Atomic-level Mechanisms Research Articles

Atomic‐Level Mechanisms of Nucleation of Pure Liquid Metals during Rapid Cooling

To obtain a material with the desired performance, the atomic-level mechanisms of nucleation from the liquid to solid phase must be understood. Although this transition has been investigated experimentally and theoretically, its atomic-level mechanisms remain debatable. In this work, the nucleation mechanisms of pure Fe under rapid cooling conditions are investigated. The local atomic packing stability and liquid-to-solid transition-energy pathways of Fe are studied using molecular dynamics simulations and first-principle calculations. The results are expressed as functions of cluster size in units of amorphous clusters (ACs) and body-centered cubic crystalline clusters (BCC-CCs). We found the prototypes of ACs in supercooled liquids and successfully divided these ACs to three categories according to their transition-energy pathways. The information obtained in this study could contribute to our current understanding of the crystallization of metallic melts during rapid cooling.

Dynamics of the F(-) + CH3I → HF + CH2I(-) Proton Transfer Reaction.

Direct chemical dynamics simulations, at collision energies Erel of 0.32 and 1.53 eV, were performed to obtain an atomistic understanding of the F(-) + CH3I reaction dynamics. There is only the F(-) + CH3I → CH3F + I(-) bimolecular nucleophilic substitution SN2 product channel at 0.32 eV. Increasing Erel to 1.53 eV opens the endothermic F(-) + CH3I → HF + CH2I(-) proton transfer reaction, which is less competitive than the SN2 reaction. The simulations reveal proton transfer occurs by two direct atomic-level mechanisms, rebound and stripping, and indirect mechanisms, involving formation of the F(-)···HCH2I complex and the roundabout. For the indirect trajectories all of the CH2I(-) is formed with zero-point energy (ZPE), while for the direct trajectories 50% form CH2I(-) without ZPE. Without a ZPE constraint for CH2I(-), the reaction cross sections for the rebound, stripping, and indirect mechanisms are 0.2 ± 0.1, 1.2 ± 0.4, and 0.7 ± 0.2 Å(2), respectively. Discarding trajectories that do not form CH2I(-) with ZPE reduces the rebound and stripping cross sections to 0.1 ± 0.1 and 0.7 ± 0.5 Å(2). The HF product is formed rotationally and vibrationally unexcited. The average value of J is 2.6 and with histogram binning n = 0. CH2I(-) is formed rotationally excited. The partitioning between CH2I(-) vibration and HF + CH2I(-) relative translation energy depends on the treatment of CH2I(-) ZPE. Without a CH2I(-) ZPE constraint the energy partitioning is primarily to relative translation with little CH2I(-) vibration. With a ZPE constraint, energy partitioning to CH2I(-) rotation, CH2I(-) vibration, and relative translation are statistically the same. The overall F(-) + CH3I rate constant at Erel of both 0.32 and 1.53 eV is in good agreement with experiment and negligibly affected by the treatment of CH2I(-) ZPE, since the SN2 reaction is the major contributor to the total reaction rate constant. The potential energy surface and reaction dynamics for F(-) + CH3I proton transfer are compared with those reported previously (J. Phys. Chem. A 2013, 117, 7162-7178) for the isoelectronic OH(-) + CH3I reaction.

Chemical dynamics simulations of the monohydrated OH(-)(H2O) + CH3I reaction. Atomic-level mechanisms and comparison with experiment.

Direct dynamics simulations, with B97-1/ECP/d theory, were performed to study the role of microsolvation for the OH(-)(H2O) + CH3I reaction. The SN2 reaction dominates at all reactant collision energies, but at higher collision energies proton transfer to form CH2I(-), and to a lesser extent CH2I(-) (H2O), becomes important. The SN2 reaction occurs by direct rebound and stripping mechanisms, and 28 different indirect atomistic mechanisms, with the latter dominating. Important components of the indirect mechanisms are the roundabout and formation of SN2 and proton transfer pre-reaction complexes and intermediates, including [CH3--I--OH](-). In contrast, for the unsolvated OH(-) + CH3I SN2 reaction, there are only seven indirect atomistic mechanisms and the direct mechanisms dominate. Overall, the simulation results for the OH(-)(H2O) + CH3I SN2 reaction are in good agreement with experiment with respect to reaction rate constant, product branching ratio, etc. Differences between simulation and experiment are present for the SN2 velocity scattering angle at high collision energies and the proton transfer probability at low collision energies. Equilibrium solvation by the H2O molecule is unimportant. The SN2 reaction is dominated by events in which H2O leaves the reactive system as CH3OH is formed or before CH3OH formation. Formation of solvated products is unimportant and participation of the (H2O)CH3OH---I(-) post-reaction complex for the SN2 reaction is negligible.

