Solvation structures for bromide ion in various solvents by extended X-ray absorption fine structure

Abstract

The Br K-edge X-ray absorption spectra have been obtained for bromide ion dissolved in various protic and aprotic solvents. The Br ⋯ O distances in protic solvents which are fixed by the hydrogen-bond interaction and Br ⋯ C in aprotic solvents by the ion–dipole interaction were determined to be 3.1–3.2 and 3.5–3.6 Å, respectively. The co-ordination numbers of Br^– were found to be widely different, 3–6, from solvent to solvent. In protic solvents the solvation number correlates with the Mayer–Gutmann acceptor number, A_N. Aprotic solvents can be divided into two groups in terms of the solvation structures. The solvation numbers in dimethylformamide, dimethylacetamide, dimethyl sulfoxide, acetone and ethyl methyl ketone are larger than those in other aprotic solvents possibly because of their bonding to Br^–through two methyl groups. The amplitude of the extended X-ray absorption fine structure (EXAFS) oscillation evaluated from the peak intensity in the Fourier-transform spectrum correlates with A_N throughout the series of solvents studied. This correlation is attributed to the similarity between the mechanism determining the amplitude of EXAFS and that of A_N.