Electron Binding Research Articles

Screening the optimal Cox/CeO2(110) (x = 1–6) catalyst for methane activation in coalbed gas

The challenges posed by energy and environmental issues have forced mankind to explore and utilize unconventional energy sources. It is imperative to convert the abundant coalbed gas (CBG) into high value-added products, i.e., selective and efficient conversion of methane from CBG. Methane activation, known as the “holy grail”, poses a challenge to the design and development of catalysts. The structural complexity of the active metal on the carrier is of particular concern. In this work, we have studied the nucleation growth of small Co clusters (up to Co6) on the surface of CeO2(110) using density functional theory, from which a stable loaded Co/CeO2(110) structure was selected to investigate the methane activation mechanism. Despite the relatively small size of the selected Co clusters, the obtained Cox/CeO2(110) exhibits interesting properties. The optimized Co5/CeO2(110) structure was selected as the optimal structure to study the activation mechanism of methane due to its competitive electronic structure, adsorption energy and binding energy. The energy barriers for the stepwise dissociation of methane to form CH3*, CH2*, CH*, and C* radical fragments are 0.44, 0.55, 0.31, and 1.20 eV, respectively, indicating that CH* dissociative dehydrogenation is the rate-determining step for the system under investigation here. This fundamental study of metal-support interactions based on Co growth on the CeO2(110) surface contributes to the understanding of the essence of Co/CeO2 catalysts with promising catalytic behavior. It provides theoretical guidance for better designing the optimal Co/CeO2 catalyst for tailored catalytic reactions.

New Manifestations of the Strong Nuclear Interactions in Solids

The discovery of global macroscopic, including optical, characteristics with the addition of one neutron made crystals LiH, LiD, Diamond, Graphene an excellent model for studying the strong nuclear interaction. New manifestations of strong interaction in solids are measurements of its dependence on the distance between nucleons, finding a neutron - electron binding energy (106 meV), which is in excellent agreement with the prediction of the theory (106,7 meV). Moreover, the creation of mass in massless Dirac fermions in graphene by strong nuclear interaction has been demonstrated for the first time.

Exploring the Electronic Topology and Binding Mechanism of Acetic Acid Adsorption on Nitrogen and Boron Co-Doped (5, 5) SWCNT

Exploring the Electronic Topology and Binding Mechanism of Acetic Acid Adsorption on Nitrogen and Boron Co-Doped (5, 5) SWCNT

Two polymorphs of a new AIEgen from transition-metal-free cross-coupling reactions: A combined experimental and crystal structure prediction study

We synthesized a new double aromatic pyrrole [2,5-bis(2,5-dimethoxyphenyl)-1H-pyrrole, abbreviated as BDMP] without using transition metals. This compound exhibits a conjugated skeleton, ideal for aggregation-induced emission luminogens (AIEgens). Two methods, Lewis acid catalysis and photoreaction, were used for synthesis. Different methods produce different crystal structures with the same molecular skeleton. The crystals show different AIE properties. The photocatalytically formed crystal (BDMP-B) is highly luminescent with a 10.4 % fluorescence yield. In contrast, the Lewis acid-catalyzed crystal (BDMP-A) has a lower fluorescence yield of 3.0 %. These differences in emission are due to varying crystal packings, confirmed by quantum calculations.To understand these differences, Crystal Structure Prediction (CSP) studies were conducted. Our goal was to find if more stable crystal packings exist. Several energy ranking approaches were used to identify the most feasible polymorphic forms. These approaches include static lattice energy, lattice binding energy, enthalpy, cohesive enthalpy, Gibbs Free Energy, cohesive Gibbs Free Energy, and high-accurate electronic binding energy from large molecule-cluster crystal packings. Our findings suggest that the most stable crystal forms can be predicted accurately using appropriate theoretical methods.This research improves our understanding of AIEgens. It shows the value of combining experimental work with both cost-effective and highly accurate theoretical methods for crystal structure analysis and prediction.

