NMR Study of Intramolecular Interactions between Aromatic Groups: Van der Waals, Charge-Transfer, or Quadrupolar Interactions? J. Am. Chem. Soc. 1998, 120, 9632−9645NMR Study of Intramolecular Interactions between Aromatic Groups: Van der Waals, Charge-Transfer, or Quadrupolar Interactions?

Abstract

Proton nuclear magnetic resonance spectroscopy has been used to investigate intramolecular interactions between different aromatic groups in a series of diesters. The materials investigated comprise two aromatic groups linked by a 2-methyl-1,3-propanedioxy spacer. This spacer permits U-shaped conformations which place the two terminal aromatic groups close together, parallel in a face-to-face arrangement. For the symmetrical diesters, 1,3-bis(9-anthracenecarbonyloxy)-2-methylpropane (A2) and 1,3-bis[(3,5-dinitrobenzoyl)oxy]-2-methylpropane (N2), neither the chemical shifts of the aromatic group protons nor the vicinal coupling constants measured in the spacer provide any evidence for a high fraction of U-shaped conformers. In both cases, the conformational distribution of the spacer is similar to that found for 1,3-diacetoxy-2-methylpropane (M2), indicating that the planar aromatic groups in A2 and N2 experience no significant mutual attractive interactions. In contrast, substantial upfield shifts are obs...