Determination of absolute configurations of chiral molecules using ab initio time-dependent Density Functional Theory calculations of optical rotation: how reliable are absolute configurations obtained for molecules with small rotations?
The absolute configuration (AC) of a chiral molecule can be determined via calculation of its specific rotation. Currently, the latter is most accurately carried out using the TDDFT/GIAO methodology. Here we examine the reliability of this methodology in determining ACs of molecules with small specific rotations. We report TDDFT/GIAO B3LYP/aug-cc-pVDZ//B3LYP/6-31G* calculations of the sodium D line specific rotations, [alpha]D, of 65 conformationally rigid chiral molecules whose experimental [alpha]D values are small (<100). The RMS deviations, sigma, of calculated and experimental [alpha]D values is 28.9. The distribution of deviations is approximately Gaussian, i.e., random. For eight molecules, more than 10% of the set, the sign of the predicted [alpha]D is incorrect. In determining an AC of a rigid molecule from [alpha]D with 95% confidence, the calculated [alpha]D value must lie within +/-2sigma of the experimental [alpha]D for one, but not both, of the possible ACs. For the 65 molecules of this study +/-2sigma is 57.8. For conformationally flexible molecules, the error bar is +/- >57.8.