The amplitude reduction factor and the cumulant expansion method: crucial factors in the structural analysis of alkoxide precursors in solution.

The transition-metal alkoxide yttrium 2-methoxyethoxide Y(OEtOMe)(3) in solution is studied as a model system of the large class of alkoxide precursors used in the sol-gel process by means of EXAFS spectroscopy. The discussion is focused on the amplitude reduction factor S (2)(0) and the cumulant expansion method. If asymmetry is present in the radial distribution function, the determination of the correct structural model can only be achieved by balancing multiple Gaussian shell fits against only one shell fit with a third cumulant C3. A method to identify the best model, based on statistical parameters of the EXAFS fit, is proposed and checked with two well-known reference compounds, Y(5)O(O(i)Pr)(13) and Y(acac)(3).3H(2)O, and applied to the structurally unknown solution of Y(OEtOMe)(3) in 2-methoxyethanol. The two references are also used to discuss the transferability of S(2)(0) values, determined from reference compounds to unknown samples. A model-free procedure to identify the correct amplitude reduction factor S(2)(0) by making use of fits with different k-weighting schemes is critically investigated. This procedure, which does not require any crystallographic data, is used for the case of Y(OEtOMe)(3) in solution, where significant differences of the amplitude reducing factor of both the oxygen and yttrium shell in comparison to the reference Y(5)O(O(i)Pr)(13) were found. With such a detailed analysis of EXAFS data, a reliable characterization of Y(OEtOMe)3 in 2-methoxyethanol by means of EXAFS spectroscopy is possible. The decameric structure unit found in solid Y(OEtOMe)(3) is not preserved, but rather, a pentameric framework similar to that in Y5O(O(i)Pr)(13) is formed.