{"id":25740,"date":"2022-11-29T15:06:41","date_gmt":"2022-11-29T15:06:41","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=25740"},"modified":"2022-11-29T15:35:29","modified_gmt":"2022-11-29T15:35:29","slug":"gaseous-carbon-the-energetics-of-two-forms-of-tetracarbon-c4-and-a-challenge","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740","title":{"rendered":"Gaseous carbon: The energetics of two forms of tetracarbon, C4 and a challenge!"},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"25740\">\n<p>The topic of dicarbon,\u00a0C<sub>2<\/sub>, has been discussed here for a few years now.\u00a0It undoubtedly would be a gas! This aspect of the species came to the fore recently<span id=\"cite_ITEM-25740-0\" name=\"citation\"><a href=\"#ITEM-25740-0\">[1]<\/a><\/span> when further experiments on a potential chemical precursor of dicarbon, the zwitterion X(+)-C\u2261C(-), showed that different variants of X(+), such as not only X=PhI(+),\u00a0but also <i>e.g.<\/i> X=dibenzothiophenium(+) appeared to generate a gaseous species, which could be trapped as &#8220;C<sub>2<\/sub>&#8221; in a solvent-free connected flask experiment.<\/p>\n<p>Part of the mystery is that C<sub>2<\/sub> itself is an extremely <b>high energy<\/b>\u00a0species, its dimerisation to C<sub>4<\/sub>\u00a0being around 107 kcal\/mol <strong>exo<\/strong>energic in free energy. Now, earlier calculations<span id=\"cite_ITEM-25740-1\" name=\"citation\"><a href=\"#ITEM-25740-1\">[2]<\/a><\/span> had revealed that the reaction of the precursor PhI(+)-C\u2261C(-) with itself can occur on a relatively low energy pathway which avoids the very high energy of C<sub>2<\/sub>. The IRC for this reaction is shown below, showing a modest barrier and a very exothermic reaction to the species PhI(+)-CCCC(-) and IPh.\u00a0<\/p>\n<p><a href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/11\/gaussian_tot_ener.svg\"><img decoding=\"async\" class=\"alignnone size-full wp-image-25747\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/11\/gaussian_tot_ener.svg\" alt=\"\" width=\"540\" \/><\/a><\/p>\n<p>Here I bring your attention to an odd feature on the IRC, in the region of -5. In this region, effectively &#8220;free C<sub>4<\/sub>&#8221; is formed (at an energy some 60 kcal\/mol lower than the reactants and 167 kcal\/mol lower than two molecules of free\u00a0C<sub>2<\/sub>), but this species is immediately trapped by a PhI to form the final products with a further decrease in energy of ~20 kcal\/mol. Suppose however, in a molecular dynamics sense, some proportion of this\u00a0&#8220;C<sub>4<\/sub>&#8221; could take a different trajectory and free itself at this point, hence escaping being trapped by PhI? This reaction would then generate what again is presumably a gaseous C<sub>4<\/sub>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-25748\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/11\/IRC.gif\" alt=\"\" width=\"502\" height=\"282\" \/><\/p>\n<p>Here I explore what might happen next, to answer the question of whether linear C<sub>4<\/sub> is\u00a0stable, or will it convert into something else? The scheme below shows some of the possible pathways, leading to the bicyclic form which I have <a href=\"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=24769\">previously discussed<\/a> extensively in terms of its stabilising aromaticity. Calculations are at the CCSD(T)\/Def2-TZVPPD gas phase level, allowing biradicals to form (FAIR Data DOI: <a href=\"https:\/\/doi.org\/10.14469\/hpc\/11956\">10.14469\/hpc\/11956<\/a>).<\/p>\n<p><sub><a href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/11\/C4a.svg\"><img decoding=\"async\" class=\"alignnone size-full wp-image-25743\" src=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/11\/C4a.svg\" alt=\"\" \/><\/a><\/sub><\/p>\n<p>You can see that C<sub>4<\/sub> is in a modest thermal well, with a free energy barrier to cyclisation of ~22 kcal\/mol. So generated at relatively low energies, it might retain its linear structure, whereas at room temperatures or higher, it will probably end up as the bicyclic aromatic species.<\/p>\n<p>The key calculation might be that dimerisation reaction shown above. Would molecular dynamics show that a proportion of the reaction allows the escape of C<sub>4<\/sub>? Would that be temperature\/pressure dependent? I am not about to try these calculations, but offers of doing so gladly accepted! But that does not necessarily solve the mystery of this reaction, alluded to above.<span id=\"cite_ITEM-25740-0\" name=\"citation\"><a href=\"#ITEM-25740-0\">[1]<\/a><\/span> Is the trapped gaseous species C<sub>2<\/sub> itself, C<sub>4<\/sub>\u00a0in some form, or indeed something else entirely?