Geometry, Vibrational Frequencies, and Ionization Energies of BeX2 (X = F, Cl, Br, and I)

Abstract

The equilibrium geometries and harmonic vibrational frequencies of BeF2, BeCl2, BeBr2, and BeI2 are calculated. For the lighter two molecules, only all-electron methods were used, whereas for the heavier two, all-electron and effective core potential (ECP) methods were used. MP2, QCISD, and B3LYP calculations were employed, and it was found that the combination of ECPs (with a suitable valence basis set) with the B3LYP density functional method is a relatively inexpensive means of obtaining reliable information on these species. All four molecules were found to be linear, in agreement with simple molecular orbital ideas. The lowest electronic states are calculated using the configuration interaction with single excitations approach. Finally the adiabatic and vertical ionization energies of the BeX2 species are calculated. For BeF2 and BeCl2, the first vertical ionization energy (VIE) was calculated at the CASSCF+MRCI level, and the lowest four VIEs were calculated at the CCSD(T) level using large basis sets: for BeCl2 comparison with the photoelectron spectrum is presented. For BeBr2 and BeI2, the lowest VIEs were calculated using a density functional approach, including spin−orbit effects. During the calculation of the adiabatic ionization energies, the equilibrium geometries of the BeX2+ species were calculated; it was found that BeF2+ and BeCl2+ had bent equilibrium geometries (corresponding to the X̃2B2 Renner−Teller component of the lowest 2Πg state, in a linear configuration), whereas BeBr2+ and BeI2+ have linear (X2Πg) equilibrium geometries. The calculated adiabatic ionization energies for BeF2 and BeCl2 are significantly higher than previously accepted values.