ABINITIO STUDY OF THE VERTICAL SPECTRUM AND IONIZATION-POTENTIALS OF SILANETHIONE (H2SIS)

The vertical electronic spectrum of H2SiS 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 X1A1(sigma-2-pi-2-n2) ground state is optimized at the MP2 level. The excited states studied include all those generated through single excitations from the sigma, pi and n MOs into the vertical pi*, sigma* and 4s, 4p Rydberg species. The lowest-lying states are 3,1(n-pi*) and 3,1(pi-pi*), 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 1A1(pi-pi*), B-1(2)(n-sigma*), B-1(1)(sigma-pi*) and 2 B-1(2)(n4s). The 0-0 origin for the strong absorption 1A1(pi-pi*) <-- X1A1 is expected near 3.0 eV. Excitations into sigma* lie below transitions into the first S(Ryd) species. The adiabatic ionization potentials (in eV) are 9.08 (X2B2, n --> infinity) and 9.84 eV (B-2(1), pi --> infinity). The ground state of H2SiS is highly polar, with a dipole moment of about 3.18 D (H2Si+S-). A comparison of the vertical spectrum of H2SiS with that of H2CO, H2CS and H2SiO 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 H2CO and H2CS the first allowed transition is of Rydberg type (n --> S(Ryd)), whereas in H2SiO and H2SiS three allowed valence transitions appear below this Rydberg excitation.