Solvent Contribution Research Articles

On spherical vapor bubble collapse in viscoelastic fluids

Cavitating vapor bubbles, occur in variety of engineering applications, employing complex fluids as operating medium. Especially in biomechanics, biomedical applications as well as in polymer processing, fluids exhibit viscoelastic properties and fundamentally different behavior than Newtonian fluids. To explain the influence of viscoelasticity on cavitation a detailed understanding of the viscoelastic influence on bubble dynamics and of the underlying mechanisms is essential. With this study we provide in-depth numerical investigations of the spherical vapor bubble collapse in viscoelastic fluids. A compressible, density-based, 3D finite-volume solver with explicit time integration is used together with a conservative, compressible formulation for the constitutive equations of three different viscoelastic models, namely the upper convected Maxwell model, the Oldroyd-B model and the simplified linear Phan-Thien Tanner model. 3D simulations of the collapse dynamics are carried out to show the viscous and viscoelastic stress development during the collapse, and its relation to the occurring deformations. Collapse behavior is investigated for various elasticity, viscosity and constitutive models. It is demonstrated that viscoelasticity fundamentally alters the collapse behavior and evolution of stresses. It is observed that viscoelastic stresses develop with a time delay proportional to elasticity and show different spatial distributions as opposed to Newtonian stresses. Viscoelasticity introduces isotropic stress components even though the spherical collapse leads to purely deviatoric (elongational) deformation. Furthermore, the distinct influence of constitutive models is illustrated and the influence of viscoelastic models with solvent contribution is explained. For the upper convected Maxwell model, we show that for increased elasticity shock wave emission can be observed depending on the applied grid resolution.

Synthesis and characterization of water-borne vanillin derived copolymers containing dynamic imine cross-links

In the effort toward the production of circular plastics, the use of lignin-derived aromatic building blocks such as vanillin is interesting due to their functionality. Nonetheless, the water-borne emulsion polymerization of acrylic monomers derived from vanillin is still lacking. Vanillin can be readily functionalized with a methacrylate group for the radical polymerization to yield polymers containing functional aldehyde side groups. The functionality can be exploited via imine formation to produce vitrimers, a class of cross-linked recyclable polymers. Besides recycling, synthesis via a water-borne emulsion polymerization is desired to reduce the negative contribution of solvents on the environment and health. In the present work, polymethacrylates based on 2-(methacryloyloxy)ethyl vanillin and 2-octyl acrylate were synthesized via a emulsion polymerization approach to produce a water-borne latex. A latex with a small particle size (49 nm) and solid content of 31 wt% was obtained. For comparative purposes, a similar polymer was synthesized via solution polymerization. Both polymers were cross-linked using a multifunctional amine to produce recyclable imine vitrimers. This research investigates the unique combination of water-borne synthesis and vitrimer polymers, and therefore contributes to future development of vitrimer latex products such as coatings and films.

Solvation Effects on Polarizability of Aromatic Fluids.

Elucidating solvation effects on polarizability in condensed phases is important for the description of the optical and dielectric behavior of high-refractive-index molecular materials. We study these effects via the polarizability model combining electronic, solvation, and vibrational contributions. The method is applied to well-characterized highly polarizable liquid precursors: benzene, naphthalene, and phenanthrene. We find that the solvation and vibrational terms are of opposite signs and cancel almost exactly for benzene, but for naphthalene and phenanthrene, a 2.5 and 5.0% decrease relative to the equilibrium electronic polarizability of the respective monomer, α1e, is predicted, respectively. The increase in electronic polarizability affects interaction polarizability of all contacts, which is the main reason for the increasing importance of solvation contribution. The calculated refractive indices agree very well with experiment for all three systems.

Environmental impact analysis of surface printing and 3D inkjet printing applications using an imine based covalent organic framework: A life cycle assessment study

