Following a Chemical Reaction on the Millisecond Time Scale by Simultaneous X-ray and UV/Vis Spectroscopy

Abstract

An innovative approach aimed at disclosing the mechanism of chemical reactions occurring in solution on the millisecond time scale is presented. Time-resolved energy dispersive X-ray absorption and UV/vis spectroscopies with millisecond resolution are used simultaneously to directly follow the evolution of both the oxidation state and the local structure of the metal center in an iron complex. Two redox reactions are studied, the former involving the transformation of Fe into two subsequent Fe species and the latter involving the more complex Fe-Fe-Fe-Fe sequence. The structural modifications occurring around the iron center are correlated to the reaction mechanisms. This combined approach has the potential to provide unique insights into reaction mechanisms in the liquid phase and represents a new powerful tool to characterize short-lived intermediates that are silent to common spectroscopic techniques.

Coupled X-ray Absorption/UV–vis Monitoring of Fast Oxidation Reactions Involving a Nonheme Iron–Oxo Complex

“…3 Recently, we have shown that time-resolved EDXAS and UV−vis spectroscopy can be used in combination, to follow fast chemical processes (halftimes from milliseconds to seconds) by the direct observation of the intermediates succeeding one another during the reaction. 4 In particular, time-resolved XAS allowed us to monitor the evolution of the oxidation state of the iron metal center present in the prototypical nonheme complex Fe II (tris(2-pyridylmethyl)amine) ([TPA•Fe II ] 2+ in Figure 1) and of the first coordination sphere around the metal itself, when the complex was subjected to the action of oxidizing species like hydrogen peroxide or peroxyacetic acid (AcOO means of perpendicular detections carried out in the probe of a stopped-flow apparatus.…”

Free-Carbon Surface for PtCu Nanoparticles: An In Situ Near Ambient Pressure X-ray Photoelectron Spectroscopy Study

“…Upon three washing cycles, the main band at 242 nm shifts to lower wavelengths (234 nm). The band characteristic of the acetic acid aqueous solution also appears at 234 nm . This may indicate either that the PVP has been completely removed and exchanged by acetate species or that the remaining PVP cannot be further removed by this method.…”

Activation of C–H bonds by a nonheme iron(iv)–oxo complex: mechanistic evidence through a coupled EDXAS/UV-Vis multivariate analysis

“…Notably, EDXAS offers the possibility of gaining simultaneous information on the time evolution of the oxidation state of the given photoabsorber and of its molecular surroundings within 5Å with a good degree of accuracy. [14][15][16] We have recently applied this technique in combination with UV-Vis spectroscopy for the identification of the succession of oxidation states and for the measurement of pseudo-first-order kinetic constants in bimolecular reactions involving nonheme iron complexes. 14,15,17 Herein, we combine multivariate statistical and theoretical analyses of timeresolved coupled EDXAS/UV-Vis data to gain a comprehensive mechanistic picture on the activation of C-H bonds in the substrates 9,10-dihydroanthracene (DHA) and diphenylmethane (Ph 2 CH 2 ) by the nonheme Fe IV -oxo complex [N4Py•Fe IV (O)] 2+ .…”

“…[14][15][16] We have recently applied this technique in combination with UV-Vis spectroscopy for the identification of the succession of oxidation states and for the measurement of pseudo-first-order kinetic constants in bimolecular reactions involving nonheme iron complexes. 14,15,17 Herein, we combine multivariate statistical and theoretical analyses of timeresolved coupled EDXAS/UV-Vis data to gain a comprehensive mechanistic picture on the activation of C-H bonds in the substrates 9,10-dihydroanthracene (DHA) and diphenylmethane (Ph 2 CH 2 ) by the nonheme Fe IV -oxo complex [N4Py•Fe IV (O)] 2+ . In particular, quantitative mechanistic and structural information for all the reaction intermediates is derived from the application of principal component analysis (PCA) and a strategy belonging to the multivariate curve resolution (MCR) family.…”