Concerted decomposition energetics

Mechanism of Nonoxidative Thermal Dehydrochlorination. This subject is still very controversial, with various workers being in favor of radical, ionic, or molecular (concerted) paths. Recent evidence for a radical mechanism has been provided by studies of decomposition energetics (52), the degradation behavior of PVC-polystyrene (53) or PVC-polypropylene (54) mixtures, and the effects of radical traps (54). Evidence for an ionic mechanism comes from solvent effects (55) and studies of the solution decomposition behavior of a model allylic chloride (56). Theoretical considerations (57,58) also suggest that an ionic (El) path is not unreasonable. Other model compound decompositions have been interpreted in terms of a concerted process (59), but differences in solvent effects led the authors to conclude that PVC degrades via a different route (59). 

Table VII. Energetics of the concerted decomposition of six-membered ring compounds. Tabulated rate constant parameters, i.e., the pre-exponential A factors and activation energies, AE s, represent the tangential value of the calculated BAC-MP4 rate constants at 600 K. AHrxd and AErxii are evaluated at 298 K. Energies are in kcal-mol l.

Lewis et al.106 calculated four possible decomposition pathways of the ot-HMX polymorph N-N02 bond dissociation, HONO elimination, C-N bond scission, and concerted ring fission. Based on energetics, it was determined that N-N02 dissociation was the initial mechanism of decomposition in the gas phase, whereas they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The more recent study of Chakraborty et al.42 using density functional theory (DFT), reported detailed decomposition pathways of p-HMX, which is the stable polymorph at room temperature. It was concluded that consecutive HONO elimination (4 HONO) and subsequent decomposition into HCN, OH, and NO are the most energetically favorable pathways in the gas phase. The results also showed that the formation of CH20 and N20 could occur preferably from secondary decomposition of methylenenitramine. 

Watson et al. reported a leading example of (3-carbon elimination observed with a well-defined metal complex [67]. Thermal decomposition of a lutetium-isobutyl complex having a vacant coordination site leads to the formation of a lutetium-methyl complex and propene by way of (3-methyl elimination, the microscopic reverse of olefin insertion. A concerted four-center transition state is proposed. This study demonstrated that (3-carbon elimination is an energetically accessible process, and provided a model for the chain transfer that occurs during propene oligomerization. 

Figure 3. Energetics of unimolecular decomposition mechanisms in RDX obtained using the ReaxFF (full lines with filled symbols) and with QM (dashed lines with open symbols). Circles represent the sequential HONO elimination, triangles show the decomposition process following homolytic N-N bond breaking (NO2 elimination), and diamonds represent the concerted ring-opening pathway.

Sometimes a molecule decomposes to give not two but three or more fragments. This concerted cleavage of two bonds in the molecule with the formation of a new molecule and two radicals occurs when it is energetically more favorable than the simple decomposition of one bond. The decomposition of perester with the cleavage of only one O—O bond requires the energy expense equal to the energy of this bond, i.e.., 140-5-160 kJ/mol. 

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