Pyridine-acetaldehyde, a molecular balance to explore the n→π* interaction.

The complex pyridine-acetaldehyde is formed through an n→π* interaction and a C-HO contact. The acetaldehyde methyl group internal rotation induces a phase-locked intermolecular oscillation along the Bürgi-Dunitz coordinate. Surprisingly, this sort of molecular balance extracts energy through the n→π* interaction to reduce the size of the internal rotation barrier.