Estudo teórico de mercúrio (II) com grupos carboxílicos de substâncias húmicas

Abstract

This work describes the mercury and a summary on toxicity of organomercury, as well as environmental importance of the HS, addressing structural aspects of the interaction between carboxylic groups from humic and fulvic acids and Hg, in two reviews. A third review is presented the Density Functional Theory (DFT) applied to theoretical study of mercury and its compounds. Environmental chemistry of Hg is extensively revised, especially the reduction process Hg2+  Hg+  Hg0. The Hg can also be found in forms organomercury, which the methylmercury (methyl-Hg, CH3Hg+) and dimethylmercury (dimethyl-Hg, CH3(Hg)2) are the most important. Structurally, the difference between the two forms of Hg is the number of methyl groups. Important tool in the consolidation of experimental data, theoretical calculations have proven that they are capable of complementing experimental results. The DFT B3LYP functional with LANL2DZ basis set are performed for the neutral structures dimethylmercury (Dimetil-Hg), methylmercury (MetHgCl), methylmercury bromide (MetHgBr), methylmercury iodide (MetHgI) and charged methylmercury (Met-Hg), methylmercury oxide ion (MetHgO-), methylmercury sulfide ion (METHgS-), methylmercury selenide ion (MetHgSe-). Additionally, the interaction of the Model of Humic Substance Molecules (MHSM) (Benzoic acid (AB), Phthalic acid (AFT), Salicylic acid (AS) e 2,4-dihydroxybenzoic acid (2,4-ADHBC) and carboxylic groups from fulvic acid (AFRS) structure proposed for the Suwannee River with Hg2+ has been investigated theoretically by using DFT method with high level B3LYP/ LANL2DZ model in order to contribute to the understanding of the behavior of Hg in nature. Findings show that the structure of methyl-Hg is more stable and less reactive than dimethyl-Hg. The bond lengths C-Hg on Met-Hg-X (X = Cl-, Br- and I-) increase with increasing volume. In the case of Met-Hg-X (X = O2-, S2- and Se2-), the bond length C-Hg remains virtually unchanged. The charge distribution analysis has revealed that the Hg oxidation tendency to reduce or anion depending on what is on. The trend of reduction/oxidation is accompanied by a greater stabilization energy (NBO) and ΔEL. The HUMO-LUMO orbital shows the structures Met-Hg-X (X = Cl-, Br- and I-) that has a larger Δε Met-Hg-X (X = O2-, S2- and Se2-). Finally, the use of B3LYP/LANL2DZ shows errors in bond length in relation to the experimental data varying between 4.74 and 6.43%. The results showed that the interaction between the carboxylic acid and Hg2+ ions indicate the occurrence of a reduction process rather than complexation in the MMSH. The interaction AB Hg ions formed with the carboxylic AB proved to be the most stable among all the MMSH-Hg. The highest values of stabilization energy of second order were observed by the molecules AB Hg e AFT Hg. Findings showed that the complexation and reduction depends on the location of the carboxyl groups in the structure of fulvic acid in the AFRS. Furthermore, the binding energy, geometry and energy gap is altered by interaction with the carboxylic groups of AFRS with Hg2+.