Partitioning Solvophobic and Dispersion Forces in Alkyl and Perfluoroalkyl Cohesion

Advances and Prospects in Understanding London Dispersion Interactions in Molecular Chemistry

London dispersion (LD) interactions are the main contribution of the attractive part of the van der Waals potential. Even though LD effects are the driving force for molecular aggregation and recognition, the role of these omnipresent interactions in structure and reactivity had been largely underappreciated over decades. However, in the recent years considerable efforts have been made to thoroughly study LD interactions and their potential as a chemical design element for structures and catalysis. This was made possible through a fruitful interplay of theory and experiment. This review highlights recent results and advances in utilizing LD interactions as a structural motif to understand and utilize intra- and intermolecularly LD-stabilized systems. Additionally, we focus on the quantification of LD interactions and their fundamental role in chemical reactions.

Molecular Wind-Up Meter for the Quantification of London Dispersion Interactions

ConspectusThe experimental quantification of interactions on the molecular level provides the necessary basis for the design of functional materials and chemical processes. The interplay of multiple parameters and the small quantity of individual interactions pose a special challenge for such endeavors. The common method is the use of molecular balances, which can exist in two different states. Thereby, a stabilizing interaction can occur in one of the states, favoring its formation and thus affecting the thermodynamic equilibrium of the system. One challenge is determining the change in this equilibrium since various analytical methods could not be applied to fast-changing equilibria. A new and promising method for quantifying molecular interactions is the use of Molecular Wind-up Meters (MWM) in which the change in kinetics, rather than the effect on thermodynamics, is investigated. An MWM is transformed with an energy input (e.g. irradiation) into a metastable state. Then, the rate of thermal transformation back to the ground state is measured. The strength of interactions present in the metastable state controls the kinetics of the back reactions, allowing direct correlation. The advantage of this approach lies in the high sensitivity (energy differences can be larger by 1 order of magnitude) and, in general, allows the use of a broader range of solvents and analytical methods. An Azobenzene-based MWM has been established as a powerful tool to quantify London dispersion interactions. London dispersion (LD) represents the attractive part of the van der Waals potential. Although neglected in the past due to its weak character, it has been shown that the influence of LD on the structure, stability, and reactivity of matter can be decisive. Especially in larger molecules, its energy contribution increases overproportionately with the number of atoms, which has sparked increasing interest in the use of so-called dispersion energy donors (DED) as a new structural element. Application of the azobenzene-based MWM not only allowed the differentiation of bulkiness, but also systematically addressed the influence of the length of n-alkyl chains. Additionally, the solvent influence on LD was studied. Based on the azobenzene MWM, an increment system has been proposed, allowing a rough estimate of the effect of a specific DED.

Empirical Model of Solvophobic Interactions in Organic Solvents

An empirical model was developed to predict organic solvophobic effects using N-phenylimide molecular balances functionalized with non-polar alkyl groups. Solution studies and X-ray crystallography confirmed intramolecular alkyl-alkyl interactions in their folded conformers. The structural modularity of the balances allowed systematic variation of alkyl group lengths. Control balances were instrumental in isolating weak organic solvophobic effects by eliminating framework solvent-solute effects. A 19 F NMR label enabled analysis across 46 deuterated and non-deuterated solvent systems. Linear correlations were observed between organic solvophobic effects and solvent cohesive energy density (ced) as well as changes in solvent-accessible surface areas (SASA). Using these empirical relationships, a model was constructed to predict organic solvophobic interaction energy per unit area for any organic solvent with known ced values. The predicted interaction energies aligned with recent organic solvophobic measurements and literature values for the hydrophobic effect on non-polar surfaces confirmed the model's accuracy and utility.

