{"id":24769,"date":"2022-03-15T09:22:20","date_gmt":"2022-03-15T09:22:20","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=24769"},"modified":"2022-03-15T11:54:33","modified_gmt":"2022-03-15T11:54:33","slug":"an-unusually-small-doubly-aromatic-molecule-c4","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=24769","title":{"rendered":"An unusually small (doubly) aromatic molecule: C4."},"content":{"rendered":"
\n

When you talk \u03c0-aromaticity, benzene is the first molecule that springs to mind.\u00a0But there are smaller molecules that can carry this property; cyclopropenylidene<\/a> (five atoms) is the smallest in terms of atom count I could think of until now, apart that is from H3<\/sub>+<\/sup> which is the smallest possible molecule that carries \u03c3-aromaticity. So here I have found what I think is an even smaller aromatic molecule containing only four carbon atoms. And it is not only \u03c0-aromatic but \u03c3-aromatic.<\/p>\n

\"\"<\/a><\/p>\n

Let me go through the analysis (using a CCSD(T)\/Def2-TZVPPD calculation, DOI: 10.14469\/hpc\/10226<\/a>). <\/p>\n

    \n
  1. Four carbons contain 16 valence electrons for bonding.<\/li>\n
  2. Eight of these are conventional, forming four C-C single bonds around the 4-ring.<\/li>\n
  3. Eight are left over, and these partition into a set of six and a set of two.<\/li>\n
  4. The set of two are in\u00a0p-\u03c0 atomic orbitals and form a 4n+2 (n=0) aromatic system<\/li>\n
  5. The set of six are in\u00a0\u03c3-sp AOs and form a 4n+2 (n=1) aromatic system.<\/li>\n
  6. The three \u03c3-MOs all contribute to the central C-C bond, particularly \u03c33 and \u03c32 in different ways.<\/li>\n
  7. \u03c32 also reminds of [1.1.1]-propellane<\/a>, where the two \u03c3-electrons are in effect external to the central\u00a0C-C bond, but spin coupled to form what might be called a \u03c3\u00a0exo<\/em>-bond<\/a>. There is also similarity to the exo<\/em> bond in C2<\/sub>.<\/li>\n
  8. The dissociation energy of the central bond can be estimated at 28 kcal\/mol from the triplet state energy.<\/li>\n<\/ol>\n\n\n\n\n\n\n\n\n\n
    Bonding MOs for C4<\/sub>.
    \nClick image to load 3D model<\/th>\n<\/tr>\n
    \u03c01<\/th>\n<\/tr>\n
    \"\"<\/td>\n<\/tr>\n
    \u03c33<\/th>\n\u03c32<\/th>\n<\/tr>\n
    \"\"<\/td>\n\"\"<\/td>\n<\/tr>\n
    \u03c31<\/th>\n<\/tr>\n
    \"\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    So this little molecule carries a lot of diversity in its chemical bonding; an ideal candidate perhaps for a tutorial in bonding theory of organic molecules?<\/p>\n

    \n

    The post has DOI: 10.14469\/hpc\/10252<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    When you talk \u03c0-aromaticity, benzene is the first molecule that springs to mind.\u00a0But there are smaller molecules that can carry this property; cyclopropenylidene (five atoms) is the smallest in terms of atom count I could think of until now, apart that is from H3+ which is the smallest possible molecule that carries \u03c3-aromaticity. So here […]<\/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":[1],"tags":[],"ppma_author":[2661],"class_list":["post-24769","post","type-post","status-publish","format-standard","hentry","category-general"],"yoast_head":"\nAn unusually small (doubly) aromatic molecule: C4. - 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=24769\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"An unusually small (doubly) aromatic molecule: C4. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"When you talk \u03c0-aromaticity, benzene is the first molecule that springs to mind.\u00a0But there are smaller molecules that can carry this property; cyclopropenylidene (five atoms) is the smallest in terms of atom count I could think of until now, apart that is from H3+ which is the smallest possible molecule that carries \u03c3-aromaticity. 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It has now been detected in the atmosphere of Titan, one of Saturn's moons and joins benzene, another aromatic molecule together with the protonated version of cyclopropenylidene, C3H3+ also\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":24881,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=24881","url_meta":{"origin":24769,"position":1},"title":"A four-atom molecule exhibiting simultaneous compliance with H\u00fcckel 4n+2 and Baird 4n selection rules for ring aromaticity.","author":"Henry Rzepa","date":"March 22, 2022","format":false,"excerpt":"Normally, aromaticity is qualitatively assessed using an electron counting rule for cyclic conjugated rings. The best known is the H\u00fcckel 4n+2 rule (n=0,1, etc) for inferring diatropic aromatic ring currents in singlet-state \u03c0-conjugated cyclic molecules\u2021 and a counter 4n rule which infers an antiaromatic paratropic ring current for the system.\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\/2022\/03\/C2B2-300x212.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":275,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=275","url_meta":{"origin":24769,"position":2},"title":"A molecule with an identity crisis: Aromatic or anti-aromatic?","author":"Henry Rzepa","date":"April 13, 2009","format":false,"excerpt":"In 1988, Wilke reported molecule 1 It was a highly unexpected outcome of a nickel-catalyzed reaction and was described as a 24-annulene with an unusual 3D shape. Little attention has been paid to this molecule since its original report, but the focus has now returned! The reason is that a\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":"A [24] annulene. Click on image for model.","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/gaytab.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1903,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1903","url_meta":{"origin":24769,"position":3},"title":"Carbobenzene: benzene with a difference","author":"Henry Rzepa","date":"April 16, 2010","format":false,"excerpt":"Some molecules, when you first see them, just intrigue. So it was with carbobenzene, the synthesis of a derivative of which was recently achieved by Remi Chauvin and co-workers (DOI: 10.1002\/chem.200601193). Two additional carbon atoms have been inserted into each of the six C-C bonds in benzene. The structure shows\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/04\/carbobenzene.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":11421,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11421","url_meta":{"origin":24769,"position":4},"title":"Six vs ten aromatic electrons?","author":"Henry Rzepa","date":"October 20, 2013","format":false,"excerpt":"Homoaromaticity is a special case of\u00a0aromaticity\u00a0in which\u00a0\u03c0-conjugation\u00a0is interrupted by a single sp3\u00a0hybridized\u00a0carbon atom (it is sometimes referred to as a suspended \u03c0-bond with no underlying \u03c3-foundation).\u00a0But consider the carbene shown below. This example comes from a recently published article which was highlighted on Steve Bachrach's blog. Here aromaticity has resulted\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":"Click for 3D","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/10\/B10-sigma.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":5632,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5632","url_meta":{"origin":24769,"position":5},"title":"A modern take on the pericyclic electrocyclic ring opening of cyclobutene.","author":"Henry Rzepa","date":"November 26, 2011","format":false,"excerpt":"Woodward and Hoffmann published their\u00a0milestone article\u00a0 \"Stereochemistry of Electrocyclic Reactions\" in 1965. This brought maturity to the electronic theory of organic chemistry, arguably started by the proto-theory of Armstrong some 75 years earlier. Here, I take a modern look at the archetypal carrier of this insight, the ring opening of\u2026","rel":"","context":"In \"Adam\"","block_context":{"text":"Adam","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=adam"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/11\/con-open.gif?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\/24769","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=24769"}],"version-history":[{"count":27,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/24769\/revisions"}],"predecessor-version":[{"id":24813,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/24769\/revisions\/24813"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=24769"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=24769"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=24769"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=24769"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}