Hydrates of trans- and gauche-difluoroacetic acids: a high-resolution microwave spectroscopic study.

Although identical in formula, trans- and gauche-difluoroacetic acids behave as different molecules in rotational spectra due to their distinct sets of rotational constants. In this study, high-resolution microwave spectra of monohydrates, dihydrates, and trihydrates of both trans- and gauche-difluoroacetic acids were recorded using a Fourier transform microwave spectrometer. Rotational and centrifugal distortion constants of these hydrates were obtained with high accuracy via fitting their microwave spectra. It was found that the subtle structural difference between the trans and gauche forms of the acid gives different tunneling motions in their monohydrates. An unusual mixing of wave functions of energetically nondegenerate conformations was observed in the monohydrate of gauche-difluoroacetic acid. Ab initio calculations using the second-order Møller-Plesset perturbation method were also performed to optimize structures and to predict magnitudes of dipole moments of these hydrates. Close agreement between experimental and theoretical rotational constants confirms the feature of hydrogen-bonded ring structures in which all incoming H(2)O molecules bind to the carboxylic group for all hydrates.