Rotational spectroscopic and theoretical study of the perfluorobutyric acid⋯formic acid complex

Perfluorinated carboxylic acids are a special class of compounds with considerable industrial applications. In this study, we applied chirped pulse and cavity-based Fourier transform microwave spectroscopy to investigate the structural and possible tunneling properties of the perfluorobutyric acid⋯formic acid complex, together with high level ab initio calculations. Only one clearly dominant conformer, featuring an eight-membered double hydrogen-bonded ring, was predicted for the hetero acid dimer and detected experimentally. The rotational constants calculated at the B3LYP-D3BJ/6-311++G(2d,p) level are in excellent agreement with the experimental ones, better than those from MP2 calculations. No tunneling splittings were detected in the experimental spectra. Detailed analyses of the non-covalent intermolecular interactions in the hetero acid dimer provide an explanation for the absence of double proton transfer, and also identify the presence of F⋯F attractive interactions.