Competition Between Intra‐ and Intermolecular Hydrogen Bonding: o ‐Anisic Acid⋅⋅⋅Formic Acid Heterodimer

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 neighbor 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.