Conformational Landscape of m-Anisic Acid and Its Complexes with Formic Acid.

Four conformers of m-anisic acid were observed in a supersonic expansion using Fourier transform microwave spectroscopy. These conformers correspond to different relative orientations of the acid and methoxy groups and have their planar skeletons stabilized by resonance. When formic acid was present in the jet, the spectra of four m-anisic acid-formic acid heterodimer conformers were detected. The complexes are formed from the interaction of formic acid with each of the four observed conformers of m-anisic acid through the typical acid-acid sequential cycle with a double O-H···O═C hydrogen bond interaction in a pseudo eight-membered ring arrangement. The four heterodimer forms retain the same four m-anisic acid conformational geometries and the same relative abundances of the m-anisic acid monomeric forms in the supersonic expansion. This proves that a kinetic mechanism dominates the formation of complexes in the jet.