Toward time-resolved laser T-jump/X-ray probe spectroscopy in aqueous solutions

Abstract

Most chemical and biochemical reactions in nature and in industrial processes are driven by thermal effects that bring the reactants above the energy barrier for reaction. In aqueous solutions, this process can also be triggered by the laser driven temperature jump (T-jump) method, in which the water vibrational (stretch, bend, or combination) modes are excited by a short laser pulse, leading to a temperature increase in the irradiated volume within a few picoseconds. The combination of the laser T-jump with X-ray spectroscopic probes would add element-specificity as well as sensitivity to the structure, the oxidation state, and the spin state of the intermediates of reactions. Here, we present preliminary results of a near infrared pump/X-ray absorption spectroscopy probe to study the ligand exchange of an octahedral aqueous Cobalt complex, which is known to pass through intermediate steps yielding tetrahedral chlorinated as final species. The structural changes of the chemical reaction are monitored with great sensitivity, even in the presence of a mild local increase in temperature. This work opens perspectives for the study of non-light-driven reactions using time-resolved X-ray spectroscopic methods.

FIG. 1.

(a) Schematic layout of the investigated chemical reaction. The equilibrium can be tuned with changes in temperature or chloride concentration, causing free chloride ions to gradually replace water molecules. In parallel, structural interconversion occurs from the hexa-octahedral to chloro-tetrahedral configuration. (b) Experimental layout of the laser T-jump/X-ray absorption spectroscopy probe. The sample jet schematically represents the closed loop wire-guided liquid jet employed in the experiment.

FIG. 2.

UV-visible spectra for a [Co²⁺] = 1 M solution. (a) Spectra at two different temperatures, T = 20 °C and T = 55 °C, [Cl⁻] = 4 M (dashed curves) and [Cl⁻] = 8 M (solid curves). (b) The effects of the temperature in the range of 30–60 °C for a [Cl⁻] = 7 M solution. The strong increase in the 600–750 nm bands is associated with the tetrahedral product formation, while the redshift of the 500 nm band is related to the formation of octahedral chlorinated intermediates.

FIG. 3.

Static UV-visible reference spectra of the [Co²⁺] = 500 mM, [Cl⁻] = 8 M water solution (blue trace), compared to pure water (green trace) and the pure Cobalt contribution (red trace, taken from the difference between the blue and the green traces). A common offset was subtracted from both the spectra, corresponding to the intensity of the pure water solution at 860 nm, where, according to Ref. , the absorbance of the water is almost negligible.

FIG. 4.

Laser T-jump/XAS probe study on a [Co²⁺] = 500 mM, [Cl⁻] = 8 M water solution. (a) Static XAS spectra were taken without the laser at 80 °C (green) and 60 °C (blue). The difference between the two static XAS spectra at T = 80 °C and T = 60 °C is displayed by the black trace (scaled by ×8) and overlaid with the pump-probe data (scaled by ×100) obtained at 7 ns by the laser-induced T-jump (red dots). The static spectra were scaled with a common factor (maximum of the T = 60 °C curve), keeping the presence of the isosbestic points. (b) Reference static X-ray absorption spectra at the Co K-edge. Blue: [Co(H₂O)₆²⁺] = 250 mM water solution at room temperature. Green: [CoCl₄][N(CH₃)₄]_2(s) pellet. Their static difference is displayed by the orange solid curve. The spectra were scaled in order to have the same integrated area below the curves. In this way, the presence of the isosbestic points is retrieved.

FIG. 5.

Assignment of spectral features to reaction intermediates. All traces were digitized as is from Ref. . In this panel, (a) NaCl = 0 M, 250 °C; (b) NaCl = 0.5 M, 250 °C; (c) NaCl = 1.5 M, 250 °C. The red solid line corresponds to (b-a) (scaled ×4), and it is ascribed to the octahedral compound mixture formation. The green solid line corresponds to (c-a), and it is ascribed to the tetrahedral compound mixture formation at the expenses of the reagent.

FIG. 6.

Schematic of the energy landscape as a function of the reaction coordinate for the investigated chemical equilibrium: changes of the energy distribution of the complexes as a function of temperature (left) and related conversion of the reagent into intermediates and products (right).