Topic
Transition state
About: Transition state is a research topic. Over the lifetime, 4978 publications have been published within this topic receiving 117965 citations. The topic is also known as: transition state of elementary reaction.
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TL;DR: It is observed that the hydroperoxyl (HO(2)) radical plays a key role in the reaction kinetics at the authors' input conditions, and atomistic simulations through ReaxFF could be a useful tool in enhancing existing continuum-scale kinetic models for prediction of hydrogen combustion kinetic conditions, which otherwise is difficult to attain through experiments.
Abstract: To investigate the reaction kinetics of hydrogen combustion at high-pressure and high-temperature conditions, we constructed a ReaxFF training set to include reaction energies and transition states relevant to hydrogen combustion and optimized the ReaxFF force field parameters against training data obtained from quantum mechanical calculations and experimental values. The optimized ReaxFF potential functions were used to run NVT MD (i.e., molecular dynamics simulation with fixed number of atoms, volume, and temperature) simulations for various H2/O2 mixtures. We observed that the hydroperoxyl (HO2) radical plays a key role in the reaction kinetics at our input conditions (T ≥ 3000 K, P > 400 atm). The reaction mechanism observed is in good agreement with predictions of existing continuum-scale kinetic models for hydrogen combustion, and a transition of reaction mechanism is observed as we move from high pressure, low temperature to low pressure, high temperature. Since ReaxFF derives its parameters from q...
87 citations
TL;DR: In this paper, the mechanism of thermal decomposition of levoglucosan (1,6-anhydro-β-d-glucopyranose) was studied using density functional theory methods.
87 citations
TL;DR: This work suggests that the vibrational distribution at the first conical intersection and the interactions among the states as the reaction proceeds between the two transition states are the origin of the population of the chemiluminescent (n,sigma*) states.
Abstract: The unimolecular chemiluminescent decomposition of unsubstituted dioxetanone was studied at the complete active space self-consistent field level of theory combined with the multistate second-order multiconfigurational perturbation theory energy correction. The calculations revealed interesting features. Two transition states, two conical intersections, and one intermediate stable biradical structure along the lowest energy reaction path were identified. It was noted that the conical intersections are found at or in very close proximity to the transition states. The first and second transition states correspond to O-O and C-C cleavages, respectively. In particular, a planar structure is supported by the (1)(sigma,sigma*) state during the O-O dissociation up to the first transition state and conical intersection. At this point the (1)(sigma,sigma*) state dissociation path bifurcates, corresponding to a torsion of the O-C-C-O angle. Simultaneously, the (1)(n,sigma*) state becomes lower in energy while still favoring a planar structure. As the lowest-energy reaction path proceeds toward the second transition state and conical intersection, the (1)(n,sigma*), (3)(n,sigma*), and (1)(sigma,sigma*) states are close in energy. This work suggests that the vibrational distribution at the first conical intersection and the interactions among the states as the reaction proceeds between the two transition states are the origin of the population of the chemiluminescent (n,sigma*) states.
87 citations
TL;DR: Compared with this third-row transition metal system with the first-row and second-row congeners, Cu+ and Ag+.
Abstract: The potential energy surface for activation of methane by the third-row transition metal cation, Au+, is studied experimentally by examining the kinetic energy dependence of this reaction using guided ion beam tandem mass spectrometry. A flow tube ion source produces Au+ primarily in its 1S0 (5d10) electronic ground state level but with some 3D (and perhaps higher lying) excited states that can be completely removed by a suitable quenching gas (N2O). Au+ (1S0) reacts with methane by endothermic dehydrogenation to form AuCH2+ as well as C-H bond cleavage to yield AuH+ and AuCH3+. The kinetic energy dependences of the cross sections for these endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D0(Au+ - CH2) = 3.70 +/- 0.07 and D0(Au+ -CH3) = 2.17 +/- 0.24. Ab initio calculations at the B3LYPHW + /6-311++G(3df,3p) level performed here show good agreement with the experimental bond energies and previous theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surface. Surprisingly, the dehydrogenation reaction does not appear to involve an oxidative addition mechanism. We also compare this third-row transition metal system with the first-row and second-row congeners, Cu+ and Ag+. Differences in thermochemistry can be explained by the lanthanide contraction and relativistic effects that alter the relative size of the valence s and d orbitals.
87 citations
TL;DR: In this paper, transition states and reaction paths between the pairs of possible isomers, methylacetylene, allene, cyclopropene, propenylidene, vinylmethylene, and cyclop-polycyclopene were determined, and the zero-point energies were calculated, and activation energies for pertinent reactions were evaluated.
Abstract: SCF, MCSCF, and CI calculations have been carried out to study thermal interconversions occurring on the singlet C{sub 3}H{sub 4} surface. Transition states and reaction paths between the pairs of possible isomers, methylacetylene, allene, cyclopropene, propenylidene, vinylmethylene, and cyclopropylidene were determined. In addition, the zero-point energies were calculated, and the activation energies for pertinent reactions were evaluated. The thermal rearrangement of allene to methylacetylene was found to proceed in four steps via vinylmethylene, cyclopropene, and propenylidene with the activation energy of 65.8 kcal/mol, which is in good agreement with the observed values of 60.5 and 63.8 kcal/mol. The same reaction paths can also apply to pyrolysis of cyclopropene, in which it undergoes conversion to methylacetylene via propenylidene more easily than to allene via vinylmethylene; the calculated activation energies are 38.1 and 43.4 kcal/mol, respectively. These are again in excellent agreement with the observed values of 37.5 and 43.3 kcal/mol. The activation energies for the allene to cyclopropylidene and the reverse conversions were calculated to be 72.2 and 10.2 kcal/mol. This indicates that cyclopropylidene may not be involved in the interconversion of allene, cyclopropene, and methylacetylene. One of the significant findings in this study is the reaction path for the cyclopropenemore » to methylacetylene conversion via propenylidene, which is less energy demanding than that via vinylmethylene. This made the calculated mechanisms in accord with the experimental data. Furthermore, we will present and discuss reaction mechanisms for pyrolysis of singly and doubly substituted cyclopropene in which this particular reaction path is expected to play a dominant role.« less
86 citations