Electronic spectra and geometries of HgX –3 in water and an assessment of various computing procedures for revealing hidden spectra

Abstract

The electronic absorption spectra, in the visible and ultraviolet regions, of HgX^–₃(X = Cl, Br or I) have for the first time been obtained, by computer techniques, free from any contributions of HgX₂ or HgX^2–₄. Four different independent methods are described and evaluated for this system and for general use. For systems where the desired spectrum is completely hidden below other bands, here for HgBr^–₃ and HgCl^–₃, a combination of two methods is needed. Additional tests to validate the computed spectra are described and advised. The spectra of HgX^–₃ were resolved into their component Gaussian bands and the transitions identified and assigned. This permitted the identification of HgCl^–₃ as planar with D_3h symmetry, and Hgl^–₃ as pyramidal, with C_3v symmetry, suggesting that the solvated species are trigonal-bipyramidal and tetrahedral, respectively. The spectrum of HgBr^–₃ appears to show features of both symmetries, but it is closer to pyramidal geometry.