Contributions of London Dispersion Forces to Solution-Phase Association Processes

ConspectusDespite their ubiquity and early discovery, London dispersion forces are often overlooked. This is due, in part, to the difficulty in assessing their contributions to molecular and polymeric structure, stability, properties, and reactivities. However, recent advances in modeling have revealed that dispersion interactions play an important role in many important chemical and biological processes. Experimental confirmation of their impact in solution has been challenging, leading to controversies about their relative importance.In the course of studying noncovalent interactions using molecular devices, our understanding and appreciation for the importance of dispersion interactions have evolved. This Account follows this intellectual journey by using examples from the literature. The goals are twofold: to describe recent advances in understanding the interaction and to provide guidance to researchers studying weak noncovalent interactions. However, first, the experimental methods for measuring the effects of dispersion interactions and the strategies for isolating their influence are described. These include the design of molecular devices to measure these weak noncovalent interactions and the strategies to disentangle the solvation, solvophobic, and dispersion components of the resulting equilibria.The literature examples are organized around five fundamental questions. (1) Do dispersion interactions have a measurable effect on solution equilibria? (2) To what extent do solvents attenuate or compensate for dispersion interactions? (3) To what extent do the solvation and solvophobic terms influence the dispersion equilibria? (4) Can we predict whether a system will form attractive dispersion or repulsive steric interactions? (5) Can the dispersion term be isolated and interrogated? We were often surprised by the answers to these questions. In each case, we describe how the systems were designed to address these questions and discuss possible interpretations of the results.While dispersion interactions in solution were weak (usually <1 kcal/mol), their influence on complexation and conformational equilibria can be observed and measured. This underscores the significance of these interactions in molecular recognition, coordination chemistry, reaction design, and catalysis. The solvent components of the dispersion equilibria can also be significant. Therefore, the isolation of the dispersion contributions from the solvation and solvophobic effects represents an ongoing challenge. The experimental studies also provide important benchmarks and offer valuable insights to help refine the next generation of computational solvent models.

Dispersion Interactions in Condensed Phases and inside Molecular Containers

ConspectusThe past decade has seen significant progress in the understanding and appreciation of the importance of London dispersion interactions (LDIs) in supramolecular systems and solutions. The Slater-Kirkwood formula relates LDIs to the molecular polarizabilities of the two interacting molecular species (α) and their interaction distance (a dependence of R-6). When advancing arguments related to intermolecular interactions, it is frequently assumed that molecules with larger molecular polarizabilities are more amenable to larger LDIs. However, arguments related to molecular polarizabilities are not always transferable to the condensed phase. In fact, the underlying bulk and molecular polarizabilities of common solvents show opposing trends. The intuitive concept that aromatic molecules are more polarizable than saturated hydrocarbons and that perfluorinated molecules are less polarizable than saturated hydrocarbons applies to the condensed phase only. When treating association phenomena in solution, where LDIs are generally very attenuated, the use of bulk polarizabilities is recommended, which are experimentally accessible through either refractive index measurements or suitable solvatochromic probes. Such probes can also be used to assess polarizabilities inside molecular container compounds, such as cucurbit[n]urils (CBn), cyclodextrins, calixarenes, and hemicarcerands. These macrocyclic cavities can have extreme microenvironments. For example, the inner concave phase of CB7 has been shown to be weakly polarizable, falling in between the gas phase and perfluorohexane; those of β-cyclodextrin and p-sulfonatocalix[4]arene have been found to be similarly polarizable as water and alkanes, respectively, and the inside of hemicarcerands displays a very large bulk polarizability, exceeding that of diiodomethane. CBn compounds are privileged molecular container compounds, which we exemplify in this Account through case studies. (1) CBn macrocycles are prime water-soluble receptors for hydrocarbons, allowing for the reduction of the binding free energies to two components: the hydrophobic effect and dispersion interactions. To understand hydrocarbon binding, we initiated the HYDROPHOBE challenge, which revealed the shortcomings of both quantum-chemical and molecular dynamics approaches. (2) The smallest CBn receptor, CB5, is uniquely suited to bind the entire noble gas series, where hydrophobic effects and dispersion interactions operate in opposite directions. CB5 was revaled to be a unique synthetic receptor for noble gases, with the dominant driving force being the recovery of the cavitation energies for the hydration of noble gases in aqueous solution. Computational methods that encounter challenges in predicting hydrocarbon affinities and trends for CB6 and CB7 perform well for noble gases binding to CB5. (3) The larger homologue, CB8, allows one to set up intermolecular interaction chambers by the encapsulation of a (first) aromatic guest, thereby tuning LDIs inside the receptor cavity. In this manner, CB8 can be modulated to preferentially bind unsaturated and aromatic rather than saturated hydrocarbons, while the unmodified cavities of the smaller macrocycles CB6 and CB7 show selective binding of saturated hydrocarbons. (4) The (charged) host-guest complexes of CBn hosts are sufficiently stable in the gas phase, allowing for the study of the influence of LDIs on inner-phase chemical reactions. These studies are particularly interesting for the theoretical analysis of isolated host-guest LDIs, as experimental and computational data are directly comparable in the gas phase due to the absence of the solvation effect.

