Ab initio MO study of structure and stability of heteronuclear trihalide anions X2Y- (X, Y = Cl, Br, or I) in the gas phase and in solution

The structure and dissociation of heteronuclear trihalide anions X2Y-(X, Y = Cl, Br, or I) in the gas phase and in solutions were investigated by ab initio MO calculations. The solvent effect was treated by our generalized Born (GB) model. The calculations including solvent effects were carried out for epsilon = 5 and 79 where epsilon is the dielectric constant of the solvent; the latter value corresponds to aqueous solution. When Y is heavier than X (Y > X), the (X-Y-X)(-) and (Y-Y-X)(-) isomers were calculated to be more stable than the (Y-X-X)(-) and (Y-X-Y)(-) isomers, respectively, both in the gas phase and in solutions; the relative stability of (X-Y-X)(-) is considerably reduced in solutions, while the relative stability of (Y-X-Y)(-) tends to be somewhat enhanced in solutions. The dissociation energies of the trihalide ions are largely reduced by solvent effects and, hence, the potential energy curves for dissociations become considerably flatter in solutions. As a result, various processes are expected to proceed easily in solutions in agreement with experimental observations. For example, the reaction between X? and Y- (Y > X) will yield (X-Y-X)(-) via (Y-X-X)(-) as an intermediate [Y-+X-2 --> (Y-X-X)(-) --> Y-X+X- --> (X-Y-X)(-)]. Though the dissociation energies in aqueous solution were overestimated, experimental trends in relative stability of trihalide ions in aqueous solution were by and large reproduced. The reactions YX+Y- --> Y-2+X-(Y > X) in aqueous solution were calculated to be exothermic in agreement with experimental equilibrium constants. (C) 1998 Elsevier Science B.V.