Ab Initio Study of Ion-Pair States of Halogen Molecules

The ion-pair states of IBr, ICl, and BrCl molecules correlating to the limits of dissociation X+(P-3(2,1,0), D-1(2)) + Y-(S-1(0)) are studied using a complete active space self-consistent field (CAS SCF), allowing for dynamic electronic correlations and spin-orbit interaction. Using experimental results, the calculated values of the equilibrium energy of the X+Y- (P-3(2)) states correlating to the lowest ion state X+(P-3(2)) are grouped in the range - 100 cm(-1), with the error of their relative location also on the order of Delta T-e similar to 100 cm(-1). For most states, the difference between the experimental and calculated values of the equilibrium internuclear distance does not calc exceed R-e(exp) - R-e(calc) = 0.02 angstrom. A comparative analysis of structural features of the ion-pair states of homonuclear and heteronuclear halogen molecules is performed using results from recent ab initio calculations for the I-2 molecule. We also discuss some issues concerning ion-pair states X+Y- with inverted charge localization, which correlate to the limits of dissociation X+Y- and are coupled with the X+Y-- states by two-electron transitions. It is noted that the structures with two stable localizations of two electrons, X+-D-Y- and X-D-Y+, where X, Y are electron affinity centers (atoms, molecules, clusters) and D is a dielectric spacer, could be of interest for microelectronics.