Lactic Acid Spectroscopy: Intra- and Intermolecular Interactions

Abstract

Lactic acid, a relevant molecule in biology and the environment, is an α-hydroxy acid with a high propensity to form hydrogen bonds, both internally and to other hydrogen-bond-accepting molecules. This work includes the novel recording of infrared spectra of gas-phase lactic acid using Fourier transform infrared spectroscopy, and the vibrational absorption features of lactic acid are assigned with the aid of computationally simulated vibrational spectra with anharmonic corrections. Theoretical chemistry methods are used to relate intramolecular hydrogen-bond strengths to the relative stability of lactic acid conformers. The formation of hydrogen-bonded lactic acid dimers and 1:1 water complexes is investigated by simulated vibrational spectra and calculated thermodynamic parameters for the lactic acid monomer and dimer and its water complex in the gas phase. The results of this study are discussed in the context of environmental chemistry with an emphasis on indoor environments.

Ambient l-lactic acid crystal polymorphism

“…While the vapor pressure of a saturated solution of LLA was in principle high enough to produce detectable aggregates, the anticipated concentration was based on a computed equilibrium constant (LLA + LLA ⇌ LLA 2 ), which may be imperfect according to the authors. Nevertheless, their failure to observe the common dimer 10 is not inconsistent with our failure to see it in crystals.…”

“…Curiously, none of the three ambient structures (I-III) that we have now are consistent with the aforementioned structures determined by vibrational spectroscopy or quantum chemical computation, [7][8][9][10] suggesting that additional minima for aggregates may be identified in solution or the gas phase, and the other modalities may be found in the solid state. LLA, therefore, underscores how difficult it may be to reason from solution structures to those found in the crystals.…”

“…Asc is not expected to be measurably populated as this conformer is short by one hydrogen bond, otherwise satisfied in the crystalline states. SsC, detected in the gas phase by microwave 15 and infrared spectroscopies, 10 is similar to conformers of forms II and III (Fig. 3C).…”

“…6 This wealth of LLA chemistry has prompted spectroscopic and computational investigations of selfassociation. [7][8][9][10] Here, we ask what story of LLA aggregation is told by crystallography?…”

“…The most recent study of dimerization of LLA in the gas phase, by FTIR spectroscopy, 10 produced no spectra that could reasonably be assigned to any R 2 2 (8) carboxylic-carboxylic dimer. While the vapor pressure of a saturated solution of LLA was in principle high enough to produce detectable aggregates, the anticipated concentration was based on a computed equilibrium constant (LLA + LLA ⇌ LLA 2 ), which may be imperfect according to the authors.…”