The problematic case of ( S )-methylthiirane: electronic circular dichroism spectra and optical rotatory dispersion

Coupled cluster and density-functional theory calculations of the electronic circular dichroism (ECD) spectra and optical rotatory dispersion (ORD) of (S)-methyloxirane and (S)-methylthiirane have been carried out for comparison to gas-phase experimental data. Coupled cluster simulations of the ECD spectra of both molecules are in excellent agreement with experiment, both for the positions of the transitions and for the signs and magnitudes of the rotational strengths. Density-functional theory yields the correct structure of the ECD spectra, but the transition energies are shifted to lower energies by up to 0.3 eV. Density-functional ORD curves of (S)-methylthiirane exhibit incorrect dispersion due to an early onset of the Cotton pole. The coupled cluster ORD curves of (S)-methylthiirane give the correct sign of the rotations, but overestimate their magnitude by up to 50% at shorter wavelengths. Although temperature-dependent harmonic vibrational corrections improve the comparison between coupled-cluster theory and experiment at 633 nm, such corrections produce an incorrect sign of the rotation at 355 nm.