Density functional theory calculations of stable isomers for trifluoroacetic acid (TFA)–(H2O)4 complexes

Stable isomers of trifluoroacetic acid (TFA) tetrahydrates, TFA–(H2O)4, have been explored by using density functional theory calculations. With the structural constraints assuming a polycyclic skeleton and homodromicity of intermolecular hydrogen bonds, structure optimization and vibrational calculations were performed for 34 isomeric structures (31 for neutral and 3 for ion-pair species, respectively) at the B971/6-311++G(3df,3pd) level. We found that the edge-sharing bicyclic “book-type” isomer is at the global minimum and more stable than the monocyclic structure by ca. 266 cm−1. We concluded that the observation of the zwitter-ionic structure in Ar matrices should be difficult, since it was predicted to be unstable by ca. 2500 cm−1 energetically.