EXAFS of bromide ion in solvents and at air/solution interface

Abstract

Solvation structures of Br − ion dissolved in 23 solvents have been studied at the Br K-edge using extended X-ray absorption fine structure (EXAFS). The results are summarized as follows. 1) Coordination bond lengths r are 0.31–0.32 nm for BrO in protic solvents and 0.35–0.36 nm for BrC in aprotic solvents. 2) The r values for water and formamide are larger than those for other protic solvents, possibly due to the strong solvent-solvent interactions of these two solvents. 3) Coordination numbers N widely differ, ranging between 3–6, from solvent to solvent. The value of N for alcohol linearly correlates with the Mayer-Gutmann's acceptor number A N. 4) In aprotic solvents containing two methyl groups per molecule, Br − is found to contact with the two methyl groups, thus the number of primary solvent molecules must be equal to half the N value. 5) EXAFS amplitude is found to correlate with A N of the solvent. To study the surface structure, a new EXAFS technique, tocal-reflection total-conversion-electron-yield EXAFS, has been developed and applied. The new technique utilizes polarized X-rays from synchrotron radiation, providing not only information on the solvation structures of ions at the aqueous solution surface but also on the preferential orientation of coordination bond at the solution surface.