- Surprising Differences of Alkane C-H Activation Catalyzed by Ruthenium Nanoparticles: Complex Surface-Substrate Recognition?
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The activation of C?H bonds of alkanes remains a major challenge for chemistry. In a series of deuteration experiments with D2 in contact with bis-(diphenylphosphino) butane (dppb) stabilized ruthenium nanoparticles (liquid substrates, 60 °C, 6 bar D2) we have observed a surprisingly large reactivity of cyclopentane as compared to cyclohexane and other alkanes. DFT calculations using a ligand-free Ru13H17 model cluster as catalyst indicate oxidative C?H cleavage of the bound substrates as rate limiting reaction step. They also indicate similar binding and activation enthalpies of reactions of cyclopentane and cyclohexane.
- Rothermel, Niels,Bouzouita, Donia,R?ther, Tobias,de Rosal, Iker,Tricard, Simon,Poteau, Romuald,Gutmann, Torsten,Chaudret, Bruno,Limbach, Hans-Heinrich,Buntkowsky, Gerd
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p. 4243 - 4247
(2018/09/14)
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- The Mechanism of Exchange of Cyclopentane with Deuterium on Metal Catalysts - Determination of Initial Product Distributions by a Monte Carlo Method
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Monte Carlo calculations have been made to derive initial product distributions for the exchange of cyclopentane with deuterium.The mechanism of one-set (one side of the ring) exchange was assumed to involve interconversion between adsorbed cyclopentyl radicals and adsorbed cyclopentene molecules.Two-set (both sides of the ring) exchange was assumed to depend on either a roll(of cyclopentene) mechanism or an αα-turnover process.Comparison of calculated distributions with a range of experimental results for Pd, Rh, Ir, Pt and Ni catalysts showed that three main processes occurred.Process I gave D1 and D2 products, process II which could be well represented by the Monte Carlo calculations gave a range of products and accounted for most of the D3- and D8-compounds, and, on some metals only, process III produced D10- and some D9-compounds.Two-set exchange occurred by the rollmechanism on Pd and by αα-turnover on Pt and probably also on Rh and Ir at higher temperatures.With Pd, Rh and Ni, the distributions provided evidence for a mechanism involving a direct dissociation of cyclopentane to adsorbed cyclopentene.
- Faro, Arnaldo C.,Kemball, Charles
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p. 3208 - 3244
(2007/10/02)
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- The relative rates of bromination of cyclohexane and cyclopentane with molecular bromine. Comparison of the reactions in solution and in vapor phase
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The realtive rates of transfer of the cyclopentadienyl radical with molecular bromine and hydrogen bromide (k2'/k-1') and with hydrogen tribromide and hydrogen bromide (k3'/k-1') have been determined.The relative transfer rates are compared with the analogous values previously reported for the reactions of cyclohexyl radicals (k2/k-1 and k3/k-1).Utilizing the values of k2'/k-1' and k2/k-1 the competitive vapor phase rates of bromine atom abstraction of hydrogen from the two substrates could be obtained.An expression using a combination of the five sets of relative rate constants was used to determine the effect of competitive cage reversal which occurs in the solution phase bromination of the two substrate radicals with caged hydrogen bromide.For two structurally similar radicals, cage reversal (internal return) was found to affect the relative rates of bromination by 30 percent, while the relative transfer rates, although differing each by a factor of two, fortuitously nearly cancel each other's effect.The effect of both internal and external reversal reactions on the relative rates of bromination of structurally dissimilar substrates, halogenated alkanes and their parent hydrocarbons, is discussed.
- Tanner, Dennis D.,Ruo, Tomoki C-S.,Takiguchi, Hideki,Guillaume, Andre
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p. 1368 - 1374
(2007/10/02)
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