Relative Energies of the Benzynes

The first contribution to benzyne chemistry produced by computational chemists concerns the relative energies of the isomeric benzynes. (Wenthold has published an excellent summary of the thermochemical properties of the benzynes.) 

Squires reported collision-induced dissociation (CID) of benzyne precursors (Reactions 5.5-5.7) to obtain their heats of formation. The heat of formation of 41 was determined to be 106 3 kcal mol in fine accord with other measurements. The values of the other two isomers were the first to be recorded A/fj298 ( 2) = 116 3 kcalmol and A/ff298 ( 2) = 128 3 kcal mol . Bergman had previously made a rough estimate of the heat of formation of 43 as 140 kcal mol , but this discrepancy with the CID results went unmentioned by Squires. 

Squires estimated the heats of formation of the benzynes with Eq. (5.5), making use of the biradical separation energy (BSE) defined in Reaction 5.8. The BSE is computed with different computational methods, and experimental heats of formation are utilized for benzene and phenyl radical. Their best estimate for the heats of formation was obtained using the correlation-consistent Cl method  

The Cl methods build upon the HF wavefunction and so there is concern that limited expansions might not be able to recover enough nondynamic correlation to provide reasonable treatment of the multiconfigurational nature of the benzynes, especially for 43. Nevertheless, CISD provides reasonable results, and the CCCl results are in excellent agreement with experiment.Both 42 and 43 are predicted to be too energetic relative to 41 with CCSD, but inclusion of triples configurations using CCSD(T) lowers this separation. 

The energies listed in Table 5.10 do not tell the whole story. A more critical examination of m-benzyne will be presented in the next section as computational results played a significant role in the characterization of this reactive species. We close this section with further discussion of problems associated with computing the energy and geometry of p-benzyne. 

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