Attractive Intermolecular Interactions Research Articles

Understanding Enantioselective Interactions by Pulling Apart Molecular Rotor Complexes.

Enantioselective interactions underpin many important phenomena from biological mechanisms to chemical catalysis. In this regard, there is great interest in understanding these effects at the molecular level. Surfaces provide a platform for these studies and aid in the long-term goal of designing heterogeneous enantiospecific interfaces. Herein we report a model system consisting of molecular rotors, one intrinsically chiral (propylene oxide) and one that becomes chiral when adsorbed on a surface (propene). Scanning tunneling microscopy (STM) measurements enable the chirality of each individual molecule to be directly visualized, and density functional theory based calculations are performed to rationalize the chiral time-averaged appearance of the molecular rotors. While there are no attractive intermolecular interactions between the molecular species themselves, when mixed together there is a strong preference for the formation of 1:1 heteromolecular pairs. We demonstrate that STM tip-induced molecular manipulations can be used to assemble these complexes, examine the chirality of each species, and thereby interrogate if their interactions are enantioselective. A statistical analysis of this data reveals that intrinsically chiral propylene oxide preferentially binds one of the enantiomers of propene with a 3:2 ratio, thereby demonstrating that the surface chirality of small nonchiral molecules can be directed with a chiral modifier. As such, this investigation sheds light onto previously reported ensemble studies in which chirally seeded layers of molecules that are achiral in the gas phase can lead to an amplification of enantioselective adsorption.

Unrecognized Intramolecular and Intermolecular Attractive Interactions between Fluorine-Containing Motifs and Ether, Carbonyl, and Amino Moieties.

In order to elucidate to what extent Coulombic and other interactions contribute to the origins of contrasteric phenomena, we have identified a significant, previously unrecognized interaction between fluorine-containing motifs and groups or molecules containing main-group heteroatoms. The axial conformers of both 2-methoxy- and 2-trifluoromethoxytetrahydropyrans preferentially adopt a rotameric arrangement in which the OCH3 and the OCF3 groups are gauche to the ring oxygen. Given that one would expect a repulsive Columbic interaction to exist between the electronegative fluorines of the CF3 group and the ring oxygen in this rotomeric orientation, this surprising result suggests that an attractive interaction exists between the CF3 group and the oxygen of the ring. The generality and origin of this interaction was examined using nonpolar CF4 to probe intermolecular interactions with systems such as dimethyl ether, trimethylamine, trimethylphosphine, and acetone. In each case there was an attractive interaction leading to formation of a complex. The attraction is not due to van der Waals forces. Rather, the fluorine lone pairs of the CF4 often act as an electron donor in these complexes leading to a transfer of charge between the reactants and formation of the complex. These previously unrecognized fluorine-heteroatom interactions likely play a significant role in the context of understanding the binding interactions of medicinally relevant molecules or pharmaceuticals possessing fluorine-containing pharmacophores with their targets.

A Better Understanding of the Unimolecular Dissociation Dynamics of Weakly Bound Aromatic Compounds at High Temperature: A Study on C6H6-C6F6 and Comparison with C6H6 Dimer.

Chemical dynamics simulations are performed to study the unimolecular dissociation of the benzene (Bz)-hexafluorobenzene (HFB) complex at five different temperatures ranging from 1000 to 2000 K, and the results are compared with that of the Bz dimer at common simulation temperatures. Bz-HFB, in comparison with Bz dimer, possesses a much attractive intermolecular interaction, a very different equilibrium geometry, and a lower average quantum vibrational excitation energy at a given temperature. Six low-frequency modes of Bz-HFB are formed by Bz + HFB association which are weakly coupled with the vibrational modes of Bz and HFB. However, this coupling is found much stronger in Bz-HFB compared to the same in the Bz dimer. The simulations are done with very good potential energy parameters taken from the literature. Considering the canonical (TST) model, the unimolecular dissociation rate constant at each temperature is calculated and fitted to the Arrhenius equation. An activation energy of 5.0 kcal/mol and a pre-exponential factor of 2.39 × 1012 s-1 are obtained, which are of expected magnitudes. The responsible vibrational mode for dissociation is identified by performing normal-mode analysis. Simulations with random excitations of high-frequency Bz and HFB modes and low-frequency inter-Bz-HFB vibrational modes of the Bz-HFB complex are also performed. The intramolecular vibrational energy redistribution (IVR) time and the unimolecular dissociation rate constants are calculated from these simulations. The latter shows good agreement with the same obtained from simulation with random excitation of all vibrational modes.

