{"id":1423,"date":"2010-01-01T12:44:52","date_gmt":"2010-01-01T11:44:52","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423"},"modified":"2011-11-28T13:21:23","modified_gmt":"2011-11-28T13:21:23","slug":"contriving-aromaticity-from-s%e2%89%a1c-triple-bonds","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423","title":{"rendered":"Contriving aromaticity from S\u2261C Triple bonds"},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"1423\">\n<p>In the <a href=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1347\" target=\"_blank\">previous post<\/a>, the molecule F<sub>3<\/sub>S-C\u2261SF<sub>3<\/sub> was found to exhibit a valence bond isomerism, one of the S-C bonds being single, the other triple, and with a large barrier (~31 kcal\/mol, \u03bd 284<em>i<\/em> cm<sup>-1<\/sup>) to interconversion of the two valence-bond forms. So an interesting extension of this phenomenon is shown below:<\/p>\n<p><div id=\"attachment_1425\" style=\"width: 315px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1425\" class=\"size-full wp-image-1425\" title=\"S3C3F6\" onclick=\"jmolInitialize('..\/Jmol\/');jmolSetAppletColor('black');jmolApplet([450,450],'load wp-content\/uploads\/2010\/01\/F6S3C3.log;frame 40;zoom 5;moveto 4 0 2 0 90 90;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 6;animation mode loop;');\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/01\/S3C3F6.jpg\" alt=\"\" width=\"305\" height=\"107\" \/><p id=\"caption-attachment-1425\" class=\"wp-caption-text\">A cyclic form of the SCS Motif. Click for 3D<\/p><\/div>If the same type of valence bond isomerism were to occur, we would now have three C\u2261S triple bonds swapping places with three CS single bonds, a sort of super version of the notation normally shown for benzene itself. If the barrier to this swapping is finite, then the interconversion shown above would be a proper equilibrium (the top arrows), but if there is no barrier, then the interconversion would be a proper resonance (the bottom double-headed arrow). Another way of posing the question is whether the so-called <a href=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=485\" target=\"_blank\">Kekul\u00e9 vibrational mode<\/a> (which in effect represents the motions implied above) has a negative force constant or a positive one respectively for the two sets of arrows shown.<\/p>\n<p>A B3LYP\/cc-pVTZ calculation (DOI: <a href=\"http:\/\/hdl.handle.net\/10042\/to-3646\">10042\/to-3646<\/a>) reveals that the optimized geometry exhibits six equal SC bonds, all 1.616\u00c5 long. Typically, a single SC bond is around 1.82\u00c5, a double 1.65\u00c5 and a triple is about 1.5\u00c5 at the same level of theory, so this C=S bond is clearly at least a double one. A NICS(0) calculation at the centroid has the value of -14.6 ppm, which indicates aromaticity. We conclude the appropriate arrow above is the bottom resonance one, rather than the top equilibrium one. This is confirmed by finding that the Kekul\u00e9 vibrational mode has a strongly positive force constant (\u03bd 1083 cm<sup>-1<\/sup>, animated in 3D model above), which contrasts with the negative value (\u03bd 284<em>i<\/em> cm<sup>-1<\/sup>) found for bond shifting in F<sub>3<\/sub>S-C\u2261SF<sub>3<\/sub> itself. Again, comparison indicates that a C\u2261S triple bond has a frequency of around 1400 cm<sup>-1<\/sup> and a double around 1200 cm<sup>-1<\/sup> (the degenerate C=S non<strong>&#8211;<\/strong>Kekul\u00e9 vibrational mode for this system is indeed calculated at around 1225 cm<sup>-1<\/sup>). So to summarise; a single F<sub>3<\/sub>S-C\u2261SF<sub>3<\/sub> unit reveals very strong bond alternation, and negative force constant (transition state) for interconversion of the two bond forms, but a cyclic form reveals the opposite behaviour, with no alternation and instead strong aromaticity.<\/p>\n<p>In part this difference in behaviour must be due to the constraints on the geometry of the cyclic form. F<sub>3<\/sub>S-C\u2261SF<sub>3<\/sub> interconverts via a highly twisted geometry with C<sub>2<\/sub> symmetry, and this twisting is not exactly possible if you create a cyclic equivalent. In part it is also due to the aromatic stabilisation energies. In the resonance above, you should be able to count a total of 12 electrons involved! Nominally, if you try to apply the 4n+2 aromaticity rule, it does not fit, until you realise that in fact you must be dealing with two sets of 6 electrons. The system in fact is a classic double-aromatic, in which six electrons circulate in the plane of the molecule (the \u03c3-set) and six above and below (the \u03c0-set; the MOs for the molecule confirm exactly this interpretation). Notice how this itself contrasts with a similarly aromatic system, the atom swapping in <a href=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1158\" target=\"_blank\">three nitrosonium cations<\/a>, where the Kekul\u00e9 mode force constant was strongly negative.<\/p>\n<p><div id=\"attachment_1438\" style=\"width: 399px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1438\" class=\"size-full wp-image-1438\" title=\"F6S3C3-elf\" onclick=\"jmolInitialize('..