Ab initio study of the vertical spectrum and ionization potentials of silanethione (H 2SiS)

Abstract

The vertical electronic spectrum of H 2SiS is investigated by using a multireference CI method and an expanded AO basis set including 4s and 4p Rydberg functions. The equilibrium geometry of the X 1A 1 (σ 2π 2n 2) ground state is optimized at the MP2 level. The excited states studied include all those generated through single excitations from the σ, π and n MOs into the vertical π*, σ* and 4s, 4p Rydberg species. The lowest-lying states are 3,1 (nπ*) and 3,1(ππ*), in the range from 2.72 to 4.55 eV. Four allowed transitions lie in the energy region from 4.55 to 6.15 eV, namely 1A 1(ππ*), 1B 2(nσ*), 1B 1(σπ*) and 2 1B 2(n4s) The 0-0 origin for the strong absorption 1A 1(ππ*)←X 1A 1 is expected near 3.0 eV. Excitations into σ* lie below transitions into the first s Ryd species. The adiabatic ionization potentials (in eV) are 9.08 (X 2B 2, n→∞) and 9.84 eV ( 2B 1, π→∞). The ground state of H 2SiS is highly polar, with a dipole moment of about 3.18 D (H 2Si +S −). A comparison of the vertical spectrum of H 2SiS with that of H 2CO, H 2CS and H 2SiO indicates a general stabilization of the valence electronic transitions relative to those of Rydberg character as C is replaced by Si. In fact, for both H 2CO and H 2CS the first allowed transition is of Rydberg type (n→s Ryd), whereas in H 2SiO and H 2SiS three allowed valence transitions appear below this Rydberg excitation.