<\/p>\n<hr \/>\n<p>This post has DOI: 10.14469\/hpc\/11959<\/p>\n<hr \/>\n<h2>References<\/h2>\n    <ol class=\"kcite-bibliography csl-bib-body\"><li id=\"ITEM-25740-0\">H.S. Rzepa, M. Arita, K. Miyamoto, and M. Uchiyama, \"A combined DFT-predictive and experimental exploration of the sensitivity towards nucleofuge variation in zwitterionic intermediates relating to mechanistic models for unimolecular chemical generation and trapping of free C\n                    &lt;sub&gt;2&lt;\/sub&gt;\n                    and alternative bimolecular pathways involving no free C\n                    &lt;sub&gt;2&lt;\/sub&gt;\", <i>Physical Chemistry Chemical Physics<\/i>, vol. 24, pp. 25816-25821, 2022. <a href=\"https:\/\/doi.org\/10.1039\/d2cp01214f\">https:\/\/doi.org\/10.1039\/d2cp01214f<\/a>\n\n<\/li>\n<li id=\"ITEM-25740-1\">H.S. Rzepa, \"Routes involving no free C\n                    &lt;sub&gt;2&lt;\/sub&gt;\n                    in a DFT-computed mechanistic model for the reported room-temperature chemical synthesis of C\n                    &lt;sub&gt;2&lt;\/sub&gt;\", <i>Physical Chemistry Chemical Physics<\/i>, vol. 23, pp. 12630-12636, 2021. <a href=\"https:\/\/doi.org\/10.1039\/d1cp02056k\">https:\/\/doi.org\/10.1039\/d1cp02056k<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> <!-- kcite-section 25740 -->","protected":false},"excerpt":{"rendered":"<p>The topic of dicarbon,\u00a0C2, has been discussed here for a few years now.\u00a0It undoubtedly would be a gas! This aspect of the species came to the fore recently when further experiments on a potential chemical precursor of dicarbon, the zwitterion X(+)-C\u2261C(-), showed that different variants of X(+), such as not only X=PhI(+),\u00a0but also e.g. X=dibenzothiophenium(+) [&hellip;]<\/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,"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":[1086],"tags":[],"ppma_author":[2661],"class_list":["post-25740","post","type-post","status-publish","format-standard","hentry","category-reaction-mechanism-2"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Gaseous carbon: The energetics of two forms of tetracarbon, C4 and a challenge! - Henry Rzepa&#039;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=25740\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Gaseous carbon: The energetics of two forms of tetracarbon, C4 and a challenge! - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"The topic of dicarbon,\u00a0C2, has been discussed here for a few years now.\u00a0It undoubtedly would be a gas! This aspect of the species came to the fore recently when further experiments on a potential chemical precursor of dicarbon, the zwitterion X(+)-C\u2261C(-), showed that different variants of X(+), such as not only X=PhI(+),\u00a0but also e.g. X=dibenzothiophenium(+) [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"article:published_time\" content=\"2022-11-29T15:06:41+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2022-11-29T15:35:29+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/11\/gaussian_tot_ener.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":"Gaseous carbon: The energetics of two forms of tetracarbon, C4 and a challenge! - Henry Rzepa&#039;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=25740","og_locale":"en_GB","og_type":"article","og_title":"Gaseous carbon: The energetics of two forms of tetracarbon, C4 and a challenge! - Henry Rzepa&#039;s Blog","og_description":"The topic of dicarbon,\u00a0C2, has been discussed here for a few years now.\u00a0It undoubtedly would be a gas! This aspect of the species came to the fore recently when further experiments on a potential chemical precursor of dicarbon, the zwitterion X(+)-C\u2261C(-), showed that different variants of X(+), such as not only X=PhI(+),\u00a0but also e.g. X=dibenzothiophenium(+) [&hellip;]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740","og_site_name":"Henry Rzepa&#039;s Blog","article_published_time":"2022-11-29T15:06:41+00:00","article_modified_time":"2022-11-29T15:35:29+00:00","og_image":[{"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/11\/gaussian_tot_ener.svg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"2 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Gaseous carbon: The energetics of two forms of tetracarbon, C4 and a challenge!","datePublished":"2022-11-29T15:06:41+00:00","dateModified":"2022-11-29T15:35:29+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740"},"wordCount":502,"commentCount":0,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740#primaryimage"},"thumbnailUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/11\/gaussian_tot_ener.svg","articleSection":["reaction mechanism"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25740","name":"Gaseous carbon: The energetics of two forms of tetracarbon, C4 and a challenge! 