Abstract
Abstract The singlet excited states of oxirane and thiirane derivatives, ethylene oxide, (R)-methyloxirane, (2S,3S)-dimethyloxirane, ethylene sulfide, (R)-methylthiirane, and (2S,3S)-dimethylthiirane, were calculated employing the symmetry-adapted cluster (SAC)/SAC-configuration interaction (CI) method. The rotatory strengths of the CD spectra were calculated in the velocity form, which is gauge-origin independent. Both the ultraviolet (UV) spectra and the circular dichroism (CD) spectra obtained with the SAC-CI method were in good agreement with the experimental spectra. In oxirane derivatives, the low-lying excited states were composed of excitations from n and σ orbitals to s, p, and d Rydberg orbitals. The excitation from the σ orbital was especially important in (2S,3S)-dimethyloxirane because of the steric effect of the methyl substitutions. However, in thiirane derivatives, the excitations to the low-lying excited states were only from the n orbital.
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