Concertedness and solvent effects in multiple proton transfer reactions: The formic acid dimer in solution

The issue of multiple proton transfer (PT) reactions in solution is addressed by performing molecular dynamics simulations for a formic acid dimer embedded in a water cluster. The reactant species is treated quantum mechanically, within a density functional approach, while the solvent is represented by a classical model. By constraining different distances within the dimer we analyze the PT process in a variety of situations representative of more complex environments. Free energy profiles are presented, and analyzed in terms of typical solvated configurations extracted from the simulations. A decrease in the PT barrier height upon solvation is rationalized in terms of a transition state which is more polarized than the stable states. The dynamics of the double PT process is studied in a low-barrier case and correlated with solvent polarization fluctuations. Cooperative effects in the motion of the two protons are observed in two different situations: when the solvent polarization does not favor the transfer of one of the two protons and when the motion of the two protons is not synchronized. This body of observations is correlated with local structural and dynamical properties of the solvent in the vicinity of the reactant. (C) 2000 American Institute of Physics. [S0021-9606(00)51121-0].

Amination and dehydration of 1,3-propanediol by hydrogen transfer: reactions of a bio-renewable platform chemical

1,3-propanediol was subjected to a range of amination conditions. The N-heterocyclic carbene piano stool complex [Cp*IrCl2(bmim)] was found to be a good catalyst for amination and dehydration in toluene or ionic liquid; product compositions could be tuned by altering the ratio of diol to amine.

Blue bottle light: lecture demonstrations of homogeneous and heterogeneous photo-induced electron transfer reactions

The classic, non-photochemical blue bottle experiment involves the reaction of methylene blue (MB) with deprotonated glucose, to form a bleached form of the dye, leuco-methylene blue (LMB), and subsequent colour recovery by shaking with air. This reaction is a popular demonstrator of key principles in kinetics and reaction mechanisms. Here it is modified so as to highlight features of homogenous and heterogeneous photoinduced electron transfer (PET) (Pure Appl. Chem., 2007, 79, 293-465) reactions, i.e. blue bottle light experiments. The homogeneous blue bottle light experiment uses methylene blue, MB, as the photo-sensitizer and triethanolamine as the sacrificial electron donor. Visible light irradiation of this system leads to its rapid bleaching, followed by the ready restoration of its original colour upon shaking away from the light source. The heterogeneous blue bottle light experiment uses titania as the photo-sensitizer, MB as a redox indicator and glucose as the sacrificial electron donor. UVA light irradiation of this system leads to the rapid bleaching of the MB and the gradual restoration of its original colour with shaking and standing. The latter 'dark' step can be made facile and more demonstrator-friendly by using platinised titania particles. These two photochemical versions of the blue bottle experiment are used to explore the factors which underpin homogeneous and heterogeneous PET reactions and provide useful demonstrations of homogeneous and heterogeneous photochemistry.

KINETIC-STUDY OF PHOTOINDUCED ELECTRON-TRANSFER REACTIONS SENSITIZED BY COLLOIDAL CDS

The kinetics of the recovery of the photoinduced transient bleaching of colloidal CdS in the presence of different electron acceptors are examined. In the presence of the zwitterionic viologen, N,N'-dipropyl-2,2'-bipyridinium disulphonate, excitation of colloidal CdS at different flash intensities generates a series of decay profiles which are superimposed when normalized. The shape of the decay curves are as predicted by a first-order activation-controlled model for a log-normal distribution of particles sizes. In contrast, the variation in flash intensity in the presence of a second viologen, N,N'-dipropyl-4,4'-bipyridinium sulphonate, generates normalized decay traces which broaden with increasing flash intensity. This behaviour is predicted by a zero-order diffusion-controlled model for a log-normal distribution of particle radii. The photoreduction of a number of other oxidants sensitized by colloidal CdS is examined and the shape of the decay kinetics interpreted via either the first- or zero-order kinetics models. The rate constants and activation energies derived using these models are consistent with the values expected for an activation- or diffusion-controlled reaction.

Analysis of DNA processing reactions in bacterial conjugation by using suicide oligonucleotides

Protein TrwC is the conjugative relaxase responsible for DNA processing in plasmid R388 bacterial conjugation. TrwC has two catalytic tyrosines, Y18 and Y26, both able to carry out cleavage reactions using unmodified oligonucleotide substrates. Suicide substrates containing a 30-Sphosphorothiolate linkage at the cleavage site displaced TrwC reaction towards covalent adducts and thereby enabled intermediate steps in relaxase reactions to be investigated. Two distinct covalent TrwC–oligonucleotide complexes could be separated from noncovalently bound protein by SDS–PAGE. As observed by mass spectrometry, one complex contained a single, cleaved oligonucleotide bound to Y18, whereas the other contained two cleaved oligonucleotides, bound to Y18 and Y26. Analysis of the cleavage reaction using suicide substrates and Y18F or Y26F mutants showed that efficient Y26 cleavage only occurs after Y18 cleavage. Strand-transfer reactions carried out with the isolated Y18–DNA complex allowed the assignment of specific roles to each tyrosine. Thus, only Y18 was used for initiation. Y26 was specifically used in the second transesterification that leads to strand transfer, thus catalyzing the termination reaction that occurs in the recipient cell.

