Structural and conformational dependence of optical rotation angles

The ability to compute and to interpret optical rotation angles of chiral m olecules is of great value in assigning relative and absolute stereochemist ry. The molar rotations for an indoline and an azetidine, as well as for me nthol and menthone, were calculated using ab inito methods and compared to the experimental values. In one case the calculated rotation angle allowed the assignment of the absolute configuration of a heterocycle of unknown st ereochemistry. The critical importance of Boltzmann averaging of conformers for reliable prediction of the optical rotation angle was established. Com parisons between static-field and time-dependent methods were made pointing to the limits and validity of the methods as electronic resonance is appro ached. A protocol analogous to population analysis was used to analyze atom ic contributions to the rotation angle in specific conformers. The combinat ion of atomic contribution maps and conformational analysis may provide an indirect tool to assist in three-dimensional structure determination.