The electronic structure of alkali bihalide molecules MX2. I. Ground states of LiF2, NaF2, LiCl2, and NaCl2 systems

The ground electronic states of the triangular alkali bihalide molecules LiFad2, NaFad2, LiClad2, and NaClad2 are investigated theoretically at the restricted−Hartree−Fock level using large Gaussian basis sets. It is argued that these MXad2 systems are stable species on the M+X+X potential energy surface and LiFad2, NaFad2, and NaClad2 are stable entities with respect to dissociation into the MX+X systems. Potential energy curves are obtained for the (X−−−X)− and M+−−−(x2)− motion in the MXad2 systems and comparison with experiment is given in terms of calculated spectroscopic properties. The vibrational assignments of Howard and Andrews for thee MX2 systems are supported by these calculations and a study is made of the influence of the M+ cation on the spectra of the constituent X2−(X 2Σu+) molecular anions.