Covalent organic frameworks (COFs) are emerging materials with structural modularity that allows their application in many fields. The aim of this work is to determine the environmental impact of using an imine based covalent organic framework (RT-COF-1) for both surface printing (Case A) and 3D inkjet printing (Case B) by applying Life Cycle Assessment (LCA) methodology. Experimental data on RT-COF-1 synthesis as well as results obtained by simulation of their precursors production, 1,3,5-tris-(4-aminophenyl) benzene (TAPB) and 1,3,5-benzenetricarbaldehyde (BTCA), are used. LCA results show that monomer synthesis is the most important contributor to environmental impacts in both case studies. On the other hand, the contribution of solvents used in Case A is also remarkable. The comparison between both case studies indicates that the environmental impacts of Case B is lower than that of Case A (reduction within 5%–65%). Finally, LCA results of Case B are compared to other materials used for 3D-printing, such as polymerizable ionic liquids (PILs). The results show that RT–COF–1 compares favourably with PILs in five of nine impact categories, being especially relevant the reductions achieved in the abiotic depletion and acidification potential (>90%), in the primary energy consumption (⁓35%) and carbon footprint (⁓50%), suggesting the potential of RT–COF–1 as 3D-printing material from an environmental perspective. This work is a first step for further research to highlight the main environmental burdens of using COF-based materials in this application.

CrystalClear: an open, modular protocol for predicting molecular crystal growth from solution.

We present a new protocol for the prediction of free energies that determine the growth of sites in molecular crystals for subsequent use in Monte Carlo simulations using tools such as CrystalGrower [Hill et al., Chemical Science, 2021, 12, 1126-1146]. Key features of the proposed approach are that it requires minimal input, namely the crystal structure and solvent only, and provides automated, rapid generation of the interaction energies. The constituent components of this protocol, namely interactions between molecules (growth units) in the crystal, solvation contributions and treatment of long-range interactions are described in detail. The power of this method is shown via prediction of crystal shapes for ibuprofen grown from ethanol, ethyl acetate, toluene and acetonitrile, adipic acid grown from water, and five polymorphs (ON, OP, Y, YT04 and R) of ROY (5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile), with promising results. The predicted energies may be used directly or subsequently refined against experimental data, facilitating insight into the interactions governing crystal growth, while also providing a prediction of the solubility of the material. The protocol has been implemented in standalone, open-source software made available alongside this publication.

Exploring the role of preferential solvation in the stability of globular proteins through the study of ovalbumin interaction with organic additives

<abstract> <p>The impact of denaturing and stabilizing osmolytes on protein conformational dynamics has been extensively explored due to the significant contribution of protein solvation to the stability, function, malfunction and regulation of globular proteins. We studied the effect of two nonspecific organic molecules, urea, which is a conventional denaturant, and dimethyl sulfoxide (DMSO), which is a multilateral organic solvent, on the stability and conformational dynamics of a non-inhibitory serpin, ovalbumin (OVA). A differential scanning microcalorimetry (DSC) experimental series conducted in the phosphate buffer solutions containing 0–30% of additives revealed the destabilizing impact of both urea and DMSO in a mild acidic media, manifested in the gradual decrease of thermal unfolding enthalpy and transition temperature. These findings differ from the results observed in our study of the mild alkaline DMSO buffered solutions of OVA, where the moderate stabilization of OVA was observed in presence of 5–10% of DMSO. However, the overall OVA interaction patterns with urea and DMSO are consistent with our previous findings on the stability and conformational flexibility of another model globular protein, α-chymotrypsin, in similar medium conditions. The obtained results could be explained by preferential solvation patterns. Positive preferential solvation of protein by urea in urea/water mixtures mainly weakens the hydrophobic interactions of the protein globule and eventually leads to the disruption of the tertiary structure within the whole range of urea concentrations. Alternatively, under certain experimental conditions in DMSO/water mixtures, positive preferential solvation by water molecules can be observed. We assume that the switch to the positive preferential solvation by DMSO, which is shown to have a soft maximum around 20–30% DMSO, could be shifted towards lower additive concentrations due to the intrinsic capability of ovalbumin OVA to convert into a heat-stable, yet flexible set of conformations that have increased the surface hydrophobicity, characteristic to molten-globule-like states.</p> </abstract>

Hydration makes a difference! How to tune protein complexes between liquid-liquid and liquid-solid phase separation.

Understanding how protein rich condensates formed upon liquid-liquid phase separation (LLPS) evolve into solid aggregates is of fundamental importance for several medical applications, since these are suspected to be hot-spots for many neurotoxic diseases. This requires developing experimental approaches to observe in real-time both LLPS and liquid-solid phase separation (LSPS), and to unravel the delicate balance of protein and water interactions dictating the free energy differences between the two. We present a vibrational THz spectroscopy approach that allows doing so from the point of view of hydration water. We focus on a cellular prion protein of high medical relevance, which we can drive to undergo either LLPS or LSPS with few mutations. We find that it is a subtle balance of hydrophobic and hydrophilic solvation contributions that allows tuning between LLPS and LSPS. Hydrophobic hydration provides an entropic driving force to phase separation, through the release of hydration water into the bulk. Water hydrating hydrophilic groups provides an enthalpic driving force to keep the condensates in a liquid state. As a result, when we modify the protein by a few mutations to be less hydrophilic, we shift from LLPS to LSPS. This molecular understanding paves the way for a rational design of proteins.

