London Dispersion in Alkane Solvents

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.

Long but Strong C-C Single Bonds: Challenges for Theory

Theoretical challenges in describing molecules with anomalously long single C-C bonds are analyzed in terms of the relative contributions of stabilizing and destabilizing intramolecular interactions. Diamondoid dimers that are stable despite the presence of C-C bonds up to 1.7 Å long, as well as other bulky molecules stabilized due to intramolecular noncovalent interactions (London dispersions) are discussed. The unexpected stability of highly crowded molecules, such as diamondoid dimers and tert-butyl-substituted hexaphenylethanes, calls for reconsideration of the "steric effect" traditionally thought to destabilize the molecule. Alternatively, "steric attraction" helps to understand bonding in sterically overloaded molecules, whose structural and energetic analysis requires a proper theoretical description of noncovalent interactions.

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.

Boosting Energy-Transfer Processes via Dispersion Interactions

The generation of open-shell intermediates under mild conditions has opened broad synthetic opportunities during this century. However, these reactive species often require a case specific and tailored tuning of experimental parameters in order to efficiently convert substrates into products. We report a general approach that can overcome these ubiquitous limitations for several visible-light promoted energy-transfer processes. The use of either naphthalene (5-20 equiv.) or simple binaphthyl derivatives (10-30 mol %) greatly increases their efficiency, giving rise to a new strategy for catalysis. The trend is consistent among different media, photocatalysts, light sources and substrates, allowing one to improve existing methods, to more easily optimize conditions for new ones, and, moreover, to disclose otherwise inaccessible reaction pathways.

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.

An NMR Spectroscopy View on London Dispersion in Catalysis: Detection, Quantification, and Application in Ion Pair and Transition Metal Catalysis

ConspectusThe energetic contribution of London dispersion (LD) can cover a broad range from very few to hundreds of kJ mol-1 for extended interaction interfaces due to its pairwise additivity. However, for a designed and successful application of LD in chemical catalysis, there are still many obstacles and questions that remain. In principle, LD can be regarded as the attractive part of the van der Waals potential. Thus, considering the whole van der Waals potential, including the repulsive part (steric repulsion), the ideal solution to the problem in catalysis would be to design compatible interaction interfaces at exactly the correct distance. In the case of a self-assembled, flexible structure arrangement, entropic contributions and solvent interactions might be detrimental. In the case of a rigid catalyst pocket, steric hindrance might not allow for large substituents that are usually applied as dispersion energy donors (DEDs). For a working catalytic system, the following question arises: how is it possible to dissect the complex interaction interfaces in terms of energetic contributions? Usually, the energetic contribution of LD to catalysis is addressed by using calculations. However, adequately computing the correct energetic contributions can be extremely challenging for a vast conformational space with all kinds of intermolecular interactions. Thus, experimental data are essential for comparison or benchmarking.Therefore, in this Account, we describe our quest for detailed experimental data obtained via NMR spectroscopy to experimentally dissect and quantify LD in catalytic systems. In addition, we address the question of whether bulky substituents used as DEDs can be used in confined catalytic pockets. With the example of Pd phosphoramidite complexes, we show how it is possible to experimentally dissect and quantify the contribution of individual interaction areas in complicated transition metal complexes. Furthermore, a correlation between conformational rigidity and heterodimer preference clearly reveals that LD can only unfold its full potential in cases where entropic contributions are minimized. This finding can also explain the small contribution of LD in flexible and solvent-exposed molecular balances. In the field of Brønsted acid catalysis, we demonstrated that LD has a strong influence on the structures, stability, and populations of confined catalytic intermediates. LD is key for populating higher aggregates such as dimers. In addition, offsets between the experimental and computational results were observed and attributed to solvent-solute dispersion interactions. We studied the delicate interplay of attractive and repulsive interactions by adding bulky DED substituents onto a substrate, which can function as a molecular balance system. Intriguingly, the effect of LD on the free substrate was straightforwardly transferred onto the highly confined intermediates. Furthermore, this effect could even be read out in the enantioselectivities of the underlying reaction. This conceptualized a general approach regarding how LD can be used beneficially in catalysis to convert from moderate/good to excellent stereoselectivities. It showcased that bulky groups such as tert-butyl must not only be regarded as occupied volumes.

