Molecular optical rotation: an evaluation of semiempirical models

Ab initio optical rotations have been obtained for allene, 1,3-dichloroallene, H2O2, and H2S2 as a function of dihedral angles, both at SCF and post-SCF levels. The results have been used to test (a) the concept that the molecular optical rotation can be written as a ∑i Mi sin θi, where θi is the dihedral angle of the ith X–A–A–X type segment and Mi is a parameter associated with that segment; and (b) Kirkwoodʹs polarizability model. The predicted ab initio optical rotation was found to depend strongly on the level of ab initio theory used, so highest levels of theory are required for accurate predictions. The optical rotation patterns obtained using high-level ab initio theory for H2O2, H2S2 and allene, as a function of dihedral angles, are found to be in line with Kirkwoodʹs polarizability model predictions but not with the ∑i Mi sin θi predictions.