Decomposition barriers

The endothermic reaction of the pentazole anion N5 with HN3 leads to an azidopentazole N8H+ with a decomposition barrier of 26 kcal mol-1 at the MP2 level. The proton of N8H+ is attached to the nitrogen atom next to the azide chain on the pentazole ring <2002PCA1872>. 

NF4+ HF2 cannot be obtained in a pure form, as it always contains some HF. ft decomposes at room temperature to form NF3, F2, and HF. The HF solution of this salt is stable at room temperature. Theoretical calculations indicate that a hypothetical NF5 has a decomposition barrier of 69kJmol but so far no experimental evidence has been found for this... 

When acetylene hydrate occurs in the equipment of an acetylene plant, especially in decomposition barriers and other safety accesories, it is less the danger of explosion than the blockage of the gas paths through the waxy mass that has to be taken into consideration. 

The decomposition barrier for the ethyl ester was found to be significantly lower, by about 75 kj moP, than that for the methyl compound, with the consequence that carboxylic acid species play a prominent role in the decomposition of ethyl esters, but not in their methyl cousins. 

Activation Parameters. Thermal processes are commonly used to break labile initiator bonds in order to form radicals. The amount of thermal energy necessary varies with the environment, but absolute temperature, T, is usually the dominant factor. The energy barrier, the minimum amount of energy that must be suppHed, is called the activation energy, E. A third important factor, known as the frequency factor, is a measure of bond motion freedom (translational, rotational, and vibrational) in the activated complex or transition state. The relationships of yi, E and T to the initiator decomposition rate (kJ) are expressed by the Arrhenius first-order rate equation (eq. 16) where R is the gas constant, and and E are known as the activation parameters. 

Figure 15.28 Decomposition of a reaction barrier into a parabola and a linear term...

Since the prefactor in Eq. (17) is a universal constant of order unity, the barrier AF / kaT is large only very close to the coexistence curve, i.e. for 5v / v /coex, while for larger 5v / the smallness of the barrier implies a very grad il transition from nucleation to spinodal decomposition.Conversely, for N x 1 where Eq. (16) holds the transition is very sharp since the barrier stays large right up to the spinodal for qo. 

This difference is the irreversible capacity loss (<2jr). Dahn and co-workers [71] were the first to correlate <21R with the capacity required for the formation of the SE1. They found that <2ir is proportional to the specific surface area of the carbon electrode and, assuming the fonnation of an Li2C03 film, calculated an SEI thickness of 45 5 A on the carbon particles, consistent with the barrier thickness needed to prevent electron tunneling [1,2]. They concluded [71] that when all the available surface area is coated with a film of the decomposition products, further decomposition ceases. 

In reviewing reported values of E for calcite decompositions, Beruto and Searcy [121] find that most are close to the dissociation enthalpy. They suggest, as a possible explanation, that if product gas removal is not rapid and complete, readsorption of C02 on CaO may establish dissociation equilibria within the pores and channels of the layer of residual phase. The rate of gas diffusion across this barrier is modified accordingly and is not characteristic of the dissociation step at the interface. 

The high values of E generally characteristic of the decomposition reactions of metal oxyhalides are widely interpreted as evidence that the initial step in anion breakdown is the rupture of the X—O bond and that the energy barrier to this reaction is not very sensitive to the properties of the cation present. Information of use in the formulation of reaction mechanisms has been obtained from radiolytic studies of oxyhalogen salts [887-889],... 

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