Asymmetry and non-adiabaticity in fragmentation of disulfide bonds upon electron capture

Although it has been generally assumed that electron attachment to disulfide derivatives leads to a systematic and significant activation of the S-S bond, we show, by using [CH(3)SSX] (X = CH(3), NH(2), OH, F) derivatives as model compounds, that this is the case only when the X substituents have low electronegativity. Through the use of MP2, QCI and CASPT2 molecular orbital (MO) methods, we elucidate, for the first time, the mechanisms that lead to unimolecular fragmentation of disulfide derivatives after electron attachment. Our theoretical scrutiny indicates that these mechanisms are more intricate than assumed in previous studies. The most stable products, from a thermodynamic viewpoint, correspond to the release of neutral molecules; CH(4), NH(3), H(2)O, and HF. However, the barriers to reach these products depend strongly on the electronegativity of the X substituents. Only for very electronegative substituents, such as OH or F, the loss of H(2)O or HF is the most favorable process, and likely the only one observed. This is possible because of two concomitant factors, 1) the extra electron for [CH(3)SSX](-) (X = OH, F) occupies a sigma*(S-X) MO, which favors the cleavage of the S-X bond, and 2) the activation barriers associated with the hydrogen transfer process to produce H(2)O and HF are rather low. Only when the substituents are less electronegative (X = H, CH(3), NH(2)) the extra electron is located in a sigma*(S-S) orbital and the cleavage of the disulfide bridge becomes the most favorable process. The intimate mechanism associated with the S-S bond dissociation process also depends strongly on the nature of the substituent. For X = H or CH(3) the process is strictly adiabatic, while for X = NH(2) it proceeds through a conical intersection (CI) associated with the charge reorganization necessary to obtain, from a molecular anion with the extra electron delocalized in a sigma*(S-S) antibonding orbital, two fragments with the proper charge localization.