{"id":14037,"date":"2015-05-22T16:16:20","date_gmt":"2015-05-22T15:16:20","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=14037"},"modified":"2023-09-16T18:11:24","modified_gmt":"2023-09-16T17:11:24","slug":"r-x%e2%89%a1x-r-g-n-lewis-100-year-old-idea","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14037","title":{"rendered":"R-X\u2261X-R: G. N. Lewis’ 100 year old idea."},"content":{"rendered":"
\n

As I have noted elsewhere<\/a>, Gilbert N. Lewis wrote a famous paper entitled “the atom and the molecule<\/em>“, the centenary of which is coming up.[1]<\/a><\/span> In a short and rarely commented\u00a0upon remark, he speculates about the shared electron pair structure of acetylene,\u00a0\u00a0R-X\u2261X-R (R=H, X=C). It could, he suggests, take up three forms. H-C:::C-H and two more which I show as he drew them. The first of these would now be called a bis-carbene<\/em> and the second a biradical<\/em>.<\/p>\n

\"\"<\/p>\n

In 1916, it was too early for Lewis to speculate what the geometries of such species might be, and in particular the C…C (or generalising, X…X) distance, and the two angles, one for each X. Well, we do not need to speculate, we can perform a search of the crystal structure database. Here it is\u00a0(R < 0.05, no errors, no disorder):<\/p>\n

\"Lewis-CC4\"<\/a><\/p>\n

A little more explanation of this 4-dimensional plot is needed:<\/p>\n

    \n
  1. The two angles are plotted as X and Y.<\/li>\n
  2. The X…X distance is plotted as colour, with red representing the longest distances and blue the shortest<\/li>\n
  3. The size of each “bin” is represented by the radius of the circle; small circles represent few examples, larger circles represent more examples in each “bin” defined by a regular range of angles.<\/li>\n<\/ol>\n

    There are one or two off-diagonal\u00a0 “outliers”, each of which probably deserves individual inspection. But dealing just with the obvious clusters, the overwhelmingly largest is for both angles of ~180\u00b0, and these are the triple bonds we know and love. As far as I know, Lewis was the first to propose a triple bond between two atoms, but if anyone reading this blog knows of an antecedent, do let me know.\u00a0The next cluster is for angles of ~109\u00b0 and these are clearly bis-carbenes.<\/em> These all occur when X \u2260 C.\u00a0There are two small clusters worthy of note; one ~130\u00b0 and one ~90\u00b0. The latter are mostly Pb-Pb and Sn-Sn, where the bonding is unhybridised pure p.<\/p>\n

    One of the limitations of searching for crystal structures is that the spin state of each molecule is never given. The biradical structure given by Lewis could well have a triplet ground state, and perhaps that might have very characteristic angles (~130\u00b0 ?). It would be great to identify a genuine example of this biradical form!<\/p>\n

    As usual, the search itself took around 10 minutes, and it provides much interesting food for thought; not bad for a 100-year-old idea!<\/p>\n

    \n

    Acknowledgments<\/h4>\n

    This post has been cross-posted in PDF format at Authorea<\/a>.<\/p>\n

    References<\/h2>\n
    1. G.N. Lewis, \"THE ATOM AND THE MOLECULE.\", Journal of the American Chemical Society<\/i>, vol. 38, pp. 762-785, 1916. https:\/\/doi.org\/10.1021\/ja02261a002<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      As I have noted elsewhere, Gilbert N. Lewis wrote a famous paper entitled “the atom and the molecule“, the centenary of which is coming up. In a short and rarely commented\u00a0upon remark, he speculates about the shared electron pair structure of acetylene,\u00a0\u00a0R-X\u2261X-R (R=H, X=C). It could, he suggests, take up three forms. H-C:::C-H and two […]<\/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":[2,565,4],"tags":[1414,1411,1395,1416,1418,1412,1417,1415,1410,1413],"ppma_author":[2661],"class_list":["post-14037","post","type-post","status-publish","format-standard","hentry","category-chemical-it","category-historical","category-interesting-chemistry","tag-carbene","tag-carbenes","tag-chemistry","tag-cluster-chemistry","tag-food","tag-functional-groups","tag-gilbert-n-lewis","tag-non-kekule-molecule","tag-organic-chemistry","tag-organic-compounds"],"yoast_head":"\nR-X\u2261X-R: G. N. Lewis' 100 year old idea. - 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=14037\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"R-X\u2261X-R: G. N. Lewis' 100 year old idea. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"As I have noted elsewhere, Gilbert N. Lewis wrote a famous paper entitled “the atom and the molecule“, the centenary of which is coming up. In a short and rarely commented\u00a0upon remark, he speculates about the shared electron pair structure of acetylene,\u00a0\u00a0R-X\u2261X-R (R=H, X=C). It could, he suggests, take up three forms. H-C:::C-H and two […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14037\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2015-05-22T15:16:20+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2023-09-16T17:11:24+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/03\/Lewis.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":"R-X\u2261X-R: G. N. 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H-C:::C-H and two […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14037","og_site_name":"Henry Rzepa's Blog","article_published_time":"2015-05-22T15:16:20+00:00","article_modified_time":"2023-09-16T17:11:24+00:00","og_image":[{"url":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/03\/Lewis.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=14037#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=14037"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"R-X\u2261X-R: G. N. 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The most ubiquitous type of carbene of recent times is the one shown below as 1, often referred to as a resonance stabilised or\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":10903,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10903","url_meta":{"origin":14037,"position":3},"title":"The butterfly effect in chemistry: bimodal bond angles.","author":"Henry Rzepa","date":"July 18, 2013","format":false,"excerpt":"This potential example of a molecule on the edge of chaos was suggested to me by a student (thanks Stephen!), originating from an inorganic tutorial. It represents a class of Mo-complex ligated by two dithiocarbamate ligands and two aryl nitrene ligands (Ar-N:). I focus on two specific examples, where R=R'\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 c3D","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/07\/PNTCMO.jpeg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":21726,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21726","url_meta":{"origin":14037,"position":4},"title":"Hydrogen bonds: carbon as an acceptor rather than as a donor?","author":"Henry Rzepa","date":"December 23, 2019","format":false,"excerpt":"A hydrogen bond donor is considered as an electronegative element carrying a hydrogen that is accepted by an atom carrying a lone pair of electrons, as in X:...H-Y where X: is the acceptor and H-Y the donor. 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