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

Abstract

ABSTRACT:The SAS bond cleavage produced upon electron attachment to disulfides was generally assumed to be an adiabatic process because the added electron occupies the r*(SAS) antibonding orbital. This is clearly the case in the parent HSSH compound, but not necessarily in XSSX 0 derivatives, where the substituents X and X 0 are different. Through the use of MS-CASPT2 calculations, we have shown that the dissociation of the SAS two-center-three-electron bond in these asymmetric XSSX 0 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 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 SAO rather than the SAS bond. Besides, the two dissociation limits CH 3 SS Á þ OH À and CH 3 SS À þ OH Á are accidentally degenerate, so the SAO 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.

Electronic Structure Methods for the Description of Nonadiabatic Effects and Conical Intersections

“…The above description can be extended beyond two states, to describe degeneracies between three electronic states. − In that case, there are five conditions that need to be satisfied to reach degeneracy, and the branching space is five-dimensional. Three-state CoIns have been proven to be important in a variety of molecular systems. ,− …”

Interaction of low-energy electrons with dimethyl sulfide and dimethyl disulfide

“…Rydberg electron transfer and negative ion photoelectron spectroscopy studies indicate that the geometrically relaxed ground states of the anionic saturated disulfides RS-SR (R = methyl, ethyl, propyl) are slightly stable (0.1 eV), i.e., the adiabatic electron affinities are slightly positive [14]. Calculations performed with different electronic structure techniques by Gámez et al [15,16] indicated that the extra electron occupies a σ * SS orbital in the DMDS anion. The DEA spectrum of DMDS was reported by Modelli et al [10] and more recently by Matias et al [11].…”

Low-energy electron interactions with dimethyl disulphide

“…Hence any CH 2 S 2 signal at higher electron energies may be below the detection limit of the apparatus. [11] may not be involved in the anion formation, since the resonances appear at substantial higher energies.…”

“…Rydberg electrontransfer spectroscopy and quantum chemical calculations on a series of saturated disulphides have shown nondissociative electron transfer at 0.2 eV [10]. Recent multireference calculations on DMDS revealed that in the ground state the extra electron occupies the σ * (S─S) molecular orbital (MO) and upper in energy, there are two states at 202 and 210 kJ mol -1 (2.094 and 2.176 eV, respectively) whose singly occupied molecular orbitals are linear combinations of two non-bonding 3p-orbitals on each of the sulphur atoms [11]. These authors also point out evidence of another state at 278 kJ mol -1 (2.881 eV) where the unpaired electron is mainly located in the σ(S─S) MO.…”