Helical Chains of Diatomic Molecules as a Model for Solid-State Optical Rotation

Optical rotation (OR) measurements are a common method for distinguishing chiral compounds, but it is not well understood how intra- and intermolecular interactions affect this electronic property. Theoretical comparisons with solution-phase measurements are hampered by the difficulty of modeling solvent effects and the isotropic averaging of the experimental observable. Solid-state OR experiments/calculations could alleviate these difficulties, but experimental measurements are challenging and computational efforts have been limited due to a lack of existing procedures. We report calculated OR tensor values for a series of helices of diatomic molecules that may serve as a benchmark in the development of general-purpose electronic structure methods to compute the optical rotation of solids, in particular, molecular crystals. We find that the OR tensors for small helix clusters show poor agreement with values converged with respect to the helix size, regardless of unit cell size. The dependence of the converged OR on the dihedral angle of homonuclear helices is well described by the Kirkwood polarizability indicating that nearest-neighbor interactions are very important, albeit not the only relevant interactions. Basis set comparisons suggest that the aug-cc-pVDZ basis is sufficient to obtain qualitatively accurate results.