The electronic structure and the optical and photoelectron spectra of carbon subsulfide

Abstract

Carbon subsulfide, the linear molecule S = C = C = C = S, has been investigated by self-consistent field-Xa-scattered wave calculations (overlapping spheres), optical (650-230 nm), and photoelectron [He(I) and He(II)] spectroscopy. The ground state valence level electronic configuration is found to be (6σg+)2(5σ u+)2(7σg+) 2(6σu+)2 (7σ u+)2(8σg+) 2(2πg)4(2πg) 4(3πu)4. Comparison of the photoelectron band positions with the calculated ionization potentials, and analysis of their vibrational structure, allows assignments to be made. The experimental ionization potentials (ev) are 3πu(9.09), 2πg(11.24) , 2πu(12.87), 8σg+ (14.47), 7σu+(14.87), 6σu+(18.2), 7σg+(19.3), 5σu+(20.5), 6σg+(21.9). Agreement between calculated and experimental ionization potentials is very good for the π levels, fair for the 8σg+ and 7σu+ levels and poor for the remaining (low lying) σ levels, although the splittings between the low lying σ levels are correctly calculated. There are two principle features in the optical spectrum, a weak band at 20.7 kcm-1 which is responsible for the orange-red color of C3S2 and a strong uv peak at 38.7 kcm-1. The 20.7 kcm-1 band is assigned as the dipole forbidden 3πu→4πu ( 1Σg or 1Δg) transition while the 38.7 kcm-1 peak is assigned as the dipole allowed 3πu→3πg (1Σu+) and 3πu→9σg+( 1Πu) transitions. Where comparison is possible, good agreement is found between calculated and observed electronic transitions. A detailed comparison is made between C3S2 and C 3O2; to facilitate this an overlapping sphere SCF-Xa-SW calculation was carried out for C3O2. The calculated charge distribution indicates that in C3S2 the sulfur atoms are positively charged while all three carbon atoms are negatively charged, with the central carbon more negative than the end carbon atoms. By contrast, in C3O2 the oxygen and central carbon atoms are negatively charged while the end carbon atoms are positively charged. In agreement with the calculated energy level diagrams C3S2 and C3O2 have similar optical spectra, but the C 3S2 absorptions are shifted to lower energy. © 1978 American Institute of Physics.