AB-INITIO MO STUDY OF STRUCTURE AND STABILITY OF X-3(-) (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 correspondin g MP2 calculations. The stability of X-3(-) relative to X-2 + X- also depends largely on the method of calculation; inclusion of polarizatio n and/or diffuse functions destabilizes X-3(-), while electron correla tion stabilizes X-3(-) significantly. The MP2/MIDI-4+ potential energ y curves in the gas phase seem to be appropriate judging from experime ntal heats of reaction. The calculated potential energy curves for X-3 (-) indicated that the stability of X-3(-) decreases as the solvent po larity increases, and increases as the atomic number of X increases. T he calculated relative stabilities of X-3(-) in water agree well with experimental formation constants of X-3(-) in aqueous solution. (C) 19 98 Elsevier Science B.V.