Context-Dependent Significance of London Dispersion

ConspectusLondon forces constitute an attractive component of van der Waals interactions and originate from transient correlated momentary dipoles in adjacent atoms. The in-depth investigation of London dispersion forces poses notable challenges, especially in solution, owing to their inherently weak and competing character. Our objective in this Account is to shed light on the context-dependent significance of London dispersion forces by contrasting our own experimental findings with those from other research endeavors. Specifically, we will explore how factors such as the choice of system and solvent can influence the apparent role of London dispersion forces in molecular recognition processes. We initiate our Account by scrutinizing the Wilcox balance, which has yielded diverse and occasionally contradictory results. Following that, we provide an overview of the role of London dispersion forces and their context-dependent variations, encompassing alkyl-alkyl, halogen-π, alkyl-π, and aromatic stacking interactions.Several experimental investigations have revealed how difficult it is to measure the significance of London dispersion in solution. Indeed, dispersion forces seldom act as the exclusive driving force in molecular recognition processes, and solvation energetics also strongly influence equilibria and kinetics. Molecular balances that bring apolar functional groups into contact have proven to be instrumental in the experimental measurement of dispersion. The intramolecular approach avoids the need to pay the entropic cost of bringing interacting groups into contact, while also enabling solvent screening. Such experimental studies have found dispersion interactions between functional groups to be very weak (<5 kJ mol-1), meaning that they frequently take backstage to electrostatic contributions and solvophobic effects and are readily damped by competitive dispersion interactions with the solvent. By using such approaches, competitive dispersion interactions with the solvent have been shown to be described by the bulk polarizability of the solvent (perfluoroalkanes have the lowest bulk polarizabilities, while carbon disulfide has one of the highest). Dispersion interactions are also strongly distance-dependent, which results in considerable context-dependent outcomes across different investigations. For example, we caution against the risk of attributing the stability of a "more sterically hindered" isomer as arising from intramolecular dispersion forces. The total energy of the system can reveal other contributions to stability, such as nonintuitive minimization of strain elsewhere in the molecule. Indeed, the delicate distance-dependent balance between sterics and London dispersion means that even subtle changes in size and geometry can lead to disparate behavior. Similarly, solvophobic effects also contribute to stabilizing contacts between bulky functional groups, which can be revealed if there is a correlation with the cohesive energy density of the solvent.

Experimental Quantification of Halogen⋅⋅⋅Arene van der Waals Contacts

Crystallographic and computational studies suggest the occurrence of favourable interactions between polarizable arenes and halogen atoms. However, the systematic experimental quantification of halogen⋅⋅⋅arene interactions in solution has been hindered by the large variance in the steric demands of the halogens. Here we have synthesized molecular balances to quantify halogen⋅⋅⋅arene contacts in 17 solvents and solvent mixtures using 1 H NMR spectroscopy. Calculations indicate that favourable halogen⋅⋅⋅arene interactions arise from London dispersion in the gas phase. In contrast, comparison of our experimental measurements with partitioned SAPT0 energies indicate that dispersion is sufficiently attenuated by the solvent that the halogen⋅⋅⋅arene interaction trend was instead aligned with increasing exchange repulsion as the halogen increased in size (ΔGX ⋅⋅⋅Ph =0 to +1.5 kJ mol-1 ). Halogen⋅⋅⋅arene contacts were slightly less disfavoured in solvents with higher solvophobicities and lower polarizabilities, but strikingly, were always less favoured than CH3 ⋅⋅⋅arene contacts (ΔGMe ⋅⋅⋅Ph =0 to -1.4 kJ mol-1 ).

