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. (C) 2013 Elsevier B.V. All rights reserved.