Sum-over-states calculation of the specific rotations of some substituted oxiranes, chloropropionitrile, ethane, and norbornenone

A sum-over-states approach has been applied to the calculation of the specific rotations of several substituted oxiranes, 2-chloropropionitrile, and 30 degrees-rotated ethane. In each case, the first few excited states proved to have only a relatively small effect on the calculated specific rotation. It was necessary to use a very large number of excited states in order to achieve convergence with the results of the more direct linear response method. However, the latter does not give information on which excited states are important in determining the specific rotation. Norbornenone is unique in that its greatly enhanced specific rotation as compared to norbornanone is associated with the low-energy n-pi* transition. The CC bond orbitals interact with the CO in the LUMO, and a density difference plot for going from the ground state to the first excited state clearly shows the perturbation of the CC.