Mechanically activated reactions

This behavior is observed in mechanically activated reactions where dislocations in the solid lattice are subject to lower activation energies and higher reaction rates [23]. This comes from the higher energy content of lattice elements that are mechanically stressed. In reactive surroundings, easier reaction and a lower apparent activation energy are observed. 

The experimental approaches which have been developed to probe the rupture of single covalent bonds, which is the first step in a mechanically activated reaction, include single molecule AFM experiments and force clamp in which the molecule is pulled at a constant velocity from the substrate, or under constant force. In the first case the force increases as the molecule is pulled from the substrate and a rupture force is measured which is the maximum force that the molecule can sustain before rupture and a drop in the force signal. In the case of force clamp experiments, a constant force is applied to determine the impact of force on reaction pathways. Sonication has also been used as a means to mechanically activate chemical reactions in solution rests where long chain polymers containing mechan-ophores are pulled apart when a cavity formed in the sonication process collapses. Experimental approaches based on these methods form the basis for molecular scale examination of mechanically activated chemical reactions. First principles approaches to date have mainly concentrated on the pulling of molecules under constant velocity, as the measured rupture forces provide a convenient quantity to compare theoretical approaches with experiment. 

In this review we have shown how investigations initially of the rupture of a covalent bond have led to the study of mechanically activated reactions and the influence of mechanical force on reaction pathways and rates of reaction. The application of mechanical force to control chemistry in a precise way is still in its infancy and we anticipate more activity in this direction in the future. The use of first principles calculations in understanding the rupture process is essential as a detailed description of the electronic structure is required for the correct description of covalent bond rupture. Calculations which determine the potential energy surface for stretching a small molecule in its ground state have evolved to consider the perturbation of a the potential surface of a molecule exposed to a constant force. First principles molecular dynamics simulations have enabled the study of bond rupture and experimental parameters which affect rupture. This determined that factors such as the length of the molecule in which the bond finds itself and the rate at which the molecule is stretched affect the... 

Hydroperoxides are more widely used as initiators in low temperature appHcations (at or below room temperature) where transition-metal (M) salts are employed as activators. The activation reaction involves electron-transfer (redox) mechanisms ... 

Toxic reactions occur by several mechanisms activation of metabolism, production of reactive intermediates and subsequent reactions with cell macromolecules, changing receptor responses, or through abnormal defence reactions. Several compounds cause toxicity by mimicking the organism s own hormones or neurotransmitters, or activating the body s endogenous receptors in some non-physiological way. ... 

Because most thermoset composites cure by a thermally activated reaction, a complicated heat transfer process occurs during solidification, the result of an exothermic cross linking reaction in the resin. The complications of thermoset resin curing are compounded by the competing mechanisms of... 

There are few studies in the literature on the kinetics and mechanism of oxidation over base metal oxides. Blumenthal and Nobe studied the oxidation of CO over copper oxide on alumina between 122 and 164°C. They reported that the kinetics is first order with respect to CO concentration, and the activation energy is 20 kcal/mole (77). Gravelle and Teichner studied CO oxidation on nickel oxide, and found that the kinetics is also first order with respect to CO concentration (78). They suggested that the mechanism of reaction is by the Eley-Rideal mechanism... 

In this reaction, P is a mechanically activated macroradical which, upon transferring part of its excess energy to a second polymer Rs —Rt, induces the degradation of the latter. 

Chain scission is the ultimate fate of a stressed bond. At some value below the critical stress for chain rupture, bond angle deformation may result in an increase in reactivity. As stated in Sect. 3.1, mechanically activated hydrolysis of polymers containing ester groups can lead to the scission of the bond this concurrent reaction should be differentiated from homolytic chain scission, for example by looking at any pH-dependence as was found to be the case during shear degradation of DNA [84]. 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

To date, synthetically useful enantioselective hydroalumination is limited to the asymmetric reductive ring-opening reaction of bicycHc ethers. In spite of the fact that further studies are necessary to get a detailed understanding of the reaction mechanism, this reaction provides a new route to various cycloalkenol derivatives, which are useful intermediates in the preparation of biologically active compounds. 

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

This reaction mode sheds new light on the mechanism of the Si-H activation reaction of the 16e complex [(7j5-H3CC5H4)Mn(CO)2 x THF] 6. 

One of the remarkable features of these 14 electron fragments, which were developed experimentally on the basis of applied MO theory considerations, is their ability to attack selected C-H and, in particular, unactivated C-Si bonds of various organosilanes. Mechanism studies of these bond activation reactions at this point suggest a new type of a-complexation in a common transition state or intermediate for both C-H and C-Si activation, which has to be further investigated in detail through experiments and by theory. 

Microwave heating has already been used in combination with some other (unconventional activation processes. Such a combination might have a synergic effect on reaction efficiencies or, at least, enhance them by combining their individual effects. Application of MW radiation to ultrasound-assisted chemical processes has been recently described by some authors [18, 19]. Mechanical activation has also been successfully combined with MW heating to increase chemical yields of several reactions [1]. 

Electronically excited states of organic molecules, acid-base properties of, 12,131 Energetic tritium and carbon atoms, reactions of, with organic compounds, 2, 201 Enolisation of simple carbonyl compounds and related reactions, 18,1 Entropies of activation and mechanisms of reactions in solution, 1,1 Enzymatic catalysis, physical organic model systems and the problem of, 11, 1 Enzyme action, catalysis of micelles, membranes and other aqueous aggregates as models of, 17. 435... 

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