Extensive ab Initio Study of the C₂O₂, C₂S₂, and C₂OS Systems: Stabilities and Singlet−Triplet Energy Gaps

Abstract

Ab initio molecular orbital theory has been used to study the three lowest open shell states of ethylenedione, C 2 O 2 , 2-thiooxoethen-1-one, C 2 OS, and ethylenedithione, C 2 S 2 . To treat the singlet and triplet states in an even-handed manner, multiconfigurational self-consistent field theory (MCSCF), which included all the important configurations for a quantitative description of these states, was used as the basis of the investigation. Further correlation effects have been included using a multireference configuration interaction (MRCI) approach. Basis sets of triple-quality with d-and f-polarization functions were employed. Equilibrium geometries were obtained from density functional theory (DFT) calculations using the B3LYP exchange correlation functional and are presented for the 3 Σ g -, 1 ∆ g states of all three molecules. Harmonic frequencies are also presented for these states and were calculated at the MCSCF and DFT levels. The reported relative energies for the states were obtained from MRCI calculations. As expected from previous work, the ground states are confirmed to be of 3 Σ g -symmetry for all molecules, but it is only for the C 2 S 2 that this state lies below the energy of the dissociation products in their own ground states. For C 2 OS, though, this energy gap is small (5.7 kcal/mol). In contrast to a number of other calculations, the 1 ∆ g states for all three molecules are also shown to be minima on the MCSCF potential energy surfaces. The 1 ∆ g states are all close to the ground states (<10 kcal/mol). Consequently, with C 2 S 2 , this singlet state also lies below the ground state of the dissociated products. The relative energies of the 1 Σ g + states at the optimized geometries for the 1 ∆ g states have also been determined.

Spectroscopy of Ethylenedione

“…[7,10,11,19] Ther elevant states involve two nearly degenerate orbitals,6 b u and 2a u (at a C 2h geometry), which are shown schematically in Figure 2A.A t linearity,they turn into the degenerate 2p u orbitals,and their population with two electrons gives rise to the 3 S g À , 1 D g ,a nd 1 S g + states of OCCO,a si ndicated in Figure 2B (right). The energy gaps shown are from the MRCI/cc-pVTZ//B3LYP/6-311 + G(2df) calculations by Talbi and Chandler.…”

“…Theaverage spacing of 417(15) cm À1 is close to the 396 cm À1 frequency of the trans-bending mode of 3 S g À OCCO,a sp redicted by MCSCF/cc-pVTZ calculations. [10] TheS 1 pedestal extends lower in energy than the triplet EA value owing to the dissociative nature of the singlet. Thea diabatic minimum of the singlet surface corresponds to the CO( 1 S + ) + CO( 1 S + ) limit, some 2.55 eV below the triplet equilibrium.…”

“…The energy gaps shown are from the MRCI/cc-pVTZ//B3LYP/6-311 + G(2df) calculations by Talbi and Chandler. [10] Whereas the singlets dissociate to 2CO( 1 S + ), this channel is inacces-sible on the triplet surface.All other dissociation channels are significantly higher in energy.…”

Polyacetylenes and Cumulenes, Potential Elements for Molecular Machines and Precursors of Carbon Clusters: A Theoretical Study

“…It was shown by recent sophisticated calculations (MCSCF, MRCI), among other points [44], that the singlet state of C 2 S 2 lies below the ground-state of the dissociation Scheme 2 product. Mass spectrometrical and theoretical studies have contributed significantly to the understanding of this area [45].…”

Laser-induced fluorescence and fluorescence depletion spectroscopy of SCCS−

“…The linear negative ion SCCS Ϫ is the most plausible candidate of the spectral carrier, since the species has two equivalent carbon atoms, for which the rotational constant is estimated to be about 0.05 cm Ϫ1 . Several theoretical works 23,26,27,29 have shown that the neutral species SCCS has the 3 ⌺ g Ϫ ground electronic state corresponding to the electronic configuration ...(7 g ) 2 (2 u ) 4 (2 g ) 4 (3 u ) 2 . When one electron is removed to form the positive ion SCCS ϩ , the ground electronic configuration should be ...(7 g ) 2 (2 u ) 4 (2 g ) 4 (3 u ) 1 , leading to the X 2 ⌸ u ͑regular͒ electronic state.…”