In Situ Structural Characterization of Ferric Iron Dimers in Aqueous Solutions: Identification of 渭-Oxo Species

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• 作者：Mengqiang Zhu ; Brendan W. Puls ; Cathrine Frandsen ; James D. Kubicki ; Hengzhong Zhang ; Glenn A. Waychunas
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：June 17, 2013
• 年：2013
• 卷：52
• 期：12
• 页码：6788-6797
• 全文大小：495K
• 年卷期：v.52,no.12(June 17, 2013)
• ISSN：1520-510X

文摘

The structure of ferric iron (Fe³⁺) dimers in aqueous solutions has long been debated. In this work, we have determined the dimer structure in situ in aqueous solutions using extended X-ray absorption fine structure (EXAFS) spectroscopy. An Fe K-edge EXAFS analysis of 0.2 M ferric nitrate solutions at pH 1.28鈥?.81 identified a Fe鈥揊e distance at 3.6 脜, strongly indicating that the dimers take the 渭-oxo form. The EXAFS analysis also indicates two short Fe鈥揙 bonds at 1.80 脜 and ten long Fe鈥揙 bonds at 2.08 脜, consistent with the 渭-oxo dimer structure. The scattering from the Fe鈥揊e paths interferes destructively with that from paths belonging to Fe(OH₂)₆³⁺ monomers that coexist with the dimers, leading to a less apparent Fe shell in the EXAFS Fourier transform. This might be a reason why the characteristic Fe鈥揊e distance was not detected in previous EXAFS studies. The existence of 渭-oxo dimers is further confirmed by M枚ssbauer analyses of analogous quick frozen solutions. This work also explores the electronic structure and the relative stability of the 渭-oxo dimer in a comparison to the dihydroxo dimer using density function theory (DFT) calculations. The identification of such dimers in aqueous solutions has important implications for iron (bio)inorganic chemistry and geochemistry, such as understanding the formation mechanisms of Fe oxyhydroxides at molecular scale.