Dispersion-Bound Isolated Dimers in the Gas Phase: Observation of the Shortest Intermolecular CH⋠⋠⋠H-C Distance via Stimulated Raman Spectroscopy.

The triphenylmethane and all-meta tert-butyl triphenylmethane dimers, (TPM)2 and (T t BuPM)2 , respectively, were studied with ionization loss stimulated Raman spectroscopy in molecular beam experiments to resolve structure sensitive vibrations. This answers the question whether the recently reported linear head-to-head arrangement in (T t BuPM)2 results from crystal packing or prevails also in the gas phase, and therefore must result from extraordinarily strong London dispersion (LD) interactions. Our study clearly demonstrates that the head-to-head arrangement is maintained even under isolated molecular beam conditions in the absence of crystal packing effects. The central Raman-active aliphatic C-D vibration of appropriately deuterated (T t BuPM)2 associated with an unusually short C-D⋅⋅⋅D-C distance exhibits a strong blue-shift compared to the undisturbed case. As the LD stabilizing tert-butyl groups are absent in (TPM)2 , it displays an approximately S6 -symmetric tail-to-tail arrangement.

Probing the Delicate Balance between Pauli Repulsion and London Dispersion with Triphenylmethyl Derivatives.

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 Δ Gd298(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 (Δ Gd298(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.

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.

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, t Bu, 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.