The electronic spectra of the mixed mercury dihalides. Part 2. Identification, equilibrium and formation constants, and assignment of transitions

Abstract

The electronic absorption spectra of HgXY (HgCll, HgBrl, and HgBrCl) in water at 20 °C are calculated over the complete wavelength range, and shown not to be the mean of those of HgX₂+ HgY₂. For the reaction HgX₂+ HgY₂⇌ 2HgXY the spectra have been derived by two different techniques, which give the same profile. Equilibrium constants (log K), independent of added Na[ClO₄], are: HgCll, 1.40 ± 0.15; HgBrl, 1.26 ± 0.10; HgBrCl, 0.70 ± 0.10. The reaction is exothermic. The replacement reaction HgX₂+ Y^–→ HgXY + X^– has not previously been studied spectroscopically to obtain the formation constants (log K), which are 0.98 ± 0.05, 0.52 ± 0.09, and 0.23 ± 0.09, respectively. Accurate molar absorption coefficients could not be computed, but the spectral profiles are identical with the other results. The formation constants have been obtained by a new graphical method. The three independent methods have shown earlier incomplete spectra to be unreliable. Further, all the equilibrium and formation constants reported are the most accurate to date. The resolved spectral bands are discussed and assigned. The absence of the expected allowed transition ¹Σ⁺→¹Σ⁺(X) is explained as due to the proximity of ¹Σ⁺(X) and ¹Δ(Y) interacting by spin–orbit coupling to produce the representation ¹Δ, to which transitions are forbidden.