A theoretical study about reactivity and spectroscopic properties of copper ions toward sulfur-containing species

Abstract

Sulfur-containing compounds have a major impact on the negative sensory quality of cachaça and other alcoholic beverages. These sulfide “off-odors” can be often removed by the presence of copper ions – a process that remains poorly understood. The DFT and CCSD(T) calculations were used to evaluate the reactivity of the copper(I) and copper(II) ions toward the volatile H2S, MeHS and EtHS species. The main results indicate that the interaction goes to the four-fold coordination model for all sulfide copper(I) compounds, whereas the five-fold coordination model was found for the copper(II) ion with the H2S and EtHS species. A distorted octahedral structure, stabilized by Jahn-Teller effect, was obtained for the copper(II) ion and MeHS species. The computed bonding energies for the copper(II)-sulfide compounds are pronouncedly larger than those for the copper(I)-sulfide structures. The electron-donor and steric effects of VSCs are responsible for differences in reactivity in the copper-sulfide systems. The UV–visible absorption spectroscopy can be used to identify and monitorate the copper-sulfide compounds under suitable conditions. A particular UV–Vis absorption profile is observed for each VSC system in the presence of Cu(I) and Cu(II) ions. The MLCT process plays an important role into the extra stabilization of the copper(II)-sulfide systems, thus favoring the reduction of VSCs in distillation process of cachaça.