Influence of arene-arene interactions on the conformation of acyclic molecules: H-1 NMR and dipole moment experimental results

Conformational analysis of derivatives of 2-methyl-1,3-propanediol is reported. The diol spacer is substituted with different aromatic residues that are linked by ether or ester functionalities. Most of the experimental evidence was obtained by H-1 NMR spectroscopy through the analysis of the vicinal coupling constants of the two ABX systems of the spacer. It is found that the main factor influencing the conformation of the molecules is the interaction between the aromatic fragments at the ends of the spacer. Thus, when the electronic character of the two residues is similar (e.g. 1-3 and 5), the molecules have high conformational flexibility. On the other hand, when the electronic character of the two residues is different (e.g. 4), the molecules possess a conformational bias, having a preference for folded conformations. These results were corroborated by NOEs and computational modelling. The folded conformation of molecules such as 4 (and others reported previously) may be due to favourably quadrupolar and van der Waals interactions between the unlike aromatic residues. The conformational energies obtained by the H-1 NMR analysis of the vicinal coupling constants were used for the calculation of the dipole moments of some molecules (1, 3, 5 and 6) and excellent agreement with the experimental values was found