Asymmetry and Non‐Adiabaticity in Fragmentation of Disulfide Bonds upon Electron Capture

Abstract

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.

Reactions of CH3SH and CH3SSCH3 with Gas-Phase Hydrated Radical Anions (H2O)n•–, CO2•–(H2O)n, and O2•–(H2O)n

“…The 3df polarization functions for S are again essential; the M06 VDE value of 1.95 eV is comparable with the corresponding NIPES value of 1.75 eV, and the M06 EA ad value of 0.15 eV is also in agreement with the bracketing value of 0.23 ± 0.21 eV, 8 the high-level G2 and G3 values of 0.17 eV, , and the fitted NIPES value of 0.08 eV . A predissociation geometry of DMDS •– ( dmds_0b C 2 h ) with the S–S distance of 4.267 Å (B3LYP) and 4.228 Å (M06) is also shown in Figure and is 1.6 kJ mol –1 (B3LYP) and 6.2 kJ mol –1 (M06) in energy higher than dmds_0a .…”

Systematic Study of the Performance of Density Functional Theory Methods for Prediction of Energies and Geometries of Organoselenium Compounds

“…In order to better understand the phenomena observed experimentally, many theoretical studies have been carried out, modeling organoselenium antioxidants − and thio- and selenouracils , by density functional methods. Recently Gámez et al − have published a series of studies on electron capture and bond cleavage in disulfides and diselenides using high-level conventional electronic structure methods.…”

Is Se–Se bond cleavage the most favourable process in electron attachment to diselenides? The importance of asymmetry

“…It seems obvious that the effect of asymmetry or the different electronegativity of the substituents attached to the Se-Se bridge, which have never been considered before, would distort the electronic density of the linkage and, consequently, influence the electron capture process and subsequent dissociation. 26 The aim of this communication is to show, precisely, that in most of the cases the Se-Se bond cleavage is neither adiabatic nor the most favourable process, in clear contrast with what was previously assumed.…”

“…Although the electron capture dissociation (ECD) has been largely investigated for disulfides [20][21][22][23][24][25] it has been only recently that its general mechanism has been fully described. 26 Not surprisingly much less attention was devoted to Se-Se bond fission by electron capture to diselenides. Some studies carried out on dimethyldiselenide 27,28 indicate that electron attachment yields mainly the breaking of the Se-Se bond since the extra electron is accommodated in the s*(Se-Se) antibonding orbital, like for disulfides.…”

“…A comparison of diselenides with disulfides 26 shows that the mechanisms associated with the A-A bond cleavage in ECD processes for [CH 3 AACH 3 ] À and [CH 3 AANH 2 ] À (A = S, Se) are alike, although the energy barriers and bond dissociation energies are higher for diselenides (Table 1). Experimentally, diselenides are better reductants than disulfides, 17,18,30 suggesting that the ejection of the extra electron must compete with ECD.…”