Fig. 15
a Definition of the torsion angles ϕ and ω used to characterise the molecular geometry of Pbtl. b Conformational energy surface of the Pbtl molecule with respect to ϕ and ω, calculated at the MP2 level of theory with the 6-31G(d,p) basis set, with the rest of the molecule optimised in 30° intervals of ϕ and ω. The data points (ϕ, ω)/(−ϕ, −ω) represent the experimental torsion angles in crystal forms of Pbtl, all of which are centrosymmetric. A, B and C are examples of characteristic conformations