Intramolecular Nitro-Aromatic Interactions Within a Molecular Torsion Balance: A Quantitative Assessment

摘要

Interactions between a nitro group and an aromatic ring have been identified as an important aspect within the solid-state structures of many energetic materials. We investigated nitro-aromatic interactions within a molecular "torsion balance", both in solid-state as well as in solution. Our balance features a Troger's base and a nitroalkene functionality substituted at 1 and 8 positions of a naphthalene ring. Because of restricted rotation around a biaryl axis, the balance can adapt two principal atropisomeric conformations: folded and unfolded. Intramolecular nitro-aromatic interactions are feasible only in the folded state. A torsion balance containing a highly electron deficient aromatic ring crystallized in the folded state leading to the formation of a short (3.4 angstrom) intramolecular NO center dot center dot center dot aromatic contact. On the other hand, with an electron rich aromatic, the balance crystallized in the unfolded conformation in which the nitro group avoids the aromatic surface. In solution, the strength of the interactions was determined to be 1 kcal/mol in different solvents. A substituent effect study showed that the interactions included a component of the electrostatic forces.