Configuration Space Analysis of the Specific Rotation of Helicenes.

In this work, we present an analysis of a series of helicene molecules to determine the driving forces for their large specific rotation, [α], and probe the effects of functionalization. The analysis is done in the configuration space of the molecular orbitals (MOs), and it allows us to decompose [α] into the component transition electric and magnetic dipoles from single MO excitations. We find that [α] for helicene molecules may be described by three sets of transitions based on the orientation of the magnetic dipole with respect to the helical axis: parallel, orthogonal, or tilted. The transitions with the magnetic dipole parallel to the helical axis, corresponding to a delocalized motion of the electron along the body of the helix, provide the largest contributions and determine the sign and magnitude of [α]. Functionalization has a complex effect on [α], which is dependent on the number of substituent groups and their electron directing strength. Furthermore, we test the [α] decomposition analysis using localized MOs (Boys and Pipek-Mezey). We show that localization schemes may be useful to simplify the interpretation of the [α] decomposition, but they are best used when the electronic transitions involve relatively small chromophoric groups.