Direct dynamics simulation of dissociation of the [CH3--I--OH]- ion-molecule complex.

Direct dynamics simulations were used to study dissociation of the [CH3--I--OH](-) complex ion, which was observed in a previous study of the OH(-) + CH3I gas phase reaction ( J. Phys. Chem. A 2013 , 117 , 7162 ). Restricted B97-1 simulations were performed to study dissociation at 65, 75, and 100 kcal/mol and the [CH3--I--OH](-) ion dissociated exponentially, in accord with RRKM theory. For these energies the major dissociation products are CH3I + OH(-), CH2I(-) + H2O, and CH3OH + I(-). Unrestricted B97-1 and restricted and unrestricted CAM-B3LYP simulations were also performed at 100 kcal/mol to compare with the restricted B97-1 results. The {CH3I + OH(-)}:{CH2I(-) + H2O}:{CH3OH + I(-)} product ratio is 0.72:0.15:0.13, 0.81:0.05:0.14, 0.71:0.19:0.10, and 0.83:0.13:0.04 for the restricted B97-1, unrestricted B97-1, restricted CAM-B3LYP, and unrestricted CAM-B3LYP simulations, respectively. Other product channels found are CH2 + I(-) + H2O, CH2 + I(-)(H2O), CH4 + IO(-), CH3(-) + IOH, and CH3 + IOH(-). The CH3(-) + IOH singlet products are only given by the restricted B97-1 simulation and the lower energy CH3 + IOH(-) doublet products are only formed by the unrestricted B97-1 simulation. Also studied were the direct and indirect atomic-level mechanisms for forming CH3I + OH(-), CH2I(-) + H2O, and CH3OH + I(-). The majority of CH3I + OH(-) were formed through a direct mechanism. For both CH2I(-) + H2O and CH3OH + I(-), the direct mechanism is overall more important than the indirect mechanisms, with the roundabout like mechanism the most important indirect mechanism at high excitation energies. Mechanism comparisons between the B97-1 and CAM-B3LYP simulations showed that formation of the CH3OH---I(-) complex is favored for the B97-1 simulations, whereas formation of the HO(-)---HCH2I complex is favored for the CAM-B3LYP simulations. The unrestricted simulations give a higher percentage of indirect mechanisms than the restricted simulations. The possible role of the self-interaction error in the simulations is also discussed. The work presented here gives a detailed picture of the [CH3--I--OH](-) dissociation dynamics and is very important for unraveling the role of [CH3--I--OH](-) in the dynamics of the OH(-)(H2O)(n=1,2) + CH3I reactions.

Mechanisms and dynamics of protonation and lithiation of ferrocene.

By elucidating the mechanism of the simplest electrophilic substitution reaction of ferrocene, it was found that the verification of the protonation reaction has been a difficulty. In the work reported here, ab initio chemical dynamics simulations were performed at B3LYP/DZVP level of theory to understand the atomic level mechanisms of protonation and lithiation of ferrocene. Protonation of ferrocene resulted in the agostic and metal-protonated forms. Trajectory calculations revealed that protonation of ferrocene occurs by exo and endo mechanisms, with exo being the major path. H(+) was found to be mobile and hopped from the Cp ring to the metal center and vice versa after the initial attack on ferrocene, with the metal-complex having a shorter lifetime. These results remove the ambiguity surrounding the mechanism, as proposed in earlier experimental and computational studies. Lithiation of ferrocene resulted in the formation of cation-π and metal-lithiated complexes. Similar to protonation, trajectory results revealed that both exo and endo paths were followed, with the exo path being the major one. In addition, lithiated-ferrocene exhibited planetary motion. The major path (exo) followed in the protonation and lithiation of ferrocene is consistent with the observations in earlier experimental studies for other hard electrophiles.

