{"id":22404,"date":"2020-05-27T09:58:41","date_gmt":"2020-05-27T08:58:41","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=22404"},"modified":"2020-06-02T13:54:34","modified_gmt":"2020-06-02T12:54:34","slug":"the-first-ever-curly-arrows-revisited-with-some-crystal-structure-mining","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404","title":{"rendered":"The first ever curly arrows. Revisited with some crystal structure mining."},"content":{"rendered":"
\n

With the current global lockdown, and students along with everyone else staying at home, I have noticed some old posts of mine are getting more attention than normal. One of these is an analysis I did in 2012 of Robinson’s original curly arrow illustration<\/a>.[1]<\/a><\/span> That and the fact that I am about to give a lecture on what I call my autobiographical journey discovering them, to our own students here (remotely of course), has prompted me to revisit my original discussion.<\/p>\n

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

Of the two modern representations of nitrosobenzene, the first corresponds to Robinson’s arrows, being an attempt to show, by resonance, that the molecule is o\/p directing towards an electrophile. Hence the accumulation of negative charge in the p-position (and other resonance structures can be drawn with it in the o-positions) encouraging electrophilic attack there. The second is the modern version, with the electron flow going in the other direction and hence discouraging electrophilic attack at the o\/p positions. All this hinges on the observation that the nitrogen lone pair, involved in the first representation, lies in the plane of the molecule and hence is orthogonal (at 90\u00b0) to the \u03c0-electrons in the benzene ring and cannot overlap with them. In the modern view, this lone pair plays no part in the resonance.\u00a0<\/p>\n

This can be tested by searching for experimental crystal structures of nitrosobenzenes. I did mention this in the original post, but showed no results. So here is the analysis, in which the plots below analyse the torsion about the phenyl–<\/strong><\/span>NO bond. You can see all the examples are either red or blue,\u2021<\/sup> which indicates torsions of ~180 or 0\u00b0. You can perceive a very nice correlation between the length of the C-N and the N-O bonds. As the latter gets shorter, the former gets longer. This only matches the second resonance shown above and not the Robinson version! Across all known crystal structures of nitrosobenzenes, the balance between these two resonance forms changes, no doubt as a result of substituents on the benzene ring.\u00a0<\/p>\n

\"\"<\/a> A different plot which now includes the angle at the nitrogen shows very little variation in that angle (113-118\u00b0), and certainly not the much larger variation implied by Robinson’s representation. As the N-O bond gets longer, so the angle at the nitrogen opens up a bit, the lone pair on the nitrogen being repulsed by the now three lone pairs on the oxygen anion. \"\"<\/a><\/p>\n

I have noted previously that such crystal structure mining can be used to capture many basic concepts in chemistry.[2]<\/a><\/span> This is a particularly clear one, discriminating between two possible forms of curly arrow. Conversely, it shows how curly arrows can be used to simply rationalise structural variations across a series of compounds.<\/p>\n

\n

\u2021<\/sup>There is one outlier (it does not appear in the plots above, since these are restricted to structures with an R-factor\u00a0<5%), that shows a linear Ph-N=O system (DOI: 10.5517\/cc108dl8<\/a>) and which may be a Robinson-like valence bond isomer of nitrosobenzene. It will be investigated further!<\/p>\n

