{"id":7721,"date":"2012-09-17T15:45:57","date_gmt":"2012-09-17T14:45:57","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7721"},"modified":"2012-09-17T16:11:56","modified_gmt":"2012-09-17T15:11:56","slug":"predicted-properties-of-a-candidate-for-a-frozen-semibullvalene-2","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7721","title":{"rendered":"Predicted properties of a candidate for a frozen semibullvalene."},"content":{"rendered":"
\n

I am following up on one unfinished thread in my previous post<\/a>; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum.<\/p>\n

\"\"<\/p>\n

The hypothesis was that such a species would be aromatic, a bis<\/em>-homoaromatic species to be precise, itself an interesting variation on the normal theme of aromaticity. The so-called computed [3,3] vibrational mode emerged as real and not imaginary. But how to go about showing that the resulting species might actually be aromatic? I started with a QTAIM characterisation of bond and ring critical points.<\/p>\n

\"\"

QTAIM analysis of the proposed frozen semibullvalene. Bond critical points in magenta. Click for 3D.<\/p><\/div>\n

The two homobonds are shown above with dashed bonds. In fact, there are no bond critical points (bcp) located between these two atoms. This of itself need not distress us; the lack of a suitable bcp has been observed previously for other such homo-aromatics[1]<\/a><\/span>, a phenomenon referred to as a no-bond bond<\/em>. But because the QTAIM does not locate a bond, there is no ring located for the aromatic system either. And in such a heavily non-planar system, the lack of an unambiguous ring centroid means that it is not clear where to place any NICS probe of aromaticity.<\/p>\n

What about the ELF technique. For M\u00f6bius homoaromaticity[2]<\/a><\/span> the homo-bond does display a locatable disynaptic ELF basin integrating to around 1-electron. No so here! So no progress yet on estimating the aromaticity.<\/p>\n

\"\"

ELF analysis for the frozen [3,3] system. Disynaptic ELF basin centroids in magenta. Click for 3D.<\/p><\/div>\n

Are there any other computable properties which might be detected experimentally? Well, I noted that the species is C2<\/sub>-symmetric and hence chiral. What about its computed electronic circular dichroism spectrum? These are shown for firstly<\/a> the frozen system, for which both C-N bonds are 2.13\u00c5 in length, and then for a distorted version<\/a> in which one C-N bond is 1.6\u00c5 and the other 2.4\u00c5. The former is more delocalised (as expected for an aromatic ring) and so has\u00a0\u03bbmax<\/sub> in a region which would give the compound a mauve colour. The latter is less delocalised and hence would have an orange-yellow colour, with \u03bbmax <\/sub>some 100nm less. The ECD spectra also have different appearances. This would be indeed an interesting way of determining changes in delocalisation, in this case possibly associated with homo-aromaticity.\u00a0<\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

