**Figure 2**
The conformers of MPD studied by quantum-chemical calculations. Each conformer is denoted by a two-symbol code (number and letter) that approximately represents the torsion angles (ψ_{1}, ψ_{2}) of the initial conformation used in the calculation. The numbers **1**, **2** and **3** correspond to ψ_{1} = 180, 300 and 60°, respectively; the letters **a**, **b** and **c** correspond to ψ_{2} = 180, 60 and 300°, respectively. [For example, **1a** is the (180°, 180°) conformer shown in Fig. 1.] To interconvert two adjacent structures, a rotation is performed about the carbon–carbon bond given in square brackets. For clarity, only C2, C3 and C4 are numbered; C1 is shown in red, while CM and C5 (which is adjacent to C4) are shown in black. |