(X)over-tilde(1)A(1), (a)over-tilde(3)B(1), and (A)over-tilde(1)B(1) 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 freq uencies, and relative energies of the (X) over tilde(1)A(1), (a) over tilde (3)B(1), and (A) over tilde(1)B(1) states of SiCl2. The ab initio results o btained have been compared with experimental values, where available. It wa s found that ab initio methods which are based on unrestricted-spin (UHF) w ave functions employing spin-unprojected energies. including the QCISD(T) a nd CCSD(T) methods and the composite methods of G1 and G2, failed to give a reliable (A) over tilde(1)B(1)-(X) over tilde(1)A(1) separation, whereas m ethods using spin-projected energies or the restricted multireference metho d MR-CISD/6-3-11G (2df) gave reliable (A) over tilde-(X) over tilde and B-X separations. The (A) over tilde(1)B(1)-(X) over tilde(1)(A) over tilde(1) and (a) over tilde(3)B(1)-(X) over tilde(1)A(1) emission spectra of SiCl2 w ere simulated, employing MP2/6-311G (2df) force constants and compared with available experimental spectra. The geometry of the (X) over tilde state w as held fixed at the geometry determined by microwave spectroscopy, and the geometries of the (a) over tilde(3)B(1) and (A) over tilde(1)B(1) states w ere adjusted via an iterative Franck-Condon analysis (IFCA) procedure until the simulated spectra matched best with the observed spectra. The IFCA der ived geometry for the (A) over tilde(1)B(1) state is r(SiCl) = 2.055 +/- 0. 008 Angstrom and theta(ClSiCl) = 119.4 degrees +/- 0.4 degrees. For the (a) over tilde(3)B(1) state, r(SiCl) = 2.041 +/- 0.005 Angstrom, while the (Cl SiCl) angle can have a value of either 115.4 degrees or 114.5 degrees, depe nding on the vibrational assignments of the experimental spectra.