{"id":1903,"date":"2010-04-16T10:44:15","date_gmt":"2010-04-16T09:44:15","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1903"},"modified":"2010-06-24T07:37:22","modified_gmt":"2010-06-24T06:37:22","slug":"carbobenzene-benzene-with-a-difference","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1903","title":{"rendered":"Carbobenzene: benzene with a difference"},"content":{"rendered":"
\n

Some molecules, when you first see them, just intrigue. So it was with carbobenzene, the synthesis of a derivative of which was recently achieved by Remi Chauvin and co-workers (DOI: 10.1002\/chem.200601193<\/a>). Two additional carbon atoms have been inserted into each of the six C-C bonds in benzene.<\/p>\n

\"\"<\/a>

Carbobenzene<\/p><\/div>\n

The structure shows two resonance forms, which remind one of Kekul\u00e9 and of course benzene itself. Counting reveals 18\u03c0-electrons in the conventional \u03c0 sense, but with a further set of 12 \u03c0-electrons located in the plane of the ring, and orthogonal to the first set. Since both could be cyclically conjugated, we can say that the first set belongs to a 4n+2 count, and should set up diatropic ring currents resulting in aromaticity, whilst the second set would belong to the 4n category, and might set up paratropic ring currents in the plane of the system. The lowest occupied molecular orbitals of each set look as follows.<\/p>\n\n\n\n
\n

\"\"

The lowest MO for the 18\u03c0-electron set. Click for 3D<\/p><\/div><\/td>\n

\n

\"\"

Lowest occupied molecular orbital for the 12\u03c0-electron set. Click for 3D<\/p><\/div><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Experimentally, the molecule is found to be aromatic. One way of quantifying this is via the so-called dissected NICS magnetic response index (DOI: 10.1021\/ol016217v<\/a>). At the ring centroid, NICS(0,1,2,3)zz<\/sub> (respectively 0,1,2,3\u00c5 above the plane of the ring) are found to be -49, -46, -38 and -28 ppm (DOI: http:\/\/10042\/to-4878<\/a>).\u00a0\u00a0The un-dissected NICS (which includes all \u03c3-current contributions) were -18, -16.6, -13 and -9 ppm. This both confirms diatropicity (for which NICS is strongly negative) and also suggests that the 12-electron \u03c0-framework is opposing the 18-electron \u03c0-framework.<\/p>\n

Another, less common way to study the aromaticity is to look at the delocalization of the electrons using the ELF technique.<\/p>\n\n\n\n
\n

\"\"

ELF function evaluated using only the 18 \u03c0 electrons. Click for 3D<\/p><\/div><\/td>\n

–<\/p>\n

\"\"

ELF function evaluated using only the 12 \u03c3-electrons. Click for 3D<\/p><\/div><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The 18-electron set bifurcate (break up into smaller basins), at the threshold of 0.87 shown above (the ELF function has a maximum of 1.0 and a minimum of 0.0), a high value which is typical of aromatic systems (benzene bifurcates at ~0.9). In contrast, the 12-electron set break up well before a value of 0.1 (shown), a low value which tends to indicate anti-aromaticity.<\/p>\n

There are many other ways of exploring the properties of such aromatic molecules, but the two above tend to suggest that carbobenzene has two personalities, one aromatic, the other antiaromatic, and with the former dominant. This gives it an interesting twist on benzene itself, and makes one wonder whether this dual Janus-like personality could be exploited in some interesting fashion.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Some molecules, when you first see them, just intrigue. So it was with carbobenzene, the synthesis of a derivative of which was recently achieved by Remi Chauvin and co-workers (DOI: 10.1002\/chem.200601193). Two additional carbon atoms have been inserted into each of the six C-C bonds in benzene. The structure shows two resonance forms, which remind […]<\/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":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[1],"tags":[214,2646,217,215,216],"ppma_author":[2661],"class_list":["post-1903","post","type-post","status-publish","format-standard","hentry","category-general","tag-aromatic-systems","tag-general","tag-http","tag-missouri","tag-remi-chauvin"],"yoast_head":"\nCarbobenzene: benzene with a difference - 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=1903\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Carbobenzene: benzene with a difference - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Some molecules, when you first see them, just intrigue. So it was with carbobenzene, the synthesis of a derivative of which was recently achieved by Remi Chauvin and co-workers (DOI: 10.1002\/chem.200601193). Two additional carbon atoms have been inserted into each of the six C-C bonds in benzene. 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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":12329,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12329","url_meta":{"origin":1903,"position":1},"title":"More (blog) connections spotted. Something new about diphenyl magnesium?","author":"Henry Rzepa","date":"April 17, 2014","format":false,"excerpt":"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\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":21925,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21925","url_meta":{"origin":1903,"position":2},"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":[]},{"id":12583,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12583","url_meta":{"origin":1903,"position":3},"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":8570,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8570","url_meta":{"origin":1903,"position":4},"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. The starting\u2026","rel":"","context":"In \"antiaromaticity\"","block_context":{"text":"antiaromaticity","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=antiaromaticity"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/12\/benzene-22.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":2046,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2046","url_meta":{"origin":1903,"position":5},"title":"Chemistry with a super-twist: A molecular trefoil knot, part 1.","author":"Henry Rzepa","date":"May 31, 2010","format":false,"excerpt":"Something important happened in chemistry for the first time about 100 years ago. A molecule was built (nowadays we would say synthesized) specifically for the purpose of investigating a theory. It was cyclo-octatetraene or (CH)8, and it was made by Willst\u00e4tter and Waser to try to find out if benzene,\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\/2010\/05\/metallatrefoil.jpg?resize=350%2C200","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\/1903","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=1903"}],"version-history":[{"count":0,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1903\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1903"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1903"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1903"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=1903"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}