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      Oriented external electric fields as mimics for probing the role of metal ions and ligands in the thermal gas-phase activation of methane

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          Abstract

          Unusual, if not unprecedented, effects of transition-metal ions and ligands are discovered when simple metal oxides or carbides activate methane in the gas phase in manners reminiscent of oriented external electric fields.

          Abstract

          In the course of combined computational and mass spectrometry-based mechanistic studies, recently we came across rather unusual, if not unprecedented, effects of transition-metal ions and ligands when simple metal oxides or carbides are subjected to thermal gas-phase reactions with methane. Interestingly, “Gedankenexperiments” demonstrate how these effects can be modeled using oriented external electric fields (OEEFs), thus expanding their predicted role as “smart reagents” (Shaik et al., Nat. Chem., 2016, 8, 1091), and further suggesting that the OEEFs may be used in controlling the adsorption/desorption behavior of methane as well as serving as a tool to explore mechanistic features.

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          Most cited references34

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          Electrostatic basis for enzyme catalysis.

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            Electrostatic catalysis of a Diels-Alder reaction.

            It is often thought that the ability to control reaction rates with an applied electrical potential gradient is unique to redox systems. However, recent theoretical studies suggest that oriented electric fields could affect the outcomes of a range of chemical reactions, regardless of whether a redox system is involved. This possibility arises because many formally covalent species can be stabilized via minor charge-separated resonance contributors. When an applied electric field is aligned in such a way as to electrostatically stabilize one of these minor forms, the degree of resonance increases, resulting in the overall stabilization of the molecule or transition state. This means that it should be possible to manipulate the kinetics and thermodynamics of non-redox processes using an external electric field, as long as the orientation of the approaching reactants with respect to the field stimulus can be controlled. Here, we provide experimental evidence that the formation of carbon-carbon bonds is accelerated by an electric field. We have designed a surface model system to probe the Diels-Alder reaction, and coupled it with a scanning tunnelling microscopy break-junction approach. This technique, performed at the single-molecule level, is perfectly suited to deliver an electric-field stimulus across approaching reactants. We find a fivefold increase in the frequency of formation of single-molecule junctions, resulting from the reaction that occurs when the electric field is present and aligned so as to favour electron flow from the dienophile to the diene. Our results are qualitatively consistent with those predicted by quantum-chemical calculations in a theoretical model of this system, and herald a new approach to chemical catalysis.
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              Chemistry with methane: concepts rather than recipes.

              Four seemingly simple transformations related to the chemistry of methane will be addressed from mechanistic and conceptual points of view: 1) metal-mediated dehydrogenation to form metal carbene complexes, 2) the hydrogen-atom abstraction step in the oxidative dimerization of methane, 3) the mechanisms of the CH(4)→CH(3)OH conversion, and 4) the initial bond scission (C-H vs. O-H) as well as the rate-limiting step in the selective CH(3)OH→CH(2)O oxidation. State-of-the-art gas-phase experiments, in conjunction with electronic-structure calculations, permit identification of the elementary reactions at a molecular level and thus allow us to unravel detailed mechanistic aspects. Where appropriate, these results are compared with findings from related studies in solution or on surfaces. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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                Author and article information

                Journal
                ICHBD9
                Dalton Transactions
                Dalton Trans.
                Royal Society of Chemistry (RSC)
                1477-9226
                1477-9234
                November 6 2018
                2018
                : 47
                : 43
                : 15271-15277
                Affiliations
                [1 ]Institut für Chemie
                [2 ]Technische Universität Berlin
                [3 ]10623 Berlin
                [4 ]Germany
                [5 ]Institute of Theoretical Chemistry
                [6 ]Institute of Chemistry
                [7 ]The Hebrew University of Jerusalem
                [8 ]91904 Jerusalem
                [9 ]Israel
                [10 ]Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology
                Article
                10.1039/C8DT03048K
                733a3e7e-bffd-451c-9296-4b778fd64027
                © 2018

                http://rsc.li/journals-terms-of-use

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