Chemical bond pressure effect

A stress that is describable by a single scalar can be identified with a hydrostatic pressure, and this can perhaps be envisioned as the isotropic effect of the (frozen) medium on the globular-like contour of an entrapped protein. Of course, transduction of the strain at the protein surface via the complex network of chemical bonds of the protein 3-D structure will result in a local strain at the metal site that is not isotropic at all. In terms of the spin Hamiltonian the local strain is just another field (or operator) to be added to our small collection of main players, B, S, and I (section 5.1). We assign it the symbol T, and we note that in three-dimensional space, contrast to B, S, and I, which are each three-component vectors. T is a symmetrical tensor with six independent elements ... 

An emerging subdiscipline of tribological simulation involves the study of tribochemical reactions—that is, reactions that are activated by pressure and shear. These reactions alter the structure of lubricants and films that are used to protect surfaces from wear. Understanding the effects of these reactions on the intended behavior of these films is of utmost importance. However, simulation studies of tribochemical reactions have been impeded by the difficulty in accurately describing changes in chemical bonding. In a limited number of cases, this can be achieved with the use of reactive FFs, as noted above, whereas in other cases, one must resort to expensive quantum chemical calculations. In this section, we will describe two studies where such methods were used to examine tribochemical reactions. 

Jenner [275] has presented a thorough description of several possible contributions to both the intrinsic and the environmental parts of the activation volumes, based on accurate experimental observation of pressure effect on reactions in solutions. The intrinsic contribution to the activation volume essentially derives from the differences in structure between the transition state and the reacting species, so it is directly related to the partial cleavage and formation of chemical bonds in the transition state. In cases where the environmental contribution is negligible, the activation volume variation gives a direct insight in the molecular mechanism [275, 280]. In this case in fact, considering... 

The distance between chemically bound atoms in many molecules is shorter than the sum of the radii of the same atoms when free, and the specific volume of the compound may be actually smaller than the total covolume of its gaseous products. If, as seems plausible, the drastic compression within the detonation front ruptures chemical bonds, many atoms suddenly expand, exerting forces like those by which solids resist compression. Such forces could result in a spike pressure much higher than the peak pressure of the non-reactive shock front, exert a brisant effect on the surroundings, and expedite the progress of the detonation wave. This view accords with observations of cases in which... 

Elementary reactions are initiated by molecular collisions in the gas phase. Many aspects of these collisions determine the magnitude of the rate constant, including the energy distributions of the collision partners, bond strengths, and internal barriers to reaction. Section 10.1 discusses the distribution of energies in collisions, and derives the molecular collision frequency. Both factors lead to a simple collision-theory expression for the reaction rate constant k, which is derived in Section 10.2. Transition-state theory is derived in Section 10.3. The Lindemann theory of the pressure-dependence observed in unimolecular reactions was introduced in Chapter 9. Section 10.4 extends the treatment of unimolecular reactions to more modem theories that accurately characterize their pressure and temperature dependencies. Analogous pressure effects are seen in a class of bimolecular reactions called chemical activation reactions, which are discussed in Section 10.5. 

Sodium borohydride solutions are a convenient, compact way to store H2. For a discussion of the theoretical amount of H2 available to a NaBEL, solution, please see Appendix A. It is more effective to store energy in the form of chemical bonds (within NaBEL,) than it is to store energy as pressure, as evidenced by the low volumetric efficiencies of pressurized H2 gas. The most critical aspect of using NaBEL, solutions as a means of H2 storage is the quantity of H2 that is contained in a given weight and volume of solution. 

In molecular crystals we assume to a first approximation that the molecules are not changed in their structure and chemical bond properties. Therefore the differences in NQR frequencies must be due to the crystal field effect. The following forces may be responsible for the stability of different crystal modifications at different temperatures or pressures ... 

Any of six factors can affect the rate (1) the nature of the reactants, (2) the temperature, (3) the presence of a catalyst, (4) the concentration of reactants in solution, (5) the pressure of gaseous reactants, and (6) the state of subdivision of solid reactants. For a reaction to occur, the atoms, molecules, or ions must come into contact with one another with enough energy to rearrange chemical bonds in some way. Increased concentration, gas pressure, or surface area of a solid tends to get the particles to collide more frequently, and increased temperature tends to get them to collide more frequently and with greater energy to accomplish more effective collisions. Catalysts work in very many different ways. 

If we pursue the model of the catalytic surface as a collection of a fixed number of sites S capable of forming chemical bonds with adsorbed species, then we may expect to find that substances which can compete for the catalytic sites may inhibit the reaction. Furthermore, the effectiveness of the inhibition will depend on the relative pressures of the two adsorbates as well as their sorption constants on the surface. Let us consider as an example the simple Langmuir-type sorption of two sorbates A and B on a surface of S° sites ... 

Addition of free water in the reaction mixture may also affect positively. However, the addition of free water cannot always be appropriate, since without sufficient chemical interaction with the substrate oxide it would not work or even can be harmful reducing the main effect of mechanical loading. Chemical interaction between solid surfaces and water under mechanical activation significantly differs from static conditions. During comminution, solids are subjected to dynamic loading that results in the extension and compression of chemical bonds. This process is believed to be similar to corrosion under pressure [29]. 

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