Unveiling the UV photofragmentation pathways of dimethyl disulfide initiated by a Rydberg excitation

Dimethyl disulfide (DMDS), one of the smallest organic molecules with an S-S bond, can serve as a model system for understanding photofragmentation in polypeptides and proteins. Prior studies using ~266 nm and ~248 nm excitation of DMDS have shed light on dissociation pathways involving the lowest excited electronic states (S1), but far less is understood about photodissociation at higher excitation energies. In this work, we characterize the excited states of DMDS with equation of motion coupled cluster theory (EOM-CCSD) and compare computed and experimental UV spectra. Through Natural Transition Orbital analysis of the excited states, we find significant Rydberg character in numerous excited states that are accessed with ~200 nm excitation. One-dimensional potential energy scans along the C-S and S-S bond coordinates reveal novel photodissociation routes resulting from ~200 nm excitation, involving excited state potential energy surfaces S1-S6. Our high-level ab-initio investigation validates and rationalizes previous experimental conclusions, including prompt S-S cleavage observed at ~266 nm, presence of competing C-S and S-S cleavage pathways, and production of excited thiomethoxy radicals after excitation at ~200 nm. Comparative benchmarking of a low cost time-dependent density function theory (TDDFT) method reveals that the CAM-B3LYP-D3 functional with diffuse aug-cc-pVDZ basis reproduces the UV spectrum and one-dimensional potential energy scans computed with EOM-CCSD, enabling its use in future non-adiabatic dynamics calculations. Calculations of spin-orbit coupling constants reveal a high likelihood of ultrafast intersystem crossing, which has not been predicted or reported to date.