Calculation of electronic circular dichroism spectra by rotating wave approximation

Calculations of the circular dichroism spectra of a few small and medium sized organic molecules are presented within the rotating wave approximation (RWA). Comparing with the conventional method based on calculation of rotatory strengths, one advantage of this approach is that information on absorption band widths and peaks heights are easily extracted. This aspect, when combined with a fitting procedure in which a Gaussian function is centred at each excitation energy, allows circular dichroism spectra to be readily simulated. Another advantage is that closed form analytical expressions for the state probability of a two-level chiral molecule interacting with a circularly polarized radiation field may be employed in the computation. The theory developed is applied to methyl substituted aziridines, dimethyloxirane and pentahelicene, whose molecular properties are computed using ab initio and time-dependent density functional theory methods. Agreement with experimental spectra is satisfactory.