Solvation of a chiral carboxylic acid: effects of hydrogen bonding on the IR and VCD spectra of α-methoxyphenylacetic acid.

Abstract

Strong hydrogen bonding to solvent molecules can significantly alter the IR and VCD spectra of a chiral solute. This can be particularly troublesome for the determination of absolute configurations, as all spatial configurations of solute-solvent clusters need to be considered explicitly in spectra calculations. With this contribution, we aim to derive general guidelines for the explicit solvation of carboxylic acids, and characterize the solute-solvent interactions of the model compound α-methoxyphenylacetic acid (MPAA) in organic solvents of different polarity. We show that, in the typical concentration range employed for VCD studies, MPAA prefers the formation of dimers in chloroform. In the other investigated solvents (acetonitrile, methanol, and dimethyl sulfoxide), hydrogen bonded solute-solvent clusters have to be considered explicitly for the spectral analysis. We discuss the origin of the solvent dependence of the VCD spectra of MPAA in detail, show which vibrational modes are most strongly affected, and that the spectral response correlates with the hydrogen bonding strength.