{"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":"2022-03-15T09:22:20","modified_gmt":"2022-03-15T09:22:20","slug":"an-unusually-small-doubly-aromatic-molecule-c4","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/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_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,"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":[4],"tags":[1524],"class_list":["post-24769","post","type-post","status-publish","format-standard","hentry","category-general","tag-general"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-6rv","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/24769","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=24769"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/24769\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=24769"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=24769"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=24769"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}