Conformers of dimers of carboxylic acids in the gas phase: A rotational study of difluoroacetic acid–formic acid

The rotational spectra of two conformers of the difluoroacetic acid–formic acid complex have been identified by pulsed jet Fourier transform microwave spectroscopy. They differ from each other depending on the trans or gauche arrangement of the terminal CHF2 group. The gauche form displays a doubling of the rotational transitions, from which the internal rotation barrier hindering the two equivalent gauche forms has been estimated to be 101.8(3) cm−1. The increase of the distance between the two monomers upon OH → OD deuteration (Ubbelohde effect) has been determined.