THEORETICAL INVESTIGATION OF THE STRUCTURES AND RELATIVE ENERGIES OF THE ISOMER PAIRS HSIS/SISH, HSIS+/SISH+, AND HSIS-/SISH-

The properties of the title molecules have been investigated by ab initio methods. The geometries were optimized at the MP2 level, and the relevant relative energies were recalculated by using a MP4 expansion and f-polarization functions. HSiS(X2A') is more stable than SiSH(X2A') by 0.18 eV. The order of stabilities is reversed for the positive ions, with SiSH+(X1A') lying 0.58 eV below HSiS+(X1-SIGMA+). The negative ions have the ground state X1A' for HSiS- and X3A" for SiSH-. Rather striking is a high value of 2.16 eV for the electron affinity of HSiS when compared with that of SiS (0.42 eV) or HCO and HCS (approximately 0.4 eV). In addition, HSiS- constitutes the first example in the literature of a HAB- species having three bound electronic states. A computed hydride affinity of 2.46 eV for the process SiS + H- --> HSiS-(X1A') corroborates a recent experimental lower-bound value of 2.24 eV for that quantity. Other properties reported here include vibrational frequencies, dipole moments, dissociation energies, ionization potentials, electron affinities, and heats of formation.