{"id":25521,"date":"2022-09-08T15:26:28","date_gmt":"2022-09-08T14:26:28","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=25521"},"modified":"2022-09-12T07:30:48","modified_gmt":"2022-09-12T06:30:48","slug":"why-does-octafluorocubane-have-such-a-high-sublimation-point","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25521","title":{"rendered":"Why does octafluorocubane have such a high sublimation point?"},"content":{"rendered":"
\n

The recently reported synthesis[1]<\/a><\/span> of octafluorocubane established a sublimation point as 168.1\u2013177.1\u00b0C (a melting point was not observed). In contrast, the heavier perfluoro-octane has an m.p. of -25\u00b0C. Why the difference? Firstly, the crystal structure is shown below, albeit as a dimer rather than a periodic lattice (click on image to obtain 3D coordinates).<\/p>\n

\"\"<\/p>\n

The distance between a fluorine and the centroid of the 4-membered carbon ring is 1.741\u00c5. Our crystallographer (thanks Andrew!) gives me the following analysis of the periodic crystal lattice:<\/p>\n

The asymmetric unit (crystal structure DOI: 10.5517\/ccdc.csd.cc29z5p5<\/a>) contains only two fluorine atoms (F1 and F2) and two carbon atoms (C3 and C4). Due to the symmetry\/special positions, the C8<\/sub>F8<\/sub> cube is formed of six C3’s, six F2’s, two C4’s and two F1’s. The 2.741\u00c5 contact comes from an F2 and hence there are six of these (and six faces). The closest F…C4(centroid) intermolecular separation for F1 is ca. 4.20\u00c5<\/em>. From the crystal structure one can indeed observe six C-F bonds of length 1.341Å and two of length 1.338Å, some 0.003Å shorter.<\/p>\n

So time for some calculations (FAIR Data DOI: 10.14469\/hpc\/11132<\/a>). The energies shown here are for the C2h<\/sub>-symmetric dimer relative to two monomers.<\/p>\n\n\n\n\n\n\n\n
Method<\/td>\n\u0394E<\/td>\n\u0394H<\/td>\n\u0394G<\/td>\nF…centroid distance, \u00c5<\/td>\n<\/tr>\n
HF\/Def2-TZVPP<\/td>\n-1.19<\/td>\n-0.01<\/td>\n+4.53<\/td>\n3.288<\/td>\n<\/tr>\n
B3LYP\/Def2-TZVPP<\/td>\n-1.17<\/td>\n-0.00<\/td>\n+4.54<\/td>\n3.176<\/td>\n<\/tr>\n
\n

B3LYP+GD3+BJ\/Def2-TZVPP<\/p>\n<\/td>\n

-5.24<\/td>\n-4.00<\/td>\n+2.87<\/td>\n2.859 (2.741 expt)<\/td>\n<\/tr>\n
\n

MP2\/Def2-TZVPP<\/p>\n<\/td>\n

-7.42<\/td>\n–<\/td>\n–<\/td>\n2.718 (2.741 expt)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The three methods were chosen as approximations to establish (a) the effect of a dispersion\/correlation correction, using the standard third-generation Grimme method and (b) the effect of more general dynamic correlations as being the difference between a Hartree-Fock calculation and DFT one. The values show the HFa<\/sup> and B3LYP-DFT as being very similar, but adding the GD3+BJ term stabilises the dimer significantly, as well as producing an F…centroid distance only a little longer than that measured. Each cube will sustain three pairs of such interactions, so the total stabilisation energy is \u00a0~15 kcal\/mol and the enthalpy stabilisation is ~12 kcal\/mol.\u00a0A periodic boundary calculation of the complete cell would certainly be an even better model of this system. Nonetheless one further test, of the trend in length between the six interacting F atoms with a ring centroid and the two that do not (exp Δ-0.003Å shorter for the latter) is also replicated by the B3LYP+GD3+BJ\/Def2-TZVPP calculation (Δ-0.006Å) which suggests the simple dimer model is not badly wrong.<\/p>\n

So from these results, it appears that the attractive interactions between molecules octafluorocubane resulting in its high sublimation temperature may not be simply electrostatic interactions (a\u00a0HF calculation would model that) or indeed of dynamic correlation (modelled by DFT methods) but a more complete electron correlation of the type normally described as dispersion and eg<\/em> available via multi-reference and\/or coupled-cluster methods. It may indeed come as a surprise that this molecule is a high melting solid because of dispersion, but the unique geometry allows an F to interact with four carbons via<\/i> such forces, and to accumulate six of these per molecule in the crystal structure. So really quite unusual.<\/p>\n

To end, it would certainly seem worthwhile to apply higher levels of theory to confirm this result, since the\u00a0GD3+BJ induced-dipole\/induced-dipole dispersion model is a relatively simple one, and as I commented in my WATOC notes<\/a>, much higher level models of this effect are now becoming available.<\/p>\n

\n

a<\/sup>As suggested by Cina Foroutan-Nejad, a commentator on the previous blog post<\/a><\/p>\n

\n

This post has DOI: 10.14469\/hpc\/11135<\/p>\n

\n

References<\/h2>\n
  1. M. Sugiyama, M. Akiyama, Y. Yonezawa, K. Komaguchi, M. Higashi, K. Nozaki, and T. Okazoe, \"Electron in a cube: Synthesis and characterization of perfluorocubane as an electron acceptor\", Science<\/i>, vol. 377, pp. 756-759, 2022. https:\/\/doi.org\/10.1126\/science.abq0516<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The recently reported synthesis of octafluorocubane established a sublimation point as 168.1\u2013177.1\u00b0C (a melting point was not observed). In contrast, the heavier perfluoro-octane has an m.p. of -25\u00b0C. Why the difference? Firstly, the crystal structure is shown below, albeit as a dimer rather than a periodic lattice (click on image to obtain 3D coordinates). The […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[],"tags":[],"ppma_author":[2661],"class_list":["post-25521","post","type-post","status-publish","format-standard","hentry"],"yoast_head":"\nWhy does octafluorocubane have such a high sublimation point? - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25521\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Why does octafluorocubane have such a high sublimation point? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The recently reported synthesis of octafluorocubane established a sublimation point as 168.1\u2013177.1\u00b0C (a melting point was not observed). In contrast, the heavier perfluoro-octane has an m.p. of -25\u00b0C. Why the difference? Firstly, the crystal structure is shown below, albeit as a dimer rather than a periodic lattice (click on image to obtain 3D coordinates). 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