Rotational isomers of lactic acid: First experimental observation of higher energy forms

Abstract

Lactic acid {2-hydroxypropionic acid [CH(CH3)OHCOOH]} monomer was studied by matrix isolation FT-IR spectroscopy and molecular orbital calculations undertaken at both DFT(B3LYP)/6-311++G(d,p) and MP2/ 6-31G(d,p) levels of approximation. The theoretical calculations predicted the conformer (SsC) with the carboxylic group and both the CCOH(alcohol) and O=CCO moieties in a cis configuration as the most stable form. In this conformer, the α-hydroxy hydrogen atom is involved in an intramolecular hydrogen bond with the carbonyl oxygen atom. The second most stable conformer (GskC) also shows a cis carboxylic group and differs from SsC in the CCOH (alcohol) and O=CCO dihedral angles, which are equal to 43.3° and 156.8° respectively. These angles are equal to -51.5° and -149.9° in the third most stable conformer (G′sk′C). These forms are characterized by showing a relatively weak intramolecular H (alcohol)⋯O(acid) hydrogen bond. In the AaT conformer, the carboxylic group is trans and the CCOH(alcohol) and O=CCO dihedral angles are 162.2° and 174.1°, respectively. This conformer shows a relatively strong H(acid)⋯O (alcohol) hydrogen bond. In consonance with the theoretical results, the matrix isolation experiments confirmed the predominance of conformer SsC in argon and xenon matrices, and provide the first experimental evidence of conformers GskC and AaT. Since the barrier for interconversion G′sk′C ↔ GskC is only ∼2 kJ mol-1, these two conformers are in equilibrium in the matrices and, at low temperature, the population of the less stable G′sk′C form is too small to enable its observation. Full assignment of the observed spectra was undertaken on the basis of comparison with the theoretical data and temperature variation studies. Water doping of matrices enabled the identification of spectral features due to weakly bound complexes of lactic acid with water.