Gnxas, A Multiple-scattering Approach To Exafs Analysis - Methodology and Applications To Iron Complexes

GNXAS, a recently developed integrated approach to the analysis of EXAFS data is presented in detail. GNXAS provides for the direct fitting of theoretical signals (calculated by utilizing the Hedin-Lundqvist complex exchange and correlation potential and spherical wave propagators) to the experimental data. GNXAS is able to calculate all the signals related to two-, three-, and four-atom correlation functions with the proper treatment of correlated distances and Debye-Waller factors. The technique is particularly well-suited for the analysis of multiple-scattering effects and thus allows for accurate determination of bond distance and angular information of second and third neighbors. Herein we report the application of GNXAS to several chemical systems of known structure. The reliability of GNXAS was evaluated on a well-ordered inorganic complex, Fe(acac)(3), as well as a lower-symmetry coordination complex with mixed ligation, Na[Fe(OH2)EDTA]. The total EXAFS signal generated by GNXAS matches closely the experimental data for both complexes, especially when all the multiple-scattering contributions were included in the theoretical signal. First neighbor distances obtained from refinement using GNXAS, as well as distances and angles for further neighbors, compared very well with crystallographic values. The angle dependence of the Fe-C-N multiple-scattering contribution in K3Fe(CN)(6) was also examined. The results indicate that GNXAS can be used to determine angles relatively accurately for Fe-C-N configurations with angles greater than about 150 degrees. These results establish the utility and reliability of the GNXAS approach and provide a reliable means to determine additional structural information from EXAFS analysis of structures of chemical interest.