The first experimental observation of the higher-energy trans conformer of trifluoroacetic acid

• Trans-TFA was observed experimentally for the first time.
• It was generated by NIR-selective vibrational excitation of cis-TFA isolated in different cryomatrices (Ar, Kr, N2).
• Tunneling decay rates were measured and compared with similar data for other carboxylic acids.
• IR spectra of cis- and trans-TFA in solid Ar, Kr and N2 were assigned. The different orientations of the CF3 and COOH groups were rationalized.

We report here the first experimental observation of the higher-energy conformer of trifluoroacetic acid (trans-TFA). The new conformer was generated by selective narrowband near-infrared vibrational excitation of the lower-energy cis-TFA conformer isolated in cryogenic matrices (Ar, Kr, N2) and shown to spontaneously decay to this latter form in the various matrix media, by tunneling. The decay rates in the different matrices were measured and compared with those of the trans conformers of other carboxylic acids in similar experimental conditions. The experimental studies received support from quantum chemistry calculations undertaken at various levels of approximation, which allowed a detailed characterization of the relevant regions of the potential energy surface of the molecule and the detailed assignment of the infrared spectra of the two conformers in the various matrices. Noteworthly, in contrast to cis-TFA that has its trifluoromethyl group eclipsed with the CO bond of the carboxylic moiety, trans-TFA has the trifluoromethyl group eclipsed with the CO bond. This unusual structure of trans-TFA results from the fact that the relative orientation of the CF3 and COOH groups in this geometry facilitates the establishment of an intramolecular hydrogen-bond-like interaction between the OH group and the closely located in-plane fluorine atom of the CF3 moiety.

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