Performance of global retention models in the optimisation of the chromatographic separation (I): Simple multi-analyte samples

Conventional retention models lead to accurate descriptions of the elution behaviour from the fitting of data for single solutes or from a set of solutes, one by one. However, the simultaneous fitting of several solutes through a regression process that separates the contributions of column and solvent from those of each solute is also possible. The result is a global retention model constituted by a set of equations with some common parameters (those associated with column and solvent), whereas others, specific to each solute, differ for each equation. This work explores the possibilities, advantages, and limitations of global models when they are applied to the optimisation of chromatographic resolution. A set constituted by 13 drugs (diuretics and β-blockers) and a training experimental design of seven multi-linear gradients are considered. Since standards for all compounds were available, the optimisation based on global models could be compared with the conventional optimisation, which is based on individual models. In their current state, global models do not predict changes in elution order, but they do allow for incorporating additional solutes (e.g., new analytes or matrix peaks) with only one new experiment. This possibility is explored by extending the model for the 13 analytes to include 26 peaks associated with a contamination in the injector. The combination of individual and global models allows an optimisation where the effects of matrix peaks on the separation of analytes can be integrated.

Critical Assessment of Data-Driven versus Heuristic Reaction Coordinates in Solution Chemistry.

Reaction coordinates are an essential ingredient of theoretical studies of rare events in chemistry and physics because they carry information about reaction mechanism and allow the computation of free-energy landscapes and kinetic rates. We present a critical assessment of the merits and disadvantages of heuristic reaction coordinates, largely employed today, with respect to coordinates optimized on the basis of reliable transition-path sampling data. We take as a test bed multinanosecond ab initio molecular dynamics simulations of chloride SN2 substitution on methyl chloride in explicit water. The computational protocol we devise allows the unsupervised optimization of agnostic coordinates able to account for solute and solvent contributions, yielding a free-energy reconstruction of quality comparable to the best heuristic coordinates without requiring chemical intuition.

Solvent Effects Enable Efficient Tandem Conversion of Cellulose and Its Monosaccharides Towards 5-Hydroxymethylfurfural.

The biomass-derived platform compound 5-hydroxymethylfurfural (HMF) has been hailed as the "Sleeping Giant" due to its promising applications, and it occupies a critical spot in the biomass upgrading roadmap. HMF is typically produced from cellulose and its monosaccharides via a complex tandem conversion with multiple steps (i. e., cellulose depolymerization, glucose isomerization, fructose dehydration, etc.). Previous investigations have confirmed the irreplaceable contribution of solvents in regulating the tandem conversion of cellulose and its monosaccharides to HMF. However, the potential effects of solvents in contributing to this multi-step tandem process have not yet been clearly elucidated. In this context, this Review aims to provide in-depth insights into the intrinsic interactions between solvent system and substrate conversion (cellulose and its monosaccharides conversion), reaction regulation (reaction activity and selectivity regulation), as well as product acquisition (humins formation inhibition and product purification). It attempts to elucidate specific solvent effects to promote a more efficient tandem conversion of cellulose and its monosaccharides towards HMF. The insights provided in this Review may contribute to a more sustainable HMF production from biomass feedstocks and a further development of greener solvent systems.

A Semi-Analytical Method for Channel and Pipe Flows for the Linear Phan-Thien-Tanner Fluid Model with a Solvent Contribution

This work presents a semi-analytical method for laminar steady-state channel and pipe flows of viscoelastic fluids using the Linear Phan-Thien-Tanner (LPTT) constitutive equation, with solvent viscosity contribution. For the semi-analytical method validation, it compares its results and two analytical solutions: the Oldroyd-B model and the simplified LPTT model (without solvent viscosity contribution). The results adopted different values of the dimensionless parameters, showing their influence on the viscoelastic fluid flow. The results include the distribution of the streamwise velocity component and the extra-stress tensor components in the wall-normal direction. In order to investigate the proposed semi-analytical method, different solutions were obtained, both for channel and pipe flows, considering different values of Reynolds number, solvent viscosity contribution in the homogeneous mixture, elongational parameter, shear parameter, and Weissenberg number. The results show that the proposed semi-analytical method can find a laminar solution using the non-Newtonian LPTT model with solvent viscosity contribution and verify the effect of the parameters in the resulting flow field.

