X-ray absorption spectroscopy of organic sulfoxides

Abstract

Organic sulfoxides, a group of compounds containing the sulfinyl S[double bond, length as m-dash]O group, are widespread in nature, important in health and disease, and used in a variety of applications in the pharmaceutical industry. We have examined the sulfur K-edge X-ray absorption near-edge spectra of a range of different sulfoxides and find that their spectra are remarkably similar. Spectra show an intense absorption peak that is comprised of two transitions; a S 1s → (S-O)σ* and a S 1s → [(S-O)π* + (S-C)σ*] transition. In most cases these are sufficiently close in energy that they are not properly resolved; however for dimethylsulfoxide the separation between these transitions increases in aqueous solution due to hydrogen bonding to the sulfinyl oxygen. We also examined tetrahydrothiophene sulfoxide using both the sulfur and oxygen K-edge. This compound has a mild degree of ring strain at the sulfur atom, which changes the energies of the two transitions so that the S 1s → [(S-O)π* + (S-C)σ*] is below the S 1s → (S-O)σ*. A comparison of the oxygen K-edge X-ray absorption near-edge spectra of tetrahydrothiophene sulfoxide with that of an unhindered sulfoxide shows little change, indicating that the electronic environment of oxygen is very similar.

Fig. 1. Comparison of the sulfur K-edge X-ray absorption spectra of a series of organic sulfoxides. (A) Shows the absorption spectra, while (B) shows the corresponding second derivative spectra. (a) Dimethylsulfoxide (50 mM aqueous solution 100 mM bis-tris-propane buffer pH 7.4), (b) dimethylsulfoxide (100 mM acetone solution), (c) l-methionine sulfoxide (50 mM aqueous solution 100 mM bis-tris-propane buffer pH 7.4), (d) n-butylsulfoxide (100 mM toluene solution), (e) di-isoallyl-sulfoxide (100 mM toluene solution), (f) methylphenylsulfoxide (100 mM toluene solution), and (g) phenyl sulfoxide (100 mM toluene solution). For (a) the spectrum of a solution of dimethylsulfide is superimposed (gray curve) to illustrate the chemical shift between the sulfide and sulfinyl functional groups. The vertical broken line at 2473.5 eV is included to guide the eye to small shifts between the spectra. The two low-energy transitions 1 and 2 that are discussed in the text are indicated on trace (a) in panel (B).

Fig. 2. The crystal structure of diphenylsulfoxide. Panel (A) shows a view of the unit cell showing the four molecules contained within, and (B) shows a view looking along the crystallographic b axis, highlighting the alignment of the SO bonds. Panel (C) shows the plane (in transparent green) defined by the a–c bisector and the crystallographic b axis used in the polarized single crystal measurements, together with the experimentally varied angle θ between the X-ray e-vector (the blue arrow in (C)) and the a–c bisector.

Fig. 3. Comparison of sulfur K-edge X-ray absorption near edge spectra for diphenylsulfoxide as a solid and in toluene solution. Panel (A) shows the absorption spectra, while (B) shows the corresponding second derivative spectra. The transitions labelled 1–4 are discussed in the text.

Fig. 4. Polarized sulfur K-edge X-ray absorption spectra of a single crystal of diphenylsulfoxide (A), with second derivative spectra (B), where θ is the angle from the SO bond rotating the crystal from the a–c bisector to the crystallographic b axis. The features marked as 1–4 show marked polarization anisotropy and are discussed in the text. The feature marked (*) grows with beam exposure and is thus most probably associated with photoreduced forms.

Fig. 5. Density functional theory simulations of the sulfur K-near-edge spectrum of dimethylsulfoxide. (a and b) Show the 0.07 electrons per a.u. isosurfaces of the excited state molecular orbitals corresponding to transitions 1 (S 1s → (S–O)σ*) (a) and 2 (S 1s → [(S–O)π* + (S–C)σ*]) (b). (c) Shows the experimental spectrum of aqueous dimethylsulfoxide from Fig. 1(a) (black line) along with simulated spectra (green lines) plus corresponding stick spectra showing the energies of computed transitions (red lines) for the number of hydrogen-bonded waters (0–4) indicated for each simulated spectrum.

Fig. 6. Density functional theory simulation of the sulfur K-near-edge spectrum of diphenylsulfoxide. The experimental spectrum (toluene solution) is shown in black, along with the simulated spectrum (green line) plus corresponding stick spectra showing the energies of computed transitions (red lines).

Fig. 7. Sulfur K-edge (A) and oxygen K-edge (B) spectra together with density functional theory simulations of the spectra of sterically hindered and unhindered sulfoxides. The sulfur K-edge spectra were measured of 100 mM toluene solutions, while the oxygen K-edge spectra were measured on solid di-n-dodecylsulfoxide and pure liquid tetrahydrothiophene sulfoxide. Tetrahydrothiophene sulfoxide (C) schematic structure (a) along with representations of the LUMO (b) and LUMO+1 (c).