Potential energy surface scan for the skeletal torsion calculated at the B3LYP/cc-pVTZ level of theory using the scan option of Gaussian 16 (Frisch et al. 2016). The energies were calculated in steps of 10° of the dihedral angle O2–C1–C6–C7 while the other structural parameters were optimized under the verytight convergence criterion. The global minimum belongs to the planar syn conformer while for the higher-energy conformer two minima can be identifiedon the surface which correspond to two equivalent skew frameworks connected with a barrier of around 1 kJ mol⁻¹ (84 cm⁻¹).