SOLVATION STRUCTURES FOR BROMIDE ION IN VARIOUS SOLVENTS BY EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE

The Br K-edge X-ray absorption spectra have been obtained for bromide ion dissolved in various protic and aprotic solvents. The Br ... 0 dis tances in protic solvents which are fixed by the hydrogen-bond interac tion and Br ... C in aprotic solvents by the ion-dipole interaction we re determined to be 3.1-3.2 and 3.5-3.6 angstrom, respectively. The co -ordination numbers of Br- were found to be widely different, 3-6, fro m solvent to solvent. In protic solvents the solvation number correlat es with the Mayer-Gutmann acceptor number, A(N). Aprotic solvents can be divided into two groups in terms of the solvation structures. The s olvation numbers in dimethylformamide, dimethylacetamide, dimethyl sul foxide, 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 str ucture (EXAFS) oscillation evaluated from the peak intensity in the Fo urier-transform spectrum correlates with A(N) throughout the series of solvents studied. This correlation is attributed to the similarity be tween the mechanism determining the amplitude of EXAFS and that of A(N ).