Thermal stability and interlayer exchange processes in heterolayers of TiOPc and PTCDA on Ag(1 1 1)

Organic–organic interfaces, in particular donor–acceptor type heterointerfaces, represent central elements of organic electronic devices. Understanding and controlling their physical properties is key to improve and modify their performance. For this purpose we have investigated the structural properties and thermal evolution of molecular heterointerface model systems. Specifically, various stacked titanyl-phthalocyanine (TiOPc)–3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) heterolayers grown on Ag(1 1 1) have been examined using Fourier transform infrared absorption spectroscopy and spot-profile analysis low-energy electron diffraction. An accurate description of the thermal evolution and prevalence of the various phases of TiOPc and PTCDA is derived from the spectral signatures of the two molecular species explored in previous studies. Exceptionally high thermal stability is found for stacked heterolayers comprising TiOPc double layers with characteristic up–down arrangement of its axial Ti–O unit and single layers of PTCDA. Thereby, interlayer exchange is negligible in a wide temperature range ( K). This is ascribed to attractive intermolecular interaction (dipole–dipole interaction, hydrogen bonding) among the TiOPc molecules oriented face-to-face within the TiOPc bilayer sheet. Stacked layers of comprising single layers of TiOPc and of PTCDA turn out to be thermally much less stable and more heterogeneous. In the course of their thermal evolution, a number of processes such as interlayer exchange, TiOPc double-layer formation and dewetting are encountered.

Isotropic Site-Site Dispersion Potential Constructed Using Quantum Chemical Calculations and a Geminal Auxiliary Basis Set

Abstract Dispersion interaction is one of the most important attractive intermolecular interactions. Because the dispersion interaction is always present, even for neutral molecules, and increases as the number of atoms in a molecule increases, accurate calculations with small computational costs are critical, especially for biosystems and condensed phase systems. In this study, we propose a site-site dispersion potential using a geminal auxiliary basis set that is local, isotropic, and free from empirical parameters. Our method correctly reproduced the Symmetry-Adapted Perturbation Theory (SAPT) data for C6 values and the dispersion energy surface between alkanes and alkenes.

Adhesion, forces and the stability of interfaces.

Weak molecular interactions (WMI) are responsible for processes such as physisorption; they are essential for the structure and stability of interfaces, and for bulk properties of liquids and molecular crystals. The dispersion interaction is one of the four basic interactions types – electrostatics, induction, dispersion and exchange repulsion – of which all WMIs are composed. The fact that each class of basic interactions covers a wide range explains the large variety of WMIs. To some of them, special names are assigned, such as hydrogen bonding or hydrophobic interactions. In chemistry, these WMIs are frequently used as if they were basic interaction types. For a long time, dispersion was largely ignored in chemistry, attractive intermolecular interactions were nearly exclusively attributed to electrostatic interactions. We discuss the importance of dispersion interactions for the stabilization in systems that are traditionally explained in terms of the “special interactions” mentioned above. System stabilization can be explained by using interaction energies, or by attractive forces between the interacting subsystems; in the case of stabilizing WMIs, one frequently speaks of adhesion energies and adhesive forces. We show that the description of system stability using maximum adhesive forces and the description using adhesion energies are not equivalent. The systems discussed are polyaromatic molecules adsorbed to graphene and carbon nanotubes; dimers of alcohols and amines; cellulose crystals; and alcohols adsorbed onto cellulose surfaces.

