0-0 bond rupture

Not all consequences of radiation exposure are deleterious. Crosslinking may be induced by irradiation to improve the mechanical behavior and degradation characteristics. For example, y-radiation is used commercially to crosslink polyethylene to enhance its resistance to softening and flow at elevated temperatmes indeed, this process may be carried out on products that have already been fabricated. 

Oxygen, ozone, and other substances can cause or accelerate chain scission as a result of chemical reaction. This effect is especially prevalent in vulcanized rubbers that have doubly bonded carbon atoms along the backbone molecular chains and that are exposed to ozone (O3), an atmospheric pollutant. One such scission reaction may be represented by 

Some of the most thermally stable polymers are the ladder polymers. For example, the ladder polymer having the structure 

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Variational RRKM theory is particularly important for imimolecular dissociation reactions, in which vibrational modes of the reactant molecule become translations and rotations in the products [22]. For CH —> CHg+H dissociation there are tlnee vibrational modes of this type, i.e. the C—H stretch which is the reaction coordinate and the two degenerate H—CH bends, which first transfomi from high-frequency to low-frequency vibrations and then hindered rotors as the H—C bond ruptures. These latter two degrees of freedom are called transitional modes [24,25]. C2Hg 2CH3 dissociation has five transitional modes, i.e. two pairs of degenerate CH rocking/rotational motions and the CH torsion. 

Aziridinones undergo two types of selective ring opening by nucleophiles <68AG(E)25). Reaction with proton-containing nucleophiles, e.g. water, alcohols, thiols, amines and mineral acids, leads exclusively to amides (339), corresponding to an C —N bond rupture. 

As we showed in Chapter 6 (on the modulus), the slope of the interatomic force-distance curve at the equilibrium separation is proportional to Young s modulus E. Interatomic forces typically drop off to negligible values at a distance of separaHon of the atom centres of 2rg. The maximum in the force-distance curve is typically reached at 1.25ro separation, and if the stress applied to the material is sufficient to exceed this maximum force per bond, fracture is bound to occur. We will denote the stress at which this bond rupture takes place by d, the ideal strength a material cannot be stronger than this. From Fig. 9.1... 

The more strain relieved by the bond rupture, the more reactive is the molecule. The substantially increased stability of [l.l.ljpropellane is due to the fact that not as much strain is relieved at the diradical stage because the diradical remains highly strained. 

Isotopic labeling experiments have established that C—O bond rupture occurs between the carbonyl carbon and oxygen substitution at the alcohol C—O bond is not involved. 

There is an intermediate mechanism between these extremes. This is a general acid catalysis in which the proton transfer and the C—O bond rupture occur as a concerted process. The concerted process need not be perfectly synchronous that is, proton transfer might be more complete at the transition state than C—O rupture, or vice versa. These ideas are represented in a three-dimensional energy diagram in Fig. 8.1. 

It is important to note that we assume the random fracture approximation (RPA) is applicable. This assumption has certain implications, the most important of which is that it bypasses the real evolutionary details of the highly complex process of the lattice bond stress distribution a) creating bond rupture events, which influence other bond rupture events, redistribution of 0(microvoid formation, propagation, coalescence, etc., and finally, macroscopic failure. We have made real lattice fracture calculations by computer simulations but typically, the lattice size is not large enough to be within percolation criteria before the calculations become excessive. However, the fractal nature of the distributed damage clusters is always evident and the RPA, while providing an easy solution to an extremely complex process, remains physically realistic. 

At high rates of strain, or when complete disentanglement cannot occur when M > M, bond rupture occurs randomly in the network and the percolation parameter p becomes dominated by chain ends such that... 

In this exercise, you will explore the bond rupture process by performing a potential energy surface scan. Run potential energy surface scans for these molecules, gradually increasing one of the C-H bond lengths, using the specified model chemistries ... 

In 1936, de Boer formulated his theory of a stressed bond which, despite its simplicity, still constitutes the basis for most models of chemical reactivity under stress [92], In order to fracture an unstressed bond which, in the absence of any vibration, is approximated by the Morse potential of Fig. 18, an energy D must be supplied. If, however, the bond is under tension due to a constant force feitt pulling on either end, the bond rupture activation energy will be decreased by an amount equivalent to the work performed by the mechanical force over the stretching distance from the equilibrium position. The bond potential energy in the presence of stress is given by ... 

In this equation, U(ct) is the activation energy for bond rupture in presence of a molecular axial stress (a), C, and C2 are constant factors. Following the rapid initial decrease of U(ct) at low levels of stress, the linearly extrapolated value U(0) to zero stress was found to be significantly lower than D ( 0.7 D with n = 100). 

Although significant insight into the process of bond rupture has been gathered from the studies on the scission kinetics, it remains desirable at this stage to carry out further experimentation at a molecular level to get information on the chain conformation at the moment of fracture. As a first step in this direction, birefringence measurements have been attempted recently in the single jet cell... 

I. Bond rupture does not involve electron transfer... 

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