A model conformational flip-flop molecular switch
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Abstract:
In a mechanic molecular switch, electron transfer processes are controlled by a conformation change of the molecule induced by the move of one of its part. The change in through ligand electronic coupling induced by the displacement or the rotation of benzene ring(s) included in a non saturated ligand are discussed. This change is used to build a flip‐flop switch constituted of an isomerizable handle connected to a bis 1,4(4 pyridyl ethynyl) benzene ligand. Isomerization of the handle produces a rotation of the central benzene. Calculated energetic and electron transfer properties of this switch are presented.Keywords:
Molecular switch
Flip-flop
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Hexane
Space velocity
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The product distributions of liquid isomerization of 2,10 epoxypinane in the presence of acid catalyst was studied.The influence of reaction temperature,reaction time and the amount of solvent on the isomerization were investigated.It is discovered that the isomerization of 2,10 epoxypinane produced mainly perilly alcohol and myrentol.Perilly alcohol can be obtained from isomerization of 2,10 epoxypinane catalyzed by NH 4NO 3 in CH 3NO 2 with 98 9% conversion and 88 1% selection.
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Alkane
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Molecular switches based on light-induced isomerization cycles are promising materials for sensors and biomedical applications. Understanding the details of the isomerization photocycle and identifying the nuclear coordinates involved in the photoreaction are relevant topics. Here we characterize the cis→trans and trans→cis isomerization of a new type of BF2-coordinated azo-dye molecular switch by pump-probe spectroscopy. While cis→trans isomerization is ultrafast and proceeds via a conical intersection, the trans→cis photocycle is more complex and involves at least three reaction channels. Finally, we employ the vibrational wavepackets accompanying isomerization to infer information on the nuclear degrees of freedom involved in the photoreaction.
Conical intersection
Molecular switch
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Photoswitchable molecular devices based on dimethyldihydropyrene (DHP) and cyclophanediene (CPD) isomers in gold junctions have been systematically studied by using first-principles calculations. The reaction pathways for the forth- and back-isomerization between DHP and CPD have been explored. It is found that along the ground state, the calculated barrier for the back-isomerization from CPD to DHP is as high as 23.2 kcal/mol. The forth- and back-isomerization on excited state was found to be much easier compared to that on the ground state. Our calculations have shown that the same conclusions about the reaction pathways can be drawn for the DHP/CPD derivatives that were experimentally studied. It is revealed that the thermal stability of the molecular switch can be significantly enhanced when certain substitutions are employed. A desirable substitution that gives a larger ON/OFF ratio and higher thermal stability is proposed for these isomeric systems. We have also found that the electrode distance has a huge impact on the electron transport properties, as well as the switching performance, of these junctions, which nicely explains some puzzling experimental observations.
Thermal Stability
Molecular switch
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The subsequent double-bond isomerization in the synthesis of dibenzocycloheptenes and their heteroaromatic analogues was investigated. In the case of biphenyls, a basic additive completely prevented an isomerization to the thermodynamic product. With electron-rich intramolecular heteroaromatic nucleophiles, the isomerization was still observed, but the kinetic product can be obtained by careful control of the reaction times in most cases. Mechanistic studies demonstrated that a slow isomerization is also possible with the gold catalyst at elevated temperatures, but much faster isomerization rates were observed with acidic additives. An observed initiation period for the gold-catalyzed isomerization indicates that not the homogenous catalyst, but a decomposition product of it may be the catalytically active species.
Kinetic control
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Isomerization of saturated hydrocarbons includes skeletal rearrangements and cis-trans isomerization of substituted cycloalkanes. Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts inducing isomerization through a carbocationic process. Bronsted and Lewis acids, acidic solids, and superacids are used in different applications. Significant efforts have been made about the fabrication, characterization, and the use of bifunctional catalysts bearing both acidic and metallic sites in isomerization. Metals are able to induce hydrogenation/dehydrogenation, whereas acid sites are involved in skeletal isomerization. Besides skeletal rearrangements, alkenes may undergo double-bond migration and cis-trans isomerization. Double-bond migration, cis-trans isomerization, and skeletal isomerization are the characteristic isomerization transformations of alkenes in the presence of acidic catalysts. Double-bond migration and cis-trans isomerization are facile transformations that can be induced under mild conditions (relatively mild acids and low temperature).
Brønsted–Lowry acid–base theory
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The effect of benzene on the isomerization of n-hexane over Pt/Ga2O3/WO3/ZrO2(PtGWZ) and Pd/Al2O3/ WO3/ZrO2(PdAWZ) catalysts were investigated comparatively.The results indicate that the activities of both PtGWZ and PdAWZ catalysts for n-hexane isomerization decrease with the increase of benzene amounts in the feed.Compared with PdAWZ catalyst,PtGWZ catalyst is less influenced by benzene.The inhibiting effect of benzene on the isomerization activities of PtGWZ is reversible and the activity for n-hexane isomerization can be fully recovered when benzene containing feed is replaced by pure n-hexane.However,the inhibiting effect of benzene on n-hexane isomerization activity of PdAWZ is irreversible.For the benzene containing feed,the deactivating effect can be observed on PdAWZ catalyst,but no deactivating effect on PtGWZ catalyst can be found.This deactivating effect is due to a rapid coke formation from cyclohaxane,which formed from benzene hydrogenation on the catalysts.The reasons for the different inhibiting effects of benzene on the isomerization activities of PtGWZ and PdAWZ catalysts are discussed.
Hexane
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Contrary to the hitherto accepted concept that every olefins undergo mutual isomerization between their cis-and trans-isomers, olefins substituted by anthracenyl and other aromatic polycyclic groups undergo only one-way isomerization from their cis-to trans-isomers, and the reverse isomerization from their trans-tocis-isomers never occurs. The mechanism for the one-way isomerization and the features to distinguish one-way or two-way isomerization is discussed to get whole view of the isomerization of olefins.
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