ORIGINAL PAPER Theoretical study of the reactions of the hydroselenyl radical (HSe ● ) with the selenenic radical (HSeO ● ) Mauricio Angel Vega-Teijido 1 & Martina Kieninger 1 & Oscar N. Ventura 1 Received: 7 April 2017 /Accepted: 15 November 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017 Abstract The formation of selenium species in some biological processes involves the generation of ionic and radical intermediates such as RSe ● , RSe - , RSeO ● , and RSeO - , among others. We performed a theoretical study of the possible mechanisms for the reaction of the two simplest Se radicals—the hydroselenyl (HSe ● ) and selenenic (HSeO ● ) radicals, in which the possible products, inter- mediates, and transition-state structures were investigated. Density functional theory (DFT) was applied at the B3LYP/6–311++ G(3df,3pd) level and the Ahlrichs Coulomb fitting basis sets were employed with an effective core potential (ECP) for both Se atoms. The same procedure was used to calculate the electronic density. All calculations were also performed using the M06-2X functional, which describes weaker bonds better than B3LYP does. In the reaction of interest, the so-called CR complex (HSe···· SeOH) is formed initially. After passing through the transition state TS1, cis-HSeSeOH is obtained as a product. If a low barrier is then overcome (passing through the transition state TS32), the trans-HSeSeOH species is obtained. The CR complex can also rearrange into the intermediate INT after overcoming the barrier presented by the transition state TS2. Additionally, the decom- position of INT to H 2 O and 1 Se 2 is possible through another transition state. This reaction is not included in this study. We also observed a second possible route for the conversion of INT to one of the HSeSeOH species; this route occurs through two pathways (with transition states TS31 and TS32). A comparison of some of the results with those obtained for sulfur analogs along the same pathways is also presented in this work. Keywords Selenium radicals . Sulfur radicals . Ahlrichs . M06 Introduction The lighter members of the chalcogen family—O, S, and Se— participate in several atmospheric and biological redox reac- tions. In particular, the atmospheric oxidation of anthropogen- ically produced sulfur is an important air pollution issue [1, 2]. It has long been known that atmospheric sulfur compounds are associated with processes such as acid rain and ozone depletion [3, 4]. Selenium is not an important component of atmospheric pollution. However, Se pollution is observed in water systems, leading to the contamination of humans, aquat- ic animals, and cattle [5, 6]. Sources of selenium pollution include the agroindustry and coal-powered industrial plants. Wastewater treatment processes are necessary to prevent the release of Se compounds, and many countries have enacted environmental legislation on Se pollution [7]. On the other hand, selenium is an essential micronutrient in humans, and supplementing the diet with small amounts of selenium is thought to help prevent diabetes and some types of cancer. Selenium deficiency has been associated with white muscle disease, which affects lambs, calves, foals, and adult horses [6]. Se is present in diverse organic and inorganic chemical systems. As it is a chalcogen, it presents some similarities to S in terms of its reactivity, and reportedly plays a role in various biological systems [8]. Most remarkably, there is an important amino acid, selenocysteine, that characterizes a This paper belongs to Topical Collection QUITEL 2016 Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00894-017-3535-1) contains supplementary material, which is available to authorized users. * Mauricio Angel Vega-Teijido mauryvg@gmail.com 1 Computational Chemistry and Biology Group-CCBG, DETEMA, Facultad de Química, Universidad de la República-UdelaR, 11800 Montevideo, Uruguay Journal of Molecular Modeling (2018) 24:3 https://doi.org/10.1007/s00894-017-3535-1