Basis Set Recommendations for DFT Calculations of Gas-Phase Optical Rotation at Different Wavelengths

Even for pure substances, the deduction of the absolute configuration is not always straightforward since there is no direct link between the magnitude and sign of the optical rotation and the absolute configuration. It would be very useful to use computations of the optical rotation to link experimentally measured optical rotations to an absolute configuration. Such electronic structure calculations of the optical rotation typically employ regular energy optimized basis sets from wave function theory, and especially the aug-cc-pVDZ basis set has been popular. Here, we have carried out extrapolation of the optical rotation to the basis set limits for nine small or medium sized molecules, using basis sets developed specifically for DFT and magnetic properties (aug-pcS-n series). We suggest that assignment of absolute configuration by comparisons between theoretical and experimental optical rotations may be improved by employing different wavelengths, and accordingly the optical rotation at two wavelengths (589.3 and 355.0 nm) has been investigated. Several fitting schemes were used to estimate the optical rotations at the basis set limit. It was found that use of the aug-cc-pVDZ basis set often leads to results that deviate significantly form the basis set limit results, especially at 355.0 nm but also at 589.3 nm. The double-ζ aug-pcS-1 basis set usually provides results which are closer to the limiting values. The basis set requirements are generally more severe at 355.0 nm, where also the aug-cc-pVTZ and 6-311++G(3fd,3dp) basis sets show significant deviations from the basis set limit results, while the aug-pcS-2 basis set always leads to results within an acceptable deviation.