Ab initio MO study of structure and stability of X3- (X=F, Cl, Br, I) in solution

The stability and dissociation of X-3(-) (X = F, Cl, Br, I) have been studied by ab initio MO/GB calculations which include solvent effects by means of the continuum model with the generalized Born (GB) formula. In the gas phase, the optimized bond angle was 180 degrees for all X-3(-) species. Symmetrical and unsymmetrical structures were obtained for X-3(-) depending on the basis set employed. In the case of Cl-3(-), for example, an unsymmetrical structure was predicted by HF/MIDI-4+* and HF/6-31+G* calculations and a symmetrical one by the corresponding MP2 calculations. The stability of X-3(-) relative to X-2 + X- also depends largely on the method of calculation; inclusion of polarization and/or diffuse functions destabilizes X-3(-), while electron correlation stabilizes X-3(-) significantly. The MP2/MIDI-4+* potential energy curves in the gas phase seem to be appropriate judging from experimental heats of reaction. The calculated potential energy curves for X-3(-) indicated that the stability of X-3(-) decreases as the solvent polarity increases, and increases as the atomic number of X increases. The calculated relative stabilities of X-3(-) in water agree well with experimental formation constants of X-3(-) in aqueous solution. (C) 1998 Elsevier Science B.V.