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Dispersion

A Gecko can walk up a smooth glass window because of the adhesion in the millions of hydrophobic setae on its toes that convey van der Waals (vdW) interactions with the surface. The attractive part of such vdW-interactions is an electron correlation (quantum mechanical many-body) effect normally referred to as London dispersion. Its overwhelming role in the formation of condensed matter has been known since the pioneering contributions of J. D. van der Waals and F. London who related dispersion to spectroscopic properties, particularly the polarizability of the interacting partners. In chemistry, dispersion helps rationalize the varying boiling points of alkanes, the greater stability of branched over linear alkanes, the π-π attractive structures of graphite and graphenes, and the mutual attraction of s-π systems. Strong covalent bonding as well as non-covalent interactions such as hydrogen- or halogen-bonding, Pauli repulsion (“steric effects”), and electrostatic multipole interactions are mainly used as “design elements” to affect chemical structures and reactivity. Extending this incomplete view is one of the key goals of the Schreiner Group, with a focus on dispersion interactions as important control elements for structure and reactivity of organic molecules. One of the major objectives is the qualitative determination and quantification of dispersion interactions utilizing a selection of particularly suitable molecular systems. A selection of our work in this arena can be found below.
Director: Univ.-Prof. Peter R. Schreiner
Dispersion

Current work:

 

DFG priority program "SPP 1807 - Control of London dispersion interactions in molecular chemistry"

 

The Role of London Dispersion Interactions in Ga‐Substituted Dipnictenes. Lijuan Song, Juliane Schoening, Christoph Wölper, Stephan Schulz and Peter R. Schreiner
Organometallics 2019, 38, 1640-1647. DOI: 10.1021/acs.organomet.9b00072

London Dispersion Interactions


We report the synthesis and structural characterization of Ga-substituted diarsene [L(EtO)GaAs]2, which is an exemplary case of a Ga-substituted dipnictene of the general type [L(X)Ga]2E2 (L = C[C(Me)N(2,6-i-Pr2-C6H3)]2, X = F, Cl, Br, I, NMe2, OEt; E = As, Sb, Bi). We examined this extended series of compounds computationally and found that attractive London dispersion interactions between the substituents on the N,N′-chelating β-diketiminate ligands as well as pnictogen−π interactions between the group 15 metal center and the ligand are key factors for the stability and the electronic structures of such types of complexes.





Syntheses, Structures and Bonding Analyses of Carbene-stabilized Stibinidenes. Julia Krüger, Christoph Wölper, Lukas John, Lijuan Song, Peter R. Schreiner, Stephan Schulz
Eur. J. Inorg. Chem. 2019, 1669–1678. DOI: 10.1002/ejic.201900167

Reactions of two equivalents of LGa {L = HC[C(Me)NAr]2; Ar = 2,6‐iPr2‐C6H3} with SbX3 (X = Cl, Br) yield double‐inserted products (LGaX)2SbX (X = Cl 1, Br 2), which were isolated at –40 °C. Warming solutions of 1 and 2 to ambient temperature results in intramolecular elimination of LGaX2 and formation of Ga‐substituted stibinidenes L(X)GaSb as reaction intermediates, which were stabilized by coordination of strong σ‐donating carbenes. Four carbene‐stabilized stibinidenes MeCAAC‐SbGa(X)L {MeCAAC = [H2C(CMe2)2NAr]C, Ar = 2,6‐iPr2‐C6H3, X = Cl 3, Br 4} and IDipp‐SbGa(X)L {IDipp = 1,3‐bis(2,6‐diisopropylphenyl)‐imidazol‐2‐ylidene, X = Cl 5, Br 6} were characterized by single‐crystal X‐ray diffraction and the nature of the bonding in 3–6 was further investigated by quantum chemical methods. The studies reveal substantial Sb–Ccarbene π‐backbonding contributions in 3 and 4, whereas the IDipp‐stabilized derivatives 5 and 6 exhibit Sb–Ccarbene single bonds. DFT computations with and without dispersion corrections reveal that dispersion interactions mainly from the bulky aryl groups decisively contribute to the stability of 3–6.

Stibinidenes





Probing the Delicate Balance between Pauli Repulsion and London Dispersion with Triphenylmethyl Derivatives. Sören Rösel, Jonathan Becker, Wesley D. Allen and Peter R. Schreiner

J. Am. Chem. Soc. 2018, 140 (43), pp 14421–14432. DOI: 10.1021/jacs.8b09145. Highlight: JACS Spotlight .

Pauli Repulsion

 

