Exchange-repulsion Interactions Research Articles

Origins of Offset-Stacking in Porous Frameworks

Parallel-displaced π-stacking in the benzene dimer and larger polycyclic aromatic hydrocarbons is driven by competition between dispersion and exchange-repulsion interactions. The present work examines whether the same is true in porous frameworks that exhibit stacking interactions, including the [18]annulene dimer, porphyrin dimer, and several models of the covalent organic framework known as COF-1. Interaction energies and their components are computed using extended symmetry-adapted perturbation theory along two-dimensional scans representing slip-stacking. As in the polycyclic aromatic hydrocarbons studied previously, we find that the van der Waals interaction potential (defined as the sum of dispersion and Pauli repulsion) drives the system into a slip-stacked geometry. Electrostatics is a relatively small component of the total interaction energy. In the case of COF-1, the van der Waals potential drives the conformational preference whether or not a solvent molecule intercalates into the framework, although the presence of the guest (mesitylene) molecule substantially limits the low-energy slip-stacking configurations that are available. Even when the COF-1 pore is empty, a modest lateral offset of ≲1.5 Å is preferred, which is small compared to the pore size.

Intermolecular interactions in clusters of ethylammonium nitrate and 1-amino-1,2,3-triazole.

The intermolecular interaction energies, including hydrogen bonds (H-bonds), of clusters of the ionic liquid ethylammonium nitrate (EAN) and 1-amino-1,2,3-triazole (1-AT) based deep eutectic propellants (DeEP) are examined. 1-AT is introduced as a neutral hydrogen bond donor (HBD) to EAN in order to form a eutectic mixture. The effective fragment potential (EFP) is used to examine the bonding interactions in the DeEP clusters. The resolution of the Identity (RI) approximated second order Møller-Plesset perturbation theory (RI-MP2) and coupled cluster theory (RI-CCSD(T)) are used to validate the EFP results. The EFP method predicts that there are significant polarization and charge transfer effects in the EAN:1-AT complexes, along with Coulombic, dispersion and exchange repulsion interactions. The EFP interaction energies are in good agreement with the RI-MP2 and RI-CCSD(T) results. The quasi-atomic orbital (QUAO) bonding and kinetic bond order (KBO) analyses are additionally used to develop a conceptual and semi-quantitative understanding of the H-bonding interactions as a function of the size of the system. The QUAO and KBO analyses suggest that the H-bonds in the examined clusters follow the characteristic hydrogen bonding three-center four electron interactions. The strongest H-bonding interactions between the (EAN)1:(1-AT)n and (EAN)2:(1-AT)n (n = 1-5) complexes are observed internally within EAN; that is, between the ethylammonium cation [EA]+ and the nitrate anion ([NO3]-). The weakest H-bonding interactions occur between [NO3]- and 1-AT. Consequently, the average strengths of the H-bonds within a given (EAN)x:(1-AT)n complex decrease as more 1-AT molecules are introduced into the EAN monomer and EAN dimer. The QUAO bonding analysis suggests that 1-AT in (EAN)x:(1-AT)n can act as both a HBD and a hydrogen bond acceptor simultaneously. It is observed that two 1-AT molecules can form H-bonds to each other. Although the KBOs that correspond to H-bonding interactions in [EA]+:1-AT, [NO3]-:1-AT and between two 1-AT molecules are weaker than the H-bonds in EAN, those weak H-bond networks with 1-AT could be important to form a stable DeEP.

The mutual interactions of RNA, counterions and water - quantifying the electrostatics at the phosphate-water interface.