References<\/h2>\n
  1. C.S.M. Allan, and H.S. Rzepa, \"Chiral Aromaticities. AIM and ELF Critical Point and NICS Magnetic Analyses of M\u00f6bius-Type Aromaticity and Homoaromaticity in Lemniscular Annulenes and Hexaphyrins\", The Journal of Organic Chemistry<\/i>, vol. 73, pp. 6615-6622, 2008. https:\/\/doi.org\/10.1021\/jo801022b<\/a>\n\n<\/li>\n
  2. C.S.M. Allan, and H.S. Rzepa, \"Chiral Aromaticities. A Topological Exploration of M\u00f6bius Homoaromaticity\", Journal of Chemical Theory and Computation<\/i>, vol. 4, pp. 1841-1848, 2008. https:\/\/doi.org\/10.1021\/ct8001915<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    I am following up on one unfinished thread in my previous post; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum. The hypothesis was that such a species would be aromatic, a bis-homoaromatic species to be precise, itself […]<\/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":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[4],"tags":[2650],"ppma_author":[2661],"class_list":["post-7721","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-pericyclic"],"yoast_head":"\nPredicted properties of a candidate for a frozen semibullvalene. - 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=7721\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Predicted properties of a candidate for a frozen semibullvalene. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"I am following up on one unfinished thread in my previous post; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum. The hypothesis was that such a species would be aromatic, a bis-homoaromatic species to be precise, itself […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7721\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2012-09-17T14:45:57+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2012-09-17T15:11:56+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/09\/semibullvalene.svg\" \/>\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=\"2 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Predicted properties of a candidate for a frozen semibullvalene. - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7721","og_locale":"en_GB","og_type":"article","og_title":"Predicted properties of a candidate for a frozen semibullvalene. - Henry Rzepa's Blog","og_description":"I am following up on one unfinished thread in my previous post; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum. 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Whilst it has a classical structure describable by a combination of Lewis-style two electron and four electron bonds, its NMR behaviour reveals it to be highly fluxional. This means that even at low temperatures, the position of these two-electron bonds rapidly shifts in the equilibrium\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/09\/CAZFUE1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":7756,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7756","url_meta":{"origin":7721,"position":1},"title":"The ten-electron homologue of semibullvalene.","author":"Henry Rzepa","date":"September 21, 2012","format":false,"excerpt":"Semibullvalene is a molecule which undergoes a facile [3,3] sigmatropic shift. So facile that it appears this equilibrium can be frozen out at the transition state if suitable substituents are used. This is a six-electron process, which leads to one of those homologous questions; what happens with ten electrons? The\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/09\/55.svg","width":350,"height":200},"classes":[]},{"id":25550,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25550","url_meta":{"origin":7721,"position":2},"title":"What is the largest angle possible at 4-coordinate carbon – 180\u00b0?","author":"Henry Rzepa","date":"September 11, 2022","format":false,"excerpt":"Four-coordinate carbon normally adopts a tetrahedral shape, where the four angles at the carbon are all 109.47\u00b0. But how large can that angle get, and can it even get to be 180\u00b0? A search of the CSD (crystal structure database) reveals a spiropentane as having the largest such angle, VAJHAP\u2026","rel":"","context":"In \"Interesting chemistry\"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=interesting-chemistry"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/09\/VAJHAP-mo20-1024x758.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":3186,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3186","url_meta":{"origin":7721,"position":3},"title":"Do marauding electrons go in packs?","author":"Henry Rzepa","date":"December 27, 2010","format":false,"excerpt":"Is there a preferred pack size for electrons on the move? Or put less flamboyantly, is there an optimum, and a maximum number of arrows (electron pairs) that one might push in revealing the mechanism of a concerted reaction? A sort of village-instinct for electrons. Consider the following (known, DOI:\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/12\/333.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":17662,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17662","url_meta":{"origin":7721,"position":4},"title":"Expanding on the curious connection between the norbornyl cation and small-ring aromatics.","author":"Henry Rzepa","date":"March 12, 2017","format":false,"excerpt":"This is another of those posts that has morphed from an earlier one noting\u00a0the death of the great chemist George Olah. The discussion about the norbornyl cation concentrated on whether this species existed in a single minimum symmetric energy well (the non-classical Winstein\/Olah proposal) or a double minimum well connected\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":17483,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17483","url_meta":{"origin":7721,"position":5},"title":"The H4 (2+) dication and its bonding.","author":"Henry Rzepa","date":"February 15, 2017","format":false,"excerpt":"This post arose from a comment attached to the post on Na2He\u00a0and relating to peculiar and rare topological features of the electron density in molecules called non-nuclear attractors. This set me thinking about other molecules that might exhibit this and one of these is shown below. The topology of the\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/02\/049-1009x1024.jpg?resize=350%2C200","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\/7721","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=7721"}],"version-history":[{"count":11,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7721\/revisions"}],"predecessor-version":[{"id":7735,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7721\/revisions\/7735"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7721"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7721"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7721"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=7721"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}