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.

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A computational investigation of the optical rotatory power of cis and trans 2-methyl-5-oxo-tetrahydro-3-furancarboxylic acids and the corresponding methyl and ethyl esters is presented. Solvent effects on both the conformational space and the rotatory power are analyzed by comparing results obtained in vacuo with those computed--using the Polarizable Continuum Model--in methanol. A comparison with experimental observations for the optical rotatory power of the title compounds in methanol is also carried out, in a few cases also for several wavelengths. Agreement between theory and experiment is in all cases excellent, in particular when solvent effects are included both in the geometry optimization and in the calculation of the OR, thus confirming the validity of the computational procedure adopted, even for this challenging family of floppy molecules.

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