AN EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE STUDY BY EMPLOYING MOLECULAR-DYNAMICS SIMULATIONS - BROMIDE ION IN METHANOLIC SOLUTION

X-ray absorption spectroscopy is widely employed in the structural ana lysis 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 v ibrations or interatomic asymmetric pair distribution functions. An ef ficient method has been recently employed in the study of the hydratio n shells of bromide and rubidium ions and brominated hydrocarbon molec ules in diluted aqueous solutions. According to this method, pair dist ribution functions [g(r)] obtained from molecular dynamics simulations can be used as relevant models in the calculation of the EXAFS signal s. Moreover, asymmetric shells modeled on the g(r) first peaks, have b een employed in the EXAFS analysis and the parameters defining the asy mmetric peaks have been optimized during the minimization procedure. I n the present paper this new procedure has been used to investigate th e coordination of Br- in methanol. The analysis of this system is part icularly interesting due to the presence of three well separated coord ination shells. We show that the inclusion of the hydrogen signal is e ssential to perform a reliable analysis. A comparison of the analysis with asymmetric and Gaussian shells shows how the accuracy of the EXAF S data analysis is improved by using asymmetric shells. (C) 1996 Ameri can Institute of Physics.