Exploring the Limits of Intramolecular London Dispersion Stabilization with Bulky Dispersion Energy Donors in Alkane Solution

We present an experimental study of a cyclooctatetraene-based molecular balance disubstituted with increasingly bulky tert-butyl (tBu), adamantyl (Ad), and diamantyl (Dia) substituents in the 1,4-/1,6-positions for which we determined the valence-bond shift equilibrium in n-hexane (hex), n-octane (oct), and n-dodecane (dod). Computations including implicit and explicit solvation support our temperature-dependent NMR equilibrium measurements indicating that the more sterically crowded 1,6-isomer is always favored, irrespective of solvent, and that the free energy is quite insensitive to substituent size.

The surface site interaction point approach to non-covalent interactions

The functional properties of molecular systems are generally determined by the sum of many weak non-covalent interactions, and therefore methods for predicting the relative magnitudes of these interactions is fundamental to understanding the relationship between function and structure in chemistry, biology and materials science. This review focuses on the Surface Site Interaction Point (SSIP) approach which describes molecules as a set of points that capture the properties of all possible non-covalent interactions that the molecule might make with another molecule. The first half of the review focuses on the empirical non-covalent interaction parameters, α and β, and provides simple rules of thumb to estimate free energy changes for interactions between different types of functional group. These parameters have been used to have been used to establish a quantitative understanding of the role of solvent in solution phase equilibria, and to describe non-covalent interactions at the interface between macroscopic surfaces as well as in the solid state. The second half of the review focuses on a computational approach for obtaining SSIPs and applications in multi-component systems where many different interactions compete. Ab initio calculation of the Molecular Electrostatic Potential (MEP) surface is used to derive an SSIP description of a molecule, where each SSIP is assigned a value equivalent to the corresponding empirical parameter, α or β. By considering the free energies of all possible pairing interactions between all SSIPs in a molecular ensemble, it is possible to calculate the speciation of all intermolecular interactions and hence predict thermodynamic properties using the SSIMPLE algorithm. SSIPs have been used to describe both the solution phase and the solid state and provide accurate predictions of partition coefficients, solvent effects on association constants for formation of intermolecular complexes, and the probability of cocrystal formation. SSIPs represent a simple and intuitive tool for describing the relationship between chemical structure and non-covalent interactions with sufficient accuracy to understand and predict the properties of complex molecular ensembles without the need for computationally expensive simulations.

Dissecting Solvent Effects on Hydrogen Bonding

The experimental isolation of H‐bond energetics from the typically dominant influence of the solvent remains challenging. Here we use synthetic molecular balances to quantify amine/amide H‐bonds in competitive solvents. Over 200 conformational free energy differences were determined using 24 H‐bonding balances in 9 solvents spanning a wide polarity range. The correlations between experimental interaction energies and gas‐phase computed energies exhibited wild solvent‐dependent variation. However, excellent correlations were found between the same computed energies and the experimental data following empirical dissection of solvent effects using Hunter's α/β solvation model. In addition to facilitating the direct comparison of experimental and computational data, changes in the fitted donor and acceptor constants reveal the energetics of secondary local interactions such as competing H‐bonds.

Surface Reconstruction of Fluoropolymers in Liquid Media

Surface reconstruction is the rearrangement of atoms or molecules at an interface in response to a stimulus, driven by lowering the overall free energy of the system. Perfluoroalkyl acrylate polymers with short side chains undergo reconstruction at room temperature when exposed to water. Here, we use contact angle aging to examine the liquid- and temperature- dependency of surface reconstruction of plasma polymerized perfluoroalkyl acrylates. We use a first order kinetic model to examine the dynamics of reconstructive processes. Our results show that, above the bulk melting point of the polymers, the contact angles of both polar and nonpolar (hydrocarbon) liquids show a time dependency well fit by the model. We conclude that surface reconstruction can be driven by the preferential segregation of hydrocarbon and fluorocarbon moieties as well as by polar interactions. This has implications in terms of using fluorocarbons to design oleophobic surfaces (and vice versa) and in terms of designing fluorocarbon and/or hydrocarbon surfaces with switchable wettability.

Enhancement of association constants of various charge-transfer complexes in siloxane solvents

The association constants of various charge-transfer complexes were evaluated in n-hexane, octamethyltrisiloxane and PDMS to investigate the solvent effect.

