Insights on the origin of the unusually large specific rotation of (1S,4S)-norbornenone.

Measurements and calculations of specific rotation are indispensable for the characterization of chiral molecules and are now performed routinely. However, the factors that determine the magnitude of this property are still not well-understood. The anomalously large specific rotation of (1S,4S)-norbornenone, an outstanding puzzle for over three decades, offers the chance to examine these factors in detail. The present work provides an explanation for the unusual behavior of this molecule in terms of interactions between chemical groups and electronic excited-state transition properties by means of ab initio density functional theory and coupled cluster theory calculations. We show that one can focus on the first excited state and examine the relative orientation of its electric and magnetic transition dipole moments. The contribution of the two transition moments of this electronic state to the specific rotation in a sum-over-states formalism reveals a constructive interaction that is possible only when the two chromophores in norbornenone (C═O and C═C) are in-plane and pointing away from each other. This is due to a small but non-negligible charge transfer between the chromophores and is consistent with recent results from Autschbach's group [Moore et al., J. Chem. Theory Comput. 2012, 8, 4336-4346]. The analysis in this work improves our understanding of this fundamental property of chiral molecules and may help in the design of other molecules with large specific rotation.