Activation Free Energy Barrier Research Articles

The role of linkers and frustrated lewis pairs catalysts in the formation of zwitterionic 1,2-anti-addition product with non-conjugated terminal diacetylenes: A computational study

This study presents a computational investigation into the mechanistic pathway and the linker units involved in forming the zwitterionic 1,2-anti-addition product of non-conjugated diacetylenes, di(propargyl)ether (DPE), di(prop-2yn-1yl)sulfane (DPS) and 1,6-Heptadiyne (HD) catalyzed by the inter-molecular phosphine/borane frustrated Lewis pairs (FLPs), i.e., PPh2[C6H3(CF3)2](P-CF)/[B(C6F5)3]([B]) and P(o-tolyl)3(P-tol)/[B(C6F5)3]([B]). The potential energy surface (PES) calculations reveal that the anti-addition of P-CF to the internal C-atoms of acetylene units is energetically more favored than that of the addition of P-tol in DPE, DPS, and HD by ∼10.0, ∼9.2, and ∼6.0 kcal/mol, respectively. The calculations performed with DPE contain “—O—,” linker unit exhibits superior reactivity than DPS and HD, which suggests the electronegativity of linkers plays a significant role and facilitates the addition of Lewis bases. The higher electronegativity of linker units enables the 1,2-addition reaction by lowering the free energy activation barriers, as observed in the DFT calculations. The Molecular Electrostatic Potential (MESP) study shows that the electrostatic interactions favor the addition of P-CF to the active acetylene positions (C5/C4/C4) of [B]-DPE/DPS/HD-π complexes than the P-tol. The Distortion/Interaction (D/I) analysis reveals that transition states involving P-CF (TS1, TS3, and TS5) exhibit more interaction energy (ΔEInt) and less distortion energies (ΔEd) than that of the P-tol (TS2, TS4, and TS6). Further, the Energy Decomposition Analysis (EDA) also rationalizes the preferential approach of the electron-deficient Lewis base over the electron-rich one on the basis of the significant contribution of orbital interaction energies (ΔEorbital) in the cases of P-CF; TS1, TS3, and TS5. This study suggests that the electronic effects of substrates and the FLPs are crucial to facilitate the desired products formed with non-conjugated terminal alkynes.

On the"Staple" Effect in the d-Glucose Acetylation by Choline-Chloride Ethylene Glycol and Choline-Chloride Urea Deep Eutectic Solvents.

This work employs hybrid quantum mechanics molecular mechanics simulations coupled with an umbrella sampling technique to investigate the acetylation of d-glucose with acetic anhydride in two deep eutectic solvents (DESs): choline-chloride ethylene glycol (ChCl/Etg) and choline-chloride urea (ChCl/U). The reaction proceeds spontaneously, with activation free energy barriers of 18.19 ± 0.15 and 19.83 ± 0.15 kcal/mol for ChCl/Etg and ChCl/U, respectively. These values align with literature data for similar reactions in ionic liquids, highlighting the potential of DESs as green reaction media for wood modifications, while minimizing lignocellulosic matrix degradation. Energy pair distribution analysis, radial distribution functions, and noncovalent interactions reveal a more solvated transition state compared to the initial reactant state within ChCl/Etg, facilitating the acetylation reaction. Interestingly, combined distribution function analysis unveils a "staple" effect in both solvents, potentially hindering efficient product separation. The work further explores hydrogen bond networks and Kamlet-Taft solvatochromic parameters to elucidate the role of solvents in the reaction mechanism. It was observed that a notable decrease in activation energy is accompanied by increasing net basicity, along with a reduction in the "staple" effect. This suggests that solvent parameters could serve as an effective screening tool for identifying novel and efficient DESs for wood modifications.