Attraction between electrophilic caps: A counterintuitive case of noncovalent interactions.

Intermolecular attractive interaction between electrophilic sites is a counterintuitive phenomenon, as the electrostatic interaction therein is repulsive and destabilizing. Here, we confirm this phenomenon in four representative complexes, using state-of-the-art quantum mechanical methods. By employing the block-localized wavefunction (BLW) method, which can turn off intermolecular charge transfer interactions, we profoundly demonstrated the significance of charge transfer interactions in these seemingly counterintuitive complexes. Indeed, after being "turned off" the intermolecular charge transfer interaction in, for example, the FNSi···BrF complex, the originally attractive intermolecular interaction turns to be repulsive. The energy decomposition approach based on the BLW method (BLW-ED) can partition the overall stability gained on the formation of intermolecular noncovalent interaction into several physically meaningful components. According to the BLW-ED analysis, the electrostatic repulsion in these counterintuitive cases is overwhelmed by the stabilizing polarization, dispersion interaction, and most importantly, the charge transfer interaction, resulting in the eventual counterintuitive overall attraction. The present study suggests that, predicting bonding sites of noncovalent interactions using only the "hole" concept may be not universally sufficient, because other significant stabilizing factors will contribute to the stability and sometimes, play even bigger roles than the electrostatic interaction and consequently govern the complex structures. © 2018 Wiley Periodicals, Inc.

Dispersion Stabilized Se/Te···π Double Chalcogen Bonding Synthons in in Situ Cryocrystallized Divalent Organochalcogen Liquids

We report the experimental observation and detailed quantum chemical analysis of the rare bifurcated type Se···π and Te···π “double” chalcogen bonded synthons observed through the in situ cryocrystallization of two room temperature organochalcogen liquids, namely, diphenylselenide and diphenyl telluride. The attractive intermolecular interactions between the heavier chalcogens and π-clouds are primarily stabilized by the dispersion forces, which is in contrast to many known model examples in the literature. Electrostatic and orbital delocalization effects also play a secondary but important role in the overall stabilization, besides providing directionality to these interactions.

On bond-critical points in QTAIM and weak interactions.

Bond critical points (BCPs) in the quantum theory of atoms in molecules (QTAIM) are shown to be a consequence of the molecular topology, symmetry, and the Poincaré-Hopf relationship, which defines the numbers of critical points of different types in a scalar field. BCPs can be induced by a polarizing field or by addition of a single non-bonded atom to a molecule. BCPs and their associated bond paths are therefore suggested not to be a suitable means of identifying chemical bonds, or even attractive intermolecular interactions. Graphical abstract Bond-critical points in QTAIM and weak interactionsᅟ.

Interfacial characterization of the molecular interactions in mixed monolayers of coumarin and phospholipids

Coumarin (CMR) is a small molecule with diverse biological functions as anti-tumor, anti-fungal, anticoagulant and antimicrobial activities. CMR is poorly water soluble molecule with low bioavailability. Lipid-based colloidal carriers play an important role in the delivery of hydrophobic compounds. Langmuir technique is an effective method to evaluate interactions between drugs and lipids. In this study, we used 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DPPG), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) lipids. Interfacial characterization was carried out based on surface pressure (π)-area (A) isotherms of pure and mixed films. Topographical analysis was performed using atomic force microscopy. The miscibility of the mixed films was dependent on their composition. The mixed films were stable due to the intermolecular attractive interactions. Our results contribute to the understanding of CMR-lipid interactions aiming to obtain stable colloidal carriers for drug delivery.

"Precipitation on Nanoparticles": Attractive Intermolecular Interactions Stabilize Specific Ligand Ratios on the Surfaces of Nanoparticles.

