Lactic acid in solution: Investigations of lactic acid self-aggregation and hydrogen bonding interactions with water and methanol using vibrational absorption and vibrational circular dichroism spectroscopies

The infrared vibrational absorption (VA) and vibrational circular dichroism (VCD) spectral features of L-(+)-lactic acid (LA) in CDCl3 solution are concentration dependent, showing evidence of oligomerization with increasing concentrations. To understand the observed spectra, geometry optimizations, vibrational frequencies, and VA and VCD intensities were evaluated for (LA)n with n=1–4 using density functional theory calculations at the B3LYP∕6-311++G(d,p), B3LYP/cc-pVTZ, and in some cases, B3LYP/aug-cc-pVTZ levels of theory. Comparisons with the experimental spectra indicate that the lowest energy LA dimer (AA), formed by two CO⋯HO hydrogen bonds, is one of the dominating species in solution at room temperature. Possible contributions from the LA trimer and tetramer are also discussed. To model the VA and VCD spectra of LA in water and in methanol, both implicit polarizable continuum model and explicit hydrogen bonding considerations were used. For explicit hydrogen bonding, geometry optimizations of the AA-(water)n and AA-(methanol)n complexes, with n=2,4,6, were performed, and the corresponding VA and VCD spectra were simulated. Comparisons of the calculated and experimental VA and VCD spectra in the range of 1000–1800cm−1 show that AA-(water)n with n=6 best reproduces the experimental spectra in water. On the other hand, AA-(methanol)n with n=2 reproduces well the experimental results taken in methanol solution. In addition, we found evidence of chirality transfer, i.e., some vibrational bands of the achiral water subunits gain VCD strength upon complexation with the chiral LA solute. The study is the first to use VCD spectroscopy to probe the structures of LA aggregates and hydrogen bonding solvation clusters in the solution phase.