\/Jmol\/');jmolSetAppletColor('black');jmolApplet([450,450],'load wp-content\/uploads\/2010\/01\/F6S3C3-elf.mol;;zoom 5;moveto 4 0 2 0 90 90;');\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/01\/F6S3C3-elf.jpg\" alt=\"\" width=\"389\" height=\"388\" \/><p id=\"caption-attachment-1438\" class=\"wp-caption-text\">ELF Analysis for F6S3C3. Click for 3D<\/p><\/div>To complete the analysis, the ELF basins (above) reveal the six SC regions to each contain 2.7 electrons, together with three carbon carbene monosynaptic basins. For comparison, a system with a high degree of SC triple character (HCS<sup>+<\/sup>) has around 3.8 in the SC region. Perhaps a better model is <a href=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1243\" target=\"_blank\">TfOSCH<\/a> (for which the carbon also has a carbene lone pair), which has 2.6e in the CS region. The carbene lone &#8220;pair&#8221; for the present molecule integrates to 2.6e each, which totals to a nice octet of electrons around each carbon and to around 7 for each S, confirming that whilst the S is <strong>hypervalent<\/strong>, its valence octet is not expanded!). This ELF picture does rather tend to confirm the original resonance structure representation shown at the top.<\/p>\n<p>All that is needed is is for someone to make this molecule to confirm its properties. Perhaps by trimerising F<sub>2<\/sub>SC, itself formed by cheletropic elimination? It is worth noting that the iso-electronic P\/N (<em>e.g.<\/em> of S\/C) analogues are very well known.<\/p>\n<div id=\"attachment_1499\" style=\"width: 192px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/01\/phosphonitriles.jpg\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1499\" class=\"size-full wp-image-1499\" title=\"phosphonitriles\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/01\/phosphonitriles.jpg\" alt=\"\" width=\"182\" height=\"144\" \/><\/a><p id=\"caption-attachment-1499\" class=\"wp-caption-text\">Phosphonitrilic compounds<\/p><\/div>\n<!-- kcite active, but no citations found -->\n<\/div> <!-- kcite-section 1423 -->","protected":false},"excerpt":{"rendered":"<p>In the previous post, the molecule F3S-C\u2261SF3 was found to exhibit a valence bond isomerism, one of the S-C bonds being single, the other triple, and with a large barrier (~31 kcal\/mol, \u03bd 284i cm-1) to interconversion of the two valence-bond forms. So an interesting extension of this phenomenon is shown below: If the same [&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":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[7,4],"tags":[16,2649,2648,2650,170],"ppma_author":[2661],"class_list":["post-1423","post","type-post","status-publish","format-standard","hentry","category-hypervalency","category-interesting-chemistry","tag-aromaticity","tag-hypervalency","tag-interesting-chemistry","tag-pericyclic","tag-south-carolina"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Contriving aromaticity from S\u2261C Triple bonds - 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=1423\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Contriving aromaticity from S\u2261C Triple bonds - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"In the previous post, the molecule F3S-C\u2261SF3 was found to exhibit a valence bond isomerism, one of the S-C bonds being single, the other triple, and with a large barrier (~31 kcal\/mol, \u03bd 284i cm-1) to interconversion of the two valence-bond forms. So an interesting extension of this phenomenon is shown below: If the same [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"article:published_time\" content=\"2010-01-01T11:44:52+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2011-11-28T13:21:23+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/01\/S3C3F6.jpg\" \/>\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=\"4 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Contriving aromaticity from S\u2261C Triple bonds - 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=1423","og_locale":"en_GB","og_type":"article","og_title":"Contriving aromaticity from S\u2261C Triple bonds - Henry Rzepa&#039;s Blog","og_description":"In the previous post, the molecule F3S-C\u2261SF3 was found to exhibit a valence bond isomerism, one of the S-C bonds being single, the other triple, and with a large barrier (~31 kcal\/mol, \u03bd 284i cm-1) to interconversion of the two valence-bond forms. So an interesting extension of this phenomenon is shown below: If the same [&hellip;]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423","og_site_name":"Henry Rzepa&#039;s Blog","article_published_time":"2010-01-01T11:44:52+00:00","article_modified_time":"2011-11-28T13:21:23+00:00","og_image":[{"url":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/01\/S3C3F6.jpg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"4 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Contriving aromaticity from S\u2261C Triple bonds","datePublished":"2010-01-01T11:44:52+00:00","dateModified":"2011-11-28T13:21:23+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423"},"wordCount":754,"commentCount":1,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/01\/S3C3F6.jpg","keywords":["aromaticity","Hypervalency","Interesting chemistry","pericyclic","South Carolina"],"articleSection":["Hypervalency","Interesting chemistry"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1423","name":"Contriving aromaticity from S\u2261C Triple bonds - 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A rather artificial example (atom-swapping between three nitrosonium cations) was used\u2026","rel":"","context":"In &quot;Hypervalency&quot;","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/12\/SF3.