Dissociation energy hydrogen compounds

As will be discussed in Chapter 13, calculated energies of one particular class of isodesmic reactions, so-called bond separation reactions, may be combined with experimental or high-quality calculated thermochemical data in order to lead directly to accurate heats of formation. These in turn can be used in whatever types of thermochemical comparisons are of interest. We start our assessment of isodesmic processes with bond separation reactions. Following this, we consider description of bond dissociation energies, hydrogenation energies and acid and base strengths in terms of isodesmic processes, that is, not as absolute quantities but expressed relative to standard compounds. 

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

Carbon-Hydrogen and Carbon-Chlorine Bond Dissociation Energies of Selected Compounds... 

Shorthand notations such as ET (electron transfer), HAT (hydrogen atom transfer), BDE (bond dissociation energy), NHE (normal hydrogen electrode), CV (cyclic voltammetry), LFP (laser flash photolysis), EPR (electron paramagnetic resonance) and KIE (kinetic isotope effect) will be used throughout the chapter. In addition, recurring chemical compounds such as TEMPO (2,2,6,6-tetramethylpiperidine-Ai-oxyl), HBT (1-hydroxyben-zotriazole), BTNO (benzotriazole-A-oxyl), HPI (iV-hydroxyphthalimide), PINO (phthal-imide-iV-oxyl), NHA (A-hydroxyacetanilide) and a few others will be referred to by means of the capital-letter acronym. 

Since the bond dissociation energy of hydrogen iodide is less than that of hydrogen bromide [DH-i, 70.5 kcal. DH-Br, 86.5 kcal. (3)], Reactions 6, 7, and 8 should occur more readily with hydrogen iodide. This would account for the fact that the promoting effect of iodine compounds on ignition is less than that of bromine compounds. 

Because of the small dissociation energies, the absorption of an infrared (IR) photon corresponding to one vibrational or even only a single rotational quantum of the diatom suffices to break the van der Waals bond. A typical example is the breakup of rare gas-hydrogen compounds,... 

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