X̃¹A₁, ã³B₁, and ù B₁ States of SiCl₂: Ab Initio Calculations and Simulation of Emission Spectra

A variety of correlated molecular orbital methods and basis sets have been employed to obtain the minimum-energy geometries, harmonic vibrational frequencies, and relative energies of the X̃1A1, ã3B1, and Ã1B1 states of SiCl2. The ab initio results obtained have been compared with experimental values, where available. It was found that ab initio methods which are based on unrestricted-spin (UHF) wave functions employing spin-unprojected energies, including the QCISD(T) and CCSD(T) methods and the composite methods of G1 and G2, failed to give a reliable Ã1B1-X̃1Ã1 separation, whereas methods using spin-projected energies or the restricted multireference method MR-CISD/6-311G (2df) gave reliable Ã-X̃ and ã-X̃ separations. The Ã1B1-X̃1Ã1 and ã3B1-X̃1A1 emission spectra of SiCl2 were simulated, employing MP2/6-311G (2df) force constants and compared with available experimental spectra. The geometry of the X̃ state was held fixed at the geometry determined by microwave spectroscopy, and the geometries of the ã3B1 and Ã1B1 states were adjusted via an iterative Franck-Condon analysis (IFCA) procedure until the simulated spectra matched best with the observed spectra. The IFCA derived geometry for the Ã1B1 state is r(SiCl) = 2.055 ± 0.008 Å and θ (ClSiCl) = 119.4° ± 0.4°. For the ã3B1 state, r(SiCl) = 2.041 ± 0.005 Å, while the (ClSiCl) angle can have a value of either 115.4° or 114.5°, depending on the vibrational assignments of the experimental spectra.