{"id":13631,"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":"2014-04-17T17:41:26","modified_gmt":"2014-04-17T16:41:26","slug":"more-blog-connections-spotted-something-new-about-diphenyl-magnesium-2","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=13631","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,"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":[6],"tags":[1171],"class_list":["post-13631","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-grignard-reagent"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-3xR","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/13631","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=13631"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/13631\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=13631"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=13631"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=13631"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}