Confining organic molecules to the surfaces of inorganic nanoparticles can induce intermolecular interactions between them, which can affect the composition of the mixed self-assembled monolayers obtained by co-adsorption from solution of two different molecules. Two thiolated ligands (a dialkylviologen and a zwitterionic sulfobetaine) that can interact with each other electrostatically were coadsorbed onto gold nanoparticles. The nanoparticles favor a narrow range of ratios of these two molecules that is largely independent of the molar ratio in solution. Changing the solution molar ratio of the two ligands by a factor of 5 000 affects the on-nanoparticle ratio of these ligands by only threefold. This behavior is reminiscent of the formation of insoluble inorganic salts (such as AgCl), which similarly compensate positive and negative charges upon crystallizing. Our results pave the way towards developing well-defined hybrid organic-inorganic nanostructures.

“Precipitation on Nanoparticles”: Attractive Intermolecular Interactions Stabilize Specific Ligand Ratios on the Surfaces of Nanoparticles

AbstractConfining organic molecules to the surfaces of inorganic nanoparticles can induce intermolecular interactions between them, which can affect the composition of the mixed self‐assembled monolayers obtained by co‐adsorption from solution of two different molecules. Two thiolated ligands (a dialkylviologen and a zwitterionic sulfobetaine) that can interact with each other electrostatically were coadsorbed onto gold nanoparticles. The nanoparticles favor a narrow range of ratios of these two molecules that is largely independent of the molar ratio in solution. Changing the solution molar ratio of the two ligands by a factor of 5 000 affects the on‐nanoparticle ratio of these ligands by only threefold. This behavior is reminiscent of the formation of insoluble inorganic salts (such as AgCl), which similarly compensate positive and negative charges upon crystallizing. Our results pave the way towards developing well‐defined hybrid organic–inorganic nanostructures.

Macromolecular crowding effects in flexible polymer solutions

Biological environments are often “crowded” due to high concentrations (300–400[Formula: see text]g/L) of macromolecules. Computational modeling approaches like Molecular Dynamics (MD), rigid-body Brownian Dynamics and Monte Carlo simulations have recently emerged, which allow to study the effects macromolecular crowding at a microscopic level and to provide complementary information to experiments. Here, we use a recently introduced multiple-conformation Monte Carlo (mcMC) approach in order to study the influence of intermolecular interactions on the structural equilibrium of flexible polyethylene glycol (PEG) polymers under self-crowding conditions. The large conformational space accessible to PEG polymers allows us to evaluate the general applicability of the mcMC approach, which describes the intramolecular degrees of freedom by a finite-size ensemble of discrete conformations. Despite the simplicity of the approach, we show that influences of intermolecular interactions on the intramolecular free energy surface can be described qualitatively using mcMC. By varying the magnitude of distinct terms in the intermolecular potential, we can further study the compensating effects of repulsive and nonspecific attractive intermolecular interactions, which favor compact and extended polymer states, respectively. We use our simulation results to derive an analytical model that describes the effects of intermolecular interactions on the stability of PEG polymer conformations as a function of the radius of gyration and the corresponding solvent accessible surface. We use this model to confirm the role of molecular surfaces for attractive interactions that can counteract excluded volume effects. Extrapolation of the model further allows for the analysis of scenarios that are not easily accessible to direct simulations as described here.

Molecular weight-dependent physisorption of non-charged poly(9,9-dioctylfluorene) onto the neutral surface of cuboidal γ-alumina in toluene