Quantifying Solvophobic Effects in Organic Solvents Using a Hydrocarbon Molecular Balance

We evaluate the use of the cohesive energy density (ced) as a quantitative descriptor for solvophobic effects in organic solvents by measuring ΔG_Z/E of the rigid Z- and E-2,2'-diethynyl-9,9'-bifluorenylidene. In line with previously employed balances, solvent-dependent changes in ΔG_Z/E are predominantly induced by solvophobic effects, leading to a strong correlation with the solvent's ced. We re-emphasize the role of ceds as quantitative descriptors of solvophobic effects of organic solvents. Our experimental findings are well supported by B3LYP-D3/def2TZVP computations.

Polarity-Tolerant Chloride Binding in Foldamer Capsules by Programmed Solvent-Exclusion

Persistent anion binding in a wide range of solution environments is a key challenge that continues to motivate and demand new strategies in synthetic receptor design. Though strong binding in low-polarity solvents has become routine, our ability to maintain high affinities in high-polarity solvents has not yet reached the standard set by nature. Anions are bound and transported regularly in aqueous environments by proteins that use secondary and tertiary structure to isolate anion binding sites from water. Inspired by this principle of solvent exclusion, we created a sequence-defined foldameric capsule whose global minimum conformation displays a helical folded state and is preorganized for 1:1 anion complexation. The high stability of the folded geometry and its ability to exclude solvent were supported by solid-state and solution phase studies. This capsule then withstood a 4-fold increase in solvent dielectric constant (ε_r) from dichloromethane (9) to acetonitrile (36) while maintaining a high and solvent-independent affinity of 10⁵ M^-1; ΔG ∼ 28 kJ mol^-1. This behavior is unusual. More typical of solvent-dependent behavior, Cl^- affinities were seen to plummet in control compounds, such as aryl-triazole macrocycles and pentads, with their solvent-exposed binding cavities susceptible to dielectric screening. Finally, dimethyl sulfoxide denatures the foldamer by putative solvent binding, which then lowers the foldamer's Cl^- affinity to normal levels. The design of this capsule demonstrates a new prototype for the development of potent receptors that can operate in polar solvents and has the potential to help manage hydrophilic anions present in the hydrosphere and biosphere.

Intramolecular London Dispersion Interactions Do Not Cancel in Solution

We present a comprehensive experimental study of a di-t-butyl-substituted cyclooctatetraene-based molecular balance to measure the effect of 16 different solvents on the equilibrium of folded versus unfolded isomers. In the folded 1,6-isomer, the two t-butyl groups are in close proximity (H···H distance ≈ 2.5 Å), but they are far apart in the unfolded 1,4-isomer (H···H distance ≈ 7 Å). We determined the relative strengths of these noncovalent intramolecular σ-σ interactions via temperature-dependent nuclear magnetic resonance measurements. The origins of the interactions were elucidated with energy decomposition analysis at the density functional and ab initio levels of theory, pinpointing the predominance of London dispersion interactions enthalpically favoring the folded state in any solvent measured.

London Dispersion in Alkane Solvents

The importance of London dispersion interactions in solution is an ongoing debate. Although the significance of dispersion for structure and stability is widely accepted, the degree of its attenuation in solution is still not properly understood. Quantitative evaluations are derived mostly from computations. Experimental data provide guidelines to include London dispersion in solution phase design. Herein, dispersive interactions were examined with an azobenzene probe. Alkyl substituents in meta positions of the azobenzene core were systematically varied and the effect on the half-lives for the thermally induced Z to E isomerization in several alkane solvents was determined. The results show that intramolecular dispersion is only marginally influenced. In solvents with low surface tension, reduced destabilizing solvent-solvent interactions increase the half-life up to 20 %. Specific individual interactions between alkyl chains on the azobenzene and those of the solvent lead to additional fluctuations of the half-lives. These presumably result from structural changes of the conformer ensemble.

