Sulfur K-edge X-ray absorption near-edge structure (XANES) spectra have been recorded and the S(1s) electron excitations evaluated by means of density functional theory-transition potential (DFT-TP) calculations to provide insight into the coordination, bonding, and electronic structure. The XANES spectra for the various species in sulfur dioxide and aqueous sodium sulfite solutions show considerable differences at different pH values in the environmentally important sulfite(IV) system. In strongly acidic (pH < ∼1) aqueous sulfite solution the XANES spectra confirm that the hydrated sulfur dioxide molecule, SO 2 (aq), dominates. The theoretical spectra are consistent with an OSO angle of ∼119° in gas phase and acetonitrile solution, while in aqueous solution hydrogen bonding reduces the angle to ∼116°. The hydration affects the XANES spectra also for the sulfite ion, SO 3 2- . At intermediate pH (∼4) the two coordination isomers, the sulfonate (HSO 3 - ) and hydrogen sulfite (SO 3 H - ) ions with the hydrogen atom coordinated to sulfur and oxygen, respectively, could be distinguished with the ratio HSO 3 - :SO 3 H - about 0.28:0.72 at 298 K. The relative amount of HSO 3 - increased with increasing temperature in the investigated range from 275 to 343 K. XANES spectra of sulfonate, methanesulfonate, trichloromethanesulfonate, and trifluoromethane- sulfonate compounds, all with closely similar S-O bond distances in tetrahedral configuration around the sulfur atom, were interpreted by DFT-TP computations. The energy of their main electronic transition from the sulfur K-shell is about 2478 eV. The additional absorption features are similar when a hydrogen atom or an electron-donating methyl group is bonded to the -SO 3 group. Significant changes occur for the electronegative trichloromethyl (Cl 3 C-) and trifluoromethyl (F 3 C-) groups, which strongly affect the distribution especially of the π electrons around the sulfur atom. The S-D bond distance 1.38(2) Å was obtained for the deuterated sulfonate (DSO 3 - ) ion by Rietveld analysis of neutron powder diffraction data of CsDSO 3 . Raman and infrared absorption spectra of the CsHSO 3 , CsDSO 3 , H 3 CSO 3 Na, and Cl 3 CSO 3 Na·H 2 O compounds and Raman spectra of the sulfite solutions have been interpreted by normal coordinate calculations. The C-S stretching force constant for the trichloromethanesulfonate ion obtains an anomalously low value due to steric repulsion between the Cl 3 C- and -SO 3 groups. The S-O stretching force constants were correlated with corresponding S-O bond distances for several oxosulfur species.
Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry