The torsional potential in 2,2′-bipyrrole revisited: High-level ab initio and DFT results

A systematic study of torsional potentials for inter-ring rotation in 2,2′-bipyrrole is accomplished. Highly accurate calculations were performed at the second-order Møller-Plesset theory (MP2) and at the very demanding coupled-cluster with single, double, and perturbatively estimated triple excitations CCSD(T) level, together with the hierarchy of (aug)-cc-pVnZ basis sets. These large-scale state-of-the-art calculations ensure virtual convergence in both N- and one-particle space. The reliability of density-functional theory (DFT) is thoroughly assessed along the whole rotational profile by root-mean-squared energy differences with respect to benchmark values. The best agreement is obtained for large fractions of Hartree-Fock (HF) exchange introduced into the functionals.