NMR-STUDY OF INTRAMOLECULAR INTERACTIONS BETWEEN AROMATIC-GROUPS - VAN-DER-WAALS, CHARGE-TRANSFER, OR QUADRUPOLAR INTERACTIONS

Proton nuclear magnetic resonance spectroscopy has been used to invest igate intramolecular interactions between different aromatic groups in a series of diesters. The materials investigated comprise two aromati c groups linked by a 2-methyl-1,3-propanedioxy spacer. This spacer per mits U-shaped conformations which place the two terminal aromatic grou ps close together, parallel in a face-to-face arrangement. For the sym metrical diesters, 1,3-bis(9-anthracenecarbonyloxy)-2-methy (A2) and 1 ,3-bis[(3,5-dinitrobenzoyl)oxy]-2-methylpropane (N2), neither the chem ical shifts of the aromatic group protons nor the vicinal coupling con stants measured in the spacer provide any evidence for a high fraction of U-shaped conformers. In both cases, the conformational distributio n of the spacer is similar to that found for 1,3-diacetoxy-2-methylpro pane (M2), indicating that the planar aromatic groups in A2 and N2 exp erience no significant mutual attractive interactions. In contrast, su bstantial upfield shifts are observed in the resonance frequencies of all aromatic protons in the anthacenyl and 3,5-dinitrophenyl groups of the unsymmetrical diester, yloxy)-3-[(3,5-dinitrobenzoyl)oxy]-2-methy lpropane (AN), relative to those for the aromatic protons of the respe ctive monoesters and symmetrical diesters. Analysis of the temperature dependence of the vicinal coupling constants indicates highly populat ed gauche states of the two central C-C bonds of the spacer chain, con sistent with a total fraction of U-shaped conformers of about 80% at a mbient temperature. Transient nuclear Overhauser effect experiments yi eld values of between 0.4 and 0.6 nm for average distances between pro tons on the dinitrophenyl ring and those on the anthracenyl ring, impl ying 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 arom atic groups. The quadrupole moments associated with the two aromatic g roups in AN have opposite sign, resulting in a significant attractive interaction when the groups are oriented face-to-face. For the symmetr ical diesters, A2 and N2, the interacting aromatic groups have identic al quadrupole moments and the interaction is repulsive in the face-to- face arrangement.