The structure and dynamics of polyanionic biomolecules, like RNA, are decisively determined by their electric interactions with the water molecules and the counterions in the environment. The solvation dynamics of the biomolecules involves a subtle balance of non-covalent and many-body interactions with structural fluctuations due to thermal motion occurring in a femto- to subnanosecond time range. This complex fluctuating many particle scenario is crucial in defining the properties of biological interfaces with far reaching significance for the folding of RNA structures and for facilitating RNA–protein interactions. Given the inherent complexity, suited model systems, carefully calibrated and benchmarked by experiments, are required to quantify the relevant interactions of RNA with the aqueous environment. In this feature article we summarize our recent progress in the understanding of the electrostatics at the biological interface of double stranded RNA (dsRNA) and transfer RNA (tRNA). Dimethyl phosphate (DMP) is introduced as a viable and rigorously accessible model system allowing the interaction strength with water molecules and counterions, their relevant fluctuation timescales and the spatial reach of interactions to be established. We find strong (up to ≈90 MV cm−1) interfacial electric fields with fluctuations extending up to ≈20 THz and demonstrate how the asymmetric stretching vibration νAS(PO2)− of the polarizable phosphate group can serve as the most sensitive probe for interfacial interactions, establishing a rigorous link between simulations and experiment. The approach allows for the direct interfacial observation of interactions of biologically relevant Mg2+ counterions with phosphate groups in contact pair geometries via the rise of a new absorption band imposed by exchange repulsion interactions at short interatomic distances. The systematic extension to RNA provides microscopic insights into the changes of the hydration structure that accompany the temperature induced melting of the dsRNA double helix and quantify the ionic interactions in the folded tRNA. The results show that pairs of negatively charged phosphate groups and Mg2+ ions represent a key structural feature of RNA embedded in water. They highlight the importance of binding motifs made of contact pairs in the electrostatic stabilization of RNA structures that have a strong impact on the surface potential and enable the fine tuning of the local electrostatic properties which are expected to be relevant for mediating the interactions between biomolecules.

Adsorption of ionic liquids on carbonaceous surfaces: The effect of curvature on selective anion⋯π and cation⋯π interactions

The investigation of interface materials is very important for the development of functional materials for energy storage applications. In this work, the microscopic adsorption mechanism of various ionic liquids (ILs) on the carbonaceous surface such as graphene sheet (G) and carbon nanotube (CNT) models are investigated using density functional theory (DFT) method. To study the interfacial interaction between ILs and different carbon surfaces, we have selected dodecabenzocoronene (DBC) as a model surface for graphene sheet and curved surface for CNT. In order to understand the adsorption mechanism and effect of curvature on the stability of the complexes, we used hydrophobic and hydrophilic ILs ([MMIm]+[X]−, where X = DCA, Cl, OTf, PF6 and BF4) at these surfaces. It is found from our calculations that the adsorption mechanism of ILs at DBC and curved surfaces is different and it depends on the existence of fluoro and non-fluoro anions in ILs. The adsorption of ILs on DBC surface is mostly dominated by dispersive nature, whereas ILs@CNT predominated by electrostatic and exchange-repulsion interactions which is confirmed by energy decomposition analysis (EDA). The selection of anion play a significant role in the stability of the complexes and also this will affect the interfacial interaction between ion-pair and surfaces. Close scrutiny of the results reveal that hydrophilic ILs are strongly adsorbed on both the surface whereas in the case of hydrophobic ILs, the cationic part of ILs ([MMIm]+) predominantly interacted with surface, except [MMIm]+[OTf]−. The oxygen atom of [OTf]− anion directly interacts with the edge of carbon surface which is confirmed through EDA, AIM and MESP analyses.

Exchange-repulsion energy in QM/EFP.

The effective fragment potential (EFP) is a quantum mechanics (QM)-based model designed to accurately describe intermolecular interactions. Hybrid QM/EFP calculations combine quantum mechanical methods with an EFP embedding to study complex systems in which many-body effects are relevant. As in EFP-only calculations, non-bonded interactions between the QM region and EFP fragments are computed as a sum of electrostatic, polarization, dispersion, and exchange-repulsion energies. The exchange-repulsion term is a computational bottleneck of the EFP calculations. Here, we present a general procedure for computing the QM/EFP exchange-repulsion interactions based on one-electron contributions to the QM Hamiltonian, by using Gaussian functions to represent localized molecular orbitals of the effective fragments. The accuracy of the exchange-repulsion and total QM/EFP interaction energies is evaluated on a diverse set of dimers, including complexes from the S22 dataset of non-covalent interactions. In most cases, the QM/EFP energies are at least as accurate as corresponding EFP energies. A simple and computationally efficient form of the introduced QM/EFP exchange-repulsion term will facilitate further developments and applications of QM/EFP methods.

