The silyl anion (SiH−3): Harmonic vibrational frequencies and infrared intensities predicted at the SCF, CISD, and CCSD levels of theory with substantial basis sets

Abstract

A b initio quantum mechanical methods have been applied to obtain harmonic vibrational frequencies and infrared intensities for the silyl anion (C3vSiH−3) in the gas phase. The best results in this study agree well with the limited experimental data of Ellison, although new experiments are required for an assessment of our predictions. The theoretical methods employed are the self-consistent field (SCF), configuration interaction with single and double excitations (CISD), and coupled cluster with single and double excitations (CCSD) methods. Basis sets used include double-zeta plus polarization (DZP), triple-zeta plus double polarization (TZ2P), and additional diffuse functions on the silicon atom [DZP+diff(Si), TZ2P+diff(Si)]. The umbrella mode of the silyl anion is most sensitive to basis set quality and electron correlation. Our best predicted vibrational frequency [CISD with TZ2P+diff(Si)] for this particular mode is 890 cm−1. At all levels of theory, the infrared (IR) intensities have been obtained. The planar transition state (D3hSiH−3) has also been studied with respect to its geometry and energy relative to the pyramidal structure.