Effects of strong and weak hydrogen bond formation on VCD spectra: a case study of 2-chloropropionic acid

The vibrational circular dichroism (VCD) spectrum of S-(-) and R-(+)-2-chloropropionic acid is thoroughly analyzed. Besides the VCD spectrum of the monomer, the dimers (stabilized by strong hydrogen bonds) and the 2-chloropropionic acid-CHCl3 complexes (stabilized by a weak hydrogen bond) are studied both experimentally (in solution and in low-temperature Ar matrix) and by quantum chemical computations. It is shown that dimer formation drastically changes, and even weak complex formation can also substantially affect the overall shape of the VCD spectrum. The present and previous results can be generalized for the practice of absolute configuration determination of carboxylic acids by VCD spectroscopy. For these measurements, if bulky groups do not block dimer formation, comparison of the computed spectra of the dimers with the experimental spectra recorded in relatively concentrated (similar to 0.1 mol dm(-3)) solutions is suggested. Our study also shows that due to the stabilization of monomers and/or the formation of weak complexes, the VCD spectrum recorded in CHCl3 is more complex and, like in the present case, can have a lower intensity than that of the spectrum recorded in CCl4. Therefore, if solubility allows, CCl4 is a much preferred solvent over CHCl3.