{"id":17122,"date":"2016-12-01T11:24:20","date_gmt":"2016-12-01T11:24:20","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=17122"},"modified":"2016-12-05T13:29:42","modified_gmt":"2016-12-05T13:29:42","slug":"long-cc-bonds","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17122","title":{"rendered":"Long C=C bonds."},"content":{"rendered":"
\n

Following on from a search for long C-C bonds, here is the same repeated for C=C double bonds.<\/p>\n

\"sq\"<\/p>\n

The query restricts the search to each carbon having just two non-metallic substituents. To avoid conjugation with these, they each are 4-coordinated; the carbons themselves are three-coordinated. Further constraints are the usual no disorder, no errors and R < 0.1 and the C=C distance > 1.4\u00c5 (the standard value is ~1.32-1.34\u00c5). The search query is deposited as DOI:\u00a010.14469\/hpc\/1959<\/a>[1]<\/a><\/span><\/p>\n

\"c_c\"<\/p>\n

The apparent longest example is LIRVEN, DOI:\u00a010.5517\/CC4R2MK<\/a>[2]<\/a><\/span> with a value of 1.589\u00c5, longer than most C-C single bonds! Closer inspection reveals the presence of lithium cations, and so the molecule bearing the C=C bond must sustain two negative charges. So this apparent C=C bond is in fact anionic, with one electron going into each of the \u03c0* orbitals, thus lengthening the CC bond.\u2021<\/sup> Not a true example of a neutral C=C bond[3]<\/a><\/span> but it now becomes interesting for what its spin state might be. Is it a biradical or a triplet for example? One to be investigated further I fancy! Another example of this type is QUKCEE[4]<\/a><\/span><\/p>\n

\"10-5517cc4r2mk-lirven\"<\/p>\n

This next FAZWIM has\u00a0a C=C length of 1.546\u00c5. It is an old structure (1986), and comes without attached hydrogen atoms. Although drawn with no hydrogens on the central C=C bond, the length suggests this molecule is simply mis-assigned.\u2020<\/sup>\"fazwin-diag\" \"fazwim\"<\/p>\n

The final example I will highlight is pretty ordinary looking and published in 2016 as a private communication; ALOVOO, DOI:\u00a010.5517\/CCDC.CSD.CC1LJSWS<\/a>[5]<\/a><\/span> with a C=C length of 1.443\u00c5. Again no obvious reason for the bond to be longer than normal.\u2021\u2020<\/sup><\/p>\n

\"10-5517ccdc-csd-cc1ljsws-alovoo\"<\/p>\n

In hunting for such unusual deviations from the norm, the most obvious explanation is normally some anomaly in the crystallographic analysis. Although the CSD (crystal structure database) is a very heavily curated resource, it seems unlikely that each deposition would be carefully inspected for its chemistry, and this must be our task here. But such anomalies can themselves point to interesting or unusual chemistry, which in \u00a0turn can be subjected to quantum computation to see if either the unusual value can be replicated or other reasons identified. \u00a0In this case, this exercise can been conducted by a human, but one can easily envisage the entire process being automated on a far larger scale. \u00a0The future?<\/p>\n

\n

\u2021<\/sup> In fact the stoichiometry shows each “double bond” is actually a di-anion, with two electrons entering each of the the \u03c0* orbitals.<\/p>\n

\u2020<\/sup>A calculation on the singlet state for the structure as drawn (\u03c9B97XD\/Def2-TZVPP, DOI: 10.14469\/hpc\/1960<\/a>) gives a bond length of 1.342\u00c5, i.e.<\/em> that expected for a double bond. The triplet state is similar in energy, but with a much longer central bond length of 1.476\u00c5, DOI: 10.14469\/hpc\/1962<\/a> but the geometry at the carbons is planar and not bent as shown above. The quintet state is 1.45\u00c5 and is again planar, doi 10.14469\/hpc\/1963<\/a>.\u00a0So calculations on FAZWIM strongly suggest the structure as shown is an error.<\/p>\n

\u2021\u2020<\/sup>The computed value is 1.324\u00c5, perfectly normal. DOI: 10.14469\/hpc\/1966<\/a>[6]<\/a><\/span><\/p>\n