Free energy reconstruction/decomposition from WHAM, force integration and free energy perturbation for an umbrella sampling simulation

Comparisons between methodologies are usually made by performing separated sampling simulations in conjunction with the corresponding estimator. Here, the accuracy and convergence of three typical approaches (WHAM, force integration, and FEP) for estimating the ion-pairing free energy in water are carefully examined in order to identify which estimator is the most efficient for a particular set of trajectories. The force integration is superior among these methods, whereas the FEP introduces relatively prominent uncertainty arising from the perturbed potential energy. The direct solute-solvent interaction term and the indirect solvent contribution accounting for the reorientation of non-spherically symmetric ions could be straightforwardly extracted using force integration and FEP.

The Role of Surface Chemistry in the Orientational Behavior of Water at an Interface.

Molecular dynamics studies have demonstrated that molecular water at an interface, with either a gas or a solid, displays anisotropic orientational behavior in contrast to its bulk counterpart. This effect has been recently implicated in the like-charge attraction problem for colloidal particles in solution. Here, negatively charged particles in solution display a long-ranged attraction where continuum electrostatic theory predicts monotonically repulsive interactions, particularly in solutions with monovalent salt ions at low ionic strength. Anisotropic orientational behavior of solvent molecules at an interface gives rise to an excess interfacial electrical potential which we suggest generates an additional solvation contribution to the total free energy that is traditionally overlooked in continuum descriptions of interparticle interactions in solution. In the present investigation we perform molecular dynamics simulation based calculations of the interfacial potential using realistic surface models representing various chemistries as well as different solvents. Similar to previous work that focused on simple model surfaces constructed by using oxygen atoms, we find that solvents at more realistic model surfaces exhibit substantial anisotropic orientational behavior. We explore the dependence of the interfacial solvation potential on surface properties such as surface group chemistry and group density at silica and carboxylated polystyrene interfaces. For water, we note surprisingly good agreement between results obtained for a simple O-atom wall and more complex surface models, suggesting a general qualitative consistency of the interfacial solvation effect for surfaces in contact with water. In contrast, for an aprotic solvent such as DMSO, surface chemistry appears to exert a stronger influence on the sign and magnitude of the interfacial solvation potential. The study carries broad implications for molecular-scale interactions and may find relevance in explaining a range of phenomena in soft-matter physics and cell biology.

Complementarity of mDSC, DMA, and DRS Techniques in the Study of Tg and Sub-Tg Transitions in Amorphous Solids: PVPVA, Indomethacin, and Amorphous Solid Dispersions Based on Indomethacin/PVPVA.

Recently, glasses, a subset of amorphous solids, have gained attention in various fields, such as polymer chemistry, optical fibers, and pharmaceuticals. One of their characteristic features, the glass transition temperature (Tg) which is absent in 100% crystalline materials, influences several material properties, such as free volume, enthalpy, viscosity, thermodynamic transitions, molecular motions, physical stability, mechanical properties, etc. In addition to Tg, there may be several other temperature-dependent transitions known as sub-Tg transitions (or β-, γ-, and δ-relaxations) which are identified by specific analytical techniques. The study of Tg and sub-Tg transitions occurring in amorphous solids has gained much attention because of its importance in understanding molecular kinetics, and it requires the combination of conventional and novel characterization techniques. In the present study, three different analytical techniques [modulated differential scanning calorimetry (mDSC), dynamic mechanical analysis (DMA), and dielectric relaxation spectroscopy (DRS)] were used to perform comprehensive qualitative/quantitative characterization of molecular relaxations, miscibility, and molecular interactions present in an amorphous polymer (PVPVA), a model drug (indomethacin, IND), and IND/PVPVA-based amorphous solid dispersions (ASDs). This is the first ever reported DMA study on PVPVA in its powder form, which avoids the contribution of solvent to the mechanical properties when a self-standing polymer film is used. A good correlation between the techniques in determining the Tg value of PVPVA, IND, and IND/PVPVA-based ASDs is established, and the negligible difference (within 10 °C) is attributed to the different material properties assessed in each technique. However, the overall Tg behavior, the decrease in Tg with increase in drug loading in ASDs, is universally observed in all the above-mentioned techniques, which reveals their complementarity. DMA and DRS techniques are used to study the different sub-Tg transitions present in PVPVA, amorphous IND, and IND/PVPVA-based ASDs because these transitions are normally too weak or too broad for mDSC to detect. For IND/PVPVA-based ASDs, both techniques show a shift of sub-Tg transitions (or secondary relaxation peaks) toward the high-temperature region from -140 to -45 °C. Thus, this paper outlines the usage of different solid-state characterization techniques in understanding the different molecular dynamics present in the polymer, drug, and their interactions in ASDs with the integrated information obtained from individual techniques.

