NMR 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 observed in the resonance frequencies of all aromatic protons in the anthacenyl and 3,5-dinitrophenyl groups of the unsymmetrical diester, 1-(9-anthracenecarbonyloxy)-3-[(3,5-dinitrobenzoyl)oxy]-2-methylpropane (AN), relative to those for the aromatic protons of the respective monoesters and symmetrical diesters. Analysis of the temperature dependence of the vicinal coupling constants indicates highly populated gauche states of the two central C−C bonds of the spacer chain, consistent with a total fraction of U-shaped conformers of about 80% at ambient temperature. Transient nuclear Overhauser effect experiments yield values of between 0.4 and 0.6 nm for average distances between protons on the dinitrophenyl ring and those on the anthracenyl ring, implying a distance of approximately 0.3 nm between the planes of the two aromatic groups. The stabilization of the U-shaped conformers in AN is rationalized in terms of quadrupole interactions between the two aromatic groups. The quadrupole moments associated with the two aromatic groups in AN have opposite sign, resulting in a significant attractive interaction when the groups are oriented face-to-face. For the symmetrical diesters, A2 and N2, the interacting aromatic groups have identical quadrupole moments and the interaction is repulsive in the face-to-face arrangement.