Rotational isomers of lactic acid: first experimental observation of higher energy forms†
Abstract
Lactic acid {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
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.