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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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TL;DR: These experiments provide new information on the nature of the underlying interactions in protein folding and chemical reactions and demonstrate the power of single-molecule techniques to identify the changes induced by a small change in hydrogen bond strength.
Abstract: We present an array of force spectroscopy experiments that aim to identify the role of solvent hydrogen bonds in protein folding and chemical reactions at the single-molecule level. In our experiments we control the strength of hydrogen bonds in the solvent environment by substituting water (H2O) with deuterium oxide (D2O). Using a combination of force protocols, we demonstrate that protein unfolding, protein collapse, protein folding and a chemical reaction are affected in different ways by substituting H2O with D2O. We find that D2O molecules form an integral part of the unfolding transition structure of the immunoglobulin module of human cardiac titin, I27. Strikingly, we find that D2O is a worse solvent than H2O for the protein I27, in direct contrast with the behaviour of simple hydrocarbons. We measure the effect of substituting H2O with D2O on the force dependent rate of reduction of a disulphide bond engineered within a single protein. Altogether, these experiments provide new information on the nature of the underlying interactions in protein folding and chemical reactions and demonstrate the power of single-molecule techniques to identify the changes induced by a small change in hydrogen bond strength.

47 citations

Journal ArticleDOI
TL;DR: Analytical expressions for each reaction were proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts, as long as the deprotonation energies are first acquired.
Abstract: In this work, quantum chemical methods were used to study propane conversion reactions on zeolites; these reactions included protolytic cracking, primary hydrogen exchange, secondary hydrogen exchange, and dehydrogenation reactions. The reactants, products, and transition-state structures were optimized at the B3LYP/6-31G* level and the energies were calculated with CBS-QB3, a complete basis set composite energy method. The computed activation barriers were 62.1 and 62.6 kcal/mol for protolytic cracking through two different transition states, 30.4 kcal/mol for primary hydrogen exchange, 29.8 kcal/mol for secondary hydrogen exchange, and 76.7 kcal/mol for dehydrogenation reactions. The effects of basis set for the geometry optimization and zeolite acidity on the reaction barriers were also investigated. Adding extra polarization and diffuse functions for the geometry optimization did not affect the activation barriers obtained with the composite energy method. The largest difference in calculated activati...

47 citations

Journal ArticleDOI
TL;DR: It was found that the metal-mediated cycloaddition reaction is concerted and takes place via transition structures that can be even more synchronous and more aromatic than their non-organometallic analogues.
Abstract: The thermal [4+3] cycloaddition reaction between allenes and tethered dienes (1,3-butadiene and furan) assisted by transition metals (Au I , Au III , Pd II , and Pt II ) was studied computationally within the density functional theory framework and compared to the analogous non-organometallic process in terms of activation barriers, synchronicity and aromaticity of the corresponding transition states. It was found that the metal-mediated cycloaddition reaction is concerted and takes place via transition structures that can be even more synchronous and more aromatic than their non-organometallic analogues. However, the processes exhibit slightly to moderately higher activation barriers than the parent cycloaddition involving the hydroxyallylic cation. The bond polarization induced by the metal moiety is clearly related to the interaction of the transition metal with the allylic π * molecular orbital, which constitutes the LUMO of the initial reactant. Finally, replacement of the 1,3-butadiene by furan caused the transformation to occur stepwise in both the non-organometallic and metal-assisted processes.

47 citations

Journal ArticleDOI
TL;DR: Detailed investigation of the mechanistic aspects of the dual gold-catalysed hydrophenoxylation of alkynes by both experimental and computational methods confirmed that phenol does not only act as a reactant, but also as a co-catalyst, lowering the energy barriers of several transition states.
Abstract: Herein we present a detailed investigation of the mechanistic aspects of the dual gold-catalysed hydrophenoxylation of alkynes, using both experimental and computational methods. The dissociation of [{Au(NHC)}2(µ-OH)][BF4] is essential to enter the catalytic cycle; this step is favored in the presence of bulky, non-coordinating counterions. Moreover, in silico studies confirmed that phenol does not only act as a reactant, but as a co-catalyst, lowering the energy barriers for several transition states. A gem-diaurated species might form during the reaction, but this lies deep within a potential energy well, and is likely to be an ‘off-cycle’ rather than an ‘in-cycle’ intermediate.

47 citations

Journal ArticleDOI
TL;DR: In this paper, it was shown that primary hydrogen isotope effects depend smoothly on transition-state geometry and yield consistently low values, kH/kD= 1−2, for sufficiently non-linear configurations.
Abstract: Calculations are reported showing that primary hydrogen isotope effects depend smoothly on transition-state geometry and yield consistently low values, kH/kD= 1–2, for sufficiently non-linear configurations. Specific calculations for 1,2-hydride shifts, borane hydrolyses, and E2C eliminations are compared with available measurements for these and for other hydrogen-transfer reactions proceeding through 3-, 4-, 5-, and 6-membered cyclic transition states. A unique temperature-independent kH/kD is noted. The low isotope effects are ascribed to the presence of a large isotopically sensitive vibration νR‡(e.g., 3500 cm.–1) in the transition state. The results are fairly insensitive to reasonable force-constant changes. The usefulness of kH/kD in defining the geometry of transition states for hydride transfer reactions is discussed.

47 citations


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