Quantifying Interactions and Solvent Effects Using Molecular Balances and Model Complexes

Where the basic units of molecular chemistry are the bonds within molecules, supramolecular chemistry is based on the interactions that occur between molecules. Understanding the "how" and "why" of the processes that govern molecular self-assembly remains an open challenge to the supramolecular community. While many interactions are readily examined in silico through electronic structure calculations, such insights may not be directly applicable to experimentalists. The practical limitations of computationally accounting for solvation is perhaps the largest bottleneck in this regard, with implicit solvation models failing to comprehensively account for the specific nature of solvent effects and explicit models incurring a prohibitively high computational cost. Since molecular recognition processes usually occur in solution, insight into the nature and effect of solvation is imperative not only for understanding these phenomena but also for the rational design of systems that exploit them.Molecular balances and supramolecular complexes have emerged as useful tools for the experimental dissection of the physicochemical basis of various noncovalent interactions, but they have historically been underexploited as a platform for the evaluation of solvent effects. Contrasting with large biological complexes, smaller synthetic model systems enable combined experimental and computational analyses, often facilitating theoretical analyses that can work in concert with experiment.Our research has focused on the development of supramolecular systems to evaluate the role of solvents in molecular recognition, and further characterize the underlying mechanisms by which molecules associate. In particular, the use of molecular balances has provided a framework to measure the magnitude of solvent effects and to examine the accuracy of solvent models. Such approaches have revealed how solvation can modulate the electronic landscape of a molecule and how competitive solvation and solvent cohesion can provide thermodynamic driving forces for association. Moreover, the use of simple model systems facilitates the interrogation and further dissection of the physicochemical origins of molecular recognition. This tandem experimental/computational approach has married less common computational techniques, like symmetry adapted perturbation theory (SAPT) and natural bonding orbital (NBO) analysis, with experimental observations to elucidate the influence of effects that are difficult to resolve experimentally (e.g., London dispersion and electron delocalization).

N-Arylimide Molecular Balances: A Comprehensive Platform for Studying Aromatic Interactions in Solution

Noncovalent interactions of aromatic surfaces play a key role in many biological processes and in determining the properties and utility of synthetic materials, sensors, and catalysts. However, the study of aromatic interactions has been challenging because these interactions are usually very weak and their trends are modulated by many factors such as structural, electronic, steric, and solvent effects. Recently, N-arylimide molecular balances have emerged as highly versatile and effective platforms for studying aromatic interactions in solution. These molecular balances can accurately measure weak noncovalent interactions in solution via their influence on the folded-unfolded conformational equilibrium. The structure (i.e., size, shape, π-conjugation, and substitution) and nature (i.e., element, charge, and polarity) of the π-surfaces and interacting groups can be readily varied, enabling the study of a wide range of aromatic interactions. These include aromatic stacking, heterocyclic aromatic stacking, and alkyl-π, chalcogen-π, silver-π, halogen-π, substituent-π, and solvent-π interactions. The ability to measure a diverse array of aromatic interactions within a single model system provides a unique perspective and insights as the interaction energies, stability trends, and solvent effects for different types of interactions can be directly compared. Some broad conclusions that have emerged from this comprehensive analysis include: (1) The strongest aromatic interactions involve groups with positive charges such as pyridinium and metal ions which interact with the electrostatically negative π-face of the aromatic surface via cation-π or metal-π interactions. Attractive electrostatic interactions can also form between aromatic surfaces and groups with partial positive charges. (2) Electrostatic interactions involving aromatic surfaces can be switched from repulsive to attractive using electron-withdrawing substituents or heterocycles. These electrostatic trends appear to span many types of aromatic interactions involving a polar group interacting with a π-surface such as halogen-π, chalcogen-π, and carbonyl-π. (3) Nonpolar groups form weak but measurable stabilizing interactions with aromatic surfaces in organic solvents due to favorable dispersion and/or solvophobic effects. A good predictor of the interaction strength is provided by the change in solvent-accessible surface area. (4) Solvent effects modulate the aromatic interactions in the forms of solvophobic effects and competitive solvation, which can be modeled using solvent cohesion density and specific solvent-solute interactions.

A Fullerene-Based Molecular Torsion Balance for Investigating Noncovalent Interactions at the C60 Surface

To investigate the nature and strength of noncovalent interactions at the fullerene surface, molecular torsion balances consisting of C₆₀ and organic moieties connected through a biphenyl linkage were synthesized. NMR and computational studies show that the unimolecular system remains in equilibrium between well-defined folded and unfolded conformers owing to restricted rotation around the biphenyl C-C bond. The energy differences between the two conformers depend on the substituents and is ascribed to differences in the intramolecular noncovalent interactions between the organic moieties and the fullerene surface. Fullerenes favor interacting with the π-faces of benzenes bearing electron-donating substituents. The correlation between the folding free energies and corresponding Hammett constants of the substituents in the arene-containing torsion balances reflects the contributions of the electrostatic interactions and dispersion force to face-to-face arene-fullerene interactions.

