Excited electronic states of carbon disulphide

Abstract

The near-ultraviolet spectrum of CS2 is studied thoroughly using high level CISTDQ and EOM-CCSD theory with DZP and TZ2P(f) basis sets. Potential energy curves of the first seven singly excited states with respect to the SCS bond angle have been predicted. The relative energy ordering of the excited states as established at the equilibrium geometries using EOM-CCSD theory is: X˜1Σ+g, < a3B2 < b˜3A2(R) < A1A2 < c˜3B2 < B˜1B2 (V) < d˜3A2 < C˜1A2, which is at odds with previous theoretical work. The bent B˜1B2 state arises from the Renner—Teller splitting of a linear 1Δu state into 1B2 and 1A2 states. In disagreement with previous experimental predictions that the 1A2 state is the lower lying component of this splitting, it is firmly established from the high level results in this research that the C˜1A2 state is the higher lying component of this Renner—Teller pair. Equilibrium geometries are determined and shown to agree qualitatively with experiment, save for the B˜1B2 state. From an analysis of the complex rotational structure of the V band the experimental geometry of the B˜1B2 state was determined to be slightly bent (re = 1.544 Å, θe = 160°), whereas the methods employed in this research predict a significantly bent geometry (re = 1.632 Å, θe = 132.1° at EOM-CCSD/TZ2P(f)). Relative transition energies, Te, are predicted, and EOM-CCSD calculations are shown to agree well with available experimental data, and show the assignment of the V and R bands to be correct while the T band is correctly assigned to the A1A2 state and not the lower lying Renner—Teller component of the 1Δu state. The high level CI methods, although yielding qualitatively correct potential energy curves, do not provide quantitatively accurate transition energies, and it is clear that even an exhaustive treatment of the eight valence π electrons is not entirely satisfactory, while EOM-CCSD is established as an accurate method for the prediction of excited state potential curves.