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":15552,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15552","url_meta":{"origin":1423,"position":1},"title":"VSEPR Theory: A closer look at trifluorothionitrile, NSF3.","author":"Henry Rzepa","date":"January 16, 2016","format":false,"excerpt":"The post on applying VSEPR (\"valence shell electron pair repulsion\") theory to the geometry of ClF3\u00a0has proved perennially popular. So here is a follow-up on another little molecue,\u00a0F3SN. As the name implies, it is often represented with an\u00a0S\u2261N bond. Here I take a look at the conventional analysis. This is\u2026","rel":"","context":"In &quot;Hypervalency&quot;","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":19102,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19102","url_meta":{"origin":1423,"position":2},"title":"Hypervalence and octet-expansion in trimethylene-\u03bb6-sulfane and related species.","author":"Henry Rzepa","date":"November 27, 2017","format":false,"excerpt":"Previously: \"Non-polar\" species such as SeMe6, SMe6, ClMe3, ClMe5 all revealed interesting properties for the Se-C, S-C or Cl-C \"single\" bonds. The latter two examples in particular hinted at internal structures for these single bonds, as manifested by two ELF basins for some of the bonds. Here I take a\u2026","rel":"","context":"In &quot;Hypervalency&quot;","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":1183,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1183","url_meta":{"origin":1423,"position":3},"title":"Multi-centre bonding in  the Grignard Reagent","author":"Henry Rzepa","date":"December 1, 2009","format":false,"excerpt":"The Grignard reaction is encountered early on in most chemistry courses, and most labs include the preparation of this reagent, typically by the following reaction: 2PhBr + 2Mg \u2192 2PhMgBr\u00a0\u2194 MgBr2 + Ph2Mg The reagent itself exists as part of an equilibrium, named after Schlenk, in which a significant concentration\u2026","rel":"","context":"In &quot;Hypervalency&quot;","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"The crystal structure of a di-aryl magnesium. Click to view 3D","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/12\/udaqiz.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":2599,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2599","url_meta":{"origin":1423,"position":4},"title":"Hypervalency: Is it real?","author":"Henry Rzepa","date":"October 16, 2010","format":false,"excerpt":"The Wikipedia page on hypervalent compounds reveals that the concept is almost as old as that of normally valent compounds. The definition there, \u00a0is \"a molecule that contains one or more\u00a0main group elements formally bearing more than eight\u00a0electrons in their\u00a0valence shells\" (although it could equally apply to e.g. transition elements\u2026","rel":"","context":"In &quot;Hypervalency&quot;","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/10\/IH7.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4479,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4479","url_meta":{"origin":1423,"position":5},"title":"Hexavalent carbon revisited (and undecavalent boron thrown in).","author":"Henry Rzepa","date":"June 26, 2011","format":false,"excerpt":"A little while ago, I speculated (blogs are good for that sort of thing) about hexavalent carbon, and noted how one often needs to make (retrospectively) obvious connections between two different areas of chemistry. That post has attracted a number of comments in the two years its been up, along\u2026","rel":"","context":"In &quot;Hypervalency&quot;","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/06\/CB11.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\/1423","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=1423"}],"version-history":[{"count":1,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1423\/revisions"}],"predecessor-version":[{"id":5688,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1423\/revisions\/5688"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1423"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1423"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1423"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=1423"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}