Electron Attachment to Diselenides Revisited: Se-Se Bond Cleavage Is Neither Adiabatic nor the Most Favorable Process

摘要

Up to now it has been generally assumed that the electron capture on diselenides XSeSeX' produces a fragmentation of the Se—Se bond. However, our high-level ab initio calculations indicate that this is the case only when the substituents X and X' attached to the diselenide bridge have low electronegativity. Also importantly, even when the two substituents are of similar electronegativity, the Se—Se bond cleavage rarely is an adiabatic process. For low-electronegative X substituents, the extra electron is placed in the σ(Se—Se) antibonding orbital, and the cleavage of the Se—Se bond is the most favorable process. However, the mechanism of this bond breaking is more intricate than previously assumed, and for asymmetric derivatives it proceeds through a conical intersection (CI). These findings emphasize the importance of using accurate ab initio calculations, rather than the usually employed density functional theory approaches, when dealing with reactions in biochemistry and organometallic chemistry, because the characterization of a CI requires the use of multireference methods to account for the mixing of states. When X is highly electronegative, the σ*(Se—X) antibonding orbital becomes highly stabilized with respect to the σ*(Se—Se) strongly favoring the cleavage of the Se-X bond, whereas the Se—Se remains practically unperturbed. Finally, when comparing the present results on diselenides with those of the disulfide analogues, it is apparent that the activation barriers and the final products of the different imimolecular reactions are higher in energy for the diselenides, in spite of the higher antioxidant strength of diselenides. This seems to indicate that the electron detachment process, less favorable for diselenides than for disulfides, competes with the electron-capture dissociation process and therefore should also be considered to explain the different antioxidant ability of these compounds.