{"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_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_hide_featured_image":false,"_jetpack_featured_image_duplicate_dismissed":0,"_jetpack_feature_clip_id":0,"_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},"jetpack_post_was_ever_published":false,"_ppma_block_editor_authors":""},"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":31727,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31727","url_meta":{"origin":7721,"position":2},"title":"How long can an N-N bond get?","author":"Henry Rzepa","date":"July 18, 2026","format":false,"excerpt":"(Some) chemists have a strange fascination with bonds between two specified atoms - more exactly how short or how long can such a bond get? I asked a slight different question of a molecule known as nitrosobenzene dimer, noting that both nitrogens were both connected to each other and carried\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":31787,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31787","url_meta":{"origin":7721,"position":3},"title":"How long can a C-C bond get? Semibullvalenes – genuine bis-homoaromatic molecules or are they [3,3] sigmatropic transition states.","author":"Henry Rzepa","date":"August 20, 2026","format":false,"excerpt":"I have in the past (around 2012 to be specific) taken an interest in a particular type of [3,3]sigmatropic pericyclic reaction called the semibullvalene rearrangement.,,. This system can apparently exhibit very long C-C bonds in the region of 2.1\u00c5, in which form it would be called a \"frozen\" transition state,\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":"","width":0,"height":0},"classes":[]},{"id":25550,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25550","url_meta":{"origin":7721,"position":4},"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":5},"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":[]}],"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","avatar_alt":"","avatar_decorative":"","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Emeritus Professor of Computational Chemistry at Imperial College London."}],"jetpack_featured_media_url":"","_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}]}}