{"id":24974,"date":"2022-04-07T13:41:18","date_gmt":"2022-04-07T12:41:18","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=24974"},"modified":"2022-04-07T16:23:31","modified_gmt":"2022-04-07T15:23:31","slug":"b2n2-a-10-electron-four-atom-molecule-displaying-both-huckel-4n2-and-baird-4n-selection-rules-for-ring-aromaticity","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=24974","title":{"rendered":"C2N2: a 10-electron four-atom molecule displaying both H\u00fcckel 4n+2 and Baird 4n selection rules for ring aromaticity."},"content":{"rendered":"
\n

The previous examples<\/a> of four atom systems displaying two layers of aromaticity illustrated how 4 (B4<\/sub>), 8 (C4<\/sub>) and 12 (N4<\/sub>) valence electrons were partitioned into 4n+2 manifolds (respectively 2+2, 6+2 and 6+6). The triplet state molecule B2<\/sub>C2<\/sub> with 6 electrons partitioned into 2\u03c0 and 4\u03c3 electrons, with the latter following Baird’s aromaticity rule.[1]<\/a><\/span>,[2]<\/a><\/span>. Now for the final missing entry; as a triplet C2<\/sub>N2<\/sub> has 10 electrons, which now partition into 4 + 6. But would that be 4\u03c0 + 6\u03c3 or 4\u03c3 + 6\u03c0? Well, in a way neither! Read on.<\/p>\n

\"\"<\/a><\/p>\n\n\n\n\n\n\n\n\n\n
Bonding MOs for C2<\/sub>N2<\/sub>.
\nClick image to load 3D model<\/th>\n<\/tr>\n
π3, 1 electron<\/th>\nπ2, 1 electron<\/th>\n<\/tr>\n
\"\"<\/td>\n\"\"<\/td>\n<\/tr>\n
\u03c33 2 electron<\/th>\nσ2, 2 electron<\/th>\n<\/tr>\n
\"\"<\/td>\n\"\"<\/td>\n<\/tr>\n
π1 2 electron<\/th>\nσ1, 2 electron<\/th>\n<\/tr>\n
\"\"<\/td>\n\"\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The calculations (\u03c9B97XD\/Def2-TZVPP and CCSD(T)\/Def2-TZVPP) are collected at FAIR DOI: 10.14469\/hpc\/10346<\/a>. These show a partitioning into\u00a05\u03c3 + 5\u03c0, a species that is not a minimum but undergoes a non-planar distortion.
\n\"\"<\/a>\n<\/p>\n

However, the first excited state (the triplet)\u00a0IS planar and is only 12.5 kcal\/mol above the planar 5+5 precursor.\u00a0It is now partitioned into\u00a06\u03c3 and 4\u03c0, with the latter conforming to Baird’s rule for open shell triplets.[1]<\/a><\/span>,[2]<\/a><\/span>\u00a0So this is unlike C2<\/sub>B2<\/sub>, which showed\u00a02\u03c0 + 4\u03c3 partitioning with the \u03c3 series following Baird’s rule. Now we have two examples in which one of the \u03c3 or the \u03c0-manifolds follow Baird’s rule and the other follows H\u00fcckel’s rule. The systems themselves are somewhat contrived, but they show the simple fun and games that can be had with these aromaticity rules.<\/p>\n

\n

This post has DOI: 10.14469\/hpc\/10350<\/a><\/p>\n

References<\/h2>\n
  1. N.C. Baird, \"Quantum organic photochemistry. II. Resonance and aromaticity in the lowest 3.pi..pi.* state of cyclic hydrocarbons\", Journal of the American Chemical Society<\/i>, vol. 94, pp. 4941-4948, 1972. https:\/\/doi.org\/10.1021\/ja00769a025<\/a>\n\n<\/li>\n
  2. M. Rosenberg, C. Dahlstrand, K. Kils\u00e5, and H. Ottosson, \"Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations\", Chemical Reviews<\/i>, vol. 114, pp. 5379-5425, 2014. https:\/\/doi.org\/10.1021\/cr300471v<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The previous examples of four atom systems displaying two layers of aromaticity illustrated how 4 (B4), 8 (C4) and 12 (N4) valence electrons were partitioned into 4n+2 manifolds (respectively 2+2, 6+2 and 6+6). The triplet state molecule B2C2 with 6 electrons partitioned into 2\u03c0 and 4\u03c3 electrons, with the latter following Baird’s aromaticity rule.,. Now […]<\/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":[],"ppma_author":[2661],"class_list":["post-24974","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry"],"yoast_head":"\nC2N2: a 10-electron four-atom molecule displaying both H\u00fcckel 4n+2 and Baird 4n selection rules for ring aromaticity. - 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=24974\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"C2N2: a 10-electron four-atom molecule displaying both H\u00fcckel 4n+2 and Baird 4n selection rules for ring aromaticity. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The previous examples of four atom systems displaying two layers of aromaticity illustrated how 4 (B4), 8 (C4) and 12 (N4) valence electrons were partitioned into 4n+2 manifolds (respectively 2+2, 6+2 and 6+6). The triplet state molecule B2C2 with 6 electrons partitioned into 2\u03c0 and 4\u03c3 electrons, with the latter following Baird’s aromaticity rule.,. 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