Do bicyclic forms of m- and p-benzyne exist?

Density functional theory calculations (restricted and unrestricted B3LYP w ith the cc-pVDZ and cc-pVTZ basis sets) have been carried out for m- and p- benzyne. For p-benzyne a closed form (bicyclic 3b) and an open form (single t diradical 3a) were located along with the transition structure that conne cts them. IRC calculations were also performed in order to ensure that the transition structure linked the two minima. Butalene (3b), though substanti ally higher in energy than the singlet diradical 3a, might be able to be is olated at low temperature because of the relatively large activation energy predicted for its conversion into the diradical 3a. In the case of m-benzy ne the DFT results with the cc-pVDZ basis set also indicated that a bicycli c (2b) and a singlet diradical (2a) were minima on the potential surface. H owever, with the cc-pVTZ basis set only the bicyclic form 2b was found to b e a minimum. The computed IR spectrum of the bicylic form (2b) was found to be in good agreement with the previously re ported experimental spectrum. Recent experimental evidence on the chemical reactivity of m-benzyne suppor ts a bicylic structure.