X-ray Absorption Spectroscopy of Aliphatic Organic Sulfides.

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 Kedge 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)sigma* and 1s -> (S-C)pi* 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)sigma* and 1s -> (S-C)pi* 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.