Conformational properties and vibrational analyses of monomeric pentafluoropropionic acid CF₃CF₂COOH and pentafluoropropionamide CF₃CF₂CONH₂

The structural stability of pentafluoropropionic acid and pentafluoropropion-amide was investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. The calculated potential energy curves of the acid were consistent with two distinct minima that correspond to cis (C=O eclipses the C-C bond) and gauche conformers, with the latter being the lower energy form at ambient temperature. The equilibrium constant for the conformational interconversion of pentafluoropropionic acid were calculated at both levels of theory and found to correspond to an equilibrium mixture of about 76 % gauche and 24% cis conformations at 298.15 K. From the calculations, pentafluoroproionamide was predicted to exist only in the gauche conformation. The vibrational frequencies of pentafluoropropionic acid in the cis and the gauche conformers and pentafluoropropionamide in the gauche structure were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental infrared data of the molecules.