The effect of a double n(O) → π∗(C = O) intramolecular interaction on the stability of 3-nitrophthalic acid

Abstract

It is not frequent that weak non-covalent interactions counteract moderate hydrogen bonds. And it is also very uncommon to observe two concurrent n → π∗ interactions, much less involving the same acceptor atom. In this work, we performed a theoretical analysis over all stable confomers of the 3-nitrophthalic acid. This compound has such a rich conformational variety, that it allowed us to compare different stabilizing and destabilizing effects as a function of a few dihedral angles. We found that the lowest-energy structure is the result of a balance between the stabilization provided by a double n → π∗ interaction, the global decrease of steric repulsions, and alteration of the electron delocalization. The contributions of these entities to the global molecular stability are coupled (i.e., all are affected when one is modified) in such a manner that the formation of a double n → π∗ interaction is preferred over the formation of a moderate hydrogen bond.