References<\/h2>\n
  1. H. Rzepa, \"Long C=C bonds\", 2016. https:\/\/doi.org\/10.14469\/hpc\/1959<\/a>\n\n<\/li>\n
  2. Matsuo, T.., Watanabe, H.., Ichinohe, M.., and Sekiguchi, A.., \"CCDC 141348: Experimental Crystal Structure Determination\", 2000. https:\/\/doi.org\/10.5517\/cc4r2mk<\/a>\n\n<\/li>\n
  3. T. Matsuo, H. Watanabe, M. Ichinohe, and A. Sekiguchi, \"Reduction of the 1,4,5,8-tetrasila-1,4,5,8-tetrahydroanthracene derivative with lithium metal. Isolation and characterization of the tetralithium salt of a tetraanion, and observation of an Si\u2013H\u22efLi+ interaction\", Inorganic Chemistry Communications<\/i>, vol. 2, pp. 510-512, 1999. https:\/\/doi.org\/10.1016\/s1387-7003(99)00136-7<\/a>\n\n<\/li>\n
  4. T. Matsuo, H. Watanabe, and A. Sekiguchi, \"A Novel Tetralithium Salt of a Tetraanion and a Dilithium Salt of a Dianion, Formed by the Reduction of the Tetrasilylethylene Moiety. Synthesis, Characterization, and Observation of an Si-H\u00b7\u00b7\u00b7Li+ Interaction\", Bulletin of the Chemical Society of Japan<\/i>, vol. 73, pp. 1461-1467, 2000. https:\/\/doi.org\/10.1246\/bcsj.73.1461<\/a>\n\n<\/li>\n
  5. M.E. Light, S. Bain, and J. Kilburn, \"CCDC 1475906: Experimental Crystal Structure Determination\", 2016. https:\/\/doi.org\/10.5517\/ccdc.csd.cc1ljsws<\/a>\n\n<\/li>\n
  6. H. Rzepa, \"ALOVOO\", 2016. https:\/\/doi.org\/10.14469\/hpc\/1966<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Following on from a search for long C-C bonds, here is the same repeated for C=C double bonds. The query restricts the search to each carbon having just two non-metallic substituents. To avoid conjugation with these, they each are 4-coordinated; the carbons themselves are three-coordinated. Further constraints are the usual no disorder, no errors and […]<\/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":true,"_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":[1745,4],"tags":[1402,557,1563,1395,1558,1016,24,1871,1899,1410,1442,734,1565],"ppma_author":[2661],"class_list":["post-17122","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","category-interesting-chemistry","tag-chemical-bond","tag-chemical-bonding","tag-chemical-nomenclature","tag-chemistry","tag-conjugated-system","tag-double-bond","tag-energy","tag-nature","tag-nonmetal","tag-organic-chemistry","tag-physical-organic-chemistry","tag-search-query","tag-substituent"],"yoast_head":"\nLong C=C bonds. - 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=17122\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Long C=C bonds. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Following on from a search for long C-C bonds, here is the same repeated for C=C double bonds. The query restricts the search to each carbon having just two non-metallic substituents. To avoid conjugation with these, they each are 4-coordinated; the carbons themselves are three-coordinated. 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But this arose from highly unusual bonding giving rise not to a single bond order but one closer to one half! How long can a\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":"long-cc","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/11\/long-cc-1024x757.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":18165,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18165","url_meta":{"origin":17122,"position":1},"title":"\u03c0-Facial hydrogen bonds to alkenes (revisited): how close can an acidic hydrogen approach?","author":"Henry Rzepa","date":"April 15, 2017","format":false,"excerpt":"Back in the early 1990s, we first discovered the delights of searching crystal structures\u00a0for unusual\u00a0bonding features. One of the first cases was a search for hydrogen bonds formed to the\u00a0\u03c0-faces of alkenes and alkynes. In those days the CSD database of crystal structures was a lot smaller (<80,000 structures; it's\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\/04\/SQ-H-pi-1024x783.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":17168,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17168","url_meta":{"origin":17122,"position":2},"title":"Molecule of the year? “CrN123”, a molecule with three different types of Cr-N bond.","author":"Henry Rzepa","date":"December 16, 2016","format":false,"excerpt":"Here is a third candidate for the C&EN \"molecule of the year\" vote. This one was shortlisted because it is the first example of a metal-nitrogen complex exhibiting single, double and triple bonds from different nitrogens to the same metal (XUZLUB has a 3D display available at DOI: 10.5517\/CC1JYY6M). Since\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":21726,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21726","url_meta":{"origin":17122,"position":3},"title":"Hydrogen bonds: carbon as an acceptor rather than as a donor?","author":"Henry Rzepa","date":"December 23, 2019","format":false,"excerpt":"A hydrogen bond donor is considered as an electronegative element carrying a hydrogen that is accepted by an atom carrying a lone pair of electrons, as in X:...H-Y where X: is the acceptor and H-Y the donor. Wikipedia asserts that carbon can act as a donor, as we saw in\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\/2019\/12\/query-1024x598.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":17543,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17543","url_meta":{"origin":17122,"position":4},"title":"Stable “unstable” molecules: a crystallographic survey of cyclobutadienes and cyclo-octatetraenes.","author":"Henry Rzepa","date":"March 5, 2017","format":false,"excerpt":"Cyclobutadiene is one of those small iconic molecules, the transience and instability of which was explained theoretically long before it was actually detected in 1965. Given that instability, I was intrigued as to how many crystal structures might have been reported for this ring system, along with the rather more\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\/134-1024x722.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":16819,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16819","url_meta":{"origin":17122,"position":5},"title":"What\u2019s in a name? Stabilised “nitrenes”.","author":"Henry Rzepa","date":"September 19, 2016","format":false,"excerpt":"I previously explored stabilized \"carbenes\" with the formal structures (R2N)2C:, concluding that perhaps the alternative ionic representation R2N+=C-NR2\u00a0might reflect their structures better. Here I take a broader look at the \"carbene\" landscape before asking the question \"what about nitrenes?\" The top row shows the compounds for which no crystal structure\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":"nitrene1","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/09\/nitrene1-1024x655.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\/17122","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=17122"}],"version-history":[{"count":11,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/17122\/revisions"}],"predecessor-version":[{"id":17141,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/17122\/revisions\/17141"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=17122"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=17122"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=17122"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=17122"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}