Electron binding energy of donors in bilayer graphene with a gate-tunable gap

Electron binding energy of donors in bilayer graphene with a gate-tunable gap

Navigating through the coordination preferences of heavy alkaline earth metals: Laying the foundations for 223Ra- and 131/135mBa-based targeted alpha therapy and theranostics of cancer

The clinical success of [223Ra]RaCl2 (Xofigo®) for the palliative treatment of bone metastases in patients with prostate cancer has highlighted the therapeutic potential of α-particle emission. Expanding the applicability of radium-223 in Targeted Alpha Therapy of non-osseous tumors is followed up with significant interest, as it holds the potential to unveil novel treatment options in the comprehensive management of cancer. Moreover, the use of barium radionuclides, like barium-131 and -135m, is still unfamiliar in nuclear medicine applications, although they can be considered as radium-223 surrogates for imaging purposes. Enabling these applications requires the establishment of chelators able to form stable complexes with radium and barium radionuclides. Until now, only a limited number of ligands have been suggested and these molecules have been primarily inspired by existing structures known for their ability to complex large metal cations. However, a systematic inspection of chelators specifically tailored to Ra2+ and Ba2+ has yet to be conducted.This work delves into a comprehensive investigation of a series of small organic ligands, aiming to unveil the coordination preferences of both radium-223 and barium-131/135m. Electronic binding energies of both metal cations to each ligand were theoretically computed via Density Functional Theory calculations (COSMO-ZORA-PBE-D3/TZ2P), while thermodynamic stability constants were experimentally determined for Ba2+-ligand complexes by potentiometry, NMR and UV–Vis spectroscopies. The outcomes revealed malonate, 2-hydroxypyridine 1-oxide and picolinate as the most favorable building blocks to design multidentate chelators. These findings serve as foundation guidelines, propelling the development of cutting-edge radium-223- and barium-131/135m-based radiopharmaceuticals for Targeted Alpha Therapy and theranostics of cancer.

Probing the Electronic Structure of [B10H10]2- Dianion Encapsulated by an Octamethylcalix[4]pyrrole Molecule.

Despite being an important closo-borate in condensed phase boron chemistry, isolated [B10H10]2- is electronically unstable and has never been detected in the gas phase. Herein, we report a successful capture of this fleeting species through binding with an octamethylcalix[4]pyrrole (omC4P) molecule to form a stable gaseous omC4P·[B10H10]2- complex and its characterizations utilizing negative ion photoelectron spectroscopy (NIPES). The recorded NIPE spectrum, contributed by both omC4P and [B10H10]2-, is deconvoluted by subtracting the omC4P contribution to yield a [B10H10]2- spectrum. The obtained [B10H10]2- spectrum consists of four major bands spanning the electron binding energy (EBE) range from 1 to 5 eV, with the EBE gaps matching excellently with the energy intervals of computed high-lying occupied molecular orbitals of the [B10H10]2- dianion. This study showcases a generic method to utilize omC4P to capture unstable multiply charged anions in the gas phase for experimental determination of their electronic structures.

Penning-trap measurement of the Q value of electron capture in 163Ho for the determination of the electron neutrino mass

The investigation of the absolute scale of the effective neutrino mass remains challenging due to the exclusively weak interaction of neutrinos with all known particles in the standard model of particle physics. At present, the most precise and least-model-dependent upper limit on the electron antineutrino mass is set by the Karlsruhe Tritium Neutrino Experiment (KATRIN) from the analysis of the tritium β-decay. Another promising approach is the electron capture in 163Ho, which is under investigation using microcalorimetry by the Electron Capture in Holmium (ECHo) and HOLMES collaborations. An independently measured Q value for this process is vital for the assessment of systematic uncertainties in the neutrino mass determination. Here we report a direct, independent determination of this Q value by measuring the free-space cyclotron frequency ratio of highly charged ions of 163Ho and 163Dy in the Penning-trap experiment PENTATRAP. Combining this ratio with atomic physics calculations of the electronic binding energies yields a Q value of 2,863.2 ± 0.6 eV c−2, which represents a more than 50-fold improvement over the state of the art. This will enable the determination of the electron neutrino mass on a sub-electronvolt level from the analysis of the electron capture in 163Ho.

Understanding the Formation and Growth of New Atmospheric Particles at the Molecular Level through Laboratory Molecular Beam Experiments.

