 0.94 ± <sub>0.20</sub><sup>0.22</sup>eV, <i>T</i><sub>0</sub> [<i>d̃</i>(<sup>1</sup>A<sub>2</sub>)] = 3.30 ± <sub>0.02</sub><sup>0.10</sup>eV, <i>T</i><sub>0</sub> [<i>B̃</i>(<sup>3</sup>A<sub>2</sub>)] = 3.58 ± <sub>0.02</sub><sup>0.10</sup>eV. The photoelectron angular distributions show significant π character in all the frontier molecular orbitals, with additional σ character in orbitals that create the <i>X̃</i>(<sup>3</sup>B) and <i>b̃</i>(<sup>1</sup>B) states upon electron detachment. These results are consistent with a quasilinear, nonplanar, doubly allylic structure of <i>X̃</i>(<sup>3</sup>B) HCCCH with both diradical and carbene character
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