Small organic compounds with S-S bonds are often used to model disulfide linkages in cysteine residues, which play an important role in the stability of proteins. As a result, studying the UV and visible photochemical dynamics of such systems is essential to understanding competing S-S and C-S photodissociation processes in proteins. We look at one of the simplest organic systems with a S-S bond, dimethyl disulfide, and decode molecular events at ultrafast timescales after irradiation with ultraviolet (UV) light. Prior experimental studies with variable excitation wavelengths have demonstrated pathways to photofragmentation involving S-S and C-S bond cleavages, but much less is known about what happens when we excite the molecule using far UV light (200 nm) to higher lying excited states with significant Rydberg character. Our goals are multifold: 1) to track the dynamics with nonadiabatic transitions between the excited states detailing the relevant electronic states leading to the competing dissociation products, 2) probe ultrafast rupture of S-S bond, 3) propose a pathway for C-S bond cleavage and estimate associated timescale.<br/><br/>We employ various theoretical approaches to study the dynamics of photoexcited dimethyl disulfide, and by comparing to results from ultrafast electron diffraction (Linac Coherent Light Source, Stanford) experiments at 200 nm, compute timescales for bond-breaking as well as branching ratios. We benchmark the performance of low-cost TDDFT methods in comparison to EOM-CCSD and arrive at the method of choice (TDDFT-CAMB3LYP) for the non-adiabatic mixed quantum-classical dynamics (NA-MQC). Two different non-adiabatic approaches are undertaken: non-adiabatic mixed quantum-classical (NA-MQC) dynamics with Trajectory surface hopping, restricted to singlet excited states and allowing only for internal conversion (IC), and NA-MQC dynamics with the inclusion of spin-orbit coupling leading to dynamics involving both singlet and triplet excited states exploring the possibility of both intersystem crossing (ISC) and IC. Calculation of spin orbit coupling constants and timescales for ISC show a viable route to photodissociation with time scale in sub-picosecond range.

Ultrafast photofragmentation of dimethyl disulphide in far-UV light: Elucidating reaction pathways with non-adiabatic mixed quantum-classical dynamics

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