Conformational landscapes of symmetrically fluorine-substituted benzoic acids I: Microwave spectroscopic and theoretical studies on 3,5-difluorobenzoic acid

We report our combined theoretical and spectroscopic studies on 3,5-difluorobenzoic acid. Using a chirped pulse Fourier transform microwave (CP-FTMW) spectrometer, we recorded and analyzed the rotational spectrum spanning the frequency range of 6 – 12.5 GHz. Quantum chemical calculations were employed to analyze the conformational changes and landscapes of 3,5-difluorobenzoic acid. These calculations focused on studying the potential energy surfaces along the CCCO and OCOH dihedral angles at the B3LYP/6-311G level. Based on the computational results, we identified the global minimum conformer 1 as well as the local minimum conformer 2. We discussed and interpreted the geometric structures of the relevant conformations, with a particular emphasis on the interactions between the carboxylic group and the substituted fluorine atoms. Furthermore, these findings were compared to benzoic acid in internal strains. In our spectral analysis, we successfully identified conformer 1 and its seven ¹³C singly substituted isotopologues. We derived highly accurate rotational constants for 3,5-difluorobenzoic acid, displaying good agreement with computational results. We established the effective structure of its ground vibrational state using Kraitchman’s equations. Similar to benzoic acid, the global minimum conformation of 3,5-difluorobenzoic acid adopts a planar structure, corroborating our computational outcomes.