The long-known, ubiquitously present, and always attractive London dispersion (LD) interaction was probed with hexaphenylethane (HPE) derivatives. A series of all-meta hydrocarbyl [Me, iPr, tBu, Cy, Ph, 1-adamantyl (Ad)]-substituted triphenylmethyl (TPM) derivatives [TPM–H, TPM–OH, (TPM–O)2, TPM] was synthesized en route, and several derivatives were characterized by single-crystal X-ray diffraction (SC-XRD). Multiple dimeric head-to-head SC-XRD structures feature an excellent geometric fit between the meta-substituents; this is particularly true for the sterically most demanding tBu and Ad substituents. NMR spectra of the iPr-, tBu-, and Cy-derived trityl radicals were obtained and reveal, together with EPR and UV–Vis spectroscopic data, that the effects of all-meta alkyl substitution on the electronic properties of the trityl scaffold are marginal. Therefore, we concluded that the most important factor for HPE stability arises from LD interactions. Beyond all-meta tBu-HPE we also identified the hitherto unreported all-meta Ad-HPE. An intricate mathematical analysis of the temperature-dependent dissociation constants allowed us to extract ΔGd298(exptl) = 0.3(5) kcal mol–1 from NMR experiments for all-meta tBu-HPE, in good agreement with previous experimental values and B3LYP-D3(BJ)/def2-TZVPP(C-PCM) computations. These computations show a stabilizing trend with substituent size in line with all-meta Ad-HPE (ΔGd298(exptl) = 2.1(6) kcal mol–1) being more stable than its tBu congener. That is, large, rigid, and symmetric hydrocarbon moieties act as excellent dispersion energy donors. Provided a good geometric fit, they are able to stabilize labile molecules such as HPE via strong intramolecular LD interactions, even in solution.

 

 

 

London Dispersion Enables the Shortest Intermolecular Hydrocarbon H•••H Contact. Sören Rösel, Henrik Quanz, Christian Logemann, Jonathan Becker, Estelle Mossou, Laura Cañadillas Delgado, Eike Caldeweyher, Stefan Grimme, and Peter R. Schreiner J. Am. Chem. Soc. 2017, 139, 7428–7431. DOI: 10.1021/jacs.7b01879
Highlights: a) Hydrogens set a short-distance record. Chem. Eng. News 2017, 95 (21), 8; b) Shortest H···H Contact between Hydrocarbon Molecules. ChemViews May 25, 2017. c) Close Encounters of the Hydrogen Kind. JACS Spotlight J. Am. Chem. Soc. 2017, 139, 7665.

London Dispersion Enables H···H Contact.gif

Neutron diffraction of tri(3,5-tert-butylphenyl)methane at 20 K reveals an intermolecular C–H···H–C distance of only 1.566(5) Å, which is the shortest reported to date. The compound crystallizes as a C3-symmetric dimer in an unusual head-to-head fashion. Quantum chemical computations of the solid state at the HSE-3c level of theory reproduce the structure and the close contact well (1.555 Å at 0 K) and emphasize the significance of packing effects; the gas-phase dimer structure at the same level shows a 1.634 Å C–H···H–C distance. Intermolecular London dispersion interactions between contacting tert-butyl substituents surrounding the central contact deliver the decisive energetic contributions to enable this remarkable bonding situation.

 

 

Sizing the Role of London Dispersion in the Dissociation of all-meta tert-Butyl Hexaphenylethane. Sören Rösel, Ciro Balestrieri, and Peter R. Schreiner*
Chem. Sci. 2017, 8, 405–410. DOI: 10.1039/C6SC02727J

The structure and dynamics of enigmatic hexa(3,5-di-tert-butylphenyl)ethane was characterized via NMR spectroscopy for the first time. Our variable temperature NMR analysis demonstrates an enthalpy–entropy compensation that results in a vanishingly low dissociation energy (ΔG298d = −1.60(6) kcal mol−1). An in silico study of increasingly larger all-meta alkyl substituted hexaphenylethane derivatives (Me, iPr, tBu, Cy, 1-Ad) reveals a non-intuitive correlation between increased dimer stability with increasing steric crowding. This stabilization originates from London dispersion as expressed through the increasing polarizability of the alkyl substituents. Substitution with conformationally flexible hydrocarbon moieties, e.g., cyclohexyl, introduces large unfavourable entropy contributions. Therefore, using rigid alkyl groups like tert-butyl or adamantyl as dispersion energy donors (DED) is essential to help stabilize extraordinary bonding situations.

London Dispersion Attraction




Uncovering Key Structural Features of an Enantioselective Peptide-Catalyzed Acylation Utilizing Advanced NMR Techniques. Eliška Procházková, Andreas Kolmer, Julian Ilgen, Mira Schwab, Lukas Kaltschnee, Maic Fredersdorf, Volker Schmidts, Raffael C. Wende, Peter R. Schreiner* and Christina M. Thiele*
Angew. Chem. Int. Ed. 2016, 55, 15754–15759. DOI: 10. 1002/anie.201608559.
Highlight: Frontispiece of communications of this issue.