Unravelling dispersion forces in liquid-phase enantioseparation. Part I: Impact of ferrocenyl versus phenyl groups

BACKGROUND

Highly ordered chiral secondary structures as well as multiple (tunable) recognition sites are the keys to success of polysaccharide carbamate-based chiral selectors in enantioseparation science. Hydrogen bonds (HBs), dipole-dipole, and π-π interactions are classically considered the most frequent noncovalent interactions underlying enantioselective recognition with these chiral selectors. Very recently, halogen, chalcogen and π-hole bonds were also identified as interactions working in polysaccharide carbamate-based selectors to promote enantiomer distinction. On the contrary, the function of dispersion interactions in this field was not explored so far.

RESULTS

The enantioseparation of chiral ferrocenes featuring chiral axis or chiral plane as stereogenic elements was performed by comparing five polysaccharide carbamate-based chiral columns, with the aim to identify enantioseparation outcomes that could be reasonably determined by dispersion forces, making available a reliable experimental data set for future theoretical studies to confirm the heuristic hypothesis. The effects of mobile phase polarity and temperature on the enantioseparation were considered, and potential recognition sites on analytes and selectors were evaluated by electrostatic potential (V) analysis and molecular dynamics (MD). In this first part, the enantioseparation of 3,3'-dibromo-5,5'-bis-ferrocenylethynyl-4,4'-bipyridine bearing two ferrocenylethynyl units linked to an axially chiral core was performed and compared to that of the analyte featuring the same structural motif with two phenyl groups in place of the ferrocenyl moieties. The results of this study showed the superiority of the ferrocenyl compared to the phenyl group, as a structural element favouring enantiodifferentiation.

SIGNIFICANCE AND NOVELTY

Even if dispersion (London) forces have been envisaged acting in liquid-phase enantioseparations, focused studies to explore possible contributions of dispersion forces with polysaccharide carbamate-based selectors are practically missing. This study allowed us to collect experimental information that support the involvement of dispersion forces as contributors to liquid-phase enantioseparation, paving the way to a new picture in this field.

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.

Probing the Size Limit of Dispersion Energy Donors with a Bifluorenylidene Balance: Magic Cyclohexyl

We report the synthesis of 14 2,2'-disubstituted 9,9'-bifluorenylidenes as molecular balances for the quantification of London dispersion interactions between various dispersion energy donors. For all balances, we measured ΔG_Z/E at 333 K using ¹H NMR in seven organic solvents. For various alkyl and aryl substituents, we generally observe a preference for the "folded" Z-isomer due to attractive London dispersion interactions. The cyclohexyl-substituted system shows the largest Z-preference in this study with ΔG_Z/E = -0.6 ± 0.05 kcal mol^-1 in all solvents, owing to the rotational freedom of cyclohexyl groups paired with their large polarizability that maximizes London dispersion interactions. On the other hand, rigid and sterically more demanding substituents like tert-butyl unexpectedly favor the unfolded E-isomer. This is a result of the close relative position in which the functional groups are positioned in this molecular balance. This close proximity is the reason for the increase of Pauli repulsion in the Z-isomers with large rigid substituents (tert-butyl, adamantyl, and diamantyl) which leads to an equilibrium shift toward the unfolded E-form. While we were able to reproduce most of our experimental trends qualitatively using contemporary computational chemistry methods, quantitative accuracy of the employed methods still needs further improvement.

