The lately reported synthesis[1] of octafluorocubane established a sublimation level as 168.1–177.1°C (a melting level was not noticed). In distinction, the heavier perfluoro-octane has an m.p. of -25°C. Why the distinction? Firstly, the crystal construction is proven under, albeit as a dimer fairly than a periodic lattice (click on on picture to acquire 3D coordinates).

The gap between a fluorine and the centroid of the four-membered carbon ring is 1.741Å. Our crystallographer (thanks Andrew!) offers me the next evaluation of the periodic crystal lattice:

The uneven unit (crystal construction DOI: 10.5517/ccdc.csd.cc29z5p5) accommodates solely two fluorine atoms (F1 and F2) and two carbon atoms (C3 and C4). As a result of symmetry/particular positions, the C₈F₈ dice is fashioned of six C3’s, six F2’s, two C4’s and two F1’s. The two.741Å contact comes from an F2 and therefore there are six of those (and 6 faces). The closest F…C4(centroid) intermolecular separation for F1 is ca. 4.20Å. From the crystal construction one can certainly observe six C-F bonds of size 1.341Å and two of size 1.338Å, some 0.003Å shorter.

So time for some calculations (FAIR Knowledge DOI: 10.14469/hpc/11132). The energies proven listed below are for the C_2h-symmetric dimer relative to 2 monomers.

The three strategies had been chosen as approximations to determine (a) the impact of a dispersion/correlation correction, utilizing the usual third-generation Grimme technique and (b) the impact of extra basic dynamic correlations as being the distinction between a Hartree-Fock calculation and DFT one. The values present the HF^a and B3LYP-DFT as being very comparable, however including the GD3+BJ time period stabilises the dimer considerably, in addition to producing an F…centroid distance solely a little bit longer than that measured. Every dice will maintain three pairs of such interactions, so the entire stabilisation power is  ~15 kcal/mol and the enthalpy stabilisation is ~12 kcal/mol. A periodic boundary calculation of the whole cell will surely be a fair higher mannequin of this method. Nonetheless one additional take a look at, of the pattern in size between the six interacting F atoms with a hoop centroid and the 2 that don’t (exp Δ-0.003Å shorter for the latter) can also be replicated by the B3LYP+GD3+BJ/Def2-TZVPP calculation (Δ-0.006Å) which suggests the easy dimer mannequin is just not badly improper.

So from these outcomes, it seems that the enticing interactions between molecules octafluorocubane leading to its excessive sublimation temperature is probably not merely electrostatic interactions (a HF calculation would mannequin that) or certainly of dynamic correlation (modelled by DFT strategies) however a extra full electron correlation of the kind usually described as dispersion and eg accessible through multi-reference and/or coupled-cluster strategies. It could certainly come as a shock that this molecule is a excessive melting strong due to dispersion, however the distinctive geometry permits an F to work together with 4 carbons through such forces, and to build up six of those per molecule within the crystal construction. So actually fairly uncommon.

To finish, it might definitely appear worthwhile to use larger ranges of concept to verify this end result, because the GD3+BJ induced-dipole/induced-dipole dispersion mannequin is a comparatively easy one, and as I commented in my WATOC notes, a lot larger stage fashions of this impact at the moment are turning into accessible.

^aAs prompt by Cina Foroutan-Nejad, a commentator on the earlier weblog submit

This submit has DOI: 10.14469/hpc/11135