Thermodynamic and kinetic models for the photolytic and hydrolytic degradation of picaridin: A computational and theoretical study

The popularity of insect repellents presents the opportunity for continuous human and environmental exposure to such chemical mixtures. Consequently, the effects of their chemical components on human health and environmental safety is significantly reliant on their stability and hence, residence times under typical environmental conditions. Hence, for the first time, a thorough density functional theoretical (DFT) study on some possible hydrolytic and photolytic degradation pathways is presented and applied in the derivation of a comprehensive kinetic model, allowing evaluation of the residence time of picaridin; a very popular insect repellent. These results show that picaridin undergoes very slow hydrolysis via two pathways which present activation free energy barriers (ΔG‡) of ca. 225 and ca. 247 kJ mol−1 due to the asynchronous formation of four-membered cyclic activated complexes in both cases. Similarly, their photolytic transformations occur via two pathways, both of which are limited by the formation of the initial radical cation.

Synthesis of Daldinans and Dynamic Stereochemistry of their Atropisomers.

The synthesis of daldinans B and E (5 and 8), 3-phenyl isoindolinones isolated from Daldinia concentrica, was achieved. The conformational properties of these isoindolinones were examined using their derivatives, revealing the presence of atropisomers. The activation free-energy barrier was estimated by the variable-temperature nuclear magnetic resonance (VT-NMR) method, and density functional theory (DFT) calculations suggested that this atropisomerization was caused by equilibration via a slow rotation around the C-3-C-8 axis.

Palladium-Catalyzed Sequential Cross-Coupling/Annulation of ortho-Vinyl Bromobenzene with Aryl Bromide: Bimetallic Pathway versus Pd(II)-Pd(IV) Pathway: A DFT Investigation.

The palladium-catalyzed sequential cross-coupling/annulation of ortho-vinyl bromobenzenes with aryl bromides generating phenanthrenes was characterized by density functional theory (DFT). The Pd(II)-Pd(IV) pathway (Path V) is shown to be less probable than the bimetallic pathway (Path I), the latter proceeding via the following six steps: oxidative addition, vinyl-C(sp2)-H activation, Pd(II)-Pd(II) transmetalation, C-C coupling, aryl-C(sp2)-H activation, and reductive elimination. The aryl-C(sp2)-H activation process acts as the rate-determining step (RDS) of the entire chemical transformation, with an activation free energy barrier of ca. 27.4-28.8 kcal·mol-1, in good agreement with the corresponding experimental data (phenanthrenes' yields of ca. 65-90% at 130 °C after 5 h of reaction). The K2CO3 additive effectively reduces the activation free energy barrier of the RDS through direct participation in the reaction while preferentially modulating the charge distributions and increasing the stability of corresponding intermediates and complexes along the reaction path. Furthermore, bonding and electronic structure analyses of the key structures indicate that the chemo- and regioselectivities of the reaction are strongly influenced by both electronic effects and steric hindrance.

Mutation at the entrance of the quinone cavity severely disrupts quinone binding in respiratory complex I

In all resolved structures of complex I, there exists a tunnel-like Q-chamber for ubiquinone binding and reduction. The entrance to the Q-chamber in ND1 subunit forms a narrow bottleneck, which is rather tight and requires thermal conformational changes for ubiquinone to get in and out of the binding chamber. The substitution of alanine with threonine at the bottleneck (AlaThr MUT), associated with 3460/ND1 mtDNA mutation in human complex I, is implicated in Leber's Hereditary Optic Neuropathy (LHON). Here, we show the AlaThr MUT further narrows the Q-chamber entrance cross-section area by almost 30%, increasing the activation free energy barrier of quinone passage by approximately 5 kJ mol−1. This severely disrupts quinone binding and reduction as quinone passage through the bottleneck is slowed down almost tenfold. Our estimate of the increase in free energy barrier is entirely due to the bottleneck narrowing, leading to a reduction of the transition state entropy between WT and MUT, and thus more difficult quinone passage. Additionally, we investigate details of possible water exchange between the Q-chamber and membrane. We find water exchange is dynamic in WT but may be severely slowed in MUT. We propose that LHON symptoms caused by 3460/ND1 mtDNA mutation are due to slowed quinone binding. This leads to an increased production of reactive oxidative species due to upstream electron backup at the FMN site of complex I, thus resulting in a mt bioenergetic defect.

