Structure of mercury(II) halides in solution and assignment of their resolved electronic spectra

Abstract

The electronic spectra of HgX₂(X = Cl, Br and I) have been measured in 23 solvents, most for the first time, the remainder to resolve discrepancies. The peak maxima correlated linearly with dielectric constant and solvent polarity (Kosower's Z value) for protic solvents, because they can solvate the linear mercury(II) halides in the equatorial position. The spectra in water and methanol were fitted to Gaussian bands, and were shown to contain no hidden bands. The resolved and observed band maxima were essentially identical, and thus all the observed bands were confidently assigned. The solvent dependence and intensities of the ¹Σ⁺_u and ¹Δ_u excited states are consistent with linear, or weakly bent upper states, contrary to some previous suggestions for HgX₂ in solution. The only exceptions were HgI₂ solutions in cyclohexane and 2,2,4-trimethylpentane: the separation between the two excited states was greater, and essentially constant, in all other solvents, and in these two solvents the ¹Δ_u state is thus assumed to be strongly bent. As the remaining dipolar aprotic solvents had at least one donor atom containing a lone pair of electrons, the solvation thus obtaining around HgX₂, while not correlating with solvent polarity, also did not alter the HgX₂ bond angle from 180°.