Atmospheric new particle formation (NPF), which exerts comprehensive implications for climate, air quality and human health, has received extensive attention. From molecule to cluster is the initial and most important stage of the nucleation process of atmospheric new particles. However, due to the complexity of the nucleation process and limitations of experimental characterization techniques, there is still a great uncertainty in understanding the nucleation mechanism at the molecular level. Laboratory-based molecular beam methods can experimentally implement the generation and growth of typical atmospheric gas-phase nucleation precursors to nanoscale clusters, characterize the key physical and chemical properties of clusters such as structure and composition, and obtain a series of their physicochemical parameters, including association rate coefficients, electron binding energy, pickup cross section and pickup probability and so on. These parameters can quantitatively illustrate the physicochemical properties of the cluster, and evaluate the effect of different gas phase nucleation precursors on the formation and growth of atmospheric new particles. We review the present literatures on atmospheric cluster formation and reaction employing the experimental method of laboratory molecular beam. The experimental apparatuses were classified and summarized from three aspects of cluster generation, growth and detection processes. Focus of this review is on the properties of nucleation clusters involving different precursor molecules of water, sulfuric acid, nitric acid and NxOy, respectively. We hope this review will provide a deep insight for effects of cluster physicochemical properties on nucleation, and reveal the formation and growth mechanism of atmospheric new particle at the molecular level.

Catalytic dechlorination of carbon tetrachloride to combustible hydrocarbons by Pd-Fe hydroxides through atomic hydrogen attack and direct electron transfer

Carbon tetrachloride (CT), a common persistent pollutant in groundwater, has attracted widespread attention from environmental researchers. The dechlorination from CT to combustible hydrocarbons not only represents the removal of contaminants, but also offers the possibility of recovering carbon resources from water. In this work, we used the coprecipitation of Fe(II) and Pd(II) to synthesize Pd-Fe hydroxides for the dechlorination of CT. When Pd and Fe dosages were 0.4 and 5 mM respectively, CT with 26 µM concentration can be fast removed within 5 min, and 43.3 % of CH4 and 11.6 % of C2H6 (mol%) were generated after 110 min. The alkaline condition was more conducive to the complete dechlorination of CT comparing with acidic and neutral conditions. During the olation process, Fe(II) as electron donor transferred electrons to Pd(II) to form Pd(0) clusters. Pd(0) active sites act on the binding of CT and transfer electron to H+/H2O to form adsorbed atomic hydrogen (Hads•). The adsorbed CT molecules on the surface of Pd-Fe hydroxides undergo continuous dechlorination to form CH4 and C2H6 through the attack of Hads• and direct electron transfer. In real groundwater applications, CT was rapidly removed, with CH4 and C2H6 yields reaching 24.5 % and 4.6 % (mol%), respectively. Encouragingly, incomplete dechlorinated products, including chloroform and dichloromethane, did not show signs of subsequent rebound even after 60 min. Furthermore, Pd-Fe hydroxides also exhibit resistance to the oxidation effects of dissolved oxygen, which demonstrates the promising application prospects.

Directional Management Self‐Additive Spacer Cations for Stable 2D Ruddlesden–Popper Perovskite Solar Cells with Efficiency over 21%

Abstract2D Ruddlesden–Popper perovskites are highly regarded materials for improving the stability of perovskite solar cells (PSCs). Wherein, self‐additive 2D perovskites have recently been proposed to provide substantial strategies for managing the crystallization kinetics and bulk defects. For a profound understanding of the formation mechanisms, herein, with selecting three self‐additive 2D perovskites as demonstrations, a comprehensive analysis of the self‐additive behavior combing experimental and theoretical calculations is conducted. Self‐additive 2D perovskites exhibit more suitable formation energies and strong interaction, which is conducive to realize the self‐additive effect to form more stable structure. As demonstrated, glycine (Gly)‐based spacer cations played a pivotal role in the nucleation and growth of 2D perovskites by adjusting the aggregation state of colloids precursor, resulting in excellent‐quality films with large average grain size (≈3 µm). Meanwhile, theoretical analysis of electronic distribution and binding energies (Eb) revealed that glycine ethyl ester (Gly‐E) perovskite possesses highly robust internal interactions, which will effectively mitigate defect formation and enhance device stability. Endowing with the above outstanding feature, Gly‐E devices exhibited an optimized PCE of 21.60%, one of the highest PCEs among all 2D RP PSCs (n ≤ 6). The findings provide a basis for the rational design of self‐additive 2D perovskites and achieving highly‐performance 2D RP PSCs.

