Covalent bond rupture

Thus, if two isoelectronic molecules, ethane, H3C-CH3, and amine-borane, H3B NH3, are compared the distinction becomes obvions the normal covalent bond ruptures homolyticaUy and the nentral species formed are CHs (paramagnetic) free radicals, withanenergy of 89.8kcal moU spent... 

Beyond Proportional Limit (Plastic Strength). As the proportional limit is exceeded (Figure 11, Region B), the stress-strain relationship is no longer linear. Stresses are now great enough to induce covalent bond rupture and permanent distortion at all three structural levels. 

At the microscopic level, stresses develop within the crystalline region of the carbohydrate microfibrils. Failure of the microfibril from stress overload causes actual covalent bond rupture and excessive microfibril disorientation. Additionally, the cell wall layers distort such that permanent microcracks occur between the various cell wall layers. Separation of the cell wall layers is soon noticeable. 

Hanson and Martin applied the same approach to investigate the rupture of isoprene and butadiene oligomers in order to investigate covalent bond rupture in rubber.Using density functional theory, they identified the point of rupture where the unrestricted solution to the Kohn-Sham electronic wave functions falls below the restricted solution. This implies that the rupture process should be heterolytic and so this concept can only be applied for instances in which radical species are formed in the initial rupture event. 

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... 

Hemicarceplex A host-guest complex of a hemicarcerand and a suitable guest. These complexes are conceptually and structurally related to carceplexes, differing only in the rate at which guest molecular entities can (slowly) dissociate. Guest dissociation from a hemicarceplex is possible without covalent bond rupture. See mechanical bond. 

Polymeric materials also experience deterioration by means of environmental interactions. However, an nndesirable interaction is specified as degradation rather than corrosion because the processes are basically dissimilar. Whereas most metallic corrosion reactions are electrochemical, polymeric degradation is physiochemical that is, it involves physical as well as chemical phenomena. Furthermore, a wide variety of reactions and adverse consequences are possible for polymer degradation. Polymers may deteriorate by sweUing and dissolution. Covalent bond rupture as a result of heat energy, chemical reactions, and radiation is also possible, typically with an attendant reduction in mechanical integrity. Because of the chemical complexity of polymers, their degradation mechanisms are not well understood. 

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of ciystallisation in both the unstressed and the stressed state. 

Boddington and Iqbal [727] have interpreted kinetic data for the slow thermal and photochemical decompositions of Hg, Ag, Na and T1 fulminates with due regard for the physical data available. The reactions are complex some rate studies were complicated by self-heating and the kinetic behaviour of the Na and T1 salts is not described in detail. It was concluded that electron transfer was involved in the decomposition of the ionic solids (i.e. Na+ and Tl+ salts), whereas the rate-controlling process during breakdown of the more covalent compounds (Hg and Ag salts) was probably bond rupture. 

We therefore draw attention to a novel technique which allows solubilization of coal without rupture of covalent bonds. This utilizes the fact that the acidity of low-rank coals, which is largely due to their high -OH contents, can be enhanced by proper choice of a medium. 

Accelerating Effect due to Phenols on the Rupture of Ether Linkages. Phenols are weak acids and polar solvent, and so often observed to enhance the thermal decomposition of covalent bond, but we could not observe any accelerating effect due to phenol on the decomposition of dibenzyl. 

It is clear that reactions suitable for use in titrimetric procedures must be stoichiometric and must be fast if a titration is to be carried out smoothly and quickly. Generally speaking, ionic reactions do proceed rapidly and present few problems. On the other hand, reactions involving covalent bond formation or rupture are frequently much slower and a variety of practical procedures are used to overcome this difficulty. The most obvious ways of driving a reaction to completion quickly are to heat the solution, to use a catalyst, or to add an excess of the reagent. In the last case, a hack titration of the excess reagent will be used to locate the stoichiometric point for the primary reaction. Reactions employed in titrimetry may be classified as acid-base oxidation-reduction complexation substitution precipitation. 

These descriptors have been widely used for the past 25 years to study chemical reactivity, i.e., the propensity of atoms, molecules, surfaces to interact with one or more reaction partners with formation or rupture of one or more covalent bonds. Kinetic and/or thermodynamic aspects, depending on the (not always obvious and even not univoque) choice of the descriptors were hereby considered. In these studies, the reactivity descriptors were used as such or within the context of some principles of which Sanderson s electronegativity equalization principle [16], Pearson s hard and soft acids and bases (HSAB) principle [17], and the maximum hardness principle [17,18] are the three best known and popular examples. 

Supramolecular chain scission differs further from covalent chain scission, because supramolecular recombination is typically the predominant fate of a ruptured chain anthropomorphically speaking, the supramolecular moieties are, by their very nature, predisposed to reassociation rather than alternative reaction pathways. This predisposition is not intrinsic to the products of covalent bond mpture, which might lead either to high-energy intermediates with nonspecific reactivity or to species that require catalyst or elevated temperature to recombine. The... 

A heterolytic rupture will, in the case of an ordinary covalent bond, lead to formation of a cation and an anion in the case of a coordinate bond, the ligand simply departs along with the electron pair it contributed to form the bond. A dissociative mechanism exhibits first-order kinetics its rate is independent of the concentration of the incoming group Z. The intermediate EX in the overall reaction ... 

Cobalt-60 produces two gamma rays of 1.17 and 133 million electron volts (MeV). Up to 30 eV are required to rupture covalent bonds and to cause ionization. 

In the first monolayer of conjugated model material, a model molecular solid or a polymer adsorbate, assume that no chemistry (covalent bonding) occurs, since, in the absence of, for example, mechanical rupturing, the bonds at the surface of the molecular film are completely satisfied. This assumption is supported by the fact that, at least for condensed molecular solids, vapor-deposited films may be re-evaporated (removed) from the surface by gentle heating in UHV. 

In the early 1990s, Brenner and coworkers [163] developed interaction potentials for model explosives that include realistic chemical reaction steps (i.e., endothermic bond rupture and exothermic product formation) and many-body effects. This potential, called the Reactive Empirical Bond Order (REBO) potential, has been used in molecular dynamics simulations by numerous groups to explore atomic-level details of self-sustained reaction waves propagating through a crystal [163-171], The potential is based on ideas first proposed by Abell [172] and implemented for covalent solids by Tersoff [173]. It introduces many-body effects through modification of the pair-additive attractive term by an empirical bond-order function whose value is dependent on the local atomic environment. The form that has been used in the detonation simulations assumes that the total energy of a system of N atoms is ... 

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