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The report presented persuasive evidence, in the form of trapping experiments, that dicarbon or C2 had been formed by the following chemical synthesis. Here I describe some\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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":23240,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23240","url_meta":{"origin":25740,"position":1},"title":"The chemical synthesis of C2: another fascinating twist to the story.","author":"Henry Rzepa","date":"January 20, 2021","format":false,"excerpt":"Last May, I wrote an update to the story sparked by the report of the chemical synthesis of C2. This species has a long history of spectroscopic observation in the gas phase, resulting from its generation at high temperatures. The chemical synthesis however was done in solution at ambient or\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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\/2021\/01\/11-dim.gif?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/01\/11-dim.gif?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/01\/11-dim.gif?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":10733,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10733","url_meta":{"origin":25740,"position":2},"title":"Is dicarbon (C2) a molecule of chemical interest?","author":"Henry Rzepa","date":"July 3, 2013","format":false,"excerpt":"C2\u00a0(dicarbon) is certainly interesting from a theoretical point of view. Whether or not it can be described as having a quadruple bond has induced much passionate discussion,,,. Its occurrence in space and in flames is also well-known. But does it have what might be called a conventional chemistry? Other highly\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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\/07\/LaOsC1.jpeg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":20933,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=20933","url_meta":{"origin":25740,"position":3},"title":"Startling bonds: revisiting C\u2a78N+, via the helium bond in N\u2261C-He+.","author":"Henry Rzepa","date":"May 27, 2019","format":false,"excerpt":"Although the small diatomic molecule known as dicarbon or C2 has been known for a long time, its properties and reactivity have really only been determined\u00a0via its very high temperature generation. My interest started in 2010, when I speculatively proposed here that the related isoelectronic species C\u2a78N+ might sustain a\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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":21009,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21009","url_meta":{"origin":25740,"position":4},"title":"Diatomics with eight valence-electrons:  formation by radioactive decay.","author":"Henry Rzepa","date":"June 2, 2019","format":false,"excerpt":"This is a follow up to my earlier post about C\u2a78N+, itself inspired by this ChemRxiv pre-print which describes a chemical synthesis of singlet biradicaloid C2 and its proposed\u00a0identification as such by chemical trapping. First row diatomics based on the iso-electronic principle of eight valence electrons include both C\u2a78N+ and\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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":21772,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21772","url_meta":{"origin":25740,"position":5},"title":"Carbon as a hydrogen bond acceptor: can dicarbon (C2) act in this manner?","author":"Henry Rzepa","date":"December 27, 2019","format":false,"excerpt":"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\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","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\/FH-CC-Stretch-1024x127.gif?resize=350%2C200&ssl=1","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\/25740","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=25740"}],"version-history":[{"count":15,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/25740\/revisions"}],"predecessor-version":[{"id":25759,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/25740\/revisions\/25759"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=25740"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=25740"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=25740"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=25740"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}