Hydrogen bond docking site competition in methyl esters

Add time:09/01/2019    Source:sciencedirect.com

The OH ⋯ O hydrogen bonds in the 2,2,2-trifluoroethanol (TFE)–methyl ester complexes in the gas phase have been investigated by FTIR spectroscopy and DFT calculations. Methyl formate (MF), methyl acetate (MA), and methyl trifluoroacetate (MTFA) were chosen as the hydrogen bond acceptors. A dominant inter-molecular hydrogen bond was formed between the OH group of TFE and different docking sites in the methyl esters (carbonyl oxygen or ester oxygen). The competition of the two docking sites decides the structure and spectral properties of the complexes. On the basis of the observed red shifts of the OH-stretching transition with respect to the TFE monomer, the order of the hydrogen bond strength can be sorted as TFE–MA (119 cm− 1) > TFE–MF (93 cm− 1) > TFE–MTFA (44 cm− 1). Combining the experimental infrared spectra with the DFT calculations, the Gibbs free energies of formation were determined to be 1.5, 4.5 and 8.6 kJ mol− 1 for TFE–MA, TFE–MF and TFE–MTFA, respectively. The hydrogen bonding in the MTFA complex is much weaker than those of the TFE–MA and TFE–MF complexes due to the effect of the CF3 substitution on MTFA, while the replacement of an H atom with a CH3 group in methyl ester only slightly increases the hydrogen bond strength. Topological analysis and localized molecular orbital energy decomposition analysis was also applied to compare the interactions in the complexes.

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