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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72 citations
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01 Jan 2009TL;DR: In this paper, the C7H7 potential energy surface was studied from first principles to determine the benzyl radical decomposition mechanism, which is in agreement with the literature evidences reporting that benzyl decomposes to hydrogen and a C 7H6 species.
Abstract: The C7H7 potential energy surface was studied from first principles to determine the benzyl radical decomposition mechanism. The investigated high temperature reaction pathway involves 15 accessible energy wells connected by 25 transition states. The analysis of the potential energy surface, performed determining kinetic constants of each elementary reaction using conventional transition state theory, evidenced that the reaction mechanism has as rate determining step the isomerization of the 1,3-cyclopentadiene, 5-vinyl radical to the 2-cyclopentene,5-ethenylidene radical and that the fastest reaction channel is dissociation to fulvenallene and hydrogen. This is in agreement with the literature evidences reporting that benzyl decomposes to hydrogen and a C7H6 species. The benzyl high-pressure decomposition rate constant estimated assuming equilibrium between the rate determining step transition state and benzyl is k1(T) = 1.44 × 1013T0.453exp(−38400/T) s−1, in good agreement with the literature data. As fulvenallene reactivity is mostly unknown, we investigated its reaction with hydrogen, which has been proposed in the literature as a possible decomposition route. The reaction proceeds fast both backward to form again benzyl and, if hydrogen adds to allene, forward toward the decomposition into the cyclopentadienyl radical and acetylene with high-pressure kinetic constants k2(T) = 8.82 × 108T1.20exp(1016/T) and k3(T) = 1.06 × 108T1.35exp(1716/T) cm3/mol/s, respectively. The computed rate constants were then inserted in a detailed kinetic mechanism and used to simulate shock tube literature experiments.
72 citations
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TL;DR: A theoretical study of the mechanism of the isomerization reaction HOC(+) --> HCO(+) is presented and it has been found that the evolution of changes in REF along the intrinsic reaction coordinate can be explained in terms of bond orders.
Abstract: A theoretical study of the mechanism of the isomerization reaction HOC+→HCO+ is presented. The mechanism was studied in terms of reaction force, chemical potential, reaction electronic flux (REF), and bond orders. It has been found that the evolution of changes in REF along the intrinsic reaction coordinate can be explained in terms of bond orders. The energetic lowering of the hydrogen assisted (catalyzed) reaction has been identified as being due to the stabilization of the H3+ transition state complex and the stepwise bond dissociation and formation of the H–O and H–C bonds, respectively.
72 citations
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TL;DR: In this paper, coupled cluster calculations at the CCSD(T)/[5s4p3d/4s3p] level of theory are reported for reactions X+H2→XH+H [X=F (1a), OH (1b), NH2 (1c), and CH3 (1d)] utilizing analytical energy gradients for geometry, frequency, charge distribution, and dipole moment calculations of reactants, transition states, and products.
Abstract: Coupled cluster calculations at the CCSD(T)/[5s4p3d/4s3p] and CCSD(T)/[5s4p3d2 f1g/4s3p2d] level of theory are reported for reactions X+H2→XH+H [X=F (1a), OH (1b), NH2 (1c), and CH3 (1d)] utilizing analytical energy gradients for geometry, frequency, charge distribution, and dipole moment calculations of reactants, transition states, and products. A careful analysis of vibrational corrections leads to reaction enthalpies at 300 K, which are within 0.04, 0.15, 0.62, and 0.89 kcal/mol of experimental values. For reaction (1a) a bent transition state and for reactions (1b) and (1c) transition states with a cis arrangement of the reactants are calculated. The cis forms of transition states (1b) and (1c) are energetically favored because of electrostatic interactions, in particular dipole–dipole attraction as is revealed by calculated charge distributions. For reactions (1a)–(1d), the CCSD(T)/[5s4p3d2 f1g/4s3p2d] activation energies at 300 K are 1.1, 5.4, 10.8, and 12.7 kcal/mol which differ by just 0.1, 1.4, ...
72 citations
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TL;DR: The experimental results presented here provide a unified picture of Norrish reactions on excited states and on the ground-state potential energy surfaces and discuss the details regarding the ion chemistry, which determines the appearance of the mass spectra that arise from ionization on the fs time scale.
Abstract: Femtosecond dynamics of Norrish type-I reactions of cyclic and acyclic ketones have been investigated in real time for a series of 13 compounds using femtosecond-resolved time-of-flight mass spectrometry. A general physical description of the ultrafast processes of ketones excited into a high-lying Rydberg state is presented. It accounts not only for the results that are presented herein but also for the results of previously reported studies. For highly excited ketones, we show that the Norrish type-I reaction is nonconcerted, and that the first bond breakage occurs along the effectively repulsive S_2 surface involving the C-C bond in a manner which is similar to that of ketones in the S_1 state (E. W.-G. Diau et al. ChemPhysChem 2001, 2, 273-293). The experimental results show that the wave packet motion out of the initial Franck-Condon region and down to the S_2 state can be resolved. This femtosecond (fs) internal conversion from the highly excited Rydberg state to the S_2 state proceeds through conical intersections (Rydberg-valence) that are accessed through the C=O stretching motion. In one of these conical intersections, the internal energy is guided into an asymmetric stretching mode. This explains the previously reported pronounced nonstatistical nature of the reaction. The second bond breakage involves an excited-state acyl radical and occurs on a time scale that is up to one order of magnitude longer than the first. We discuss the details regarding the ion chemistry, which determines the appearance of the mass spectra that arise from ionization on the fs time scale. The experimental results presented here, aided by the theoretical work reported in paper III, provide a unified picture of Norrish reactions on excited states and on the ground-state potential energy surfaces.
72 citations