Synergy of Electrostatic and van der Waals Interactions in the Adhesion of Epoxy Resin with Carbon-Fiber and Glass Surfaces

Abstract The adhesion between epoxy resin and carbon fiber is investigated by using pair interaction energy decomposition analysis (PIEDA), by which the adhesive interaction energy and adhesive force can be partitioned into the electrostatic, exchange-repulsion, charge-transfer, and van der Waals (dispersion) contributions. The three stabilizing electrostatic, charge-transfer, and dispersion effects are correlated with the destabilizing exchange-repulsion effect. The surface structures of carbon fiber are modeled by the basal face, the armchair-edge structure, and the OH-functionalized armchair-edge structure of graphite. The surface of α-cristobalite (covered with OH groups), which can be viewed as a good model of a hydrophilic glass surface, is also studied. Adhesive properties of the model interfaces are evaluated on the basis of the binding energy of the resin with the carbon and glass surfaces and the adhesive force acting at the interfaces in terms of energy decomposition. PIEDA calculations demonstrate that only dispersion interactions can substantially work in the hydrophobic surfaces of the basal face and armchair-edge structures. This is a direct consequence of the electrostatic and charge-transfer interactions being cancelled by the exchange-repulsion interactions. On the other hand, both electrostatic and dispersion interactions are significant in the OH-functionalized surfaces of the armchair edge of graphite and α-cristobalite.

Symmetry-Adapted Perturbation Theory Energy Analysis of Alkyl Fluorine-Aromatic Interactions in Torsion Balance Systems.

Symmetry-adapted perturbation theory (SAPT) calculations are carried out to elucidate the intermolecular interactions present between fluorinated and nonfluorinated alkyl chain groups and aromatic π systems in the folded and unfolded conformers of Wilcox torsion balance systems. The calculations predict the folded conformers to be 2.0-2.3 kcal/mol more stable than the unfolded conformers, with the preference for the folded conformer being greater in the fluorinated alkyl chain case. We also establish that a simple electrostatic analysis, based on atomic charges, is inadequate for understanding the conformational preferences of these systems. In the folded conformers, there are sizable charge penetration contributions that are not recovered by point charge models. Additionally, the SAPT analysis reveals that exchange-repulsion interactions make a significant contribution to the relative stability of the folded and unfolded conformer.

Theoretical study of the interaction in HCCH...X– (X = F, Cl, Br, I) hydrogen-bonded anion–molecule complexes and calculation of the complex formation energy within an electrostatic model

An electrostatic model is proposed to calculate the formation energy ΔЕ of linear HCCH…Х– (X = F, Cl, Br, I) hydrogen-bonded anionic complexes between a halide ion and an acetylene molecule. The potential energy surface (PES) in this model includes four kinds of interaction in the HCCH…Х– system, namely, electrostatic (Coulombic), polarization, dispersive, and exchange repulsion interactions. An analytical expression for the formation energy ΔЕ of the complexes is obtained, and the PES parameters necessary for calculating ΔЕ are determined. The calculated ΔЕ values are in good agreement with ab initio data from the literature. The relative contributions from the different types of interactions to ΔЕ are estimated and analyzed. The electrostatic (Coulombic) interaction makes the main contribution to the formation energy of HCCH…Х– clusters.

Rubrene: The Interplay between Intramolecular and Intermolecular Interactions Determines the Planarization of Its Tetracene Core in the Solid State.

Rubrene is one of the most studied molecular semiconductors; its chemical structure consists of a tetracene backbone with four phenyl rings appended to the two central fused rings. Derivatization of these phenyl rings can lead to two very different solid-state molecular conformations and packings: One in which the tetracene core is planar and there exists substantive overlap among neighboring π-conjugated backbones; and another where the tetracene core is twisted and the overlap of neighboring π-conjugated backbones is completely disrupted. State-of-the-art electronic structure calculations show for all isolated rubrene derivatives that the twisted conformation is more favorable (by -1.7 to -4.1 kcal mol(-1)), which is a consequence of energetically unfavorable exchange-repulsion interactions among the phenyl side groups. Calculations based on available crystallographic structures reveal that planar conformations of the tetracene core in the solid state result from intermolecular interactions that can be tuned through well-chosen functionalization of the phenyl side groups and lead to improved intermolecular electronic couplings. Understanding the interplay of these intramolecular and intermolecular interactions provides insight into how to chemically modify rubrene and similar molecular semiconductors to improve the intrinsic materials electronic properties.

