Optical rotation calculation of a highly flexible molecule: the case of paraconic acid.

The absolute configuration of (S)-(-)-paraconic acid is correctly assigned on the basis of ab initio calculations of the specific optical rotation (OR) at the sodium D line, carried out both in vacuum and in methanol. Density functional theory (DFT) and Møller-Plesset second-order perturbation theory (MP2) are used to determine the most stable conformational structures, whose OR values are then calculated using DFT linear response theory and London atomic orbitals. The total OR is obtained by averaging these values using the population fractions determined from Boltzmann's statistics. The total OR of the MP2 structures has the correct sign both in vacuum and in solution, whereas only the solvent-relaxed DFT structures correctly reproduce the experimental sign. The strong solvent effect on the total OR is shown to arise primarily due to the variations in the relative energies of the various conformations.