{"id":11681,"date":"2013-11-23T20:51:12","date_gmt":"2013-11-23T20:51:12","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=11681"},"modified":"2014-01-16T09:47:43","modified_gmt":"2014-01-16T09:47:43","slug":"caesium-trifluoride-could-it-be-made","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11681","title":{"rendered":"Caesium trifluoride: could it be made?"},"content":{"rendered":"
\n

Mercury (IV) tetrafluoride<\/a> attracted much interest when it was reported in 2007[1]<\/a><\/span> as the first instance of the metal being induced to act as a proper transition element (utilising d-electrons for bonding) rather than a post-transition main group metal (utilising just s-electrons) for which the HgF2<\/sub> dihalide would be more normal (“Is mercury now a transition element?”[2]<\/a><\/span>). Perhaps this is the modern equivalent of transmutation! Well, now we have new speculation about how to induce the same sort of behaviour for caesium; might it form CsF3<\/sub> (at high pressures) rather than the CsF we would be more familiar with.[3]<\/a><\/span>\u00a0Here I report some further calculations inspired<\/a> by this report.<\/p>\n

The argument goes something like this. Xenon difluoride<\/a>\u00a0(XeF2<\/sub>) is a well-known stable compound of xenon. Caesium comes immediately after xenon in the periodic table (electron shell properties [Xe<\/strong>].6s<\/strong>1<\/sup>) and so Cs+<\/sup> would be iso-electronic with Xe. If the latter can form a stable difluoride (and higher), why not Cs? A neutral compound following this line of argument would therefore be\u00a0CsF3<\/sub>.<\/p>\n

So here comes a calculation. I used a large basis set (Def2-QZVPPD basis for Cs), with a pseudopotential describing 46 core electrons (including the two d-shells) and a further 8 in the 5s\/p shell to make up the [Xe] core + 1 extra in the 6s shell) using the \u03c9B97XD functional to obtain the geometry, shown below.[4]<\/a><\/span><\/p>\n

\"Click

Click for 3D and normal modes<\/p><\/div>\n

All 3N-6 normal vibrational modes are real, which indicates it is a proper minimum in the potential energy surface. A QTAIM analysis shows that \u03c1(r) at the bond critical points is pretty respectable for bonds (below). The QTAIM-derived average number of electrons on Cs is 7.4, which gives a charge of 1.6 on the Cs , thus involving the 5p shell as well as the 6s.<\/p>\n

\"CsF3-AIM\"<\/p>\n

The most obvious question is what is the free energy change when the species dissociates to CsF and F2<\/sub>? This is exo-energic by 25.1 kcal\/mol[5]<\/a><\/span>, not as large as you might have thought! Well, what about the barrier to such dissociation? The transition state for this process is shown below, delightfully asymmetric![6]<\/a><\/span> and this gives a free energy barrier of 33.9 kcal\/mol (the IRC smoothly defines this reaction[7]<\/a><\/span>,[8]<\/a><\/span>).<\/p>\n

\"Click

Click for animation<\/p><\/div>\n

We may conclude from this brief foray that in CsF3<\/sub>\u00a0caesium would indeed deserve to be called a p-block element, although this is not quite as good as it sounds. Take a look for example at the highest occupied molecular orbital. It certainly involves the p-block, but this orbital is in fact anti-bonding, and the Cs-F bonds derive their strength from the lower occupied orbitals.<\/p>\n

\"Click

HOMO. Click for 3D<\/p><\/div>\n

 <\/p>\n

\"Click

HOMO-6. Click for 3D<\/p><\/div>\n

And the final take-home message. The report of this molecule[3]<\/a><\/span> suggests it could be stable under high pressure. Here, the free energy barrier to dissociation is calculated to be indeed high, which implies that if made it could be kinetically quite stable even under normal pressures (in an inert matrix where it would be prevented from reacting with itself).<\/p>\n

POSTSCRIPT:<\/strong> The LUMO below is also antibonding<\/em>, but is mostly 6s on Cs. If one force-populates this by a double excitation from the \u00a0HOMO, the resulting state is higher in energy (the wavefunction is stable to both singlet and triplet excitations). Which shows that Cs utilises (antibonding) 5p rather than (antibonding) 6s in this species.<\/p>\n