Accurate First Principles Model Potentials for Intermolecular Interactions

The general effective fragment potential (EFP) method provides model potentials for any molecule that is derived from first principles, with no empirically fitted parameters. The EFP method has been interfaced with most currently used ab initio single-reference and multireference quantum mechanics (QM) methods, ranging from Hartree-Fock and coupled cluster theory to multireference perturbation theory. The most recent innovations in the EFP model have been to make the computationally expensive charge transfer term much more efficient and to interface the general EFP dispersion and exchange repulsion interactions with QM methods. Following a summary of the method and its implementation in generally available computer programs, these most recent new developments are discussed.

Buckyplates and Buckybowls: Examining the Effects of Curvature on π–π Interactions

π-π interactions are integral to many areas of chemistry, biochemistry, and materials science. Here we use electronic structure theory to analyze how π-π interactions change as the π-systems are curved in model complexes based on coronene and corannulene dimers. Curvature redistributes electronic charge in the π-cloud and creates a dipole moment in these systems, leading to enhanced intermolecular electrostatic interactions in the concave-convex (nested) geometries that are the focus of this work. Curvature of both monomers also has a geometric effect on the interaction by decreasing the average C-C distance between monomers and by increasing the magnitude of both favorable London dispersion interactions and unfavorable exchange-repulsion interactions. Overall, increasing curvature in nested π-π interactions leads to more favorable interaction energies regardless of the native state of the monomers, except at short distances where the most highly curved systems are less favorable as exchange repulsion terms begin to dominate the interaction.

Interpretation of Intermolecular Geometric Isotope Effect in Hydrogen Bonds: Nuclear Orbital plus Molecular Orbital Study

The intermolecular geometric isotope effect (GIE) in hydrogen bond A-X···B (X = H and D) is investigated theoretically using the nuclear orbital plus molecular orbital (NOMO) theory. To interpret the GIE in terms of physically meaningful energy components such as electrostatic and exchange-repulsion interactions, the reduced variational space self-consistent-field method is extended to the NOMO scheme. The intermolecular GIE is analyzed as a two-stage process: the intramolecular bond shrinkage and the intermolecular bond elongation. According to the isotopic shifts of energy components described by the NOMO/MP2 method, the intermolecular GIE is approximately interpreted as a process reducing the exchange-repulsion interaction after the decrease of electrostatic interaction.

Improving the Point-Charge Description of Hydrogen Bonds by Adaptive Force Matching

The MP2f water force field, recently developed using the adaptive force matching method, is improved significantly after the introduction of a short-range repulsion term. This term is designed to address the deficiency of point-charge models at short charge-charge separations. It may also provide a more realistic description of hydrogen bonds by capturing exchange-repulsion interactions. When compared with MP2f, the new MP2f_hb force field predicts properties of liquid water in much better agreement with experiments. The calculated site-site radial distribution functions are in good agreement with X-ray diffraction data corroborating the argument that point-charge models with only Lennard-Jones repulsion underestimate the nearest neighbor O-O distance. The incorporation of the new short-range repulsion term is argued to be important for any force field model that uses point charges to model electrostatics.

Accurate ab initio Calculations of Methane Dimer Interaction Energies and Molecular Dynamics Simulation of Fluid Methane

