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We report the generation of gold nanoparticles (AuNPs) from the aqueous solution of chloro(2,2',2″-terpyridine)gold(III) ion ([Au(tpy)Cl] 2+ ) through X-ray radiolysis and optical excitation at a synchrotron. The original purpose of the experiment was to investigate the photoinduced structural changes of [Au(tpy)Cl] 2+ upon 400 nm excitation using time-resolved X-ray liquidography (TRXL). Initially, the TRXL data did not show any signal that would suggest structural changes of the solute molecule, but after an induction time, the TRXL data started to show sharp peaks and valleys. In the early phase, AuNPs with two types of morphology, dendrites, and spheres, were formed by the reducing action of hydrated electrons generated by the X-ray radiolysis of water, thereby allowing the detection of TRXL data due to the laser-induced lattice expansion and relaxation of AuNPs. Along with the lattice expansion, the dendritic and spherical AuNPs were transformed into smaller, raspberry-shaped AuNPs of a relatively uniform size via ablation by the optical femtosecond laser pulse used for the TRXL experiment. Density functional theory calculations confirm that the reduction potential of the metal complex relative to the hydration potential of X-ray-generated electrons determines the facile AuNP formation observed for [Au(tpy)Cl] 2+ .

Ionic species often play important roles in chemical reactions occurring in water and other solvents, but it has been elusive to determine the solvent-dependent molecular structure with atomic resolution. The triiodide ion has a molecular structure that sensitively changes depending on the type of solvent and its symmetry can be broken by strong solute-solvent interaction. Here, by applying pump-probe x-ray solution scattering, we characterize the exact molecular structure of I(3)(-) ion in water, methanol, and acetonitrile with subangstrom accuracy. The data reveal that I(3)(-) ion has an asymmetric and bent structure in water. In contrast, the ion keeps its symmetry in acetonitrile, while the symmetry breaking occurs to a lesser extent in methanol than in water. The symmetry breaking of I(3)(-) ion is reproduced by density functional theory calculations using 34 explicit water molecules, confirming that the origin of the symmetry breaking is the hydrogen-bonding interaction between the solute and solvent molecules.

100 ps time-resolved X-ray solution-scattering capabilities have been developed using multilayer optics at the beamline NW14A, Photon Factory Advanced Ring, KEK. X-ray pulses with an energy bandwidth of DeltaE/E = 1-5% are generated by reflecting X-ray pulses (DeltaE/E = 15%) through multilayer optics, made of W/B(4)C or depth-graded Ru/C on silicon substrate. This tailor-made wide-bandwidth X-ray pulse provides high-quality solution-scattering data for obtaining photo-induced molecular reaction dynamics. The time-resolved solution scattering of CH(2)I(2) in methanol is demonstrated as a typical example.

Most chemical reactions occur in solution, and complex interactions between solute and solvent influence the rich chemistry of these processes. To track time-dependent processes in such reactions, researchers often use time-resolved spectroscopy. In these experiments, an optical pulse (pump) initiates a reaction, and another time-delayed optical pulse (probe) monitors the progress of the reaction. However, because of the wavelength range of the probe light used in these experiments, from infrared to ultraviolet, researchers cannot directly determine detailed structural information such as the bond lengths and bond angles of reaction intermediates. In addition, not all intermediates might be sensitive to the spectroscopic signal chosen for the experiment. This Account describes time-resolved X-ray liquidography (TRXL), a technique that overcomes these problems. In this technique, we replace the optical probe with the diffraction of hard X-ray pulses emitted from a synchrotron source. In TRXL, diffraction signals are sensitive to all chemical species simultaneously. In addition, each chemical species has a characteristic diffraction signal, a fingerprint, that we calculate from its three-dimensional atomic coordinates. Because, X-rays scatter from all atoms in the solution sample, including both the solute and the solvent, the analysis of TRXL data can track not only the reaction pathways of the solute molecules but also the solvent behavior and the solute-solvent arrangement, thus providing a global picture of the reactions. We have used TRXL to study structural dynamics and spatiotemporal kinetics of many molecular systems including diatomic molecules, haloalkanes, organometallic complexes, and protein molecules over timescales from picoseconds to milliseconds. We have observed that TRXL data adds to and, in some cases, contradicts results from time-resolved spectroscopy. For example, TRXL has shown that the reaction intermediates upon C-I bond dissociation in C(2)H(4)I(2) and C(2)F(4)I(2) have completely different structures and corresponding subsequent reaction pathways, underscoring the dramatic effect of the fluorine substitution. We have also used TRXL to identify a new reaction intermediate of the photolysis of Ru(3)(CO)(12) that has no bridging carbonyl groups. Though not detected by time-resolved infrared spectroscopy, this intermediate predominates based on the TRXL data. In looking at the quaternary conformational changes of hemoglobin, TRXL analysis suggests a faster transition than was suggested by optical spectroscopy. The time resolution of TRXL is currently limited by the X-ray pulse width available from synchrotron sources ( approximately 100 ps). The resolution should improve to 100 fs or better with X-ray free electron lasers. With this higher resolution, real time observation of ultrafast chemical events such as bond-breaking and bond-making will be possible.

