Resonance energies from thermochemistry

As well as the major causes listed above, a number of more specific reasons have resulted in much work in some special cases. These include studies resulting directly or indirectly from an interest in the kinetics and thermochemistry of the reactions of methylene. In some cases, interest in reaction mechanisms of some alicyclic compounds in the liquid phase has led to gas phase work. Finally in some instances the pyrolytic studies have allowed estimates of resonance energy and strain energy to be made. 

Now the energy of form A+B can be estimated by electrostatics (Chap. 12), and the energy of the hypothetical completely covalent structure A B can be estimated from thermochemistry (Chap. 9). The stability of the actual molecule AB (its heat of formation) may be measured (also described in Chap. 9). When comparisons are made between the experimental stability of AB and the hypothetical stabilities of the two extreme structures, the molecule itself proves more stable than either extreme form. This, as will be remembered, must be so if the resonance treatment has been used correctly. 

The radical abstraction activation energy (Ei) was estimated from the thermochemistry (Ai/° i = 12.6 kcal.mole ) and a back abstraction activation energy of, = 9 kcal.mole Other estimates follow from the initiation reaction thermodynamics and by analogies with similar reactions . The chain carrier is the resonance stabilized, 2-methyl ally radical, and this should give the slowest (maximally inhibited) chain. From the steady state, one obtains... 

As noted previously, ion chemistry experiments can also give information about the thermochemistry of neutral molecules. Through judicious use of ion cyclotron resonance experiments54, a 0 K bond energy for Me—Au equal to 207 15 kJmol-1 has been found. From the sublimation enthalpy of gold6 (—366 kJmol-1), we find reaction 24 is exothermic by ca 159 kJ mol-1, essentially identical to that for the corresponding reaction for silver. 

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