Two‐ and three‐state conical intersections in the electron capture dissociation of disulfides: The importance of multireference calculations

Abstract

AbstractThe SS bond cleavage produced upon electron attachment to disulfides was generally assumed to be an adiabatic process because the added electron occupies the σ*(SS) antibonding orbital. This is clearly the case in the parent HSSH compound, but not necessarily in XSSX′ derivatives, where the substituents X and X′ are different. Through the use of MS‐CASPT2 calculations, we have shown that the dissociation of the SS two‐center‐three‐electron bond in these asymmetric XSSX′ compounds requires the interaction of at least two states, in order to localize the extra electron in one of the fragments upon dissociation. This is actually the case for the CH3SSNH2 derivative, where the most favorable dissociation process locates the extra electron at the SCH3 moiety, the SCH3· + SNH dissociation limit being 66 kJ mol−1 higher in energy. The situation is still more complex when one of the substituents is an OH group, because, in this particular case, the most favorable process is the dissociation of the SO rather than the SS bond. Besides, the two dissociation limits CH3SS· + OH− and CH3SS− + OH· are accidentally degenerate, so the SO bond fission involves a three‐state conical intersection. This constitutes the first example of a three‐state crossing computed by accurate ab initio calculations involved in a nonphotochemical reaction. These findings highlight the necessity of using multireference approaches to appropriately describe the electron capture dissociation of disulfide bridges. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3316–3323, 2011