Gel polymer dominated ion charging mechanisms within graphene nanochannels

Revealing microscopic real-time ion transport behavior is critically-important for understanding the charging mechanisms of polymer electrolytes. Herein polymer dominated ion charging mechanism of LiCl/polyvinyl alcohol (PVA) gel polymer electrolytes within graphene nanochannels is observed by combined electrochemical quartz crystal microbalance (EQCM) and molecule dynamics (MD) simulations. This is substantially different with the conventional aqueous electrolytes, demonstrating the dominated role of PVA polymer (including polymer-ion and polymer-electrode interactions) in mediating the charging mechanisms. Specifically, in-situ EQCM measurement reveals that –OH polar groups of PVA exhibit more significant hindrance to cations than anions, yielding the charging mechanisms from Li+ adsorption/desorption (at 0 wt % PVA) to Li+/Cl− ion exchange dominated transport (at 40 wt % PVA). Furthermore, ionic flux is quantitatively characterized based on the non-equilibrium thermodynamics, and the charging resistance is revealed via combining constant potential method and equivalent electric circuits. More importantly, increasing PVA concentration remarkably affects the thermodynamic properties (e.g., dielectric property, angle distribution and number density), while almost identical capacitance is recognized because of higher dielectric constant of PVA molecules. By solving Poisson equation, this is further interpreted and invalidated by quantifying the contributions of ions, solvents and PVA molecules to the electric potentials.

Note on the physical basis of spatially resolved thermodynamic functions

ABSTRACT The spatial resolution of extensive thermodynamic functions is discussed. A physical definition of the spatial resolution based on a spatial analogy of partial molar quantities is advocated, which is shown to be consistent with how hydration energies are typically spatially resolved in the molecular simulation literature. It is then shown that, provided the solvent is not at a phase transition, the spatially resolved entropy function calculated by first-order grid inhomogeneous solvation theory [Nguyen CN, Young TK, Gilson MK. Grid inhomogeneous solvation theory: hydration structure and thermodynamics of the miniature receptor cucurbit [7] uril. J Chem Phys. 2012;137(4):044101] satisfies the definition rigorously, whereas that calculated by grid cell theory [Gerogiokas G, Calabro G, Henchman RH, et al. Prediction of small molecule hydration thermodynamics with grid cell theory. J Chem Theory Comput. 2014;10(1):35–48] most likely does not. Moreover, for an ideal gas in an external field, the former theory is shown consistent whereas the latter is not. Finally, consistent with the proposed definition and with the case of an ideal gas in an external field, we derive an approximate expression for the solvent contribution to the free energy of solvation in the limit of infinite dilution from the spatial variation of the density around the solute.

NMR‐Based Cross‐Link Densities in EPDM and EPDM/ULDPE Blend Materials and Correlation with Mechanical Properties

AbstractThe role of cross‐linking in dictating the microstructural and mechanical properties in ethylene‐propylene‐diene‐monomer rubber (EPDM) and EPDM/ULDPE blends cross‐linked by different sulfur amounts is investigated by solid‐state1H time‐domain NMR spectroscopy and tensile‐tests. Analyses of spin‐spin relaxation time (T2), by combining free‐induction decay (FID), magic‐sandwich echo‐FID, and Hahn‐echo experiments demonstrate a reduction in crystal‐amorphous interface regions of pure ultralow‐density polyethylene (ULDPE) upon curative addition. The blends demonstrate a complete loss of these fractions due to curative‐induced plasticization and solvation by polyethylene segments of EPDM. Cross‐link densities, quantified by the magnitude of residual dipolar coupling constant (Dres), arising from topological restrictions to segmental motions, are measured by multiple‐quantum experiments. The entanglement‐dominated EPDMs demonstrate a significant reduction in ultimate tensile properties with increasingDres. The analogous blends yield similarDresvalues up to 0.36 phr of free sulfur. Thereafter, a deviation from the cross‐linking trend of the EPDMs is observed with the blends approaching a cross‐linking limit, thus emphasizing the migration of additives to the amorphous phase of the ULDPE. From the additional contributions of solvation and complex entanglement scenarios in the blends, restoration and even significant enhancement in ultimate tensile strength are achieved. Limitations in applying the popular Mooney–Rivlin analysis are also briefly discussed.