Degradation of Cd-yellow Paints: ab initio study of the adsorption of oxygen and water on {10.0} CdS surface

The cadmium yellow paints (CdS) used in impressionist and modernist paintings in early 1900s are undergoing several deterioration processes, including whitening and discoloration. A relevant effect produced at the surface of the paintings is the growth of discolored crusts, formed mainly by white globular hydrated cadmium sulfate (CdSO4*nH2O) and cadmium carbonate (CdCO3). Recent studies ascribe to an initial photo-oxidation process of CdS the input for the formation of such whitish compounds. In order to understand, at atomic level, the oxidation and hydration mechanisms of these whitish globules, we present the early stages of a theoretical study of the interaction between the hexagonal {10.0} surface of CdS and O2 and H2O molecules to simulate the combined effects of exposure to air and humidity. For this purpose, we adopted a first principles method within the framework of the Density Functional Theory (DFT) in the Generalized Gradient Approximation (GGA-PBE) with the use of ultrasoft pseudopotentials.

In Silico understanding of the cyclodextrin–phenanthrene hybrid assemblies in both aqueous medium and bacterial membranes

The explicit-solvent molecular dynamic (MD) simulation and adaptive biased forces (ABF) methods were employed to systemically study the structural and thermodynamic nature of the β-cyclodextrin (βCD) monomer, phenanthrene (Phe) monomer, and their inclusion complexes in both the aqueous and membrane environments, aiming at clarifying the atomic-level mechanisms underlying in the CD-enhanced degradation of polycyclic aromatic hydrocarbons (PAHs) by bacteria. Simulations showed that βCD and Phe monomers could associate together to construct two distinctive assemblies, i.e, βCD1–Phe1 and βCD2–Phe1, respectively. The membrane-involved equilibrium simulations and the data of potential of mean forces (PMFs) further confirmed that Phe monomer was capable of penetrating through the membranes without confronting any large energy barrier, whereas, the single βCD and βCD-involved assemblies were unable to pass across the membranes. These observations clearly suggested that βCD only served as the carrier to enhance the bioavailability of Phe rather than the co-substrate in the Phe biodegradation process. The Phe-separation PMF profiles indicated that the maximum of the Phe uptake by bacteria would be achieved by the “optimal” βCD:Phe molar ratio, which facilitated the maximal formation of βCD1–Phe1 inclusion and the minimal construction of βCD2–Phe1 complex.

Effects of Interfacial Bonding on Friction and Wear at Silica/Silica Interfaces

Static friction between amorphous silica surfaces with a varying number of interfacial siloxane (Si–O–Si) bridges was studied using molecular dynamic simulations. Static friction was found to increase linearly with the applied normal pressure, which can be explained in the framework of Prandlt–Tomlinson’s model. Friction force was found to increase with concentration of siloxane bridges, but with a decreasing gradient, with the latter being due to interactions between neighboring siloxane bridges. In addition, we identified atomic-level wear mechanisms of silica. These mechanisms include both transfer of individual atoms accompanied by breaking interfacial siloxane bridges and transfer of atomic cluster initialized by rupturing of surface Si–O bonds. Our simulations showed that small clusters are continually formed and dissolved at the sliding interface, which plays an important role in wear at silica/silica interface.

Point defects in garnet-type solid electrolyte (c-Li7La3Zr2O12) for Li-ion batteries

Using ab-initio density-functional theory (DFT) methods, the atomic structure and electronic properties of one of the most promising family of solid electrolytes for Li-ion battery applications, lanthanum oxides with a garnet-type structure (c-Li7La3Zr2O12) are studied. The Li-ion (Li+) defects including Li/Li+ vacancies, interstitials, and vacancy–interstitial pair defect formation energy within the Li7La3Zr2O12 supercell are systematically investigated. This study is essential to understand the defect chemistry and the Li+ conductivity mechanisms. Our results indicate that the Li+ vacancy defects are thermodynamically more favorable than interstitial Li+ defects. This work will therefore be helpful to elucidate the atomic level mechanisms of Li defect formation in order to improve the ionic conductivity for future Li-ion battery applications.

Interaction between voids and grain boundaries in UO2 by molecular-dynamics simulation

This work uses atomic-level simulations to analyze the interactions of voids with a grain boundary (GB) in UO2, the ubiquitous fuel material for light water reactors. Specifically, the high-temperature interactions of a (310) Σ5 tilt GB structure with voids of diameter 1.8nm are analyzed. We find that the GB tends to move towards the void when they are within a few nm of each other. With increasing temperature, GB migration from greater distances toward to the void is predicted to take place. Both GB pinning to the void and void dissolution at the GB take place. The atomic-level mechanisms and the energetics associated with these processes are characterized.

