Determination of Intermolecular Hydrogen Bonded Conformers of α-Aryloxypropanoic Acids Using Density Functional Theory Predictions of Vibrational Absorption and Vibrational Circular Dichroism Spectra.

Abstract

The density functional theoretical predictions of vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra for monomeric chiral α-aryloxypropanoic acids are found to be in serious disagreement with corresponding experimental spectra. This disagreement made it difficult to establish the predominant conformations and absolute configuration of chiral α-aryloxypropanoic acids. Since carboxylic acids tend to form dimers through intermolecular hydrogen bonding in solution, such hydrogen bonding should be either eliminated in the experimental measurements or modeled properly in the theoretical calculations for achieving agreement between theoretical predictions and experimental observations. Esterification eliminates hydrogen bonding in solution, but this approach does not provide information about the conformations of the acids. As a consequence, the option to properly model the dimers of α-aryloxypropanoic acids and to evaluate the dimer conformations in reproducing the experimental spectra becomes important. Here we report a method for obtaining the dimer conformations of α-aryloxypropanoic acids using density functional theory and evaluate their reliability by comparing the predicted and experimental VA and VCD spectra. The population weighted predicted VA and VCD spectra of dimers matched well with the corresponding experimental spectra in solution, thereby indicating the predominant dimer conformers and absolute configuration of α-aryloxypropanoic acids.