Do Bicyclic Forms ofm- andp-Benzyne Exist?

Abstract

Density functional theory calculations (restricted and unrestricted B3LYP with 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 (singlet diradical 3a) were located along with the transition structure that connects them. IRC calculations were also performed in order to ensure that the transition structure linked the two minima. Butalene (3b), though substantially higher in energy than the singlet diradical 3a, might be able to be isolated at low temperature because of the relatively large activation energy predicted for its conversion into the diradical 3a. In the case of m-benzyne the DFT results with the cc-pVDZ basis set also indicated that a bicyclic (2b) and a singlet diradical (2a) were minima on the potential surface. However, with the cc-pVTZ basis set only the bicyclic form 2b was found to be a minimum. The computed IR spectrum of the bicylic form (2b) was found to be in good agreement with the previously reported experimental spectrum. Recent experimental evidence on the chemical reactivity of m-benzyne supports a bicylic structure.