Importance of electronegativity differences and surface structure in molecular dissociation reactions at transition metal surfaces

The dissociative adsorption of N-2 has been studied at both monatomic steps and flat regions on the surfaces of the 4d transition metals from Zr to Pd. Using density functional theory (DFT) calculations, we have determined and analyzed the trends in both straight reactivity and structure sensitivity across the periodic table. With regards to reactivity, we find that the trend in activation energy (Ea) is determined mainly by a charge transfer from the surface metal atoms to the N atoms during transition state formation, namely, the degree of ionicity of the N-surface bond at the transition state. Indeed, we find that the strength of the metal-N bond at the transition state (and therefore the trend in Ea) can be predicted by the difference in Mulliken electronegativity between the metal and N. Structure sensitivity is analyzed in terms of geometric and electronic effects. We find that the lowering of Ea due to steps is more pronounced on the right-hand side of the periodic table. It is found that for the early transition metals the geometric and electronic effects work in opposition when going from terrace to step active site. In the case of the late 4d metals, however, these effects work in combination, producing a more marked reduction in Ea.

Organometallic hydrogen transfer and dehydrogenation catalysts for the conversion of bio-renewable alcohols

Organometallic hydrogen transfer and dehydrogenation provide straightforward atom efficient routes from alcohols to a variety of chemical products. The potential of these reactions to enable the conversion of biomass to value added chemicals is discussed, with reference to the products that can be prepared from aliphatic alcohols in good isolated yield.

Optimization of heat transfer characteristics, flow distribution, and reaction processing for a microstructured reactor/heat-exchanger for optimal performance in platinum catalyzed ammonia oxidation

The present work is focused on the demonstration of the advantages of miniaturized reactor systems which are essential for processes where potential for considerable heat transfer intensification exists as well as for kinetic studies of highly exothermic reactions at near-isothermal conditions. The heat transfer characteristics of four different cross-flow designs of a microstructured reactor/heat-exchanger (MRHE) were studied by CFD simulation using ammonia oxidation on a platinum catalyst as a model reaction. An appropriate distribution of the nitrogen flow used as a coolant can decrease drastically the axial temperature gradient in the reaction channels. In case of a microreactor made of a highly conductive material, the temperature non-uniformity in the reactor is strongly dependent on the distance between the reaction and cooling channels. Appropriate design of a single periodic reactor/heat-exchanger unit, combined with a non-uniform inlet coolant distribution, reduces the temperature gradients in the complete reactor to less than 4degreesC, even at conditions corresponding to an adiabatic temperature rise of about 1400degreesC, which are generally not accessible in conventional reactors because of the danger of runaway reactions. To obtain the required coolant flow distribution, an optimization study was performed to acquire the particular geometry of the inlet and outlet chambers in the microreactor/heat-exchanger. The predicted temperature profiles are in good agreement with experimental data from temperature sensors located along the reactant and coolant flows. The results demonstrate the clear potential of microstructured devices as reliable instruments for kinetic research as well as for proper heat management in the case of highly exothermic reactions. (C) 2002 Elsevier Science B.V. All rights reserved.

Reactivity difference between protolytic forms of some macrocyclic chromium(III) complexes in ligand substitution and electron transfer processes

The review provides insight into the mechanism of ligand substitution and electron transfer (from chromium(III) to iron(III)) by comparison of the reactivity of some tetraazamacrocyclic chromium(III) complexes in the conjugate acid-base forms. Use of two geometrical isomers made possible to estimate the influence of geometry and protolytic reactions in trans and cis position towards the leaving group on the rate enhancement. Studies on the reaction rates in different media demonstrated the role played by outer sphere interactions in a monodentate ligand substitution. (C) 2009 Published by Elsevier B.V.