Hydrostatic pressure effects on deformation mechanisms of nanocrystalline fcc metals

A series of large-scale molecular dynamics (MD) simulations have been performed to investigate hydrostatic pressure effects, and the interplay between pressure and grain size, on the flow stress and the related atomic-level deformation mechanisms in nanocrystalline (NC) Cu. The strength of NC Cu increases with increasing hydrostatic pressures for all grain sizes studies in the present paper (3–15nm). The critical grain size for maximum strength first shifts towards lower values with increasing hydrostatic pressure (0–5GPa), and then shifts towards higher values as the hydrostatic pressure becomes even higher (5–80GPa). Below the critical hydrostatic pressure, the dislocation behaviors increase with increasing hydrostatic pressure for all grain sizes and the dependency of effective modulus as a function of hydrostatic pressure is almost the same for all grain sizes, which should lead to the position shifting of maximum strength towards lower grain sizes. Above the critical hydrostatic pressure, the dislocation behaviors start to decrease with increasing hydrostatic pressure for small grain sizes, and continue to increase with increasing hydrostatic pressure for large grain sizes. The slopes of effective modulus as a function of hydrostatic pressure increase slightly with increasing grain size above the critical hydrostatic pressure. The position shifting of maximum strength towards larger grain sizes at large hydrostatic pressure should be attributed to these two observations. Moreover, GB thickening is observed to increase monotonically with increasing pressure for all grain sizes, and the NC Cu with 3nm grain size has the trend to become amorphous state under hydrostatic pressure of 80GPa, which gives a new way to produce crystalline-to-amorphous transition. The findings in the present study should provide insights to the potential applications of NC metals under extreme environments.

Temperature Dependence of the OH–+ CH3I Reaction Kinetics. Experimental and Simulation Studies and Atomic-Level Dynamics

Direct dynamics simulations and selected ion flow tube (SIFT) experiments were performed to study the kinetics and dynamics of the OH(-) + CH3I reaction versus temperature. This work complements previous direct dynamics simulation and molecular beam ion imaging experiments of this reaction versus reaction collision energy (Xie et al. J. Phys. Chem. A 2013, 117, 7162). The simulations and experiments are in quite good agreement. Both identify the SN2, OH(-) + CH3I → CH3OH + I(-), and proton transfer, OH(-) + CH3I → CH2I(-) + H2O, reactions as having nearly equal importance. In the experiments, the SN2 pathway constitutes 0.64 ± 0.05, 0.56 ± 0.05, 0.51 ± 0.05, and 0.46 ± 0.05 of the total reaction at 210, 300, 400, and 500 K, respectively. For the simulations this fraction is 0.56 ± 0.06, 0.55 ± 0.04, and 0.50 ± 0.05 at 300, 400, and 500 K, respectively. The experimental total reaction rate constant is (2.3 ± 0.6) × 10(-9), (1.7 ± 0.4) × 10(-9), (1.9 ± 0.5) × 10(-9), and (1.8 ± 0.5) × 10(-9) cm(3) s(-1) at 210, 300, 400, and 500 K, respectively, which is approximately 25% smaller than the collision capture value. The simulation values for this rate constant are (1.7 ± 0.2) × 10(-9), (1.8 ± 0.1) × 10(-9), and (1.6 ± 0.1) × 10(-9) cm(3)s(-1) at 300, 400, and 500 K. From the simulations, direct rebound and stripping mechanisms as well as multiple indirect mechanisms are identified as the atomic-level reaction mechanisms for both the SN2 and proton-transfer pathways. For the SN2 reaction the direct and indirect mechanisms have nearly equal probabilities; the direct mechanisms are slightly more probable, and direct rebound is more important than direct stripping. For the proton-transfer pathway the indirect mechanisms are more important than the direct mechanisms, and stripping is significantly more important than rebound for the latter. Calculations were performed with the OH(-) quantum number J equal to 0, 3, and 6 to investigate the effect of OH(-) rotational excitation on the OH(-) + CH3I reaction dynamics. The overall reaction probability and the probabilities for the SN2 and proton-transfer pathways have little dependence on J. Possible effects on the atomistic mechanisms were investigated for the SN2 pathway and the probability of the direct rebound mechanism increased with J. However, the other atomistic mechanisms were not appreciably affected by J.