Reconciling Electrostatic and n→π* Orbital Contributions in Carbonyl Interactions

Interactions between carbonyl groups are prevalent in protein structures. Earlier investigations identified dominant electrostatic dipolar interactions, while others implicated lone pair n→π* orbital delocalisation. Here these observations are reconciled. A combined experimental and computational approach confirmed the dominance of electrostatic interactions in a new series of synthetic molecular balances, while also highlighting the distance-dependent observation of inductive polarisation manifested by n→π* orbital delocalisation. Computational fiSAPT energy decomposition and natural bonding orbital analyses correlated with experimental data to reveal the contexts in which short-range inductive polarisation augment electrostatic dipolar interactions. Thus, we provide a framework for reconciling the context dependency of the dominance of electrostatic interactions and the occurrence of n→π* orbital delocalisation in C=O⋅⋅⋅C=O interactions.

Exploring London Dispersion and Solvent Interactions at Alkyl-Alkyl Interfaces Using Azobenzene Switches

Interactions on the molecular level control structure as well as function. Especially interfaces between innocent alkyl groups are hardly studied although they are of great importance in larger systems. Herein, London dispersion in conjunction with solvent interactions between linear alkyl chains was examined with an azobenzene-based experimental setup. Alkyl chains in all meta positions of the azobenzene core were systematically elongated, and the change in rate for the thermally induced Z→E isomerization in n-decane was determined. The stability of the Z-isomer increased with longer chains and reached a maximum for n-butyl groups. Further elongation led to faster isomerization. The origin of the intramolecular interactions was elaborated by various techniques, including 1 H NOESY NMR spectroscopy. The results indicate that there are additional long-range interactions between n-alkyl chains with the opposite phenyl core in the Z-state. These interactions are most likely dominated by attractive London dispersion. This work provides rare insight into the stabilizing contributions of highly flexible groups in an intra- as well as an intermolecular setting.

Polarizability and isotope effects on dispersion interactions in water

True understanding of dispersion interaction in solution remains elusive because of difficulty in the precise evaluation of its interaction energy. Here, the effect of substituents with different polarizability on dispersion interactions in water is discussed based on the thermodynamic parameters determined by isothermal titration calorimetry for the formation of discrete aggregates from gear-shaped amphiphiles (GSAs). The substituents with higher polarizability enthalpically more stabilize the nanocube, which is due to stronger dispersion interactions and to the hydrophobic effect. The differences in the thermodynamic parameters for the nanocubes from the GSAs with CH3 and CD3 groups are also discussed to lead to the conclusion that the H/D isotope effect on dispersion interactions is negligibly small, which is due to almost perfect entropy-enthalpy compensation between the two isotopomers.

Interplay between n→π* Interactions and Dynamic Covalent Bonds: Quantification and Modulation by Solvent Effects

Orbital donor-acceptor interactions play critical roles throughout chemistry, and hence, their regulation and functionalization are of great significance. Herein we demonstrate for the first time the investigation of n→π* interactions through the strategy of dynamic covalent chemistry (DCC), and we further showcase its use in the stabilization of imine. The n→π* interaction between donor X and acceptor aldehyde/imine within 2-X-2'-formylbiphenyl derivatives was found to significantly influence the thermodynamics of imine exchange. The orbital interaction was then quantified through imine exchange, the equilibrium of which was successfully correlated with the difference in natural bond orbital stabilization energy of n→π* interactions of aldehyde and its imine. Moreover, the examination of solvent effects provided insights into the distinct feature of the modulation of n→π* interaction with aprotic and protic solvents. The n→π* interaction involving imine was enhanced in protic solvents due to hydrogen bonding with the solvent. This finding further enabled the stabilization of imine in purely aqueous solution. The strategies and results reported should find application in many fields, including molecular recognition, biological labeling, and asymmetric catalysis.