Light one-electron molecular ions within the finite-basis-set method for the two-center Dirac equation

During the last decades outstanding results on the precision description of light diatomic molecular compounds have been achieved. The most advanced calculations of electron binding energies have been realized mainly in the framework of the nonrelativistic approach with a consistent account of relativistic and radiative QED corrections. Recently, it has been shown that methods based on the Dirac equation are also suitable for obtaining highly accurate results in simple light molecules. In this paper, we present a completely relativistic method and discuss its application to the description of diatomic systems. In particular, the electronic spectra of the light one-electron quasi-molecular compounds H-H+, He+-He2+ and He+-H+ are analyzed. For this purpose, the two-center Dirac equation is solved by a dual-kinetic balanced finite-basis-set method for axially symmetric systems, called A-DKB. This method allows for a complete relativistic consideration of the electron at fixed inter-nuclear distances. A comparison of the obtained results with the nonrelativistic and relativistic calculations presented in the literature is performed. Without pursuing the goal of high accuracy calculations, the advantages and disadvantages of the approach, as well as possible applications of the method, are discussed in detail.

Optical Manipulation of Bipolarons in a System with Nonlinear Electron-Phonon Coupling.

We investigate full quantum mechanical evolution of two electrons nonlinearly coupled to quantum phonons and simulate the dynamical response of the system subject to a short spatially uniform optical pulse that couples to dipole-active vibrational modes. Nonlinear electron-phonon coupling can either soften or stiffen the phonon frequency in the presence of electron density. In the former case, an external optical pulse tuned just below the phonon frequency generates attraction between electrons and leads to a long-lived bound state even after the optical pulse is switched off. It originates from a dynamical modification of the self-trapping potential that induces a metastable state. By increasing the pulse frequency, the attractive electron-electron interaction changes to repulsive. Two sequential optical pulses with different frequencies can switch between attractive and repulsive interaction. Finally, we show that the pulse-induced binding of electrons is shown to be efficient also for weakly dispersive optical phonons, in the presence anharmonic phonon spectrum and in two dimensions.

Different mechanisms of A-site and B-site high entropy effect on radiation tolerance of pyrochlores

The properties of high entropy pyrochlore have been studied extensively, but there is no consistent conclusion on its radiation tolerance. Besides, the mechanism of the high entropy effect on the radiation tolerance of pyrochlore is still unclear. In this work, combined with experiments and calculations of pyrochlores with similar cationic radius ratios, the A-site and B-site high entropy effects on structural evolution under irradiation are analyzed. In situ irradiation experiments were carried out on A-site high entropy pyrochlores such as (La0.2Nd0.2Sm0.2Gd0.2Er0.2)2Zr2O7, B-site Gd2(Ti1/3Sn1/3Zr1/3)2O7, and ternary pyrochlore Sm2Zr2O7 and Gd2Sn2O7 for comparison. The A-site high entropy pyrochlore can maintain a stable structure under high fluence irradiation like corresponding ternary pyrochlore, demonstrated by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), energy dispersive spectroscopy (EDS) mapping and Raman spectrum. The additional irradiation experiments on A-site high entropy pyrochlores (La1/3Nd1/3Gd1/3)2Zr2O7 and (Nd1/3Sm1/3Gd1/3)2Zr2O7 also confirm the similarity under irradiation between A-site high entropy and ternary pyrochlores. However, the B-site high entropy pyrochlore Gd2(Ti1/3Sn1/3Zr1/3)2O7 becomes amorphous at exceptionally low irradiation fluences, indicating a significantly distinct radiation tolerance compared with the A-site high entropy. The difference between the A-site and B-site high entropy effect is analyzed from cationic lattice distortion, bond strength, and inner electron binding energy by first-principles calculations. The results reveal the role and mechanism of the high entropy effect in pyrochlores and lay a foundation for material design and future applications.

New-Generation Electron-Propagator Methods for Molecular Electron-Binding Energies.

