Transition thermal activation

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

Transition from Thermal Activation to Viscous Drag... 

Transition from thermal activation to viscous drag occurs when... 

J.N. Johnson and D.L. Tonks, Dynamic Plasticity in Transition from Thermal Activation to Viscous Drag, in Shock Compression of Condensed Matter— 1991 (edited by S.C. Schmidt, R.D. Dick, J.W. Forbes, and D.G. Tasker), Elsevier Science, Amsterdam, 1992, pp. 371-378. 

Here W(a,(i) is the p a transition probability for which we accept the conventional thermally activated atomic exchange model . Below we briefly review several recent works on the general formulation of this approach and on its applications to studies of alloy phase transformatious. 

Whilst temperature coefficients suggest modest potential differences, these calculations do not take into account the large potential changes that can occur when thermal effects allow transition from active to passive states. 

Quantum tunnelling in chemical reactions can be visualised in terms of a reaction coordinate diagram (Figure 2.4). As we have seen, classical transitions are achieved by thermal activation - nuclear (i.e. atomic position) displacement along the R curve distorts the geometry so that the... 

Figure 2.4. Reaction coordinate diagram for a simple chemical reaction. The reactant A is converted to product B. The R curve represents the potential energy surface of the reactant and the P curve the potential energy surface of the product. Thermal activation leads to an over-the-barrier process at transition state X. The vibrational states have been shown for the reactant A. As temperature increases, the higher energy vibrational states are occupied leading to increased penetration of the P curve below the classical transition state, and therefore increased tunnelling probability.

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