The thiocarbonyl chromophore. A time-dependent density-functional study

Abstract

Density-functional linear response theory calculations allow a very satisfactory description of the spectral characteristics of thiocarbonyl compounds in the ultraviolet and visible region. The n→π ∗- and π→π ∗-type absorption bands are reasonably well reproduced by TD-DFT using the hybrid HF/DFT functional B3-LYP with the valence double-zeta basis set 6-31+G ∗. The absolute mean deviation is 0.20 eV for n→π ∗ and π→π ∗ transitions. The spectral shifts of the absorption bands with substitution are well reproduced. A third absorption band in the near-ultraviolet region is interpreted by TD-DFT and CASSCF/CASPT2 calculations. In the case of bis(trimethylsilyl)thioketone the theoretical predictions were grossly at variance with the experimental finding. According to the TD-DFT calculations and to empirical considerations the experimental result is wrong. The n→π ∗ absorption band is expected in the near-infrared rather than in the visible region. This conclusion is supported by calculations of spectral absorptions and 13C chemical shifts of related compounds.