Photochemistry of HgBr2 in methanol investigated using time-resolved X-ray liquidography

Abstract

We investigate the photoinduced dissociation of HgBr(2) in methanol and the ensuring structural dynamics of the photo-products over a time span from 100 ps to 1 μs after photolysis at 267 nm by using time-resolved X-ray liquidography (TRXL). By making use of the atomic-level structural sensitivity of X-ray scattering and the superb 100 ps time resolution of X-ray pulses from a 3rd-generation synchrotron, the structural dynamics of a chemical reaction in solution can be directly monitored. The measured time-dependent X-ray solution scattering signals, analyzed using global-fitting based on DFT calculations and MD simulations, show that photoexcited HgBr(2) dissociates via both two-body (HgBr + Br) and three-body (Hg + Br + Br) dissociation pathways with a ∼2 : 1 branching ratio. Following dissociation, the photoproducts recombine via three reactions involving Br species: (1) Hg + Br, (2) HgBr + Br, and (3) Br + Br. The associated rate constants and branching ratios are determined from the global-fitting analysis. Also, we examine the energy dissipation from reacting solute molecules and relaxation of excited molecules to solvent bath accompanying the temperature rise of 0.54 K. Compared to a previous TRXL study of the photodissociation of HgI(2), the results of this work suggest that the photodissociation pathway of HgBr(2) is different from that of HgI(2), which dissociates predominantly via two-body dissociation, at least to within the currently available time resolution of ∼100 ps. In addition, the error analysis of the fit parameters used in the global-fitting are discussed in detail with a comparison of various error estimation algorithms.

Global Reaction Pathways in the Photodissociation of I3− Ions in Solution at 267 and 400 nm Studied by Picosecond X‐ray Liquidography

“…Details of the fitting and error analysis are given in the Experimental Section and in our previous work. [13,14,18] The results of fitting analysis for the TRXL data obtained with 400 nm excitation are shown in Figure 6. The difference scattering curve at 100 ps is presented as an example because À photodissociation in methanol with 400 nm laser excitation in a) q-space and b) r-space.…”

“…[1][2][3][4][5][6] However, in the typical time-resolved spectroscopic techniques, the observables are related to opti-cal resonances of the molecules and cannot be correlated directly with the changes in atomic arrangement of the molecules. As an alternative, replacement of the optical probe by structural probes, such as X-ray absorption [7][8][9][10] or X-ray diffraction, [11][12][13][14][15][16][17][18][19][20][21][22] allows us to directly obtain the detailed structural information including the structure of short-lived solute species and solvent rearrangement around the solutes. Based on this idea, time-resolved X-ray liquidography (TRXL) or so-called time-resolved X-ray solution scattering has been applied to various solution-phase molecular systems ranging from small molecules [11][12][13][14][15][16][17][18][19] and proteins [20][21][22] to nanoparticles [23][24][25] and protein-nanoparticle complexes, [26] thereby revealing the reaction pathways and intermediates of the reactions.…”

“…Thus, TRXL is well suited for studying the entire pathways of a reaction. [11][12][13][14][15][16][17][18][19][20][21][22][39][40][41][42][43] In this work, we investigated the change of reaction mechanism of I 3 À photodissociation depending on the excitation wavelength by directly measuring the structures of transient solute species, the cage environment, and the solvent hydrodynamics by using TRXL.…”

Singular value decomposition as a tool for background corrections in time-resolved XFEL scattering data

“…The general theory and ideas underlying TR-WAXS investigations of structural dynamics in solution-state photochemistry has been developed over the past two decades and is described in detail elsewhere [12][13][14]. Briefly, in TR-WAXS experiments, the sample of interest is excited by a short laser pulse, and after a time delay Dt, a short X-ray pulse probes the structure by measuring the scattering intensity I as a function of scattering vector Q.…”

Model-independent particle species disentanglement by X-ray cross-correlation scattering

“…It is often of interest to extend the perspective from pure samples to mixtures of more than one particle species or, similarly, of more configurations of the same particle. In case of dynamical studies, for instance, it is implicit that a number of data sets are collected at different time points, corresponding to different mixture compositions45678910. The angular means of the X-ray intensities can then be processed using the singular value decomposition algorithm111213 (SVD), which serves as a noise filter and yields the number the particle species present in the system.…”