Void nucleation at dislocation boundaries aided by the synergy of multiple dislocation pile-ups

Void nucleation is of great significance in understanding ductile fracture. Recent experiments have shown that voids are nucleated via vacancy condensation and dislocation boundaries are the main nucleation sites. However, it is unclear what role is played exactly by dislocation boundaries in promoting void nucleation. Here we propose a new mechanism for dislocation boundary-induced void nucleation and develop a corresponding model based on the classical nucleation theory and vacancy diffusion theory. The model suggests that void nucleation is mainly influenced by hydrostatic stress, temperature, and relative vacancy concentration, whose contributions are systematically studied. It is also suggested that the vacancy formation energy and the interaction energy of hydrostatic stress and vacancy, which are absent in the previous models and introduced in ours, exhibit a clear tendency to lower the activation free energy barrier. Analysis of the nucleation kinetic suggests that the growth rate of void depends on the vacancy diffusion coefficient and vacancy concentration; the higher the values of these parameters, the faster the growth rate of the void. The kinetic feasibility of the newly proposed mechanism is examined using three-dimensional discrete dislocation dynamics simulations. The results predict that the size of incipient voids nucleated at the dislocation boundary is ∼35 nm, which is consistent with the experimental characterization value of ∼50 nm. Finally, when the relaxation of the dislocation boundary is considered, the synergistic effect is weakened.

DFT Studies on the Mechanisms of Carboamination/Diamination of Unactivated Alkenes Mediated by Pd(IV) Intermediates.

Density functional theory (DFT) calculations have been employed to investigate the mechanism of carboamination and diamination of unactivated alkenes mediated by Pd(IV) intermediates. Both reactions share a common Pd(IV) intermediate, serving as the starting point for either the carboamination or the diamination pathway. The formation of this Pd(IV) intermediate encompasses a transition state that substantially impacts the turnover frequency (TOF) of catalytic cycles, with an apparent activation free-energy barrier of 26.1 kcal mol-1. Carboamination of unactivated alkenes proceeds through the coordination of a toluene molecule, C-H activation, inner reductive elimination, and the separation of the carboamination product from this intermediate, while diamination of unactivated alkenes involves the formation of the ion nucleophile, SN2 attack, and the separation of the diamination product. A comparison of the free-energy profiles for carboamination and diamination of unactivated alkenes can elucidate the origin of the chemoselectivity, and Bader's atoms in molecules (AIM) wave function analyses have been performed to analyze the contributions of the outer C-N bonding in the diamination process.

Synthesis, Reaction Mechanism, DNA Cleavage and Antitumor Properties of Naphthoquinone‐Based Enediynes

AbstractUnder physiological conditions, natural enediynes antibiotics generate free radicals through a cycloaromatization reaction. The resulting highly reactive radicals can abstract hydrogen atoms on the phosphate backbone of DNA, leading to high cytotoxicity. On the other hand, anthraquinone‐fused compounds such as daunomycin and adriamycin can insert into DNA through planar aromatic ring structures, resulting in strong DNA damage. To this end, a series of naphthoquinone‐based enediynes were designed and synthesized by adjusting the type of terminal alkyne to modulate their water solubility and radical generation property. Theoretical calculations suggest that the naphthoquinone‐based enediyne generates radicals through a rearrangement (1,3‐proton transfer) followed by Myers‐Saito cyclization processes, with a much lower activation free energy barrier than Bergman cyclization (24.2 kcal/mol vs 39.8 kcal/mol). Differential scanning calorimetry and electron paramagnetic resonance results showed that naphthoquinone‐based enediynes undergo cycloaromatization at relatively low temperatures, resulting in the generation of highly reactive radical species. The enediynes with optimal water solubility displayed strong DNA cleavage ability at micromolar level and high cytotoxicity towards Hela cells, providing a half‐inhibitory concentration of 8.19 μM.

Ceramide-1-phosphate transfer protein enhances lipid transport by disrupting hydrophobic lipid-membrane contacts.

