An extended x-ray absorption fine structure study by employing molecular dynamics simulations: Bromide ion in methanolic solution

Abstract

X-ray absorption spectroscopy is widely employed in the structural analysis of disordered systems. In the standard extended x-ray absorption fine structure (EXAFS) analysis the coordination of the photoabsorber is usually defined by means of Gaussian shells. It is known that this procedure can lead to significant errors in the determination of the coordination parameters for systems which present anharmonic thermal vibrations or interatomic asymmetric pair distribution functions. An efficient method has been recently employed in the study of the hydration shells of bromide and rubidium ions and brominated hydrocarbon molecules in diluted aqueous solutions. According to this method, pair distribution functions [g(r)] obtained from molecular dynamics simulations can be used as relevant models in the calculation of the EXAFS signals. Moreover, asymmetric shells modeled on the g(r) first peaks, have been employed in the EXAFS analysis and the parameters defining the asymmetric peaks have been optimized during the minimization procedure. In the present paper this new procedure has been used to investigate the coordination of Br− in methanol. The analysis of this system is particularly interesting due to the presence of three well separated coordination shells. We show that the inclusion of the hydrogen signal is essential to perform a reliable analysis. A comparison of the analysis with asymmetric and Gaussian shells shows how the accuracy of the EXAFS data analysis is improved by using asymmetric shells.