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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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Journal ArticleDOI
TL;DR: In this article, the intramolecular tautomerism of some acetyl derivatives, CH3COX (X = H, BH2, CH 3, NH2, OH, F, Cl, CN, NC).
Abstract: Ab initio molecular orbital calculations have been performed on the intramolecular tautomerism of some acetyl derivatives, CH3COX (X = H, BH2, CH3, NH2, OH, F, Cl, CN, NC). All stationary points and the keto, enol, and transition structures were optimized at the HF/6-31G* and MP2(full)/6-31G* levels of theory and confirmed by frequency calculations. Single-point calculations at MP4(FC)/6-311++G**//MP2(Full)/6-31G* were also carried out for all stationary points. The intrinsic reaction coordinate (IRC) for the tautomeric processes were traced to connect the transition structures and the corresponding substituted tautomeric pairs. The natural bond orbital (NBO) analyses on the transition states show that the interactions of the lone pair electrons on the oxygen atom and the σ* C−H bonds have a significant effect on their stabilities, which consequently affects the activation energies of the tautomeric processes. The energy barriers calculated at various levels of theory are reported for each tautomeric inte...

39 citations

Journal ArticleDOI
TL;DR: In this article, an ab initio molecular orbital theory has been applied to a study of the hydrogen abstractions of phenolic antioxidants in the chain process of autoxidation, and the optimum structures of phenols, of peroxides, and of those compounds in the transition states were obtained with a Hartree-Fock/STO-3G basis set.
Abstract: An ab initio molecular orbital theory has been applied to a study of the hydrogen abstractions of phenolic antioxidants in the chain process of autoxidation. The optimum structures of phenols, of peroxides, and of those compounds in the transition states were obtained with a Hartree–Fock/STO-3G basis set. From the values of enthalpy (ΔH) and activation energy (Ea) obtained, it was found that the rates of the reaction of peroxyl radical with phenolic antioxidant were faster than those with organic substrate in the propagation, and that the effect of the aromatic ring of the antioxidants not only stabilized a product state but also decreased an energy level in the transition state. The para-substituent effect that an electron-releasing substituent increased the antioxidant activity, whereas an electron-withdrawing one decreased it, was recognized. The relationship between ΔH and Ea values followed the Evans–Polanyi rule. The transition states in the hydrogen abstractions with lower Ea values prefer reactant...

39 citations

Journal ArticleDOI
TL;DR: The predictions reveal that the catalytic process results in considerable energy savings and the most favorable pathway of the reaction is through a general-base-catalyzed neutral stepwise mechanism.
Abstract: Density functional and ab initio methods were applied in examining the possible mechanistic pathways for the reaction of methyl benzoate with ammonia. Transition state structures and energies were determined for concerted and neutral stepwise mechanisms. The theoretical results show that the two possible pathways have similar activation energies. The general base catalysis of the process was also examined. The predictions reveal that the catalytic process results in considerable energy savings and the most favorable pathway of the reaction is through a general-base-catalyzed neutral stepwise mechanism. The structure and transition vectors of the transition states indicate that the catalytic role of ammonia is realized by facilitating the proton-transfer processes. Comparison of the energetics of the aminolysis for methyl benzoate and methyl formate shows the more favorable process to be that for the aliphatic ester. The differing reactivity of the two esters is explained in terms of the electrostatic pote...

39 citations

Journal ArticleDOI
TL;DR: In this paper, density functional B3LYP calculations have been employed to investigate potential energy surfaces for the reactions of scandium, nickel, and palladium oxides with methane.
Abstract: Density functional B3LYP calculations have been employed to investigate potential energy surfaces for the reactions of scandium, nickel, and palladium oxides with methane. The results show that NiO and PdO are reactive toward methane and can form molecular complexes with CH4 bound by 9−10 kcal/mol without a barrier. At elevated temperatures, the dominant reaction channel is direct abstraction of a hydrogen atom by the oxides from CH4 with a barrier of ∼16 kcal/mol leading to MOH (M = Ni, Pd) and free methyl radical. A minor reaction channel is an insertion into a C−H bond to produce CH3MOH molecules via transition states lying 19−20 kcal/mol above the initial reactants. For instance, for PdO, the rate constant of the hydrogen abstraction channel evaluated using the transition state theory for the 300−1000 K temperature range, kmethyl = 7.12 × 10-11 exp(−17 329/RT) cm3 s-1 molecule-1, is 2−3 orders of magnitude higher than the insertion rate constant and the branching ratio for the PdOH + CH3 products is 9...

39 citations

Journal ArticleDOI
TL;DR: It was found that charge analyses supported the evidences of MEP, and the obtained optimized geometric parameters and vibrational frequencies were observed to be in good agreement with the available experimental results.

39 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202364
2022136
2021148
2020155
2019145
2018147