To understand how polymers physisorb onto solid surfaces, we investigated the physisorption behavior of non-charged, semiflexible poly(9,9-dioctylfluorene) (PF8) with three different number-average degrees of polymerization (DPn) as photoluminescent and chromophoric probes onto cuboidal γ-alumina in toluene at 5, 25, and 50 °C. PF8 revealed noticeable DPn and temperature dependencies in its physisorption behaviors. Molecular mechanics (MM)/molecular dynamics (MD) simulations [consistent valence force field (CVFF)] and Moller–Plesset second-order perturbation theory (MP2) with 6-31 G(d,p) calculations suggested that the PF8 in toluene has multiple interactions from CH/π to C–H/O interactions on the (110) surface of γ-alumina. The competition between multiple intermolecular CH/π and C–H/O interactions was crucial for the spontaneous physisorption of PF8 to occur in the presence of a solvent quantity of toluene. Calculations by time-dependent density functional theory (TD-DFT) with Becke three parameter Lee-Yang-Par (B3LYP) method and 6–31 G(d,p) basis set of a model fluorene 9-mer indicated that the π–π* absorption wavelength largely depends on the regularity of the dihedral angles between fluorene rings, while the intensity and spectral width of the π–π* absorption band are largely influenced by the regularity of the dihedral angles. Solution-phase physisorption systems are a result of the inherent nature of several competitive weak intermolecular interactions coexisting among the polymers, surface, and solvents. Noticeable molecular weight and temperature dependency for physisorption behavior of semiflexible, non-charged poly(9,9-n-dioctylfluorene) (PF8) onto cuboidal γ-alumina in toluene were found. MM/MD simulations (CVFF) and quantum mechanical (MP2/6-31G(d,p)) calculations suggested that PF8 swaps interacting toluene for (110) surface of γ-alumina by changing the interaction from CH/π interactions to C-H/O interactions of γ-alumina in toluene. The competition between multiple intermolecular CH/π and C-H/O interactions was crucial whether the spontaneous physisorption of PF8 occurs in place of solvent quantity of toluene. The solution-phase physisorption systems should be the consequence of several weak attractive intermolecular interactions coexisting between polymers, surface, and solvents

Entropy-Driven Diastereoselectivity Improvement in the Paternò-Büchi Reaction of 1-Naphthyl Aryl Ethenes with a Chiral Cyanobenzoate through Remote Alkylation.

The precise stereocontrol of photocycloaddition reactions is still a significant challenge owing to their mechanistic complexity and the involvement of highly reactive and short-lived intermediates. Attempts have hitherto been made through structural modifications, mostly by introducing steric conflicts, to increase the difference between the enthalpic barriers. Herein, we show that entropy plays a crucial role in influencing the diastereoselectivity of a Paternò-Büchi reaction. Remote meta alkylation of the donor caused nominal changes in its photophysical properties as well as those of the exciplexes derived thereof. Nevertheless, the diastereomeric excess of the oxetane product was greatly improved by about 40 %. This enhancement, which is not accompanied by any significant changes in the photophysical properties, is difficult to rationalize by conventional enthalpic control concepts based on repulsive steric and/or attractive intermolecular interactions as well as electronic perturbations. Differential activation parameters and compensatory enthalpy-entropy relationships revealed that the diastereoselectivity enhancement is not simply enthalpic but also entropic in origin.

New fabrication approach to develop a high birefringence photo-crosslinked film based on a sulfur-containing liquid crystalline molecule with large temperature dependence of birefringence

ABSTRACTWe present a new fabrication approach to achieve a high birefringence film by means of photopolymerization based on an alkylthio-containing rod-like liquid crystalline molecule exhibiting large temperature dependence of birefringence. We designed a new reactive mesogen having alkylthio linkages (BPM–S). It was found that BPM–S had a larger increment of birefringence with decreasing temperature, relative to commercially available alkoxy analog LC242. This result could be thought to be due to enhanced intermolecular attractive interaction for an alkylthio mesogen implied by the proximity of laterally neighboring molecules and cybotactic nematic tendency based on wide-angle X-ray measurement. The uniaxially-aligned photo-polymerized film for BPM–S showed higher birefringence than that for LC 242.

Cluster Size and Quinary Structure Determine the Rheological Effects of Antibody Self-Association at High Concentrations.