Minimizing side reactions in chemoenzymatic dynamic kinetic resolution: organometallic and material strategies

Chemoenzymatic dynamic kinetic resolution (DKR) of rac-1-phenyl ethanol into R-1-phenylethanol acetate was investigated with emphasis on the minimization of side reactions. The organometallic hydrogen transfer (racemization) catalyst was varied, and this was observed to alter the rate and extent of oxidation of the alcohol to form ketone side products. The performance of highly active catalyst [(pentamethylcyclopentadienyl) IrCl2(1-benzyl,3-methyl-imidazol-2-ylidene)] was found to depend on the batch of lipase B used. The interaction between the bio- and chemo-catalysts was reduced by employing physical entrapment of the enzyme in silica using a sol-gel process. The nature of the gelation method was found to be important, with an alkaline method preferred, as an acidic method was found to initiate a further side reaction, the acid catalyzed dehydration of the secondary alcohol. The acidic gel was found to be a heterogeneous solid acid.

Universal tight binding model for chemical reactions in solution and at surfaces. I. Organic molecules

As is now well established, a first order expansion of the Hohenberg-Kohn total energy density functional about a trial input density, namely, the Harris-Foulkes functional, can be used to rationalize a non self consistent tight binding model. If the expansion is taken to second order then the energy and electron density matrix need to be calculated self consistently and from this functional one can derive a charge self consistent tight binding theory. In this paper we have used this to describe a polarizable ion tight binding model which has the benefit of treating charge transfer in point multipoles. This admits a ready description of ionic polarizability and crystal field splitting. It is necessary in constructing such a model to find a number of parameters that mimic their more exact counterparts in the density functional theory. We describe in detail how this is done using a combination of intuition, exact analytical fitting, and a genetic optimization algorithm. Having obtained model parameters we show that this constitutes a transferable scheme that can be applied rather universally to small and medium sized organic molecules. We have shown that the model gives a good account of static structural and dynamic vibrational properties of a library of molecules, and finally we demonstrate the model's capability by showing a real time simulation of an enolization reaction in aqueous solution. In two subsequent papers, we show that the model is a great deal more general in that it will describe solvents and solid substrates and that therefore we have created a self consistent quantum mechanical scheme that may be applied to simulations in heterogeneous catalysis.

CoO nanoparticles embedded in three-dimensional nitrogen/sulfur co-doped carbon nanofiber networks as a bifunctional catalyst for oxygen reduction/evolution reactions

High-performance and low-cost bifunctional electrocatalysts play crucial roles in oxygen reduction and evolution reactions. Herein, a novel three-dimensional (3D) bifunctional electrocatalyst was prepared by embedding CoO nanoparticles into nitrogen and sulfur co-doped carbon nanofiber networks (denoted as CoO@N/S-CNF) through a facile approach. The carbon nanofiber networks were derived from a nanostructured biological material which provided abundant functional groups to nucleate and anchor nanoparticles while retaining its interconnected 3D porous structure. The composite possesses a high specific surface area and graphitization degree, which favors both mass transport and charge transfer for electrochemical reaction. The CoO@N/S-CNF not only exhibits highly efficient catalytic activity towards oxygen reduction reaction (ORR) in alkaline media with an onset potential of about 0.84 V, but also shows better stability and stronger resistance to methanol than Pt/C. Furthermore, it only needs an overpotential of 1.55 V to achieve a current density of 10 mA cm^-2, suggesting that it is an efficient electrocatalyst for oxygen evolution reaction (OER). The ΔE value (oxygen electrode activity parameter) of CoO@N/S-CNF is calculated to be 0.828 V, which demonstrates that the composite could be a promising bifunctional electrocatalyst for both ORR and OER.

Reactions of atomic and molecular ions with acetone, 1,1,1- trifluoroacetone, and hexafluoroacetone: An investigation of the effects of molecular structure on the dynamics and kinetics of ion-molecule reactions

A selected ion flow tube study of the reactions of a series of gas-phase atomic cations (S⁺, Xe⁺, O⁺, Kr⁺, N⁺, Ar⁺ and Ne⁺) and molecular ions (SF _n⁺ (n = 1-5), CF_n⁺ (n = 1-3), CF₂Cl⁺, H₃O⁺, NO⁺, N ₂O⁺, CO₂⁺, CO⁺, and N₂⁺) spanning a large range of recombination energies (6.3-21.6 eV), with acetone, 1,1,1-trifluoroacetone, and hexafluoroacetone has been undertaken with the objective of exploring the nature of the reaction ion chemistry as the methyl groups in acetone are substituted for CF₃. The reaction rate coefficients and product ion branching ratios for all 66 reactions, measured at 298 K, are reported. The experimental reaction rate coefficients are compared to theoretically calculated collisional values. Several distinct reaction processes were observed among the large number of reactions studied, including charge transfer (non-dissociative and dissociative), abstraction, ion-molecule associations and, in the case of the reactions involving the reagent ion H₃O⁺, proton transfer.