Optical rotation from coupled cluster and density functional theories
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- Institutt for kjemi 
Chiral compounds are of fundamental importance in medical chemistry, biochemistry, and industry. Optical rotation, which is one of the most well-known optical responses of chiral molecules to an electromagnetic field, is a valuable tool in discerning between two enantiomers of a chiral molecule through the determination of their absolute configurations. This thesis presents optical rotation calculations for a broad range of chiral compounds by employing both coupled cluster models and density functional theory. First, the optical rotation was calculated for seven rigid chiral molecules using coupled cluster singles-doubles (CCSD), the second-order approximation (CC2), and time-dependent density functional theory (TDDFT) by employing two functionals B3LYP and CAM-B3LYP with the aug-cc-pVXZ (X = D, T, or Q) basis sets. The calculations demonstrated that the CAM-B3LYP functional predicts optical rotations with minimum deviations compared to the CCSD method at λ = 355 and 589.3 nm. In addition, our results illustrated that the augcc- pVDZ basis set provides optical rotations in good agreement with the larger basis sets for molecules not having a small-angle optical rotation at λ = 589.3 nm. Also, several two-point inverse power extrapolations to the basis set limit were performed for the optical rotation using the CC2 model at λ = 355 and 589.3 nm. Compared to the results in the basis set limit, i.e., the aug-cc-pV6Z basis set, a twopoint inverse power extrapolation for the aug-cc-pVTZ and aug-cc-pVQZ basis sets with n = 5 as the exponent gives results close to optical rotations obtained using the aug-cc-pV5Z basis set. Next, the optical rotation was studied for 14 pyrrole molecules containing both carbon and sulfur atoms as stereocenters. The TDDFT/CAM-B3LYP and CC2 approaches were used with the aug-cc-pVDZ basis set for calculating the optical rotation at λ = 589 nm. The investigations indicated that both methods give similar results for both sign and magnitude of the optical rotations, consistent with experiments, although several conformers for four molecules needed to be studied to reproduce the experimental signs. Similar methods were employed for calculating the optical rotation of a large group of fluorinated alcohol, amine, amide, and ester compounds at λ = 589 nm. The comparison of CAM-B3LYP and CC2 results to experiments illustrated that both methods are able to reproduce the experimental optical rotations for both sign and magnitude. Several conformers for molecules containing the benzyloxy and naphthalene groups needed to be considered to obtain consistent signs with experiments. Implicit and explicit solvent effects on the optical rotation were investigated for several conformers of four fluorinated molecules (chosen from our previous work) containing the 1-naphthalene or 4-(benzyloxy)phenyl groups at the stereocenter. The optical rotation was obtained through the polarizable continuum model (PCM) and a microsolvation approach in conjunction with PCM using the TDDFT/ CAMB3LYP method and the aug-cc-pVDZ basis set at λ = 589 nm. For the first group of molecules, the reduction of absolute deviations between computational and experimental results shows improvements in the optical rotation by considering both the implicit and explicit contributions of a solvent. For molecules containing the 4-(benzyloxy)phenyl group, the microsolvation model reproduces the sign of experimental optical rotations, where the signs were opposite to experiments for the most stable conformers in the gas phase. In the microsolvation model, one dihedral angle was found to have a large effect on the optical rotation sign. Our studies demonstrated that a microsolvation approach in conjunction with PCM predicts both sign and magnitude of optical rotations in agreement with experiments.