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.

Selectivity of Surface Defects for the Activation of Supported Metal Atoms: Acetylene Cyclotrimerization on Pd1/MgO

Abstract: We report on the results of density functional cluster model calculations on thermodynamical and kinetical aspects of the acetylene cyclotrimerization reaction occurring on single Pd atoms deposited with soft-landing techniques on MgO(100) thin films. The different elementary steps of the reaction as well as the transition states involved have been investigated in detail for Pd atoms adsorbed on different sites of MgO to understand the role of the substrate in this reaction. The analysis of the complete reaction path indicates that only basic defect sites such as neutral and charged oxygen vacancies (F and F+ centers) located at the MgO terraces can activate supported Pd atoms for this process.

Theoretical studies of the O+H2 reaction

Abstract: ab initio theoretical calculations of the potential energy surfaces and rates of reaction of the O+H2 reactions are reported. (AIP)

Formation pathways of ethynyl-substituted and cyclopenta-fused polycyclic aromatic hydrocarbons

Abstract: Two novel classes of polycyclic aromatic hydrocarbons (PAH), those with ethynyl substituents (ethynyl-PAH) and those with externally fused five-membered rings (cyclopenta-fused PAH or CP-PAH), have recently been identified in the products of a variety of fuels and combustion/pyrolysis environments. However, the recently developed capacity for identifying these compounds has raised new questions about preferential reaction pathways. Specifically, across various fuels and operating conditions, experimentally observed products are (1) CP-PAH, which result from C 2 H 2 addition to an aryl radical, followed by cyclization to a cyclopenta ring and (2) ethynyl-PAH, which result from C 2 H 2 addition to locations on the aryl radical where cyclization is not possible. We have never observed ethynyl-PAH resulting from C 2 H 2 addition to an aryl radical at a point where cyclization into a five-membered ring is possible. To explain this behavior, we have performed AM1 semiempirical quantum chemical computations with group correction in order to examine the potential energy surfaces of the reaction pathways that lead to ethynyl-PAH and CP-PAH. We have performed computations for the parent aryl radical, possible ethynyl-PAH products, possible CP-PAH products, as well as intermediates and transition states, for C 2 H 2 addition to naphthalene, anthracene, phenanthrene, acenaphthylene, fluoranthene, and pyrene. Possible CP-PAH products are acenaphthylene, aceanthrylene, acephenanthrylene, pyracylene, cyclopenta[ cd ]fluoranthene, and cyclopenta[ cd ]pyrene. In all cases, we have found that, although energy differences between ethynyl-PAH isomers are very small (∼1 kcal/mol), the experimentally observed ethynyl-PAH is always the lowest energy isomer. Furthermore, the observed preference for cyclization to CP-PAH over formation of an ethynyl-PAH can be explained by the significantly lower energy barrier (23 vs. 36 kcal/mol) for the cyclization reactions. Finally, we have determined that, while not prohibited, the isomerization of ethynyl-PAH to CP-PAH requires significantly higher energy than the aryl-vinyl cyclization reactions, and therefore is not expected to make a significant contribution to the product distribution. These results are sufficiently consistent that the computation of reaction pathway energy surfaces can be used to identify likely ethynyl-PAH and CP-PAH products from the addition of C 2 H 2 to much larger parent PAH.

Use of DFT-based reactivity descriptors for rationalizing radical reactions: A critical analysis

Abstract: Hydrogen abstraction by C 2 H, OH, CH 3 , CF 3 , C 2 H 3 , and C 2 H 5 radicals from methane and propene and addition reactions of these radicals with substituted propenes have been investigated by using BHandHLYP/6-311G-(d,p) level of theory. Transition states for all these reactions have been located. The reactivity of different radicals and substrates toward hydrogen abstraction and radical addition reactions has been critically analyzed by using density functional theory based reactivity descriptors, namely, local softness and electronegativity. The regiochemistry of the radical addition reaction has also been explained from the local softness values of the potential addition sites.