Small molecule‐mediated control of hydroxyapatite growth: Free energy calculations benchmarked to density functional theory

The unique, plate-like morphology of hydroxyapatite (HAP) nanocrystals in bone lends to the hierarchical structure and functions of bone. Proteins enriched in phosphoserine (Ser-OPO3) and glutamic acid (Glu) residues have been proposed to regulate crystal morphology; however, the atomic-level mechanisms remain unclear. Previous molecular dynamics studies addressing biomineralization have used force fields with limited benchmarking, especially at the water/mineral interface, and often limited sampling for the binding free energy profile. Here, we use the umbrella sampling/weighted histogram analysis method to obtain the adsorption free energy of Ser-OPO3 and Glu on HAP (100) and (001) surfaces to understand organic-mediated crystal growth. The calculated organic-water-mineral interfacial energies are carefully benchmarked to density functional theory calculations, with explicit inclusion of solvating water molecules around the adsorbate plus the Poisson-Boltzmann continuum model for long-range solvation effects. Both amino acids adsorb more strongly on the HAP (100) face than the (001) face. Growth rate along the [100] direction should then be slower than in the [001] direction, resulting in plate-like crystal morphology with greater surface area for the (100) than the (001) face, consistent with bone HAP crystal morphology. Thus, even small molecules are capable of regulating bone crystal growth by preferential adsorption in specific directions. Furthermore, Ser-OPO3 is a more effective growth modifier by adsorbing more strongly than Glu on the (100) face, providing one possible explanation for the energetically expensive process of phosphorylation of some proteins involved in bone biomineralization. The current results have broader implications for designing routes for biomimetic crystal synthesis.

Molecular dynamics based study and characterization of deformation mechanisms near a crack in a crystalline material

Modeling crack propagation in crystalline materials is a challenging enterprise due to complexities induced by the interaction of the crack with various deformation mechanisms such as dislocation, micro twin, stacking faults etc.. As a first step toward the development of physics-based models of deformation in the presence of a crack, this paper proposes a comprehensive approach based on molecular dynamics simulations of a crystalline material with an embedded crack. The MD-based framework invokes a sequence of four tasks to accomplish the overall goal, viz. (i) MD simulation, (ii) characterization of atomic-level crack and deformation mechanisms, (iii) quantification of atomic-level deformation mechanisms and crack, and (iv) response analysis. Effective characterization methods like CNA, DXA and deformation gradient analysis followed by quantification are able to delineate the crack length/opening, dislocation structure and microtwins at a high resolution. Interactions of the crack with the dislocation networks and microtwins under mode I loading conditions are investigated for different lattice orientations. Crystal orientation has significant effect on the mechanisms activation and evolution. An important study is made through partitioning of the total energy into recoverable elastic energy, defect energy and inelastic dissipation, and correlating them with deformation characteristics such as dislocation density and twin volume fraction. Finally, a simple mechanistic model of deformation is developed, which associates dislocation density evolution with the stress–strain response in a crystalline material in the presence of a crack. Results show good quantitative agreement of material softening and hardening behavior with direct MD simulation results. The model can be further used to estimate the range of strain-rates that may be applied for physically meaningful MD simulations.

Indirect Dynamics in a Highly Exoergic Substitution Reaction

The highly exoergic nucleophilic substitution reaction F(-) + CH3I shows reaction dynamics strikingly different from that of substitution reactions of larger halogen anions. Over a wide range of collision energies, a large fraction of indirect scattering via a long-lived hydrogen-bonded complex is found both in crossed-beam imaging experiments and in direct chemical dynamics simulations. Our measured differential scattering cross sections show large-angle scattering and low product velocities for all collision energies, resulting from efficient transfer of the collision energy to internal energy of the CH3F reaction product. Both findings are in strong contrast to the previously studied substitution reaction of Cl(-) + CH3I [Science 2008, 319, 183-186] at all but the lowest collision energies, a discrepancy that was not captured in a subsequent study at only a low collision energy [J. Phys. Chem. Lett. 2010, 1, 2747-2752]. Our direct chemical dynamics simulations at the DFT/B97-1 level of theory show that the reaction is dominated by three atomic-level mechanisms, an indirect reaction proceeding via an F(-)-HCH2I hydrogen-bonded complex, a direct rebound, and a direct stripping reaction. The indirect mechanism is found to contribute about one-half of the overall substitution reaction rate at both low and high collision energies. This large fraction of indirect scattering at high collision energy is particularly surprising, because the barrier for the F(-)-HCH2I complex to form products is only 0.10 eV. Overall, experiment and simulation agree very favorably in both the scattering angle and the product internal energy distributions.

