Explicit Solvation of Carboxylic Acids for Vibrational Circular Dichroism Studies: Limiting the Computational Efforts without Losing Accuracy

On the basis of a comprehensive analysis of experimental and theoretical IR and vibrational circular dichroism (VCD) spectra, we make suggestions for solvation schemes for carboxylic acids. More specifically, we have studied two chiral carboxylic acids in solvents of different polarity and hydrogen bonding capabilities and verify previously proposed solute–solvent structures for their general applicability. Explicit solvation with acetonitrile-d₃ is shown to be most important for carboxylic acid groups directly attached to a stereocenter, while dimethylsulfoxide-d₆ should always be considered explicitly in spectra analysis. In order to circumvent the need to consider dimerization with calculations on the full homodimer in nonpolar solvents such as chloroform-d₁, we quantitatively evaluate the quality of truncated models. Methanol-d₄ is concluded to be the most challenging solvent for VCD studies of carboxylic acids, as the acid strength affects the hydrogen bonding strength to methanol and thus significantly determines the degree and effect of solvation. With the help of matrix-isolation IR spectroscopy, we also characterize the monomeric species of α-phenylpropionic acid.