Cellular distributions of the sphingolipid ceramide-1-phosphate (C1P) impact essential biological processes. C1P levels are spatiotemporally regulated by ceramide-1-phosphate transfer protein (CPTP), which efficiently shuttles C1P between organelle membranes. Yet, how CPTP rapidly extracts and inserts C1P into a membrane remains unknown. Here, we devise a multiscale simulation approach to elucidate biophysical details of CPTP-mediated C1P transport. We find that CPTP binds a membrane poised to extract and insert C1P and that membrane binding promotes conformational changes in CPTP that facilitate C1P uptake and release. By significantly disrupting a lipid's local hydrophobic environment in the membrane, CPTP lowers the activation free energy barrier for passive C1P desorption and enhances C1P extraction from the membrane. Upon uptake of C1P, further conformational changes may aid membrane unbinding in a manner reminiscent of the electrostatic switching mechanism used by other lipid transfer proteins. Insertion of C1P into an acceptor membrane, eased by a decrease in membrane order by CPTP, restarts the transfer cycle. Most notably, we provide molecular evidence for CPTP's ability to catalyze C1P extraction by breaking hydrophobic C1P-membrane contacts with compensatory hydrophobic lipid-protein contacts. Our work, thus, provides biophysical insights into how CPTP efficiently traffics C1P between membranes to maintain sphingolipid homeostasis and, additionally, presents a simulation method aptly suited for uncovering the catalytic mechanisms of other lipid transfer proteins.

N-Heterocyclic carbene: thiazolylidene–Cu(I) complexes: microwave-assisted synthesis and use as catalyst in A3 reaction

A simple method involving microwave-assisted reaction of 3-benzylthiazolium/benzothiazolium bromides with cuprous oxide has been developed for the synthesis of four new N-heterocyclic carbene (NHC): thiazolylidene/benzothiazolylidene–Cu(I) complexes. Structures of the complexes could be established on the basis of the IR, 1H, and 13C NMR studies and elemental analysis. A representative NHC:benzothiazolylidene–Cu(I) complex was used as catalyst for the microwave-assisted A3 reaction of phenylacetylene with a secondary amine and aldehyde to obtain new substituted propargylamines in excellent yields. A theoretical investigation of the model A3 reaction at the DFT level reveals initial coordination of the alkyne with the NHC–Cu(I) complex followed by its attack on the iminium ion generated from the reaction of secondary amine with aldehyde. In the presence of the catalyst, the activation free energy barrier (ΔG#) is lowered by ca. 3 kcal mol−1.

Computational study on the mechanisms of inhibition of SARS-CoV-2 Mpro by aldehyde warheads based on DFT.

SARS-CoV-2 main protease, Mpro, plays a crucial role in the virus replication cycle, making it an important target for antiviral research. In this study, a simplified model obtained through truncation is used to explore the reaction mechanism of aldehyde warhead compounds inhibiting Mpro at the level of density functional theory. According to the calculation results, proton transfer (P_T)-nucleophilic attack (N_A) is the rate-determining step in the entire reaction pathway. The water molecule that plays a catalytic role occupies the oxyanion hole, which is unfavorable for the aldehyde warhead to approach the Cys145 SH. Through a hypothetical study of substituting the main chain NH with methylene, it is further confirmed that the P_T-N_A is a proton transfer-dominated process accompanied by a nucleophilic attack reaction. In this process, the oxyanion hole serves only to stabilize the aldehyde oxygen anion and therefore does not have a significant impact on the activation free energy barrier of the step. Our research results provide a unique perspective for understanding the covalent inhibition reaction of the Mpro active site. This study also offers theoretical guidance for the design of new Mpro covalent inhibitors.

The treatment of dispersion terms for solution systems.

