Conformational Equilibria and Large-Amplitude Motions in Dimers of Carboxylic Acids: Rotational Spectrum of Acetic Acid-Difluoroacetic Acid

We report the rotational spectra of two conformers of the acetic acid-difluoroacetic acid adduct (CH3COOH-CHF2COOH) and supply information on its internal dynamics. The two conformers differ from each other, depending on the trans or gauche orientation of the terminal -CHF2 group. Both conformers display splittings of the rotational transitions, due to the internal rotation of the methyl group of acetic acid. The corresponding barriers are determined to be V-3(trans)=99.8(3) and V-3(gauche)=90.5(9)cm(-1) (where V-3 is the methyl rotation barrier height). The gauche form displays a further doubling of the rotational transitions, due to the tunneling motion of the CHF2 group between its two equivalent conformations. The corresponding B-2 barrier is estimated to be 108(2) cm(-1) . The increase in the distance between the two monomers upon OH -> OD deuteration (the Ubbelohde effect) is determined.