We report direct structural evidence of the bridged radical (CH2ICH2.) in a polar solution, obtained using time-resolved liquid-phase x-ray diffraction. This transient intermediate has long been hypothesized to explain stereo-chemical control in many association and/or dissociation reactions involving haloalkanes. Ultrashort optical pulses were used to dissociate an iodine atom from the haloethane molecule (C2H4I2) dissolved in methanol, and the diffraction of picosecond x-ray pulses from a synchrotron supports the following structural dynamics, with approximately 0.01 angstrom spatial resolution and approximately 100 picosecond time resolution: The loss of one iodine atom from C2H4I2 leads to the C-I-C triangular geometry of CH2ICH2.. This transient C2H4I then binds to an iodine atom to form a new species, the C2H4I-I isomer, which eventually decays into C2H4 + I2. Solvent dynamics were also extracted from the data, revealing a change in the solvent cage geometry, heating, and thermal expansion.

The time-resolved diffraction signal from a laser-excited solution has three principal components: the solute-only term, the solute-solvent cross term, and the solvent-only term. The last term is very sensitive to the thermodynamic state of the bulk solvent, which may change during a chemical reaction due to energy transfer from light-absorbing solute molecules to the surrounding solvent molecules and the following relaxation to equilibrium with the environment around the scattering volume. The volume expansion coefficient α for a liquid is typically ∼1×10-3 K-1, which is about 1000 times greater than for a solid. Hence solvent scattering is a very sensitive on-line thermometer. The decomposition of the scattered x-ray signal has so far been aided by molecular dynamics (MD) simulations, a method capable of simulating the solvent response as well as the solute term and solute/solvent cross terms for the data analysis. Here we present an experimental procedure, applicable to most hydrogen containing solvents, that directly measures the solvent response to a transient temperature rise. The overtone modes of OH stretching and CH3 asymmetric stretching in liquid methanol were excited by near-infrared femtosecond laser pulses at 1.5 and 1.7 μm and the ensuing hydrodynamics, induced by the transfer of heat from a subset of excited CH3OH* to the bulk and the subsequent thermal expansion, were probed by 100 ps x-ray pulses from a synchrotron. The time-resolved data allowed us to extract two key differentials: the change in the solvent diffraction from a temperature change at constant density, seen at a very short time delay ∼100 ps, and a term from a change in density at constant temperature. The latter term becomes relevant at later times ∼1 μs when the bulk of liquid expands to accommodate its new temperature at ambient pressure. These two terms are the principal building blocks in the hydrodynamic equation of state, and they are needed in a self-consistent reconstruction of the solvent response during a chemical reaction. We compare the experimental solvent terms with those from MD simulations. The use of experimentally determined solvent differentials greatly improved the quality of global fits when applied to the time-resolved data for C2H4I2 dissolved in methanol. [ABSTRACT FROM AUTHOR]

A time-resolved x-ray-diffraction experiment is presented that aims to study the recombination of laser-dissociated iodine molecules dissolved in CCl4. This process is monitored over an extended time interval from pico- to microseconds. The variations of atom-atom distances are probed with a milliangstrom resolution. A recent theory of time-resolved x-ray diffraction is used to analyze the experimental data; it employs the correlation function approach of statistical mechanics. The most striking outcome of this study is the experimental determination of time-dependent I–I atom-atom distribution functions. The structure of the CCl4 solvent changes simultaneously; the solvent thus appears as a reaction partner rather than an inert medium hosting it. Thermal expansion of the system is nonuniform in time, an effect due to the presence of the acoustic horizon. One concludes that a time-resolved x-ray diffraction permits real-time visualization of solvent and solute motions during a chemical reaction. [ABSTRACT FROM AUTHOR]