DFT calculations of reaction mechanisms in solution have always been a hot topic, especially for transition-metal-catalyzed reactions, in which the traditional DFT-D3 method has been extensively employed. The overestimation of the dispersion from the traditional DFT-D3 method leads to a quite low activation free-energy barrier, so it is worth finding a proper way to deal with the dispersion for solution systems. The solvent-solute dispersion is also important for solution systems, and thus it should be calculated together with the solute dispersion. The newly generated solute-solute dispersion energy should be shared equally with the newly formed cavity between two interacting species; therefore, only half of the solute-solute and solvent-solute dispersion terms belong to the solute molecule. The detailed treatment of dispersion correction for solution systems has been fully addressed, and this method has been confirmed with the examples of ligand exchange reactions and catalytic reactions.

Inhibitory Effect of Lanthanides on Native Lipid Flip-Flop

The influence of ytterbium ions (Yb3+), a commonly used paramagnetic NMR chemical shift reagent, on the physical properties and flip-flop kinetics of dipalmitoylphosphatidylcholine (DPPC) planar supported lipid bilayers (PSLBs) was investigated. Langmuir isotherm studies revealed that Yb3+ interacts strongly with the phosphate headgroup of DPPC, evidenced by the increases in shear and compression moduli. Using sum-frequency vibrational spectroscopy, changes in the acyl chain ordering and phase transition temperature were also observed, consistent with Yb3+ interacting with the phosphate headgroup of DPPC. The changes in the physical properties of the membrane were also observed to be concentration dependent, with more pronounced modification observed at low (50 μM) Yb3+ concentrations compared to 6.5 mM Tb3+, suggesting a cross-linking mechanism between adjacent DPPC lipids. Additionally, the changes in membrane packing and phase transition temperatures in the presence of Tris buffer suggested that a putative Yb(Tris)3+ complex forms that coordinates to the PC headgroup. The kinetics of DPPC flip-flop in the gel and liquid crystalline (lc) phases were substantially inhibited in the presence of Yb3+, regardless of the Yb3+ concentration. Analysis of the flip-flop kinetics under the framework of transition state theory revealed that the free energy barrier to flip-flop in both the gel and lc phases was substantial increased over a pure DPPC membrane. In the gel phase, the trend in the free energy barrier appeared to follow the trend in the shear moduli, suggesting that the Yb3+-DPPC headgroup interaction was driving the increase in the activation free energy barrier. In the lc phase, activation free energies of DPPC flip-flop in the presence of 50 μM or 6.5 mM Yb3+ were found to mirror the free energies of TEMPO-DPPC flip-flop, leading to the conclusion that the strong interaction between Yb3+ and the PC headgroup was essentially manifested as a headgroup charge modification. These studies illustrate that the presence of the lanthanide Yb3+ results in significant modification to the lipid membrane physical properties and, more importantly, results in a pronounced inhibition of native lipid flip-flop.

A computational mechanistic study of the hydrolytic degradation of three common pyrethroid insecticides

The hydrolytic evolution mechanisms of three pyrethroid insecticides: Imiprothrin, deltamethrin and tetramethrin, commonly applied in bug sprays, were investigated via density functional theoretical (DFT) modelling. Whereas for deltamethrin, hydrolysis terminates after cleavage of the ester linkage in the starting compound via an asynchronously formed four membered cyclic transition state, for the other two compounds, the final transition state result in the release of formaldehyde and a cyclic amide via a five membered cyclic activated complex. Furthermore, though the mechanisms for all compounds are limited by the ester hydrolysis step; the first for deltamethrin and tetramethrin, the hydrolytic evolution for imiprothrin commences with a water catalysed tautomerization process. The limiting activation free energy barrier, for all three compounds: ΔGǂ = 221–290 kJ mol−1, is such that their residence time in air should be several months; the shortest of which would be that for deltamethrin followed by tetramethrin.

