Sulfur K-edge X-ray absorption spectroscopy and time-dependent density functional theory of arsenic dithiocarbamates

Abstract

S K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT) calculations were performed on a series of As[S₂CNR₂]₃ complexes, where R₂ = Et₂, (CH₂)₅ and Ph₂, to determine how dithiocarbamate substituents attached to N affect As[S₂CNR₂]₃ electronic structure. Complimentary [PPh₄][S₂CNR₂] salts were also studied to compare dithiocarbamate bonding in the absence of As. The XAS results indicate that changing the orientation of the alkyl substituents from trans to cis (R₂ = Et₂vs. (CH₂)₅) yields subtle variations whereas differences associated with a change from alkyl to aryl are much more pronounced. For example, despite the differences in As 4p mixing, the first features in the S K-edge XAS spectra of [PPh₄][S₂CNPh₂] and As[S₂CNPh₂]₃ were both shifted by 0.3 eV compared to their alkyl-substituted derivatives. DFT calculations revealed that the unique shift observed for [PPh₄][S₂CNPh₂] is due to phenyl-induced splitting of the π* orbitals delocalized over N, C and S. A similar phenomenon accounts for the shift observed for As[S₂CNPh₂]₃, but the presence of two unique S environments (As–S and As⋯S) prevented reliable analysis of As–S covalency from the XAS data. In the absence of experimental values, DFT calculations revealed a decrease in As–S orbital mixing in As[S₂CNPh₂]₃ that stems from a redistribution of electron density to S atoms participating in weaker As⋯S interactions. Simulated spectra obtained from TDDFT calculations reproduce the experimental differences in the S K-edge XAS data, which suggests that the theory is accurately modeling the experimental differences in As–S orbital mixing. The results highlight how S K-edge XAS and DFT can be used cooperatively to understand the electronic structure of low symmetry coordination complexes containing S atoms in different chemical environments.