Four conformers of the heterodimer o-anisic acid – formic acid, formed in a supersonic expansion, have been probed by Fourier transform microwave spectroscopy. Two of these forms have the typical double intermolecular hydrogen bond cyclic structure. The other two show the o-anisic acid moiety bearing a trans-COOH arrangement supported by an intramolecular O-H··O bond to the neighbour methoxy group. In these conformers, formic acid interacts with o-anisic acid mainly through an intermolecular O-H···O hydrogen bond either to the O-H or to the C=O moieties, reinforced by other weak interactions. Surprisingly, the most abundant conformer in the supersonic expansion is the complex in which the o-anisic acid is in trans arrangement with the formic acid interacting with the O-H group. Such a trans-COOH arrangement in which the intramolecular hydrogen bond dominates over the usually observed double intermolecular hydrogen bond interaction has never been observed previously in an acid-acid dimer.