Ab initio calculations and reduced density gradient analyses of the structure and energetics of hydrated calcium fluoride and calcium carbonate

We studied microhydrated calcium fluoride, calcium carbonate and their ions at the MP2/6-311++G** level of theory and found that water–water non-covalent interactions destabilize the solvation shell, and are compensated by cooperative hydrogen bonds.

Contributions of van der Waals Interactions and Hydrophobic Attraction to Molecular Adhesions on a Hydrophobic MoS2 Surface in Water

Pushing the boundaries of the investigation of hydrophobic attraction (HA) to the molecular scale readily ensures the collection of experimental results free of secondary effects, thereby facilitating the unraveling of the underlying mechanism by providing clean experimental results that truly reflect the hydrophobic attraction. Regardless of the feasibility of this approach, investigations using this promising method are stagnant due to the difficulties in determining the individual contributions of HA and van der Waals (vdW) interactions at the molecular scale. Here, a novel approach was proposed for the first time to determine the individual contributions of vdW interactions and HA by studying the single-molecule adhesion forces of a neutral oligo ethylene glycol methacrylate copolymer on a MoS₂ crystal exposed to different water chemistry. The anisotropic surface properties of MoS₂ enabled the partitioning of vdW interactions and hydrophobic attraction in total single-molecule adhesion forces and also enabled determining the contribution of electrostatic interaction (ESI). When the presence of ESI is excluded, the study of single-molecule adhesion forces using single-molecule force spectroscopy (SMFS) revealed that the contribution of vdW interactions to total molecular interactions was smaller than 9 pN. The strong single-molecule adhesion forces of oligo ethylene glycol copolymer on the hydrophobic basal surface of MoS₂ demonstrated that HA plays a dominant role with contribution up to 89% to the total single-molecule adhesion force. By utilizing the derived theoretical model, we quantified the individual contribution of each fundamental interaction under a variety of conditions. This study proposed a facile approach to quantitatively clarify the roles of vdW interactions and HA at the molecular scale, which may help assist future experimental and theoretical investigations of hydrophobic (solvophobic) effects and vdW interactions in aqueous solutions.

Size-dependent rate acceleration in the silylation of secondary alcohols: the bigger the faster

Relative rates for the reaction of secondary alcohols carrying large aromatic moieties with silyl chlorides carrying equally large substituents have been determined in organic solvents. Introducing thoroughly matching pairs of big dispersion energy donor (DED) groups enhanced rate constants up to four times, notably depending on the hydrogen bond donor ability of the solvent. A linear correlation between computed dispersion energy contributions to the stability of the silyl ether products and experimental relative rate constants was found. These results indicate a cooperation between solvophobic effects and DED-groups in the kinetic control of silylation reactions.

Exploring the Role of Solvent on Carbohydrate-Aryl Interactions by Diffusion NMR-Based Studies

Carbohydrate-protein interactions play an important role in many molecular recognition processes. An exquisite combination of multiple factors favors the interaction of the receptor with one specific type of sugar, whereas others are excluded. Stacking CH-aromatic interactions within the binding site provide a relevant contribution to the stabilization of the resulting sugar-protein complex. Being experimentally difficult to detect and analyze, the key CH-π interaction features have been very often dissected using a variety of techniques and simple model systems. In the present work, diffusion NMR spectroscopy has been employed to separate the components of sugar mixtures in different solvents on the basis of their differential ability to interact through CH-π interactions with one particular aromatic cosolute in solution. The experimental data show that the properties of the solvent did also influence the diffusion behavior of the sugars present in the mixture, inhibiting or improving their separation. Overall, the results showed that, for the considered monosaccharide derivatives, their diffusion coefficient values and, consequently, their apparent molecular sizes and/or shapes depend on the balance between solute/cosolute as well as solute/solvent interactions. Thus, in certain media and in the presence of the aromatic cosolute, the studied saccharides that are more suited to display CH-π interactions exhibited a lower diffusion coefficient than the noncomplexing sugars in the mixture. However, when dissolved in another medium, the interaction with the solvent strongly competes with that of the aromatic cosolute.

Semi-quantitative evaluation of molecular meshing via surface analysis with varying probe radii

Molecular meshing in molecular recognition and assembly can be assessed by plotting the distribution of contact surfaces against the contact distance.