The energetics of the X2 + X- → X3- equilibrium (X = Cl, Br, I) in aqueous and nonaqueous solution

The magnitude of the stability constants (K) for the formation Of X-3(-) from X- and X-2 (X = Cl, Br, I) in aqueous and nonaqueous solution depends strongly on both X and solvent. Thermodynamic arguments establish that for aqueous solutions this is not the result of differing bond strengths in the X-3(-) anion but rather depends on the solvation energies of the relevant anions and halogens. The experimental results for K in different organic solvents can also be rationalized in terms which emphasise solvation effects. Electrospray ionization mass spectrometry (ESI-MS) has been used to qualitatively probe relative bond strengths in the absence of solvent.