AbstractIntermolecular interaction potentials of the methane dimers have been calculated for 12 symmetric conformations using the Hartree-Fock (HF) self-consistent theory, the second-order M�ller-Plesset (MP2) perturbation theory, and the coupled-cluster with single and double and perturbative triple excitations (CCSD(T)) theory. The HF calculations yield unbound potentials largely due to the exchange-repulsion interaction. In MP2 and CCSD(T) calculations, the basis set effects on the repulsion exponent, the equilibrium bond length, the binding energy, and the asymptotic behavior of the calculated intermolecular potentials have been thoroughly studied. We have employed basis sets from the Slater-type orbitals fitted with Gaussian functions, Pople�s medium size basis sets to Dunning�s correlation consistent basis sets. With increasing basis size, the repulsion exponent and the equilibrium bond length converge at the 6-31G** basis set and the 6-311++G(2d, 2p) basis set, respectively, while a large basis set (aug-cc-pVTZ) is required to converge the binding energy at a chemical accuracy (˜0.01 kcal/mol). We used the BSSE corrected results that systematically converge to the destined potential curve with increasing basis size. The binding energy calculated and the equilibrium bond length using the CCSD(T) method are close to the results at the basis set limit. For molecular dynamics simulation, a 4-site potential model with sites located at the hydrogen atoms was used to fit the ab initio potential data. This model stems from a hydrogen-hydrogen repulsion mechanism to explain the stability of the dimer structure. MD simulations using the ab initio PES show good agreement on both the atom-wise radial distribution functions and the self-diffusion coefficients over a wide range of experimental conditions.

Intermolecular potentials of the methane dimer calculated with Møller-Plesset perturbation theory and density functional theory

We have calculated the intermolecular interaction potentials of the methane dimer at the minimum-energy D(3d) conformation using the Hartree-Fock (HF) self-consistent theory, the correlation-corrected second-order Møller-Plesset (MP2) perturbation theory, and the density functional theory (DFT) with the Perdew-Wang (PW91) functional as the exchange or the correlation part. The HF calculations yield unbound potentials largely due to the exchange-repulsion interaction. In the MP2 calculations, the basis set effects on the repulsion exponent, the equilibrium bond length, the binding energy, and the asymptotic behavior of the calculated intermolecular potentials have been thoroughly studied. We have employed basis sets from the Slater-type orbitals fitted with Gaussian functions (STO-nG) (n=3-6) [Quantum Theory of Molecular and Solids: The Self-Consistent Field for Molecular and Solids (McGraw-Hill, New York, 1974), Vol. 4], Pople's medium size basis sets of Krishnan et al. [J. Chem. Phys. 72, 650 (1980)] [up to 6-311++G(3df,3pd)] to Dunning's correlation consistent basis sets [J. Chem. Phys. 90, 1007 (1989)] (cc-pVXZ and aug-cc-pVXZ) (X=D, T, and Q). With increasing basis size, the repulsion exponent and the equilibrium bond length converge at the 6-31G** basis set and the 6-311++G(2d,2p) basis set, respectively, while a large basis set (aug-cc-pVTZ) is required to converge the binding energy at a chemical accuracy (approximately 0.01 kcal/mol). Up to the largest basis set used, the asymptotic dispersion coefficient has not converged to the destined C6 value from molecular polarizability calculations. The slow convergence could indicate the inefficacy of using the MP2 calculations with Gaussian-type functions to model the asymptotic behavior. Both the basis set superposition error (BSSE) corrected and uncorrected results are presented to emphasize the importance of including such corrections. Only the BSSE corrected results systematically converge to the destined potential curve with increasing basis size. The DFT calculations generate a wide range of interaction patterns, from purely unbound to strongly bound, underestimating or overestimating the binding energy. The binding energy calculated using the PW91PW91 functional and the equilibrium bond length calculated using the PW91VP86 functional are close to the MP2 results at the basis set limit.

Intermolecular potentials from supermolecule and monomer calculations

The use of monomer properties to improve supermolecule calculations of intermolecular potentials is reviewed. For Van der Waals dimers, the MP2 supermolecule method is too inaccurate for most purposes, and the CCSD(T) supermolecule method requires too much computer time for large molecules. Using perturbation theory to analyse the MP2 supermolecule energy shows that the second-order dispersion energy is the main source of the inaccuracy. It is shown that the dispersion energy can be improved by using more accurate dispersion energy coefficients which can be obtained from monomer frequency-dependent polarizabilities. The supermolecule MP2 electrostatic and exchange-repulsion interaction energies can also be recalculated or scaled to a higher level of theory, using monomer charge densities. Applying these corrections to the MP2 supermolecule energy does not require much additional computer time, and gives potential energy surfaces with comparable accuracy to supermolecule CCSD(T) calculations. Possible extensions of the method to different supermolecule methods and to larger molecules are discussed.

