Ring cleavage dissociations

Excited electronic states have also been considered to explain NR dissociations of heterocyclic radicals in which low-energy losses of hydrogen atoms compete with high energy ring-cleavage dissociations. The latter reactions were interpreted as starting from excited electronic states of the radicals [27,28]. Formation of excited electronic states upon collisional electron transfer has also been studied with smaller molecular systems, e.g. CHn [45], 02 [46], and H3 [47,48],... 

Cyclohexylamino-l,2,3,4-selenatriazole (29) dissociated into hydrazoic acid and isoselenocyanate on attempted isolation (equation 13) (82UP42800). Monosubstituted 5-amino-l,2,3,4-thiatriazoles also undergo this type of ring cleavage, although only in the presence of base (see Section 4.28.2.4.1). 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

Experimental values of bond dissociation enthalpies are scarce compared with the data available for standard enthalpies of formation. This is not surprising because most chemical reactions that have been studied thermochemically involve the cleavage and the formation of several bonds. The measured standard reaction enthalpies are thus enthalpy balances of various bond dissociation enthalpies, whose individual values are often unknown. Consider, for example, reaction 5.10, where the arene ring in (q6-bcnzene)chromium tricarbonyl is replaced by three carbonyl ligands. The enthalpy of this reaction at 298.15 K,... 

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