Structure of m-Benzyne

Comparing the geometries and energetics of these two isomers requires appropriate treatment of their respective electronic natnres. 

While most of the computational methods indicate that just a single isomer exits on the PES, that being 42, conflrmation of this prediction requires comparison to experiment. The IR spectra of 42 has been obtained in a number of ways UV photolysis of matrix-isolated 45 or flash vacuum pyrolysis of 46, 

In the first of these experiments, an intense band was observed at 547 cm , with weaker bands at 751, 824, and 936 cm . AU of these matched well with 

Kraka and Cremer answered Hess criticisms by first arguing that the UB3LYP description of 42 includes appreciable triplet contamination. When this is corrected for using the sum method, the open ring isomer 42 is now more stable than 44. Furthermore, they note that Hess misassigned the experimental IR spectrum and there are actually only two bands in the 1200-1600 cm region. 

TABLE 5.12 Comparison of Spectroscopic Data of 42 (CCSD(T)) and 44 (B3LYP) with Experiment 

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Based on the experimental IR spectrum and appropriate computations, it is clear that the structure of m-benzyne is that of 42. The bicyclic isomer 44 is an artifact of computational methods that fail to adequately account for ihe multiconfigurational nature of the wavefunction for 42. 

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