A new generation of electron-propagator methods for the calculation of electron binding energies has surpassed its antecedents with respect to accuracy, efficiency, and interpretability. No adjustable parameters are introduced in these fully ab initio procedures. Numerical tests versus several databases of valence, vertical electron binding energies of closed-shell molecules and atoms have been performed. Easily interpreted self-energy approximations with cubic arithmetic scaling produce mean absolute errors (MAEs) of 0.2 and 0.3 eV for electron detachments and attachments, respectively. The most accurate explicitly renormalized methods with fifth-power arithmetic scaling yield MAEs below 0.1 eV for detachments and attachments. Approximate renormalization leads to more efficient fifth-power alternatives for electron detachments that achieve similar accuracy with fewer bottleneck operations. Composite protocols generate excellent predictions versus highly accurate basis-extrapolated standards and experiments. The validity of the diagonal self-energy approximation and the accuracy of the approximate renormalizations are confirmed. The success of these perturbative methods based on canonical Hartree-Fock orbitals rests on a Hermitized, intermediately normalized superoperator metric. The results of all of the new-generation calculations may be analyzed in terms of final-state orbital relaxation and differential correlation effects.

Observation of a super-tetrahedral cluster of acetonitrile-solvated dodecaborate dianion via dihydrogen bonding

We launched a combined negative ion photoelectron spectroscopy and multiscale theoretical investigation on the geometric and electronic structures of a series of acetonitrile-solvated dodecaborate clusters, i.e., B12H122−·nCH3CN (n = 1–4). The electron binding energies of B12H122−·nCH3CN are observed to increase with cluster size, suggesting their enhanced electronic stability. B3LYP-D3(BJ)/ma-def2-TZVP geometry optimizations indicate each acetonitrile molecule binds to B12H122− via a threefold dihydrogen bond (DHB) B3–H3 ⁝⁝⁝ H3C–CN unit, in which three adjacent nucleophilic H atoms in B12H122− interact with the three methyl hydrogens of acetonitrile. The structural evolution from n = 1 to 4 can be rationalized by the surface charge redistributions through the restrained electrostatic potential analysis. Notably, a super-tetrahedral cluster of B12H122− solvated by four acetonitrile molecules with 12 DHBs is observed. The post-Hartree–Fock domain-based local pair natural orbital- coupled cluster singles, doubles, and perturbative triples [DLPNO-CCSD(T)] calculated vertical detachment energies agree well with the experimental measurements, confirming the identified isomers as the most stable ones. Furthermore, the nature and strength of the intermolecular interactions between B12H122− and CH3CN are revealed by the quantum theory of atoms-in-molecules and the energy decomposition analysis. Ab initio molecular dynamics simulations are conducted at various temperatures to reveal the great kinetic and thermodynamic stabilities of the selected B12H122−·CH3CN cluster. The binding motif in B12H122−·CH3CN is largely retained for the whole halogenated series B12X122−·CH3CN (X = F–I). This study provides a molecular-level understanding of structural evolution for acetonitrile-solvated dodecaborate clusters and a fresh view by examining acetonitrile as a real hydrogen bond (HB) donor to form strong HB interactions.

Asymmetric response of transition metal cationic orbitals to applied electric field

Understanding the quantum mechanical mechanisms underlying atomic/ionic interfacial processes and phenomena, particularly their dependence on the electronic orbital rearrangement of atoms/ions in an external electric field, remains a significant challenge. This study investigated the asymmetric response of transition metal (TM) cationic orbitals when subjected to an applied electric field. Quantum mechanical calculations were employed to quantify the newly formed hybrid orbitals and evaluate the corresponding orbital energies of the TM cations. Analysis of the quantitative contribution of asymmetric orbital hybridization to TM–surface interactions showed a significant change in orbital energy and increased effective charges of TM cations at the charged surface. This asymmetric response, induced by a negative external electric field generated from the structural charges of clay minerals (e.g., montmorillonite), repels electrons from the outer-shell orbital. This repulsion consequently increases the electron binding energy of the inner-shell orbitals, leading to new surface reactions, polarization-enhanced induction force, and polarization-induced covalent bonding between the TM cations and the charged surface. Our theoretical predictions regarding TM–clay mineral interactions are consistent with the experimental observations of TM cation adsorption. This finding has significant implications for the adsorptive removal of TM cations from wastewaters and for enhancing the catalytic efficiency of TM–surface catalysts. The unique physical and chemical characteristics exhibited by TMs at charged particle surfaces, resulting from their asymmetric response, can play pivotal roles in environmental and chemical engineering.

Deprotonated sulfamic acid and its homodimers: Does sulfamic acid adopt zwitterion during cluster growth?

