Dispersion stabilized molecules
Distinct from steric hindrance, dispersion stabilization has only recently been considered in depth by organic and inorganic chemists after earlier gaining prominence in protein science and supramolecular chemistry.
[2] Although quantum mechanical in nature, the energy of dispersion interactions can be approximated classically, showing a R−6 dependence on the distance between two atoms.
[1] Dispersion stabilization is often signified by atomic contacts below their van der Waals radii in a molecule's crystal structure.
[5] Although there is controversy about accepting bond critical points as evidence for net attractive interactions, AIM analysis has been invoked by different research groups to show dispersion effects in a variety of molecules.
More recently, computational analysis has shown the formation of the tBu substituted dimer to be stabilized through dispersion interactions.
Beyond just the tBu substituted hexaphenylethane, Schriener and coworkers have synthesized new molecules with "dispersion energy donors" to form both long C-C bonds and short H•••H contacts.
The effect of dispersion stabilization was further probed with a series of meta-substituted hexaphenylethane molecules substituted with Me, iPr, tBu, Cy, and adamantyl groups.
[11] Of these molecules, only the tBu and adamantyl analogs were observed to form the head-to-head dimer, showing the sensitivity of dispersion stabilization to rigid, polarizable substituents.
When the (3,5-tBu2H3C6)3CH molecule dimerizes to form [(3,5-tBu2H3C6)3CH]2, stabilizing interactions between tBu groups bring the central pair of hydrogens to a contact distance of 1.566Å as determined by neutron diffraction.
[7]Researchers have posited that the stability of the bulky hydrocarbon tetra(tert-butyl)tetrahedrane is in part from dispersion forces.
The compounds display high stability for a formally 4+ oxidation state metal center, which has traditionally been attributed to unfavorable β-elimination.
[15] Computational work has determined that the close norbornyl contacts are worth -45.9 kcal/mol of energy, providing significant stabilization to the molecule.
[16] The crystal structure of the silylated plumbylene dimer has different geometries about each Pb atom, indicating that the molecule forms a singular donor-acceptor interaction.
The authors suggest through DFT calculations that dispersion forces between bulky trimethylsilyl groups determine the dimer's conformation.Dispersion has been implicated in stabilizing a Ga-substituted doubly bonded dipnictenes of the form [L(X)Ga]2E2 where E = As, Sb, Bi and L = C[C(Me)N(2,6-iPr2-C6H3).
Through computational analysis, the authors identified 9 H-H interactions that each provide -0.7 kcal/mol of energy, overcoming the steric penalty of bringing the tBu groups together.