References<\/h2>\n
  1. \"Forthcoming events\", Journal of the Society of Chemical Industry<\/i>, vol. 43, pp. 1295-1298, 1924. https:\/\/doi.org\/10.1002\/jctb.5000435208<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"Discovering More Chemical Concepts from 3D Chemical Information Searches of Crystal Structure Databases\", Journal of Chemical Education<\/i>, vol. 93, pp. 550-554, 2015. https:\/\/doi.org\/10.1021\/acs.jchemed.5b00346<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    With the current global lockdown, and students along with everyone else staying at home, I have noticed some old posts of mine are getting more attention than normal. One of these is an analysis I did in 2012 of Robinson’s original curly arrow illustration. That and the fact that I am about to give a […]<\/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":[1745,2327],"tags":[],"ppma_author":[2661],"class_list":["post-22404","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","category-curl-arrows"],"yoast_head":"\nThe first ever curly arrows. Revisited with some crystal structure mining. - 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=22404\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The first ever curly arrows. Revisited with some crystal structure mining. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"With the current global lockdown, and students along with everyone else staying at home, I have noticed some old posts of mine are getting more attention than normal. One of these is an analysis I did in 2012 of Robinson’s original curly arrow illustration. That and the fact that I am about to give a […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2020-05-27T08:58:41+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2020-06-02T12:54:34+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/05\/robinson.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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The first ever curly arrows. Revisited with some crystal structure mining. - Henry Rzepa'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=22404","og_locale":"en_GB","og_type":"article","og_title":"The first ever curly arrows. Revisited with some crystal structure mining. - Henry Rzepa's Blog","og_description":"With the current global lockdown, and students along with everyone else staying at home, I have noticed some old posts of mine are getting more attention than normal. One of these is an analysis I did in 2012 of Robinson’s original curly arrow illustration. That and the fact that I am about to give a […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404","og_site_name":"Henry Rzepa's Blog","article_published_time":"2020-05-27T08:58:41+00:00","article_modified_time":"2020-06-02T12:54:34+00:00","og_image":[{"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/05\/robinson.svg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"3 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"The first ever curly arrows. Revisited with some crystal structure mining.","datePublished":"2020-05-27T08:58:41+00:00","dateModified":"2020-06-02T12:54:34+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404"},"wordCount":523,"commentCount":1,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404#primaryimage"},"thumbnailUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/05\/robinson.svg","articleSection":["crystal_structure_mining","Curly arrows"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404","name":"The first ever curly arrows. Revisited with some crystal structure mining. - Henry Rzepa's Blog","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#website"},"primaryImageOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404#primaryimage"},"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404#primaryimage"},"thumbnailUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/05\/robinson.svg","datePublished":"2020-05-27T08:58:41+00:00","dateModified":"2020-06-02T12:54:34+00:00","author":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"breadcrumb":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404#breadcrumb"},"inLanguage":"en-GB","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404"]}]},{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404#primaryimage","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/05\/robinson.svg","contentUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/05\/robinson.svg"},{"@type":"BreadcrumbList","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22404#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog"},{"@type":"ListItem","position":2,"name":"The first ever curly arrows. Revisited with some crystal structure mining."}]},{"@type":"WebSite","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#website","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/","name":"Henry Rzepa's Blog","description":"Chemistry with a twist","potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"en-GB"},{"@type":"Person","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281","name":"Henry Rzepa","image":{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g370be3a7397865e4fd161aefeb0a5a85","url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","contentUrl":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","caption":"Henry Rzepa"},"description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London.","sameAs":["https:\/\/orcid.org\/0000-0002-8635-8390"],"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?author=1"}]}},"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/pDef7-5Pm","jetpack-related-posts":[{"id":7234,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7234","url_meta":{"origin":22404,"position":0},"title":"The first ever curly arrows.","author":"Henry Rzepa","date":"July 20, 2012","format":false,"excerpt":"I was first taught curly arrow pushing in 1968, and have myself taught it to many a generation of student since. But the other day, I learnt something new. Nick Greeves was kind enough to send me this link\u2021to the origin of curly arrow pushing in organic chemistry, where the\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":27114,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=27114","url_meta":{"origin":22404,"position":1},"title":"The 100th Anniversary year of Curly Arrows.","author":"Henry Rzepa","date":"June 14, 2024","format":false,"excerpt":"Chemists now use the term \"curly arrows\" as a language to describe the electronic rearrangements that occur when a (predominately organic) molecule transforms to another - the so called chemical reaction. It is also used to infer, via valence bond or resonance theory, what the mechanistic implications of that reaction\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":"","src":"","width":0,"height":0},"classes":[]},{"id":7267,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7267","url_meta":{"origin":22404,"position":2},"title":"The first ever curly arrows. And now for something completely different.","author":"Henry Rzepa","date":"July 21, 2012","format":false,"excerpt":"The discussion appended to the post on curly arrows is continued here. Recollect the curly arrow diagram (in modern style) derived from\u00a0Robinson's original suggestion: The pertinent point is that the angle subtended at the nitrogen atom evolves from being bent (~115\u00b0) on the left, to linear (180\u00b0) on the right.\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/nitroso-C2v.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":22445,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22445","url_meta":{"origin":22404,"position":3},"title":"Curly arrows in the 21st Century. Proton-coupled electron transfers.","author":"Henry Rzepa","date":"June 10, 2020","format":false,"excerpt":"One of the most fascinating and important articles dealing with curly arrows I have seen is that by Klein and Knizia on the topic of C-H bond activations using an iron catalyst. These are so-called high spin systems with unpaired electrons and the mechanism of C-H activation involves both double\u2026","rel":"","context":"In "Curly arrows"","block_context":{"text":"Curly arrows","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2327"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/06\/TS.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":28849,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=28849","url_meta":{"origin":22404,"position":4},"title":"Mechanism of the dimerisation of Nitrosobenzene.","author":"Henry Rzepa","date":"June 14, 2025","format":false,"excerpt":"I am in the process of revising my annual lecture to first year university students on the topic of \"curly arrows\". I like to start my story in 1924, when Robert Robinson published the very first example as an illustration of why nitrosobenzene undergoes electrophilic bromination in the para position\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.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\/2025\/06\/trans.gif?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":19999,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19999","url_meta":{"origin":22404,"position":5},"title":"Early “curly” (reaction) arrows. Those of Ingold in 1926.","author":"Henry Rzepa","date":"August 22, 2018","format":false,"excerpt":"In 2012, I wrote a story of the first ever reaction curly arrows, attributed to Robert Robinson in 1924. At the time there was a great rivalry between him and another UK chemist, Christopher Ingold, with the latter also asserting his claim for their use. As part of the move\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":"","src":"","width":0,"height":0},"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\/22404","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=22404"}],"version-history":[{"count":12,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/22404\/revisions"}],"predecessor-version":[{"id":22444,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/22404\/revisions\/22444"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=22404"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=22404"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=22404"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=22404"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}