1. Optimal parameters for analysis of XES emission spectrum
Angelique Amado | Feifei Li | Department of Chemistry and Biochemistry, Las Cruces, NM
Figures 1 and 2 are only two examples of the optimized parameters for the emission spectrum of a Co-dmg ligand (Figure 1) and an acac ligand (figure 2). Figure1 parameters are two peak with a freely floated shape and figure 2 is a three peak 0.5 locked shape.
Table 1 categorizes varying complex parameters in the form of area sums as a method of emission magnitude comparison among orders of metalloenzymes.
The table is divided by number of peaks that correspond to either a locked shape at 0.5 or “freely floated” optimization with no restrictions on parameters.
Figures 3 and 4 illustrate a visual representation of the structure of both the acetylacetonate ligand (figure 3) and the reaction with a metal ion to dimethylglyoxime (figure 4)
It becomes necessary to organize the curve parameters of the XES emission data into categorizes based on the optimization parameters in order to accurately asses the differences in bond activation and/or orbital transition each ligand posses and/or undergoes.
Introduction
All curve fitting was done using the Fityk version software. The data consists of a normalized intensity versus energy (eV) plotted curves with a complete range of 7610 eV to 7730 eV. The “mainline” curve parameters range from 7610 eV to 7680 eV, while the VtC region has a domain of 7680 eV to 7730 eV. The function used to optimize curve parameters is Psuedovoigt.
Kβ X-ray emission spectroscopy (XES) is an emerging method to quantify and understand processes such as bond activation and protonation in order to better discern the mechanisms undergone in a reaction.
In past attempts to trace reactive intermediates, traditional spectroscopy has not proven fully capable to capture the authentic nature of these intermediates. Other methods such as vibrational and EPR techniques for spectroscopic analysis are able to quantitatively assess the nature of pronation and orbital transitions, but X-ray methods of spectroscopy exceed the conventional techniques due to the fact that it is not spin dependent.§
This particular focus being explored has the possibility to extract information about the roles of metal catalysts and metalloenzymes in a variety of settings, specifically, biological systems.§,²
XES data analysis is sensitive enough that it possesses the capability to describe properties of activation, particle movement, and geometry as a tool for interpreting the catalytic function of metal complexes attached to varying ligands.
Although the data collected corresponds to a series of cobalt complexes and assigned ligands, the methods are applicable to a variety of transition metals.³
X-ray emission spectroscopy acts as a direct probe into the orbital transitions undergone with reference to the primary metal complex. At a higher energy level the valence to core (VtC) region provides information about ligand identities.³
Results and Discussion
Conclusion
Figure 1- VtC region of JP1-Cl2
Figure 2- VtC region FL1- acac 2nd 4
XES techniques are sensitive enough to probe metal to ligand interactions.
The VtC region of the plot is much more apt to the XES method of spectral analysis.
A locked shape parameter at 0.5 poses as an optimal model for consistency and analysis of data. Additionally, increasing the number of peaks will provide more accurate data, but it is necessary to optimize the curve to have the fewest amount of peaks to achieve accurate parameters.
Analysis of area sum, peak intensity, energy range, and curve shape provide a plethora of information about the nature of compared ligands interacting with a metal complex, and give insight to the mechanisms behind the reaction processes.
Objective
References
The direction of this analysis is to explore the best fit parameters for the Kβ emission (XES) spectrum data obtained for various cobalt complexes, predominately, in the valence-to-core (VtC) region of the spectrum.
¹ Pollock, C. Development of Kβ X-ray emission spectroscopy as a probe of chemical and biological catalysis. Doctoral dissertation
² Pollock, C. J.; DeBeer, S. Valence-to-core X-ray emission spectroscopy: a sensitive probe of the nature of a bound ligand. Am. Chem. Soc
³ Pollock, C. J.; Grubel, K.; Holland, P. L.; Debeer, S. Experimentally Quantifying Small-Molecule Bond Activation Using Valence-to-Core X-ray Emission Spectroscopy. Am. Chem. Soc
§ Lassalle-kaiser, B.; Iii, T. T. B.; Krewald, V.; Kern, J.; Martha, A. Supporting Information : Experimental and Computational X-ray Emission Spectroscopy as a Direct Probe of Protonation States in Oxo-Bridged Mn IV -Dimers Relevant to Redox-active. Inorganic Chem
¨ Rees, J. A.; Martin-Diaconescu, V.; Kovacs, J. A.; DeBeer, S. X-ray Absorption and Emission Study of Dioxygen Activation by a Small-Molecule Manganese Complex. Am. Chem. Soc. 2015,
Methodology
Table 2- Area sums for Co-dmg, Co-amine, Co-acac, and “others”
The XES emission data was collected at the Cornell High Energy Synchrotron Source (CHESS). Raw data was examined for the possibility of radiation damage; averaged, then energy-converted from angle space to energy space, and normalized. The data was organized into three categories of ligands which include: dimethylglyoxime (dmg), amine & acetylacetonate (acac), and “others.” Calibration was done using Co foil in order to provide a reference of the emission plot for the metal complex.
Figure 4- example of dimethylglyoxime ligand reaction with a Ni metal ion.
Figure 3- example of acetylacetonate ligand attached to a Ni metal ion