Tilting the Balance: London Dispersion Systematically Enhances Enantioselectivities in Brønsted Acid Catalyzed Transfer Hydrogenation of Imines

London dispersion (LD) is attracting more and more attention in catalysis since LD is ubiquitously present and cumulative. Since dispersion is hard to grasp, recent research has concentrated mainly on the effect of LD in individual catalytic complexes or on the impact of dispersion energy donors (DEDs) on balance systems. The systematic transfer of LD effects onto confined and more complex systems in catalysis is still in its infancy, and no general approach for using DED residues in catalysis has emerged so far. Thus, on the example of asymmetric Brønsted acid catalyzed transfer hydrogenation of imines, we translated the findings of previously isolated balance systems onto confined catalytic intermediates, resulting in a systematic enhancement of stereoselectivity when employing DED-substituted substrates. As the imine substrate is present as Z- and E-isomers, which can, respectively, be converted to R- and S-product enantiomers, implementing tert-butyl groups as DED residues led to an additional stabilization of the Z-imine by up to 4.5 kJ/mol. NMR studies revealed that this effect is transferred onto catalyst/imine and catalyst/imine/nucleophile intermediates and that the underlying reaction mechanism is not affected. A clear correlation between ee and LD stabilization was demonstrated for 3 substrates and 10 catalysts, allowing to convert moderate-good to good-excellent enantioselectivities. Our findings conceptualize a general approach on how to beneficially employ DED residues in catalysis: they clearly showcase that bulky alkyl residues such as tert-butyl groups must be considered regarding not only their repulsive steric bulk but also their attractive properties even in catalytic complexes.

Influence of intermolecular interactions on the infrared complex indices of refraction for binary liquid mixtures

This paper investigates the accuracy of deriving the composite optical constants of binary mixtures from only the complex indices of refraction of the neat materials. These optical constants enable the reflectance spectra of the binary mixtures to be modeled for multiple scenarios (e.g., different substrates, thicknesses, volume ratios), which is important for contact and standoff chemical detection. Using volume fractions, each mixture's complex index of refraction was approximated via three different mixing rules. To explore the impact of intermolecular interactions, these predictions are tested by experimental measurements for two representative sets of binary mixtures: (1) tributyl phosphate combined with n-dodecane, a non-polar medium, to represent mixtures which primarily interact via dispersion forces and (2) tributyl phosphate and 1-butanol to represent mixtures with polar functional groups that can also interact via dipole-dipole interactions, including hydrogen bonding. The residuals and the root-mean-square error between the experimental and calculated index values are computed and demonstrate that for miscible liquids in which the average geometry of the cross-interactions can be considered isotropic (e.g., dispersion), the refractive indices of the mixtures can be modeled using composite n and k values derived from volume fractions of the neat liquids. Conversely, in spectral regions where the geometry of the cross-interactions is more restricted and anisotropic (e.g., hydrogen bonding), the calculated n and k values vary from the measured values. The impact of these interactions on the reflectance spectra are then compared by modeling a thin film of the binary mixtures on an aluminum substrate using both the measured and the mathematically computed indices of refraction.

Azobenzene‐Substituted Triptycenes: Understanding the Exciton Coupling of Molecular Switches in Close Proximity

Herein, we report a series of azobenzene‐substituted triptycenes. In their design, these switching units were placed in close proximity, but electronically separated by a sp³ center. The azobenzene switches were prepared by Baeyer–Mills coupling as key step. The isomerization behavior was investigated by ¹H NMR spectroscopy, UV/Vis spectroscopy, and HPLC. It was shown that all azobenzene moieties are efficiently switchable. Despite the geometric decoupling of the chromophores, computational studies revealed excitonic coupling effects between the individual azobenzene units depending on the connectivity pattern due to the different transition dipole moments of the π→π* excitations. Transition probabilities for those excitations are slightly altered, which is also revealed in their absorption spectra. These insights provide new design parameters for combining multiple photoswitches in one molecule, which have high potential as energy or information storage systems, or, among others, in molecular machines and supramolecular chemistry.

