{"id":12560,"date":"2014-05-28T09:28:57","date_gmt":"2014-05-28T08:28:57","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=12560"},"modified":"2014-05-28T09:47:32","modified_gmt":"2014-05-28T08:47:32","slug":"benzene-an-oscillation-or-a-vibration","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560","title":{"rendered":"Benzene: an oscillation or a vibration?"},"content":{"rendered":"
\n

In the preceding post<\/a>, a nice discussion broke out about Kekul\u00e9’s 1872 model for benzene.[1]<\/a><\/span> This model has become known as the oscillation hypothesis<\/em> between two extreme forms of benzene (below). The discussion centered around the semantics of the term oscillation<\/em> compared to vibration<\/em> (a\u00a0synonym or not?) and the timescale implied by each word. The original article is in german, but more significantly, obtainable only with difficulty. Thus I cannot access[1]<\/a><\/span> the article directly since my university does not have the appropriate “back-number” subscription.\u2021<\/sup> So it was with delight that I tracked down an English translation in a journal that I could<\/em> easily access.[2]<\/a><\/span> Here I discuss what I found (on pages 614-615, the translation does not have its own DOI).<\/p>\n

\"The<\/a>

The bent bond formula<\/p><\/div>\n

The translation is by no other than Henry Armstrong, whose own contributions I have documented elsewhere<\/a>. The pertinent points (it’s a long explanation) seem to be:<\/p>\n

    \n
  1. Kekul\u00e9 does not use the word oscillation anywhere. This seems to have been added by subsequent commentators.<\/li>\n
  2. He does describe the atoms as being in continuous movement, actually using the very modern term intramolecular motion<\/em> (as translated of course).<\/li>\n
  3. He also describes this motion as returning to a mean position of equilibrium, and the separate atoms as possessing rectilinear motion, striking and recoiling against adjacent partners.<\/li>\n
  4. He finally concludes by describing at some length what happens during\u00a0two units of time involving what we would regard as one\u00a0complete vibration to return the atoms to their starting point. This description is couched in words, and refers to what we would now call a normal (vibrational) mode evolving in time. You can see that written description below for yourself (in translation). It IS quite verbose; if ever a case could be made for replacing 1000 words with one picture, this is it!<\/li>\n<\/ol>\n
    \"The<\/a>

    Armstrong’s translation<\/p><\/div>\n

    Perhaps I can attempt to replace the (1000?) words above with that one picture\u00a0(below). Here, I think Kekul\u00e9 does manage to complicate things by including a hydrogen (h) as part of his scheme. Carbon C1 is described as contacting C2, and then immediately a hydrogen (although since he does not number the hydrogens it is not absolutely clear he means the hydrogen on carbon 2 at this stage). The modern equivalent below shows relatively little motion from the (light) hydrogen atoms, and certainly no obvious contact between e.g.<\/em> C1 and any hydrogen other than the one it is bonded to.<\/p>\n

    \"1318\"<\/a>
    \nWe now replace the description above by using far more concise vectors to describe the movement of the atoms with respect to time. And of course Kekul\u00e9 had no real idea of how long his cycle took (only that it must be short as inferred from the laboratory observation of not being able to isolate geometric isomers, perhaps shorter than 100 seconds?); we now know that it is about 10-14<\/sup> s. Commentators to this day describe this as Kekul\u00e9’s oscillation hypothesis, but since Kekul\u00e9 did not use the term at all but did use (thrice) the word vibration<\/em>\u00a0we really should call it his vibration hypothesis, <\/em>as indeed Paul Schleyer noted in his comment on the original post.<\/p>\n

    \n

    \u2021<\/sup>There is little doubt that historical researches have become severely endangered by the increasing lack of access to older issues of many journals. In some cases, older can mean as little as ten years!<\/p>\n