\"CsF3-LUMO\"<\/p>\n

References<\/h2>\n
  1. X. Wang, L. Andrews, S. Riedel, and M. Kaupp, \"Mercury Is a Transition Metal: The First Experimental Evidence for HgF<sub>4<\/sub>\", Angewandte Chemie International Edition<\/i>, vol. 46, pp. 8371-8375, 2007. https:\/\/doi.org\/10.1002\/anie.200703710<\/a>\n\n<\/li>\n
  2. M. Miao, \"Caesium in high oxidation states and as a p-block element\", Nature Chemistry<\/i>, vol. 5, pp. 846-852, 2013. https:\/\/doi.org\/10.1038\/nchem.1754<\/a>\n\n<\/li>\n
  3. H.S. Rzepa, \"Gaussian Job Archive for CsF3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.861029<\/a>\n\n<\/li>\n
  4. H.S. Rzepa, \"Gaussian Job Archive for CsF3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.861030<\/a>\n\n<\/li>\n
  5. H.S. Rzepa, \"Gaussian Job Archive for CsF3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.861038<\/a>\n\n<\/li>\n
  6. H.S. Rzepa, \"Gaussian Job Archive for CsF3\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.861047<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Mercury (IV) tetrafluoride attracted much interest when it was reported in 2007 as the first instance of the metal being induced to act as a proper transition element (utilising d-electrons for bonding) rather than a post-transition main group metal (utilising just s-electrons) for which the HgF2 dihalide would be more normal (“Is mercury now a […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_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},"jetpack_post_was_ever_published":false},"categories":[7,4],"tags":[152,24,206,1155,157,74,1154,142],"ppma_author":[2661],"class_list":["post-11681","post","type-post","status-publish","format-standard","hentry","category-hypervalency","category-interesting-chemistry","tag-animation","tag-energy","tag-free-energy-barrier","tag-free-energy-change","tag-metal","tag-pence","tag-post-transition-main-group-metal","tag-potential-energy-surface"],"yoast_head":"\nCaesium trifluoride: could it be made? - 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=11681\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Caesium trifluoride: could it be made? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Mercury (IV) tetrafluoride attracted much interest when it was reported in 2007 as the first instance of the metal being induced to act as a proper transition element (utilising d-electrons for bonding) rather than a post-transition main group metal (utilising just s-electrons) for which the HgF2 dihalide would be more normal (“Is mercury now a […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11681\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-11-23T20:51:12+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2014-01-16T09:47:43+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/11\/CsF31.jpg\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Caesium trifluoride: could it be made? 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Here I play a little\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"Pt-cbd","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/05\/Pt-cbd.jpeg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":16619,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16619","url_meta":{"origin":11681,"position":1},"title":"Pyrophoric metals + the mechanism of thermal decomposition of magnesium oxalate.","author":"Henry Rzepa","date":"March 19, 2017","format":false,"excerpt":"A pyrophoric metal is one that burns spontaneously in oxygen; I came across this phenomenon as a teenager doing experiments at home. Pyrophoric iron for example is prepared by heating anhydrous iron (II) oxalate in a sealed test tube (i.e. to 600\u00b0 or higher). When the tube is broken open\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/155-1024x363.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10801,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10801","url_meta":{"origin":11681,"position":2},"title":"Is CLi6 hypervalent?","author":"Henry Rzepa","date":"July 5, 2013","format":false,"excerpt":"A comment made on the previous post on the topic of hexa-coordinate carbon cited an article entitled \"Observation of hypervalent CLi6\u00a0by Knudsen-effusion mass spectrometry\" by Kudo as a amongst the earliest of evidence that such species can exist (in the gas phase). It was a spectacular vindication of the earlier\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"Click for 3D","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/07\/CLi6-Lp.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":12880,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12880","url_meta":{"origin":11681,"position":3},"title":"Full circle. Stereoisomeric transition states for [1,4] pericyclic shifts.","author":"Henry Rzepa","date":"August 18, 2014","format":false,"excerpt":"This post, the fifth in the series, comes full circle. I started off by speculating how to invert the stereochemical outcome of an electrocyclic reaction by inverting a bond polarity. This led to finding transition states for BOTH outcomes with suitable substitution, and then seeking other examples. Migration in homotropylium\u2026","rel":"","context":"In "pericyclic"","block_context":{"text":"pericyclic","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=559"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":19499,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19499","url_meta":{"origin":11681,"position":4},"title":"Never mind main group “hypervalency”, what about transition metal “hypervalency”?","author":"Henry Rzepa","date":"March 18, 2018","format":false,"excerpt":"I have posted often on the chemical phenomenon known as hypervalency, being careful to state that as defined it applies just to \"octet excess\" in main group elements. But what about the next valence shell, occurring in transition metals and known as the \"18-electron rule\"? You rarely hear the term\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2018\/03\/NiPP-987x1024.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":23588,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23588","url_meta":{"origin":11681,"position":5},"title":"Two new reality-based suggestions for molecules with a metal M\u2a78C quadruple bond.","author":"Henry Rzepa","date":"May 8, 2021","format":false,"excerpt":"Following from much discussion over the last decade about the nature of C2, a diatomic molecule which some have suggested sustains a quadruple bond between the two carbon atoms, new ideas are now appearing for molecules in which such a bond may also exist between carbon and a transition metal\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/05\/Screenshot-702-300x63.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11681","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=11681"}],"version-history":[{"count":27,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11681\/revisions"}],"predecessor-version":[{"id":11888,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11681\/revisions\/11888"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=11681"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=11681"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=11681"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=11681"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}