An environmental pseudopotential approach to molecular interactions: Implementation in MOLPRO.

We present the implementation into the MOLPRO package of a model for the interaction of a central system with its surrounding environment. The properties of a target system enclosed by a noncovalently bound environment or solvent are modeled as those of a system embedded into the effective pseudopotential arising from the exact electrostatic Coulomb potential and the approximated exchange-repulsion potential. For the latter we use the charge-density overlap model, which relates the exchange-repulsion interaction energy between two species with the overlap of their ground-state electron charge densities. The solutions of the modified Hartree-Fock equations for the target system are obtained self-consistently. This way the exchange-induction effects arising from the converged electron-charge density of the embedded system are implicitly included. Inclusion of the correlation effects is provided by the use of post-Hartree-Fock and density-functional techniques available in the MOLPRO package. The computational and conceptual advantages provided by this approach are shown in the calculation of the dipole polarizabilities of halide and chalcogenide anions in different environments.

INTERMOLECULAR FORCES IN AGGREGATES OF ASPHALTENES AND RESINS

ABSTRACT In order to determine the forces acting in the asphaltene and resin molecular aggregates, an analysis of the origin of the intermolecular interactions in organic molecules was made. The results showed that the forces present in the aggregates of asphaltenes and resins were only those originated in the van der Waals, Coulombic (electrostatic), charge transfer and induction plus the exchange–repulsion interaction. No other forces were found to be significant in these aggregates that are formed by molecules containing well-known atomic groups. Molecular orbital calculations in resins and model asphaltene suggested that charge transfer is small or negligible in their aggregates. Although the H bonds present could be quite strong, other intermolecular interactions should not be disregarded in the analysis of the energy of their aggregates.

Structure of the chlorobenzene–argon dimer: Microwave spectrum and ab initio analysis

The rotational spectra of the Cl35 and Cl37 isotopes of the chlorobenzene–argon van der Waals dimer have been assigned using Fourier transform microwave spectroscopy techniques. Rotational constants and chlorine nuclear quadrupole coupling constants were determined which confirm that the complex has Cs symmetry. The argon is over the aromatic ring, shifted from a position above the geometrical ring center towards the substituted carbon atom, and at a distance of about 3.68 Å from it. This distance is 0.1–0.2 Å shorter than the similar distance in the benzene–argon and fluorobenzene–argon complexes. Experimental results are confirmed and explained with the help of second-order Mo/ller–Plesset perturbation calculations using a VDZP+diff basis set. The complex binding energy of the chlorobenzene–argon complex is 1.28 kcal/mol (fluorobenzene–argon, 1.17; benzene–argon, 1.12 kcal/mol) reflecting an increase in stability caused by larger dispersion interactions when replacing one benzene H atom by F or by Cl. The structure and stability of Ar⋅C6H5–X complexes are explained in terms of a balance between stabilizing dispersion and destabilizing exchange repulsion interactions between the monomers.

Flexing analysis of ethane internal rotation energetics

A flexing analysis of the ethane barrier energy in terms of structural (ΔEstruct), steric exchange (ΔEsteric), and hyperconjugative charge-transfer (ΔEdeloc) energy contributions has been carried out using natural bond orbitals. No evidence is found for the view that the ethane staggered equilibrium geometry or the C–C bond expansion that accompanies rotation results from steric exchange repulsion interactions. The analysis shows that ΔEstruct and ΔEdeloc have very different stereoelectronic dependencies, but that the ΔEsteric and ΔEdeloc dependencies are antagonistic. All of their contributions are strongly affected by the C–C bond expansion, with the result that the barrier mechanism cannot be understood without taking into account their different relaxation dependencies. Neglect of C–C expansion leaves the charge-transfer interactions paramount by subduing the steric and structural contributions. These interactions are found to be an important determinant for the expansion. The strong expansion dependence found for ΔEstruct is largely controlled by weakening of the C–C bond, and to a lesser extent by concomitant strengthening of the C–H bonds. Most of this dependence can be mimicked by C–C expansion in the absence of methyl torsion indicating that C–C bond weakening does not arise from the symmetry change accompanying ethane torsion.