The question of how nonspecific reversible intermolecular protein interactions affect solution rheology at high concentrations is fundamentally rooted in the translation of nanometer-scale interactions into macroscopic properties. Well-defined solutions of purified monoclonal antibodies (mAbs) provide a useful system with which to investigate the manifold intricacies of weak protein interactions at high concentrations. Recently, characterization of self-associating IgG1 antibody (mAb2) solutions has established the direct role of protein clusters on concentrated mAb rheology. Expanding on our earlier work with three additional mAbs (mAb1, mAb3, and mAb4), the observed concentration-dependent static light scattering and rheological data present a substantially more complex relationship between protein interactions and solution viscosity at high concentrations. The four mAb systems exhibited divergent correlations between cluster formation (size) and concentrated solution viscosities dependent on mAb primary sequence and solution conditions. To address this challenge, well-established features of colloidal cluster phenomena could be applied as a framework for interpreting our observations. The initial stages of mAb cluster formation were investigated with small-angle X-ray scattering (SAXS) and ensemble-optimized fit methods, to uncover shifts in the dimer structure populations which are produced by changes in mAb interaction modes and association valence under the different solution conditions. Analysis of mAb average cluster number and effective hydrodynamic radii at high concentrations revealed cluster architectures can have a wide range of fractal dimensions. Collectively, the static light scattering, SAXS, and rheological characterization demonstrate that nonspecific and anisotropic attractive intermolecular interactions produce antibody clusters with different quinary structures to regulate the rheological properties of concentrated mAb solutions.

Rheology of self-nucleated poly(ɛ-caprolactone) melts

This work presents novel findings on the rheological properties of self-nucleated polymer melts. Different polycaprolactones (PCL) with varying molar masses were investigated with shear rheometry, assessing the viscoelastic functions at different temperatures, reached either from the melt or from the semi-crystalline state. The latter condition produces a self-nucleated melt, in which residual clusters of unclear nature enhance the re-crystallization process, while a homogeneous melt is obtained with the first protocol. Striking differences are observed in melts with or without self-nuclei. The Newtonian viscosity and flow activation energy increase up to one order of magnitude with respect to the fully equilibrated melt. The plateau modulus also shows a remarkable increment, indicating an apparent decrease of the molecular weight between entanglements. The results demonstrate the existence of an additional attractive intermolecular interaction between chain segments in self-nucleated PCL melts, able to provide “extra” physical entanglements which progressively dissolve as the temperature is raised. Self-nucleated propene/ethylene copolymers analyzed in previous work display much weaker effects. The apparent discrepancy is discussed in light of the differences in intermolecular interactions and melting enthalpies between the two polymers.

Quaterrylene molecules on Ag(111): self-assembly behavior and voltage pulse induced trimer formation.

The self-assembly behavior of quaterrylene (QR) molecules on Ag(111) surfaces has been investigated by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. It is found that the QR molecules are highly mobile on the Ag(111) surface at 78 K. No ordered assembled structure is formed on the surface with a sub-monolayer coverage up to 0.8 monolayer due to the intermolecular repulsive interactions, whereas ordered molecular structures are observed at one monolayer coverage. According to our DFT calculations, charge transfer occurs between the substrate and the adsorbed QR molecule. As a result, out-of-plane dipoles appear at the interface, which are ascribed to the repulsive dipole-dipole interactions between the QR molecules. Furthermore, due to the planar geometry, the QR molecules exhibit relatively low diffusion barriers on Ag(111). By applying a voltage pulse between the tunneling gap, immobilization and aggregation of QR molecules take place, resulting in the formation of a triangle-shaped trimer. Our work demonstrates the ability of manipulating intermolecular repulsive and attractive interactions at the single molecular level.

Electric-field-controlled phase transition in a 2D molecular layer

Self-assembly of organic molecules is a mechanism crucial for design of molecular nanodevices. We demonstrate unprecedented control over the self-assembly, which could allow switching and patterning at scales accessible by lithography techniques. We use the scanning tunneling microscope (STM) to induce a reversible 2D-gas-solid phase transition of copper phthalocyanine molecules on technologically important silicon surface functionalized by a metal monolayer. By means of ab-initio calculations we show that the charge transfer in the system results in a dipole moment carried by the molecules. The dipole moment interacts with a non-uniform electric field of the STM tip and the interaction changes the local density of molecules. To model the transition, we perform kinetic Monte Carlo simulations which reveal that the ordered molecular structures can form even without any attractive intermolecular interaction.