Gas-Phase Thermochemistry of Polyhalide Anions

A flowing afterglow−tandem mass spectrometer has been used to determine 0 K bond strengths in three hypervalent polyhalide ions: D (Cl 2 −Cl - ) = 99 ± 5 kJ/mol, D (Br 2 −Br - ) = 127 ± 7 kJ/mol, and D (Br 2 −Br 3 - ) = 40 ± 7 kJ/mol. These bond strengths are close to the previously measured values D (I 2 −I - ) = 126 ± 6 kJ/mol and D (I 2 −I 3 - ) = 49 ± 6 kJ/mol. In contrast, Cl 5 - and F 3 - are not formed at room temperature in the flowing afterglow. Solvation energies for the trihalides derived from this work and literature thermochemistry are inversely correlated with the size of the ion, in reasonable agreement with the predictions of the Born model. The electron affinities EA(Cl 3 ) = 4.60 ± 0.09 eV and EA(Br 3 ) = 4.55 ± 0.10 eV can also be derived. The gas-phase bond strengths are more consistent with the three center-four electron bond model than with the expanded octet model.