    References<\/h2>\n
    1. A. Kekul\u00e9, \"Ueber einige Condensationsproducte des Aldehyds\", Justus Liebigs Annalen der Chemie<\/i>, vol. 162, pp. 77-124, 1872. https:\/\/doi.org\/10.1002\/jlac.18721620110<\/a>\n\n<\/li>\n
    2. \"Organic chemistry\", Journal of the Chemical Society<\/i>, vol. 25, pp. 605, 1872. https:\/\/doi.org\/10.1039\/js8722500605<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      In the preceding post, a nice discussion broke out about Kekul\u00e9’s 1872 model for benzene. This model has become known as the oscillation hypothesis between two extreme forms of benzene (below). The discussion centered around the semantics of the term oscillation compared to vibration (a\u00a0synonym or not?) and the timescale implied by each word. The […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_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},"jetpack_post_was_ever_published":false},"categories":[565],"tags":[165,1175,1133],"ppma_author":[2661],"class_list":["post-12560","post","type-post","status-publish","format-standard","hentry","category-historical","tag-henry-armstrong","tag-kekule-vibration","tag-paul-schleyer"],"yoast_head":"\nBenzene: an oscillation or a vibration? - 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=12560\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Benzene: an oscillation or a vibration? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"In the preceding post, a nice discussion broke out about Kekul\u00e9’s 1872 model for benzene. This model has become known as the oscillation hypothesis between two extreme forms of benzene (below). The discussion centered around the semantics of the term oscillation compared to vibration (a\u00a0synonym or not?) and the timescale implied by each word. The […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2014-05-28T08:28:57+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2014-05-28T08:47:32+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/05\/2.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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Benzene: an oscillation or a vibration? - 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=12560","og_locale":"en_GB","og_type":"article","og_title":"Benzene: an oscillation or a vibration? - Henry Rzepa's Blog","og_description":"In the preceding post, a nice discussion broke out about Kekul\u00e9’s 1872 model for benzene. This model has become known as the oscillation hypothesis between two extreme forms of benzene (below). The discussion centered around the semantics of the term oscillation compared to vibration (a\u00a0synonym or not?) and the timescale implied by each word. The […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560","og_site_name":"Henry Rzepa's Blog","article_published_time":"2014-05-28T08:28:57+00:00","article_modified_time":"2014-05-28T08:47:32+00:00","og_image":[{"url":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/05\/2.jpg","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=12560#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Benzene: an oscillation or a vibration?","datePublished":"2014-05-28T08:28:57+00:00","dateModified":"2014-05-28T08:47:32+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560"},"wordCount":587,"commentCount":10,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/05\/2.jpg","keywords":["Henry Armstrong","Kekule vibration","Paul Schleyer"],"articleSection":["Historical"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560","name":"Benzene: an oscillation or a vibration? - 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=12560#primaryimage"},"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/05\/2.jpg","datePublished":"2014-05-28T08:28:57+00:00","dateModified":"2014-05-28T08:47:32+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=12560#breadcrumb"},"inLanguage":"en-GB","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560"]}]},{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560#primaryimage","url":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/05\/2.jpg","contentUrl":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/05\/2.jpg"},{"@type":"BreadcrumbList","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12560#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog"},{"@type":"ListItem","position":2,"name":"Benzene: an oscillation or a vibration?"}]},{"@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\/pDef7-3gA","jetpack-related-posts":[{"id":12546,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12546","url_meta":{"origin":12560,"position":0},"title":"Benzene. As you have never seen it represented before!","author":"Henry Rzepa","date":"May 18, 2014","format":false,"excerpt":"Continuing my european visits, here are two photos from Bonn. First, a word about how the representation of benzene evolved, attributed to Kekul\u00e9. Above is his first effort, made in 1865. This one above is better, offered in 1866. But whilst what we now know as the double bond (C=C)\u2026","rel":"","context":"In "Historical"","block_context":{"text":"Historical","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=565"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":12583,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12583","url_meta":{"origin":12560,"position":1},"title":"Kekul\u00e9’s vibration: A modern example of its use.","author":"Henry Rzepa","date":"June 6, 2014","format":false,"excerpt":"Following the discussion here of Kekul\u00e9's suggestion of what we now call a vibrational mode (and which in fact now bears his name), I thought I might apply the concept to a recent molecule known as [2.2]paracyclophane. The idea was sparked by Steve Bachrach's latest post, where the \"zero-point\" structure\u2026","rel":"","context":"In "Historical"","block_context":{"text":"Historical","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=565"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":21407,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21407","url_meta":{"origin":12560,"position":2},"title":"Does Kekulene have Kekul\u00e9 vibrational modes? Yes!","author":"Henry Rzepa","date":"October 19, 2019","format":false,"excerpt":"Increasingly, individual small molecules are having their structures imaged using STM, including cyclo[18]carbon that I recently discussed. The latest one receiving such treatment is Kekulene. As with cyclo[18]carbon, the point of interest was which of the two resonance structures shown below most closely resembled the measured structure. The one on\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2019\/10\/b3-1423-B2u-1024x575.gif?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":12115,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12115","url_meta":{"origin":12560,"position":3},"title":"Aromatic electrophilic substitution. A different light on the bromination of benzene.","author":"Henry Rzepa","date":"March 12, 2014","format":false,"excerpt":"My previous post related to the aromatic electrophilic substitution of benzene using as electrophile phenyl diazonium chloride. Another prototypical reaction, and again one where benzene is too inactive for the reaction to occur easily, is the catalyst-free bromination of benzene to give bromobenzene and HBr.\u00a0 The \"text-book\" mechanism involves nucleophilic\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":"br2+benzene","src":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/03\/br2+benzene.svg","width":350,"height":200},"classes":[]},{"id":9917,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9917","url_meta":{"origin":12560,"position":4},"title":"Concerted vs stepwise (Meisenheimer) mechanisms for aromatic nucleophilic substitution.","author":"Henry Rzepa","date":"March 25, 2013","format":false,"excerpt":"My two previous explorations of aromatic substitutions have involved an electrophile (NO+ or Li+). Time now to look at a nucleophile, representing nucleophilic aromatic substitution. The mechanism of this is thought to pass through an intermediate analogous to the Wheland for an electrophile, this time known as the Meisenheimer complex.\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 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/03\/trinitro.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":21176,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21176","url_meta":{"origin":12560,"position":5},"title":"Cyclo[18]carbon: The Kekul\u00e9 vibration calculated and hence a mystery!","author":"Henry Rzepa","date":"August 30, 2019","format":false,"excerpt":"I have discussed the vibration in benzene known as the Kekul\u00e9 mode in other posts, the first of which was all of ten years ago. It is a stretching mode that lengthens three of the bonds in benzene (a [6]-annulene) and shortens the other three, thus leading to a cyclohexatriene\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2019\/08\/C18.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\/12560","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=12560"}],"version-history":[{"count":12,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/12560\/revisions"}],"predecessor-version":[{"id":12575,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/12560\/revisions\/12575"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=12560"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=12560"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=12560"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=12560"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}