The structure of meta-benzyne revisited - A close look into sigma-bond formation

A detailed density fuctional theory (DFT) and ab initio quantum chemical in vestigation of meta-benzyne (1) is presented with a focus on the distance o f the radical centers C1 and C3. Energy profiles for the cyclization of the biradical form (1a) to give the highly strained bicyclic anti-Bredt olefin (1b) are calculated employing four different functionals (B3LYP, B3PW91, B LYP, BPW91) as well as different ab initio methods (HF, MP2, CASSCF) in com bination with two different basis sets (cc-pVDZ, cc-pVTZ). To judge the per formance of the different methods, high-level single-point calculations (CC SD(T)/cc-pVTZ, CASPT2/cc-pVTZ, and CAS(8,8)-CISD+Q/cc-pVTZ) are carried out for a large number of structures along the cyclization coordinate. These c alculations show that only one minimum energy structure exists for meta-ben zyne and that the C1C3 separation is 205 +/- 5 pm. The topology of the PES as well as the equilibrium geometry strongly depend on the level of theory applied. Hybrid DFT methods overestimate bonding between the radical center s, pure GGA methods perform significantly better, and the BLYP fuctional ap pears to be the most suitable one for aromatic meta-biradicals. Despite the large distance of the, radical centers in 1, the biradical character is lo w (19-32% depending on the definition of this quantity) and therefore neith er la nor lb is an appropriate representation of meta-benzyne. NBO populati on and topological analysis of the electron density distribution reveal tha t the best way to describe the electronic structure of this molecule is a s igma -allylic system in which primarily the antibonding C2H7 orbital partic ipates in the interaction of the radical lobes.