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Monday, March 27, 2023

Why does octafluorocubane have such a excessive sublimation level?

The not too long ago 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 relatively 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!) provides me the next evaluation of the periodic crystal lattice:

The uneven unit (crystal construction DOI: 10.5517/ccdc.csd.cc29z5p5) comprises solely two fluorine atoms (F1 and F2) and two carbon atoms (C3 and C4). Because of the symmetry/particular positions, the C8F8 dice is shaped 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 Information DOI: 10.14469/hpc/11132). The energies proven listed below are for the C2h-symmetric dimer relative to 2 monomers.

Methodology ΔE ΔH ΔG F…centroid distance, Å
HF/Def2-TZVPP -1.19 -0.01 +4.53 3.288
B3LYP/Def2-TZVPP -1.17 -0.00 +4.54 3.176


-5.24 -4.00 +2.87 2.859 (2.741 expt)


-7.42 2.718 (2.741 expt)

The three strategies have 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 normal dynamic correlations as being the distinction between a Hartree-Fock calculation and DFT one. The values present the HFa and B3LYP-DFT as being very related, however including the GD3+BJ time period stabilises the dimer considerably, in addition to producing an F…centroid distance solely a bit longer than that measured. Every dice will maintain three pairs of such interactions, so the overall stabilisation power is  ~15 kcal/mol and the enthalpy stabilisation is ~12 kcal/mol. A periodic boundary calculation of the whole cell would definitely be a fair higher mannequin of this technique. Nonetheless one additional check, of the development 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 be replicated by the B3LYP+GD3+BJ/Def2-TZVPP calculation (Δ-0.006Å) which suggests the straightforward dimer mannequin is just not badly fallacious.

So from these outcomes, it seems that the enticing interactions between molecules octafluorocubane leading to its excessive sublimation temperature might not be 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 by way of multi-reference and/or coupled-cluster strategies. It might 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 by way of such forces, and to build up six of those per molecule within the crystal construction. So actually fairly uncommon.

To finish, it could actually appear worthwhile to use increased ranges of idea to substantiate this consequence, for the reason that GD3+BJ induced-dipole/induced-dipole dispersion mannequin is a comparatively easy one, and as I commented in my WATOC notes, a lot increased stage fashions of this impact at the moment are turning into accessible.

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

This submit has DOI: 10.14469/hpc/11135

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