{"id":21772,"date":"2019-12-27T06:57:13","date_gmt":"2019-12-27T06:57:13","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=21772"},"modified":"2019-12-27T06:57:13","modified_gmt":"2019-12-27T06:57:13","slug":"carbon-as-a-hydrogen-bond-acceptor-can-dicarbon-c2-act-in-this-manner","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=21772","title":{"rendered":"Carbon as a hydrogen bond acceptor: can dicarbon (C2) act in this manner?"},"content":{"rendered":"
\n

In the previous post<\/a>, I showed that carbon can act as a hydrogen bond acceptor (of a proton) to form strong hydrogen bond complexes. Which brings me to a conceptual connection: can singlet dicarbon form such a hydrogen bond?\u00a0
\n\"\"<\/a><\/p>\n

Dicarbon can be variously represented as above. The first form shows it as a bis-carbene, with an unbonded lone pair of electrons at each end of a carbon double bond. The middle form has emerged in the last ten years or so as a serious alternative to describing the singlet state structure. \u00a0It contains a so-called triple endo<\/em>–<\/strong>bond and one further much weaker exo<\/strong><\/em>-bond (indicated separately by the symbol above the bond), referred to for simplicity as quadruple-bonded dicarbon. The third form would be a triplet biradical with triple bonded carbon. The species is known to be a singlet ground state with a significant excitation energy to the triplet. One can then ask the question: would either of these singlet state species be capable of being a hydrogen bond acceptor?<\/p>\n

Time for calculations, at the CCSD(T)\/Def2-TZVPP level using HF as the hydrogen bond donor (to enable advantage to be taken of the axial symmetry), data DOI: 10.14469\/hpc\/6554<\/a>.<\/p>\n

    \n
  1. The singlet quadruple bonded form emerges as 32 kcal\/mol higher in total energy than the singlet dicarbene.<\/li>\n
  2. The quadruple bonded form shows no sign of forming a hydrogen bond. The geometry optimisation curve is shown below followed by the final geometry (\u00c5).\n

    \u00a0\"\"<\/a><\/p>\n

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

  3. The bis-carbene form \u00a0(calculated by a double electron excitation, orbitals 10 to 12 and 11 to 15) DOES form such a complex. The hydrogen bond length (2.04\u00c5) is exactly that found from the crystal structures of the shortest such bonds.\n

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

  4. Two \u00a0of the normal vibrational modes of this species are shown below, being respectively the H…C and C=C stretches (153 and 1394 cm-1<\/sup>). \u00a0\u00a0\"\"<\/a> \"\"<\/a><\/li>\n<\/ol>\n

    So dicarbon CAN form a short hydrogen bond to a donor such as HF, but only in its excited singlet state, which is some 32 kcal\/mol above the quadruple-bonded form. Perhaps because of that fourth bond, the hydrogen bonding ability of this species is entirely inhibited. We have gotten to the point I wanted to reach; an experimental prediction that if singlet dicarbon can ever be trapped in a very inert matrix at very low temperatures in the presence of a hydrogen bond donor, it will not<\/strong> form a hydrogen bond to that donor. That is going to be a difficult experiment, but at least the prediction is out there as a challenge!<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    In the previous post, I showed that carbon can act as a hydrogen bond acceptor (of a proton) to form strong hydrogen bond complexes. Which brings me to a conceptual connection: can singlet dicarbon form such a hydrogen bond?\u00a0 Dicarbon can be variously represented as above. The first form shows it as a bis-carbene, with […]<\/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":[6],"tags":[1526],"class_list":["post-21772","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-interesting-chemistry"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-5Fa","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21772","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=21772"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21772\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=21772"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=21772"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=21772"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}