Atomic-Level Bond Breaking

Ultimately, fracture results from the breaking of atomic bonds. For a brittle solid, the balancing of the energy release rate G and the dissipative processes associated with the creation of new free surface is played out explicitly on an atom by atom basis if one carries out a molecular dynamics simulation of the relevant atomic-level processes. A number of calculations illustrate the level to which such calculations can be pushed using parallel versions of molecular dynamics codes. An especially beautiful sequence of snapshots from the deformation history of a solid undergoing fracture is shown in fig. 12.33. The key point illustrated by 

We begin by examining the types of mixed atomistic/continuum strategies discussed earlier (see section 12.3.4) with special emphasis on how these methods have addressed the factors that determine whether an atomically sharp crack tip will cleave or emit dislocations. As shown in fig. 12.34, the lattice Green function 

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The situation with some forms of biological deterioration is somewhat different. Where the agent is macrobiological, as in the case of rodents, insects, and marine borers, the attack is physical in nature, such as by gnawing or boring. The attack is not at the atomic or molecular level. Any breaking of molecular bonds such as in polymer chain shortening is thus accidental. The attack may be said to be at the material s structural level, not the polymer molecule level. 

The harmonic approximation is only valid for small deviations of the atoms from their equilibrium positions. The most obvious shortcoming of the harmonic potential is that the bond between two atoms can not break. With physically more realistic potentials, such as the Lennard-Jones or the Morse potential, the energy levels are no longer equally spaced and vibrational transitions with An > 1 are no longer forbidden. Such transitions are called overtones. The overtone of gaseous CO at 4260 cm (slightly less than 2 x 2143 = 4286 cm ) is an example. 

Aim for the isohypsic condition of no net change in oxidation level for atoms involved in bond-making and bond-breaking reaction steps (HI = 0). 

The +C(SH)3 cation and the radical dication derived from it have been the subject of high-level calculations.67 The ability of two adjacent sulfur atoms to stabilize cations, anions, and radicals makes these species usefiil for relating bond-breaking and electron-transfer energies.68,69 Electrophilicity parameters for the dithiocarbenium ions (27) have been worked out,70 and the stabilities of the cations (28), (29) and (30) have been estimated using PM3 calculations.71 Cation (31) can be captured by solvent or azide ion, or it may ring close to (32), which subsequently alkylates another (31) cation as shown.72... 

Chemical reactions, the transformation of matter at the atomic level, are distinctive features of chemistry. They include a series of basic processes from the transfer of single electrons or protons to the transfer of groups of nuclei and electrons between molecules, that is, the breaking and formation of chemical bonds. These processes are of fundamental importance to all aspects of life in the sense that they determine the function and evolution in chemical and biological systems. 

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