{"id":12329,"date":"2014-04-17T17:41:26","date_gmt":"2014-04-17T16:41:26","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=12329"},"modified":"2014-04-18T07:52:22","modified_gmt":"2014-04-18T06:52:22","slug":"more-blog-connections-spotted-something-new-about-diphenyl-magnesium","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12329","title":{"rendered":"More (blog) connections spotted. Something new about diphenyl magnesium?"},"content":{"rendered":"
\n

I have just noticed unexpected links between two old posts, one about benzene<\/a>, one about diphenyl magnesium<\/a>\u00a0and\u00a0a link to August Kekul\u00e9.\u2020<\/sup><\/p>\n

\"Click

Click for 3D<\/p><\/div>\n

The post about benzene dealt with the apparently simple issue of why all the C-C bonds are of equal length. The answer, in brief is purely because of the \u03c3-electrons. If the \u03c0-electrons had their way, benzene would have three short and three long C-C bonds. Experimentally, this is detected by the value of a particular stretching vibration known as the Kekul\u00e9 mode,\u2020<\/sup> which stretches three C-C bonds in the ring and shortens the other three. In benzene this is depressed from an expected value of ~1600 cm-1<\/sup> to ~1318\u2021<\/sup> due to an effect called \u03c0-distortivity.[1]<\/a><\/span><\/p>\n

The second post dealt with the unusual structure of Ph2<\/sub>Mg, which is a bridging dimer of (one of the) species present in solution for the Grignard reagent. In this structure, all the C-C lengths in each phenyl ring are very close to the natural value in benzene itself (~1.405-1.411\u00c5), which suggests that despite the Mg and the bridged coordination for two of the phenyl rings, all four appear to retain aromaticity. Shown above are the displacement vectors at their extreme for one of four Kekul\u00e9 modes calculated for this system using \u03c9B97XD\/6-311G(d,p)[2]<\/a><\/span>. Two have the value 1296 and two 1298 cm-1<\/sup>. All four are lower than the value for benzene itself.<\/p>\n

What does this mean? That the\u00a0\u03c0-electrons in Ph2<\/sub>Mg are more distortive than in benzene itself. If weakening the \u03c0-system (by say promoting two bonding electrons in benzene into \u03c0*<\/sup> MOs to form a quintet state<\/a>) elevates the Kekul\u00e9 mode wavenumber to almost its expected normal<\/i> value, one may ask if\u00a0the converse is true?\u00a0Is the\u00a0\u03c0-system and hence\u00a0aromaticity stronger for a molecule where the\u00a0Kekul\u00e9 mode is even more depressed than in benzene itself? I pose the question, but do not answer it here.\u2020<\/sup><\/p>\n

\n

\u2020<\/sup>Kekul\u00e9 will be celebrated on the occasion of the inaugural event on 9 May 2014 in Bonn Germany to keep “Historic Sites of Chemistry”.<\/p>\n

\u2021<\/sup> Calculated using \u03c9B97XD\/6-311G(d,p) with a mass weighting for the hydrogens of 0.001 to avoid coupling perturbations with CCH wags.<\/p>\n

References<\/h2>\n
  1. S. Shaik, A. Shurki, D. Danovich, and P.C. Hiberty, \"A Different Story of \u03c0-DelocalizationThe Distortivity of \u03c0-Electrons and Its Chemical Manifestations\", Chemical Reviews<\/i>, vol. 101, pp. 1501-1540, 2001. https:\/\/doi.org\/10.1021\/cr990363l<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"Gaussian Job Archive for C24H20Mg2\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.1002073<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    I have just noticed unexpected links between two old posts, one about benzene, one about diphenyl magnesium\u00a0and\u00a0a link to August Kekul\u00e9.\u2020 The post about benzene dealt with the apparently simple issue of why all the C-C bonds are of equal length. The answer, in brief is purely because of the \u03c3-electrons. If the \u03c0-electrons had […]<\/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":[4],"tags":[1172],"ppma_author":[2661],"class_list":["post-12329","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-grignard-reagent"],"yoast_head":"\nMore (blog) connections spotted. Something new about diphenyl magnesium? - 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=12329\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"More (blog) connections spotted. Something new about diphenyl magnesium? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"I have just noticed unexpected links between two old posts, one about benzene, one about diphenyl magnesium\u00a0and\u00a0a link to August Kekul\u00e9.\u2020 The post about benzene dealt with the apparently simple issue of why all the C-C bonds are of equal length. The answer, in brief is purely because of the \u03c3-electrons. 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Something new about diphenyl magnesium? - 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=12329","og_locale":"en_GB","og_type":"article","og_title":"More (blog) connections spotted. Something new about diphenyl magnesium? - Henry Rzepa's Blog","og_description":"I have just noticed unexpected links between two old posts, one about benzene, one about diphenyl magnesium\u00a0and\u00a0a link to August Kekul\u00e9.\u2020 The post about benzene dealt with the apparently simple issue of why all the C-C bonds are of equal length. The answer, in brief is purely because of the \u03c3-electrons. 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Generations of chemists have learnt early on in their studies of the subject that these two representations of where the electron pairs in benzene might be located (formally called electronic resonance or valence bond forms) each contribute ~50% to the\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":"The Kekule structures of benzene.","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/06\/benzene.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1183,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1183","url_meta":{"origin":12329,"position":1},"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 "Hypervalency"","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. 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This four-carbon molecule has\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.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/10\/JODLIX.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8570,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8570","url_meta":{"origin":12329,"position":3},"title":"The mechanism of the Birch reduction. Sequel to benzene reduction.","author":"Henry Rzepa","date":"December 5, 2012","format":false,"excerpt":"I noted briefly in discussing why Birch reduction of benzene gives 1,4-cyclohexadiene (diagram below) that the geometry of the end-stage pentadienyl anion was distorted in the presence of the sodium cation to favour this product. This distortion actually has some pedagogic value, and so I elaborate this here. 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The structure shows\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/04\/carbobenzene.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":21925,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21925","url_meta":{"origin":12329,"position":5},"title":"The singlet and open shell higher-spin states of [4], [6] and [8]-annulenes and their Kekul\u00e9 vibrational modes","author":"Henry Rzepa","date":"March 11, 2020","format":false,"excerpt":"In 2001, Shaik and co-workers published the first of several famous review articles on the topic\u00a0A Different Story of \u03c0-Delocalization. The Distortivity of \u03c0-Electrons and Its Chemical Manifestations. The main premise was that the delocalized \u03c0-electronic component of benzene is unstable toward a localizing distortion and is at the same\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\/2020\/03\/CBD-Eu.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\/12329","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=12329"}],"version-history":[{"count":19,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/12329\/revisions"}],"predecessor-version":[{"id":12350,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/12329\/revisions\/12350"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=12329"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=12329"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=12329"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=12329"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}