X-ray Absorption Spectroscopy of Aliphatic Organic Sulfides

Abstract

Organic sulfides, sometimes called thioethers, are important in a variety of materials with diverse roles in biology and the environment. They also contribute a significant proportion of the sulfur in fossil fuels. We have studied a range of aliphatic sulfides using a combination of sulfur K-edge X-ray absorption spectroscopy and density functional theory calculations. We show that the sulfur K-edge near-edge X-ray absorption spectra of aliphatic organic sulfides comprise two intense transitions in the near-edge spectrum, which can be assigned as 1s → (S-C)σ* and 1s → (S-C)π* transitions. These transitions are found to change in a systematic manner in sterically hindered sulfides composed of four-, five- and six-membered rings. Both the 1s → (S-C)σ* and 1s → (S-C)π* transitions are sensitive to the presence of strain in the C-S-C angle, shifting to lower values with more strained ring systems. Steric effects can give obtuse C-S-C angles, which are predicted to cause the two transitions to converge to the same energy and even cross over at very obtuse angles.

Sulfur K-Edge X-ray Absorption Spectroscopy of Aryl and Aryl–Alkyl Sulfides

“…The higher energy intense transition shows substantial S–C σ* character (LUMO+3), also involving the aromatic ring. The two intense groups of transitions are related to those of the alkyl sulfides, but with the order reversed . A similar phenomenon has been noted by Mijovilovich et al for benzyl sulfide and benzyl phenyl sulfide …”

“…Sulfur K-edge X-ray absorption spectroscopy (XAS) has been used previously to probe the sulfur speciation in complex specimens, including fossil fuels. − Our previous work has demonstrated the technique’s sensitivity to a series of thiophenes including solution tautomers, , to organic disulfides, and to a range of different cyclic and acyclic alkyl sulfides . Here we report upon the sulfur K-edge XAS of both aryl and aryl–alkyl sulfides and show how these differ from those of the alkyl sulfides …”

“…Spectra were recorded by measuring total X-ray fluorescence by means of a large-area PIN diode (Canberra Industries, Meriden, CT, USA), screened from visible light through use of an aluminized Mylar window on its face, as well as darkening the experimental enclosure. Sequential dilutions, used to test for fluorescence self-absorption artifacts , as previously described, − demonstrated that spectra were essentially free from self-absorption artifacts. Calibration of the incident X-ray energy was achieved with reference to the spectrum of solid anhydrous sodium thiosulfate (Na 2 S 2 O 3 ) assuming the energy of the lowest energy K-edge absorption peak to be the literature value of 2469.2 eV .…”

“…Figure1. Sulfur K-edge X-ray absorption spectra of aryl and aryl− alkyl organic sulfides in toluene solution, in comparison with the spectrum of a typical unstrained alkyl sulfide (n-butyl sulfide) 20. …”

Pulsed Electron-Electron Double Resonance (PELDOR) and Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy in Bioanalysis

“…Functional group Oxidation state (Waldo et al 1991;Xia et al 1998;Vairavamurthy 1998) (George et al 2014;Pickering et al 2016;Cotelesage et al 2017).…”