The Formic Acid−Trifluoroacetic Acid Bimolecule. Gas-Phase Infrared Spectrum and Computational Studies

Abstract

The gas-phase FT-IR spectrum of the formic acid−trifluoroacetic acid (FA−Tfa) hydrogen-bonded complex, or bimolecule, was obtained by numerical analysis of the FT-IR spectrum of a mixture of FA and Tfa vapors. Nineteen out of 24 vibrational modes predicted by ab initio frequency calculations to occur in the mid-IR range (400−4000 cm-1) were observed as well-defined absorbance peaks, with the other five occurring as complex or overlapped regions. Several hydrogen-bond-influenced vibrations of each monomer were identified, including CO stretching and COH in-plane and OH out-of-plane bending. These occurred at 1701, 1403, and 871 cm-1, respectively, for FA, and at 1774, 1325, and 942 cm-1, respectively, for Tfa. The hydrogen bond donated by Tfa in the bimolecule appears to be stronger than that in the Tfa dimer, while the FA-donated hydrogen bond is weaker than that in the FA dimer. Geometry optimization and vibrational frequency calculations were carried out at 21 levels of theory up to B3LYP/aug-cc-PVDZ. All levels of theory predicted an unsymmetrical complex, with hydrogen bond distances of 1.608 Å and 1.706 Å donated by Tfa and FA, respectively, and the corresponding O···O distances of 2.620 Å and 2.704 Å (B3LYP/aug-cc-PVDZ). These values differ from the symmetrical, or nearly symmetrical, structure derived from microwave spectra (Costain and Srivastava, J. Chem. Phys. 1964, 41, 1620−1627; Martinache et al. Chem. Phys. 1990, 148, 129−140). In agreement with previous experimental findings, the bimolecule was predicted to be more stable than either homodimer, with a calculated ΔHcomplexation of −14.2 kcal/mol (B3LYP/aug-cc-PVDZ). Mulliken population analysis predicted a polar complex with a transfer of 0.02−0.03 protons from Tfa to FA and a predicted dipole moment of 2.3−2.4 D, depending on the level of theory. The combined spectroscopic and computational evidence indicates that in this complex the Tfa-donated hydrogen bond is strengthened more than the FA-donated hydrogen bond is weakened.