Solvent-Dependent Structural Dynamics in the Ultrafast Photodissociation Reaction of Triiodide Observed with Time-Resolved X-ray Solution Scattering

Abstract

Resolving the structural dynamics of bond breaking, bond formation, and solvation is required for a deeper understanding of solution-phase chemical reactions. In this work, we investigate the photodissociation of triiodide in four solvents using femtosecond time-resolved X-ray solution scattering following 400 nm photoexcitation. Structural analysis of the scattering data resolves the solvent-dependent structural evolution during the bond cleavage, internal rearrangements, solvent-cage escape, and bond reformation in real time. The nature and structure of the reaction intermediates during the recombination are determined, elucidating the full mechanism of photodissociation and recombination on ultrafast time scales. We resolve the structure of the precursor state for recombination as a geminate pair. Further, we determine the size of the solvent cages from the refined structures of the radical pair. The observed structural dynamics present a comprehensive picture of the solvent influence on structure and dynamics of dissociation reactions.

Figure 1

(A) Schematic representation of the experimental setup. The XFEL pulse and laser pump pulse (400 nm, 70 fs fwhm) are overlapped at the sample position. Scattering patterns are recorded with the octal MPCCD detector. (B) The time-dependent difference scattering signals for the four solvents studied (top left to bottom right: acetonitrile, water, ethanol, and methanol) are presented. The data covers time delays from −0.1 to 500 ps and a q-range of 0.5 to 4.5 Å^–1.

Figure 2

Sketch of the triiodide molecule with all structural parameters used to describe the structure. In the structural refinement, , , and α were optimized.

Figure 3

Comparison of modeled and experimental data for I₃^– in acetonitrile. The top row shows data for isotropic (ΔS₀) and the bottom row, for anisotropic (ΔS₂). (A) Experimental difference scattering. (B) Modeled data. (C) Both model (black solid lines) and experimental data (red dots) for selected time points.

Figure 4

Optimized parameters from the structural refinement for I₃^– in acetonitrile. The two panels present the optimized structural parameters (top) and population dynamics (bottom). A linear scale is used for the first 1 ps to highlight the fastest dynamics; later time points are presented on a logarithmic scale. Amplitudes as defined in eq 5.

Figure 5

Time dependent amplitudes of the GP (crosses) and NG (filled dots) for the four solvents (from top to bottom: acetonitrile, water, ethanol, methanol). The GP amplitudes are modeled with a biexponential decay, presented as a solid line. The lifetimes of the GP in the different solvents are given in the plot.

Figure 6

Left: Evolution of the distance , illustrating the bond dissociation. From the initial increase, the speed of dissociation, v_d, is estimated (for ethanol, the data point at t = 0.4 ps was excluded from the speed estimate). Right: Radial distribution functions of the solvent O atoms (C for acetonitrile) around the I atoms in their ground state I₃^– structure. Solvents from top to bottom: acetonitrile, water, ethanol, and methanol.

Figure 7

Scheme of the reaction mechanism of the photodissocation and recombination of I₃^–.