Beyond Steric Crowding: Dispersion Energy Donor Effects in Large Hydrocarbon Ligands

Interactions between sterically crowded hydrocarbon-substituted ligands are widely considered to be repulsive because of the intrusion of the electron clouds of the ligand atoms into each other's space, which results in Pauli repulsion. Nonetheless, there is another interaction between the ligands which is less widely publicized but is always present. This is the London dispersion (LD) interaction which can occur between atoms or molecules in which dipoles can be induced instantaneously, for example, between the H atoms from the ligand C-H groups.These LD interactions are always attractive, but their effects are not as widely recognized as those of the Pauli repulsion despite their central role in the formation of condensed matter. Their relatively poor recognition is probably due to the relative weakness (ca. 1 kcal mol^-1) of individual H···H interactions owing to their especially strong distance dependence. In contrast, where there are numerous H···H interactions, a collective LD energy equaling several tens of kcal mol^-1 may ensue. As a result, in some molecules the latent importance of the LD attraction energies emerges and assumes a prominence that can overshadow the Pauli effects (e.g., in the stabilization of high-oxidation-state transition-metal alkyls, inducing disproportionation reactions, or in the stabilization of otherwise unstable bonds).Despite being known for over a century, the accurate quantification of individual H···H LD effects in molecular species is a relatively recent phenomenon and at present is based mainly on modified DFT calculations. A few leading reviews summarized these earlier studies of the C-H···H-C LD interactions in organic molecules, and their effects on the structures and stabilities were described. LD effects in sterically crowded inorganic and organometallic molecules have been recognized.The author's interest in these LD effects arose fortuitously over a decade ago during research on sterically crowded heavier main-group element carbene analogues and two-coordinate, open-shell (d¹-d⁹) transition-metal complexes where counterintuitive steric effects were observed. More detailed explanations of these effects were provided by dispersion-corrected DFT calculations in collaboration with the groups of Tuononen and Nagase (see below).This Account describes our development of these initial results for other inorganic molecular classes. More recently, the work has led us to move to the planned inclusion of dispersion effects in ligands to stabilize new molecular types with theoretical input from the groups of Vasko and Grimme (see below). Our approach sought to use what Grimme has described as dispersion effect donor (DED) groups (i.e., spatially close-lying, densely packed substituents either as ligands (e.g., -C₆H₂-2,4,6-Cy₃, Cy = cyclohexyl) or as parts of ligands (e.g., a Cy substituent) that produce relatively large dispersion energies to stabilize these new compounds.We predict that the future design of sterically crowding hydrocarbon ligands will include the consideration and incorporation of LD effects as a standard methodology for directed use in the attainment of new synthetic targets.

An Incremental System To Predict the Effect of Different London Dispersion Donors in All‐ meta ‐Substituted Azobenzenes

Predictive models based on incremental systems exist for many chemical phenomena, thus allowing easy estimates. Despite their low magnitude in isolated systems London dispersion interactions are ubiquitous in manifold situations ranging from solvation to catalysis or in biological systems. Based on our azobenzene system, we systematically determined the London dispersion donor strength of the alkyl substituents Me, Et, iPr up to tBu. Based on this data, we were able to implement an incremental system for London dispersion for the azobenzene scheme. We propose an equation that allows the prediction of the effect of change of substituents on London dispersion interactions in azobenzenes, which has to be validated in similar molecular arrangements in the future.

Preferential solvation of meso-methyl BODIPYs with pyridine via pseudo-hydrogen-bonds

This study explored unexpected pseudo-hydrogen bond interactions between meso-methyl BODIPYs and pyridine or acridine. NMR spectral evidence indicated that the meso-methyl group and BF₂ core of BODIPYs formed C-H⋯N and C-H⋯F-B pseudo-hydrogen bonds with pyridine, respectively. The weak binding strength was attributed to the preferential solvation of pyridine in the vicinity of meso-methyl BODIPYs in cyclohexane. The observations were explained by the formation of pseudo-hydrogen bonds based on the quantum theory of atoms in molecules (QTAIM) formalism. In contrast, acridine binds to BODIPY with a moderate binding strength. QTAIM formalism suggested the existence of the complementary pseudo-hydrogen bonds, which superficially seemed to rationalise the experimental observations. However, extensive NMR experiments have found no discrete geometry for the complex, indicating considerable geometric freedom. This discrepancy suggests that the static pictures based on the QTAIM analyses conflict with the enthalpy-entropy compensation principle in essential thermodynamics.

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.

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).