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 CO 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 ΔH complexation 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.

Characterization of phenomena occurring at the interface of chiral conducting surfaces

“…Trifluoroacetic acid (CF 3 COOH) has been chosen because its IR spectrum exhibits strong stretchings at 1188 cm À1 (-CF 3 ) and 1774 cm À1 (-COOH). 57 ATR-FTIR spectrum has been recorded after electropolymerization of 1 on Pt electrode in order to form a chiral surface poly(1)-Pt with pendant chiral center (leucine). Fig.…”

“…4). 57 The presence of bands characteristic to CF 3 COOH come from the strong hydrogen bond interactions between CF 3 COOH and the chiral surface poly(1)-Pt. Furthermore, in the presence of CF 3 COOH, poly(1)-Pt was reduced and turns orange (undoped state).…”

A fast empirical GAFF compatible partial atomic charge assignment scheme for modeling interactions of small molecules with biomolecular targets

“…Hydrogen bonding between nucleic acid bases are studied extensively and so are the organic acid dimers 94–106, 112. We have chosen nitrogen bases methylated in the first position for pyrimidines [1‐mUra (U), [1‐mThy (T)], and [1‐mCyt (C)] and in the ninth position for purines [9‐mAde (A)] and [9‐mGua (G)], six organic acid dimers and four other small molecule dimers (Figure 4).…”

“…We have chosen nitrogen bases methylated in the first position for pyrimidines [1‐mUra (U), [1‐mThy (T)], and [1‐mCyt (C)] and in the ninth position for purines [9‐mAde (A)] and [9‐mGua (G)], six organic acid dimers and four other small molecule dimers (Figure 4). Gas phase experimental binding affinity of these six methylated base pairs and other 10 small molecule dimers are reported in the literature 94–106. A comparison of the calculated dimer energy with experiment is given in Table 5.…”

“…HF/6‐31G* and MP2/6‐31+G** energies for the organic dimerswere obtained from ref 87,. while experimental values of nucleic acid base pairs and 10 small molecule dimers were obtained from refs 94–106,. and CCSD(T)/CBS energies of the amino acids side chain pairs were adapted from ref 92…”