Exploring the conversion mechanism of formaldehyde to CO2 and H2 catalyzed by bifunctional ruthenium catalysts: A DFT study

Bifunctional catalysts have a wide range of applications in chemical hydrogen storage. However, the versatile structure and complex influencing factors of catalysts still limit the current mechanistic understanding. Herein, a theoretical study based on density functional theory calculation is performed to illuminate the mechanistic preference of the conversion from formaldehyde to CO2 and H2 catalyzed by bifunctional ruthenium catalysts. The computational results indicate: (1) In contrast to the previously proposed mechanism, the catalyst is involved in the formaldehyde hydrolysis reaction and effectively reduces the activation free energy barrier. (2) Metal ligand cooperation mechanism is preferred instead of the metal-centred mechanism due to the stronger interaction between substrate and dual active sites on the bifunctional catalyst. (3) The catalyst with the OH group exhibits better catalytic activity compared with the NH group due to appropriate catalytic driving force, which is governed by the intrinsic electronic effects of the Lewis basic functional ligand. The pKa value can be used as a reliable descriptor to evaluate the catalytic activity of the functional ligand. Our study highlights the advantages of bifunctional catalysts in dehydrogenation-related reactions and proposes feasible strategies for the regulation of bifunctional catalyst activity, which is expected to provide new inspiration for future bifunctional catalyst design.

Conversion of CO2 to cyclic carbonates by metal-ethylenediamine complexes in ionic liquid: A DFT mechanistic study

To curtail the accumulation of CO2 concentration in the atmosphere, different strategies have been adopted at present. Among them, one of the possible methods of reducing CO2 level involves transforming it into useful organic compounds. In this line, converting CO2 to cyclic carbonates is a topic of interest as cyclic carbonates are commercially important chemicals. Recently, Honores and co-workers have experimentally prepared cyclic carbonates from cycloaddition of CO2 and ethylene oxide/propylene oxide/Epichlorohydrin catalyzed by Co-ethylenediamine (en) complexes using [BMIm][BF4] ionic liquid as solvent. This work explores the complete reaction mechanism of the cycloaddition reaction at B3LYP/LANL2DZ(6−31G*) level of density functional theory. Two mechanist routes were computed: first route deals with the reaction mechanism without the presence of ionic liquid and second route deals with the ionic liquid involved reaction mechanism. In both the routes, the ring closure step was calculated to be the rate-determining step with an activation barrier in the range of 47.87–52.23 kcal/mol in the absence of ionic liquid and 48.82 to 35.29 kcal/mol in the presence of ionic liquid. Interestingly, the involvement of ionic liquids in the reaction mechanism was found to remarkably reduce the activation free energy barriers by 2.12–19.31 kcal/mol in all the steps along the proposed mechanism. Geometrical analysis indicated that the ability of ionic liquids to form intermolecular hydrogen bonding interaction with the catalyst and substrate is the reason behind the decrease in the barriers. In order to quantify the strength of the hydrogen bonding interactions, NBO analysis was performed on key transition state geometries and nB-F → σ*H-N hydrogen bonding interactions between anionic part of IL and en ligand framework were found to possess significant second-order stabilization energies. Overall, this study sheds light on the mechanistic details of the cycloaddition reaction of CO2 and epoxide and also substantiates that the ionic liquids are efficient green solvents which can be employed in similar catalytic reactions to achieve more efficiency.

Reduction Mechanism of Alcohols Contents Caused by Acids During Oil-Paper Insulation Aging

This work investigates which acid can mainly lead to the reduction of alcohols contents in insulating oil during transformer oil-paper insulation aging. In terms of the activation free energy barrier theory, the microscopic mechanisms for the preferential reactions of methanol and ethanol with formic, acetic, naphthenic and stearic acids are analyzed. It is revealed that the decrease in alcohols concentrations in the middle and late stages of transformer oil-paper insulation aging is mainly caused by the reaction between alcohols and naphthenic acid. The results are cross-validated by utilizing the frontier molecular orbital theory and accelerated thermal aging experiments. Additionally, it is observed that methanol is more susceptible to the effect of acids than ethanol in the middle and late stages of aging. The obtained conclusions, in this paper, could provide a theoretical basis for using alcohols in oil as aging indicators to characterize the full-stage aging status of transformer paper insulation.

XNgNSi (X = HCC, F; Ng = Kr, Xe, Rn): A New Class of Metastable Insertion Compounds Containing Ng-C/F and Ng-N Bonds and Possible Isomerization therein.