Computational Investigation of Structural Basis for Enhanced Binding of Isoflavone Analogues with Mitochondrial Aldehyde Dehydrogenase.

Isoflavone compounds are potent inhibitors against mitochondrial aldehyde dehydrogenase (ALDH2) for the treatment of alcoholism and drug addiction, and an in-depth understanding of the underlying structural basis helps design new inhibitors for enhanced binding. Here, we investigated the binding poses and strengths of eight isoflavone analogues (including CVT-10216 and daidzin) with ALDH2 via computational methods of molecular docking, molecular dynamics (MD) simulation, molecular mechanics Poisson–Boltzmann surface area (MM-PBSA), steered MD, and umbrella sampling. Neither the Vina scoring of docked and MD-sampled complexes nor the nonbonded protein–inhibitor interaction energy from MD simulations is able to reproduce the relative binding strength of the inhibitors compared to experimental IC50 values. Considering the solvation contribution, MM-PBSA and relatively expensive umbrella sampling yield good performance for the relative binding (free) energies. The isoflavone skeleton prefers to form π–π stacking, π–sulfur, and π–alkyl interactions with planar (Phe and Trp) or sulfur-containing (Cys and Met) residues. The enhanced inhibition of CVT-10216 originates from both end groups of the isoflavone skeleton offering strong van der Waals contacts and from the methylsulfonamide group at the 4′ position by hydrogen bonding (HB) with neighboring receptor residues. These results indicate that the hydrophobic binding tunnel of ALDH2 is larger than the isoflavone skeleton in length and thus an extended hydrophobic core is likely a premise for potent inhibitors.

Ozone episodes during and after the 2018 Chinese National Day holidays in Guangzhou: Implications for the control of precursor VOCs

The impact of reducing industrial emissions of volatile organic compounds (VOCs) on ozone (O3) pollution is of wide concern particularly in highly industrialized megacities. In this study, O3, nitrogen oxides (NOx) and VOCs were measured at an urban site in the Pearl River Delta region during the 2018 Chinese National Day Holidays and two after-holiday periods (one with ozone pollution and another without). O3 pollution occurred throughout the 7-day holidays even industrial emissions of VOCs were passively reduced due to temporary factory shutdowns, and the toluene to benzene ratios dropped from ∼10 during non-holidays to ∼5 during the holidays. Box model (AtChem2-MCM) simulations with the input of observation data revealed that O3 formation was all VOC-limited, and alkenes had the highest relative incremental reactivity (RIR) during the holiday and non-holiday O3 episodes while aromatics had the highest RIR during the non-pollution period. Box model also demonstrated that even aromatics decreased proportionally to levels with near-zero contributions of industrial aromatic solvents, O3 concentrations would only decrease by less than 20% during the holiday and non-holiday O3 episodes and ozone pollution in the periods could not be eliminated. The results imply that controlling emissions of industrial aromatic solvents might be not enough to eliminate O3 pollution in the region, and more attention should be paid to anthropogenic reactive alkenes. Isoprene and formaldehyde were among the top 3 species by RIRs in all the three pollution and non-pollution periods, suggesting substantial contribution to O3 formation from biogenic VOCs.

Visualizing Chemical Reactions with X-rays

The recombination of laser-dissociated iodine molecules dissolved in CCl4 is explored by time-resolved x-ray diffraction. The x-raypulses employed in our experiments were generated by the ESRF synchrotron in Grenoble. The solvent contribution to the measured signals was eliminated using appropriate experimental procedures. Motions of iodine atoms were then studied from 200 ps to 10 ps. Different relaxation processes are shown to operate in this time domain. It is proved that the iodine recombination follows two reaction paths, taking place in the electronic states X and A/A' of I2, respectively. In spite of widely different experimental approaches, laser optical and x-ray studies provide a similar picture of this prototype reaction.