We present a joint experimental and computational study on the geometric and electronic structures of deprotonated sulfamic acid (SA) clusters [(SA)n–H]− (n = 1, 2) employing negative ion photoelectron spectroscopy and high-level ab initio calculations. The photoelectron spectra provide the vertical/adiabatic detachment energy (VDE/ADE) of the sulfamate anion (SM−) H2N●SO3− at 4.85 ± 0.05 and 4.58 ± 0.08 eV, respectively, and the VDE and ADE of the SM−●SA dimer at 6.41 ± 0.05 and 5.87 ± 0.08 eV, respectively. The significantly increased electron binding energies of the dimer confirm the enhanced electronic stability upon the addition of one SA molecule. The CCSD(T)-predicted VDEs/ADEs agree excellently with the experimental data, confirming the identified structures as the most stable ones. Two types of dimer isomers possessing different hydrogen bonding (HB) motifs are identified, corresponding to SM− binding to a zwitterionic SA (SM−●SAz) and a canonical SA (SM−●SAc), respectively. Two N–H⋯O HBs and one superior O–H⋯O HB are formed in the lowest-lying SM−●SAc, while SM−●SAz has three moderate N–H⋯O HBs, with the former being 4.71 kcal/mol more stable. Further theoretical analyses reveal that the binding strength advantage of SM−●SAc over SM−●SAz arises from its significant contributions of orbital interactions between fragments, illustrating that sulfamate strongly interacts with its parent SA acid and preferably chooses the canonical SA in the subsequent cluster formations. Given the prominent presence of SA, this study provides the first evidence that the canonical dimer model of sulfamic acid should exist as a superior configuration during cluster growth.

Theoretical Investigation on excited dipole bound states of alkali-containing diatomic anions

Abstract Information about electronic excited states of molecular anions plays an important role in investigating the electron attachment and detachment processes. Here we present a high-level theoretical study on the electronic structures of twelve alkali-metal-containing diatomic anions MX- (MX = LiH, LiF, LiCl, NaF, NaCl, NaBr, RbCl, KCl, KBr, RbI, KI, and CsI). The equation-of-motion electron-attachment coupled-cluster singles and doubles (EOM-EA-CCSD) method is carried out to calculate the electron binding energies (EBEs) of 10 electronic excited states of each of the twelve molecule anions. With addition of different s-/p-/d- diffusion functions in the basis set, we have identified possible excited dipole bound states (DBSs) of each anion. With the investigation of EBEs on the twelve MXs with dipole moment (DM) up to 12.1 D, we evaluate the dependence of the number of anionic excited DBSs on molecular DM. The results indicate that, there exists at least two or three DBSs of anions with the molecular DM larger than 7 D and a molecule with DM > 10 D can sustain a π-DBS of the anion. Our study would shed some implications on the excited DBSs electronic states of alkali-metal-containing diatomic molecules.

Temperature-Dependent Kinetics for the Reactions of Fen- (n = 2-17) and FexNiy- (x + y = 3-9) with O2: Comparison of Pure and Mixed Metal Clusters with Relevance to Meteor Radio Afterglows and Surface Oxidation.

Rate constants and product branching fractions were measured from 300-600 K for Fen- + O2 (n = 2-17) and for 300-500 K for FexNiy- + O2 (x + y = 3-9) using a selected-ion flow tube (SIFT) apparatus. Rate constants for 46 species are reported. All rate constants increased with increasing temperature, and several were in excess of the Langevin-Gioumousis-Stevenson (LGS) capture rate at elevated temperatures. As with previously studied transition metal anion oxidation reactions, the collision limit is treated as the sum of the LGS limit along with a hard-sphere contribution, allowing for determination of activation energies. These values are compared to each other along with previous results for Nin-. Measured rate constants for all three series (Fen-, Nin-, and FexNy-) vary over a relatively narrow range (1-5 × 10-10 cm3 s-1 at 300 K) being at least 15% of the collision rate constant. All reaction rate constants increase with temperature, described by small activation energies of 0.5-4 kJ mol-1. The data are consistent with an anticorrelation between the electron binding energy and rate constant, previously noted in other systems. The Fen- reaction produces a larger population of higher energy electrons than do the Nin- reactions, with FexNiy- producing an intermediate amount. The results suggest that the overall rate constant is limited by a small energetic barrier located at a large internuclear distance where electrostatic forces dominate, causing the potentials to be similar across systems, while the product formation is determined by the shorter-range, valence portion of the potential, which varies widely between systems.