 

Enantioselective Peptide-Catalyzed Acylation

 

We report on a detailed NMR spectroscopic study of the catalyst-substrate interaction of a highly enantioselective oligopeptide catalyst that is used for the kinetic resolution of trans-cycloalkane-1,2-diols via monoacylation. The extraordinary selectivity has been rationalized by molecular dynamics as well as density functional theory (DFT) computations. Herein we describe the conformational analysis of the organocatalyst studied by a combination of nuclear Overhauser effect (NOE) and residual dipolar coupling (RDC)-based methods that resulted in an ensemble of four final conformers. To corroborate the proposed mechanism, we also investigated the catalyst in mixtures with both trans-cyclohexane-1,2-diol enantiomers separately, using advanced NMR methods such as T1 relaxation time and diffusion-ordered spectroscopy (DOSY) measurements to probe molecular aggregation. We determined intramolecular distance changes within the catalyst after diol addition from quantitative NOE data. Finally, we developed a pure shift EASY ROESY experiment using PSYCHE homodecoupling to directly observe intermolecular NOE contacts between the trans-1,2-diol and the cyclohexyl moiety of the catalyst hidden by spectral overlap in conventional spectra. All experimental NMR data support the results proposed by earlier computations including the proposed key role of dispersion interaction.

 

 

 

London Dispersion Decisively Contributes to the Thermodynamic Stability of Bulky NHC-Coordinated Main Group Compounds. J. Philipp Wagner and Peter R. Schreiner
J. Chem. Theory Comput. 2016, 12, 231–237. DOI: 10.1021/acs.jctc.5b01100

 

London Dispersion Decisively Contributes to the Thermodynamic Stability.gif

 

We evaluated the dispersion stabilization of a series of seemingly reactive main group compounds coordinated to bulky N-heterocyclic carbene ligands. We computed the thermochemistry of hypothetical isodesmic exchange reactions of these ligands with their unsubstituted parent systems employing the B3LYP/6-311G(d,p) level of theory with and without dispersion corrections. The energy difference between these two approaches gave dispersion corrections of 30 kcal mol–1 and more. We therefore conclude that London dispersion contributes critically to the thermodynamic stabilities of these compounds. As such, these core–shell structures undergo reactions of the reactive core as long as the dispersion stabilization is conserved.

 

 

London Dispersion in Molecular Chemistry–Reconsidering Steric Effects.  J. Philipp Wagner and Peter R. Schreiner
Angew. Chem. Int. Ed. 2015, 54, 12274–12296. DOI: 10.1002/anie.201503476. Highlights: a) Frontispiz in the journal; b) oted as very important paper (top 5% of all Angewandte publications); b) Steven Bachrach, Computational Organic Chemistry, January 4, 2016; c) Listed as hot paper (by Thomson Reuters Web of Science) published in the past two years and receiving enough citations to place it in the top 0.1% of all papers in chemistry.

 

LD Steric EffectsLondon dispersion, which constitutes the attractive part of the famous van der Waals potential, has long been underappreciated in molecular chemistry as an important element of structural stability, and thus affects chemical reactivity and catalysis. This negligence is due to the common notion that dispersion is weak, which is only true for one pair of interacting atoms. For increasingly larger structures, the overall dispersion contribution grows rapidly and can amount to tens of kcal mol−1. This Review collects and emphasizes the importance of inter- and intramolecular dispersion for molecules consisting mostly of first row atoms. The synergy of experiment and theory has now reached a stage where dispersion effects can be examined in fine detail. This forces us to reconsider our perception of steric hindrance and stereoelectronic effects. The quantitation of dispersion energy donors will improve our ability to design sophisticated molecular structures and much better catalysts.

 

 

 

The Self-Association of Graphane is Driven by London Dispersion and Enhanced Orbital Interactions. Changwei Wang, Yirong Mo, J. Philipp Wagner, Peter R. Schreiner, Eluvathingal D. Jemmis, David Danovich, and Sason Shaik
J. Comput. Theory Chem. 2015, 11, 1621–1630. DOI: 10.1021/acs.jctc.5b00075

 

The Self-Association of Graphane.gif

 

We investigated the nature of the cohesive energy between graphane sheets via multiple CH···HC interactions, using density functional theory (DFT) including dispersion correction (Grimme’s D3 approach) computations of [n]graphane σ dimers (n = 6–73). For comparison, we also evaluated the binding between graphene sheets that display prototypical π/π interactions. The results were analyzed using the block-localized wave function (BLW) method, which is a variant of ab initio valence bond (VB) theory. BLW interprets the intermolecular interactions in terms of frozen interaction energy (ΔEF) composed of electrostatic and Pauli repulsion interactions, polarization (ΔEpol), charge-transfer interaction (ΔECT), and dispersion effects (ΔEdisp). The BLW analysis reveals that the cohesive energy between graphane sheets is dominated by two stabilizing effects, namely intermolecular London dispersion and two-way charge transfer energy due to the σCH → σ*HC interactions. The shift of the electron density around the nonpolar covalent C–H bonds involved in the intermolecular interaction decreases the C–H bond lengths uniformly by 0.001 Å. The ΔECT term, which accounts for ∼15% of the total binding energy, results in the accumulation of electron density in the interface area between two layers. This accumulated electron density thus acts as an electronic “glue” for the graphane layers and constitutes an important driving force in the self-association and stability of graphane under ambient conditions. Similarly, the “double faced adhesive tape” style of charge transfer interactions was also observed among graphene sheets in which it accounts for ∼18% of the total binding energy. The binding energy between graphane sheets is additive and can be expressed as a sum of CH···HC interactions, or as a function of the number of C–H bonds.