Polarized XANES of Co(III)(NH3)6 molecular crystals

Abstract

Polarized single crystal x-ray absorption spectra have been measured for Co(NH,) (ClO,),Cl:KCl. These spectra show pronounced orientation dependence fior several features on the high energy side of the Co K absorption edge. The Co in this salt has a tri onally distorted octahedral structure with N-Co-N angles of 89.5O and 90.5 8 , hence first shell scatterers cannot account for the observed orientation dependence. As we show below, the orientation dependent features can be interpreted in terms of a singlescattering EXAFS model involvin hydrogen-bonded to the Co(NH,), & scattering from a perchlorate which is ion. Co(NH,),(ClO.,),Cl:KCl was prepared by the method of Wilson and Solomon [l] and smgle crystals were oriented by standard diffraction methods. In initial experiments, the Co(NH3)03+ molecules were oriented so that both the propagation (k) and polarization (e) vectors were contained in one of the CoN4 planes. The crystal was rotated about e *k and XANES spectra were measured every 15 degrees for a total of thirteen different orientations of e. In a second experiment, the polarization vector was oriented either parallel or perpendicular to the crystallographic 3-fold axis. In order to compare different orientations it was necessary to develop a procedure for the precise, reproducible normalization of experimental XANES spectra. In brief, this method 121 involves subtraction of a low order polynomial from the data followed by multiplication by a scale factor. The polynomial and scale factor are adjusted to maximize the agreement, both below and above the edge, with tabulated x-ray absorption cross section.