Recently, astronomically important silaisocyanoacetylene (HCCNSi) possessing a large dipole moment has been detected for the first time with the help of crossed molecular beam experiments. Quantum chemical computations at higher levels of theory have also been performed to characterize the transient species. In this study, we have analyzed the equilibrium geometry, stability, reactivity, and energetics as well as the nature of bonding in the noble gas (Ng) inserted HCCNSi compound. We have also considered its F analogue to understand the influence of the most electronegative atom in the compound. Metastable behavior of the XNgNSi compounds (X = HCC, F; Ng = Kr-Rn) is examined by calculating thermochemical parameters like free energy change (ΔG) and zero-point-energy-corrected dissociation energy (D0) at 298 K for all possible two-body (2B) and three-body (3B) (both neutral as well as ionic) dissociation channels using coupled-cluster theory [CCSD(T)] in addition to density functional theory (DFT) as well as second order Møller-Plesset perturbation theory (MP2). The set of predicted compounds is found to be endergonic in nature, having high positive free energy change suggesting the thermochemical stability of the compounds except for the 2B Ng-release paths. Though thermodynamically feasible, they are kinetically protected with very high activation free energy barriers. Interestingly, the release of Ng from the parent moiety XNgNSi produces the XSiN isomer, by 180° flipping of the NSi moiety. This can also be seen in the dynamical simulation carried out with the help of atom-centered density matrix propagation (ADMP) technique at 2000K for 1 ps. The bonding in Ng-C, Ng-F, and Ng-N bonds of the studied compounds is analyzed and described with the aid of natural bond orbital (NBO), topological parameters computed using atoms-in-molecules theory (AIM), energy decomposition analysis (EDA), and adaptive natural density partitioning (AdNDP) methods. The natural charge distribution on the constituent atoms suggests that the compounds can be partitioned into both ways of representations, viz., neutral radical as well as ionic fragments. Lastly, the reactivity of the compounds is scrutinized using certain reactivity descriptors calculated within the domain of conceptual density functional theory (CDFT).

Surface Diffusion of Underpotential‐Deposited Lead Adatoms on Gold Nanoelectrodes

AbstractA simple and readily applicable voltammetric approach is described to characterize and measure the site‐hopping surface diffusion of underpotential‐deposited (UPD) metal adatoms at nanoelectrodes. UPD refers to the deposition of atoms on foreign metal supports at potentials lower than those predicted by Nernst law for a bulk deposition. Despite its importance in several fields of catalysis, advanced nanofabrication or even atomic nanoengineering by atomic layer epitaxy, diffusion of UPD adatoms is difficult to observe at micro‐ and macroelectrodes. In fact, at electrodes of usual dimensions, UPD adatoms surface diffusion is masked by other electrochemical phenomena of greater relative amplitudes. Conversely, at nanowires electrodes sealed in glass, only an extremely small fraction of the surface of the wires is exposed to the electrolyte solution and is rapidly loaded. This allows the spillage of UPD adatoms onto the much larger area of the nanowire rod which is immune to Faradaic reactions due to its isolation from the electrochemical solution by the glass casing. Therefore, surprisingly for a UPD process, voltammetric peaks currents and integral desorption charges primarily reflect these diffusional surface processes so that the integral charges vary linearly with the inverse of the square root of the scan rate. This is easily observable and measurable with usual bench‐level electrochemical instrumentation. In this work, by using nanoelectrodes with diameters between 90 and 260 nm, we were able to establish the major involvement of this site‐hopping surface diffusion of UPD Pb adatoms on polycrystalline gold (Au) and characterize it quantitatively. The equivalent surface diffusion coefficient of UPD Pb adatoms was determined to be ∼4.4×10−11 cm2 s−1 at room temperature, corresponding to a Gibbs free energy activation barrier of ca. 18.72 kJ mol−1 (i. e., 0.19 eV per Pb adatom) for the reaction of inter‐sites exchange of Pb adatoms on a polycrystalline Au surface.