Ab initio calculations of optical rotation
Coupled cluster (CC) and density functional theory (DFT) are highly regarded as robust quantum chemical methods for accurately predicting a wide variety of properties, such as molecular structures, thermochemical data, vibrational spectra, etc., but there has been little focus on the theoretical prediction of optical rotation. This property, also referred to as circular birefringence, is inherent to all chiral molecules and occurs because such samples exhibit di erent refractive indices for left- and right- circularly polarized light. This thesis focuses on the theoretical prediction of this chiroptic property using CC and DFT quantum chemical models. Several small chiral systems have been studied, including (S)-methyloxirane, (R)-epichlorohydrin, (R)-methylthiirane, and the conformationallyexible molecules, (R)-3-chloro-1-butene and (R)-2-chlorobutane. All predicted results have been compared to recently published gas-phase cavity ringdown polarimetry data. When applicable, well-converged Gibbs free energy di erences among confomers were determined using complete-basis-set extrapolations of CC energies in order to obtain Boltzmann-averaged speci c rotations. The overall results indicate that the theoretical rotation is highly dependent on the choice of optimized geometry and basis set (di use functions are shown to be extremely important), and that there is a large di erence between the CC and DFT predicted values, with DFT usually predicting magnitudes that are larger than those of coupled cluster theory.