Dynamics of the F− + CH3Cl → Cl− + CH3F SN2 reaction on a chemically accurate potential energy surface

Though bimolecular nucleophilic substitution (SN2) reactions play a fundamental role in chemistry, chemically accurate full-dimensional global analytical potential energy surfaces (PESs) have not been developed for these systems. These PESs govern the motion of the atoms in a chemical reaction; thus, the knowledge of the PES is essential to study the dynamics and atomic-level mechanisms. Here, we report a full-dimensional ab initio PES for the F− + CH3Cl → Cl− + CH3F reaction, the PES has an estimated average accuracy of about 0.5 kcal mol−1. Quasiclassical trajectories on this PES reveal that the direct rebound mechanism dominates at high collision energies, whereas the reaction is mainly indirect at low collision energies, where the formation of long-lived hydrogen-bonded and C3v ion–dipole entrance-channel complexes play a major role in the dynamics. A direct stripping mechanism is also found at large impact parameters resulting in significant forward scattering, whereas the direct rebound mechanism scatters towards backward directions. At high collision energies the reaction can be controlled by orienting the reactants into a reactive F− + H3CCl orientation, whereas at low collision energies the initial orientation is not always maintained, because the long-range ion–dipole interactions efficiently steer the reactants into a reactive orientation even if F− initially approaches the non-reactive side of CH3Cl. Mode-specific vibrational distributions show that the reaction produces vibrationally hot CH3F molecules with excited CF stretching, especially at low collision energies.

Biophysical Probes Reveal a “Compromise” Nature of the Methyl-lysine Binding Pocket in L3MBTL1

Histone lysine methylation (Kme) encodes essential information modulating many biological processes including gene expression and transcriptional regulation. However, the atomic-level recognition mechanisms of methylated histones by their respective adaptor proteins are still elusive. For instance, it is unclear how L3MBTL1, a methyl-lysine histone code reader, recognizes equally well both mono- and dimethyl marks but ignores unmodified and trimethylated lysine residues. We made use of molecular dynamics (MD) and free energy perturbation (FEP) techniques in order to investigate the energetics and dynamics of the methyl-lysine recognition. Isothermal titration calorimetry (ITC) was employed to experimentally validate the computational findings. Both computational and experimental methods were applied to a set of designed "biophysical" probes that mimic the shape of a single lysine residue and reproduce the binding affinities of cognate histone peptides. Our results suggest that, besides forming favorable interactions, the L3MBTL1 binding pocket energetically penalizes both methylation states and has most probably evolved as a "compromise" that nonoptimally fits to both mono- and dimethyl-lysine marks.

Atomistic mechanisms of lithium insertion in amorphous silicon

Understanding the lithium–silicon alloying behavior is essential for achieving maximum charge capacity in the negative electrodes of lithium-ion batteries. Our atomistic simulations show that in amorphous silicon with a disordered network structure, inserted lithium atoms can find equilibrium positions in the interstices of big rings. Alternatively, lithium is incorporated into the network by the destruction and reformation of smaller rings. These atomic-level mechanisms are characterized by using the network topology measure of ring statistics, which are correlated to the lithiation responses of silicon electrodes. The results reveal the influence of lithium concentrations on the electro-chemical–mechanical behavior of silicon. Implications on the reversibility and dynamics of the lithiation process are discussed.

F− + CH3I → FCH3 + I− Reaction Dynamics. Nontraditional Atomistic Mechanisms and Formation of a Hydrogen-Bonded Complex

Ion imaging experiments and direct chemical dynamics simulations were performed to study the atomic-level dynamics for the F− + CH3I → FCH3 + I− SN2 nucleophilic substitution reaction at 0.32 eV collision energy. The simulations reproduce the product energy partitionings and the velocity scattering angle distribution measured in the experiments. The simulations reveal that the substitution reaction occurs by two direct atomic-level mechanisms, that is, rebound and stripping, and an indirect mechanism. Approximately 90% of the indirect events occur via a prereaction F−···HCH2I hydrogen-bonded complex. This mechanism may play an important role for other F− SN2 reactions due to the strong electronegativity of fluorine. The average product energy partitioning for the F− + CH3I indirect mechanism agrees with the prediction of PST, even though a FCH3···I− postreaction complex is not formed.

Atomic Level Mechanisms of Solid-State Dewetting in Thin Metal Films Deposited on Silicon (100) Substrates

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.