{"id":26009,"date":"2023-05-21T14:42:35","date_gmt":"2023-05-21T13:42:35","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=26009"},"modified":"2023-05-22T06:44:00","modified_gmt":"2023-05-22T05:44:00","slug":"tunable-aromaticity-an-unrecognized-new-aromatic-molecule","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=26009","title":{"rendered":"Tunable aromaticity? An unrecognized new aromatic molecule?"},"content":{"rendered":"
\n

Some time ago in 2010, I showed a chemical problem<\/a> I used to set during university entrance interviews. It was all about pattern recognition and how one can develop a hypothesis based on this. In that instance, it involved recognising that a cyclic molecule which appeared to have the cyclohexatriene benzene-aromatic pattern 1<\/strong> was in fact a trimer of carbon dioxide. Perhaps small amounts of this aromatic molecule exist in solutions of fizzy drinks? Analysing these patterns occupied about 10-20 minutes of an interview, and although you might think I was posing a difficult challenge, many students successfully rose to it! Now I revisit, but with a slightly better reality check on a related molecule 2 <\/strong>(cyanuric acid).<\/p>\n

. \"\"<\/a><\/p>\n

As many as 58 examples of crystal structures of 1,3,5-triazinane-2,4,6-trione 2<\/strong> (cyanuric acid) are known, often with a co-adduct. Cyanuric acid is in effect a cyclic trimer of isocyanic acid rather than of carbon dioxide. These examples tend to be planar, with a mean C-N ring distance of ~1.37\u00c5 and a C-O distance of 1.22\u00c5.\u00a0<\/p>\n

\"\"<\/p>\n

Two outliers stand out, both from a very recently published article, being a co-adduct with melamine (1,3,5-triazine-2,4,6-triamine).[1]<\/a><\/span> QACSUI02<\/a> exhibits a shorter C-N distance of ~1.33\u00c5 but a longer C-O distances of 1.32\u00c5 and have a symmetrical patten of hydrogen bonds to the six receptors of the central unit. Could this correspond more closely to the cyclohexatriene resonance structures shown to the left of the diagram at the top? The first task is to see if these bond lengths can be replicated using calculation (often a useful procedure to check that the crystal structure is correct). For this purpose, the structure below was chosen as the starting point for various models, using an \u03c9B97XD\/Def2-TZVPP model.<\/p>\n

\"\"<\/p>\n\n\n\n\n\n\n\n\n\n\n
Model<\/th>\nC-N distance<\/th>\nC-O distance<\/th>\n<\/tr>\n
QACSUI02<\/a> (crystal structure)<\/td>\n1.331<\/strong><\/td>\n1.318<\/strong><\/td>\n<\/tr>\n
\u03c9B97XD\/Def2-TZVPP<\/a> as single layer<\/td>\n1.3678<\/td>\n1.2185<\/td>\n<\/tr>\n
\u03c9B97XD\/Def2-TZVPP<\/a> three layers<\/td>\n1.365<\/td>\n1.218<\/td>\n<\/tr>\n
\u03c9B97XD\/Def2-TZVPP<\/a> no H-bonds<\/td>\n1.3816<\/td>\n1.2002<\/td>\n<\/tr>\n
\n
\n<\/td>\n<\/tr>\n
XAKSOU<\/a> (crystal structure)<\/td>\n1.367<\/strong><\/td>\n1.208<\/strong><\/td>\n<\/tr>\n
\u03c9B97XD\/Def2-TZVPP<\/a>\u00a0<\/td>\n1.3670<\/td>\n1.2213<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

This creates a mystery. The calculated bond lengths show that whilst H-bonding to the central ring decreases the C-N length by 0.014\u00c5 and increases the C-O length by 0.017\u00c5, this effect is nowhere near large enough to match the apparent lengths in the crystal structure, where a C-N effect of ~0.037\u00c5 would be needed.<\/p>\n

Another system XAKSOU has been reported where discrete LiCl units replace the hydrogen the H-bonds formed to melamine above.[2]<\/a><\/span> A Li is coordinated to the carbonyl oxygen instead of a hydrogen bond, and a chloride anion from another molecule in the unit cell replaces the H-bond to nitrogen.<\/p>\n

\"\"<\/p>\n

In the computed model, an intramolecular Cl-H hydrogen bond is used as the model, resulting in similar C-N lengths as the crystal structure (one which does not match the lengths in the outlying crystal structure QACSUI02<\/a>)<\/p>\n

\"\"<\/p>\n

So the final question to ask is whether this latter structure is aromatic. NICS(0)\/(1) values of -2.8\/-1.1ppm<\/a> are computed, which suggests very little aromaticity (aromatic values would be -10 to -20 pm). So it does not seem as if aromaticity can be tuned into cyanuric acid 2 <\/strong>by polarising both the NH and CO units with ionic\/H-bond interactions so that the aromatic cyclohexatriene motif is better favoured over the 1,3,5-triazinane-2,4,6-trione non-aromatic resonance form. Are there any other examples where aromatically tunable molecules might be possible?<\/p>\n

References<\/h2>\n
  1. K. Song, H. Yang, B. Chen, X. Lin, Y. Liu, Y. Liu, H. Li, S. Zheng, and Z. Chen, \"The facile implementing ternary resistive memory in graphite-like melamine-cyanuric acid hydrogen-bonded organic framework with high ternary yield and environmental tolerance\", Applied Surface Science<\/i>, vol. 608, pp. 155161, 2023. https:\/\/doi.org\/10.1016\/j.apsusc.2022.155161<\/a>\n\n<\/li>\n
  2. O. Shemchuk, D. Braga, L. Maini, and F. Grepioni, \"Anhydrous ionic co-crystals of cyanuric acid with LiCl and NaCl\", CrystEngComm<\/i>, vol. 19, pp. 1366-1369, 2017. https:\/\/doi.org\/10.1039\/c7ce00037e<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Some time ago in 2010, I showed a chemical problem I used to set during university entrance interviews. It was all about pattern recognition and how one can develop a hypothesis based on this. In that instance, it involved recognising that a cyclic molecule which appeared to have the cyclohexatriene benzene-aromatic pattern 1 was in […]<\/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":[1745],"tags":[],"ppma_author":[2661],"class_list":["post-26009","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining"],"yoast_head":"\nTunable aromaticity? An unrecognized new aromatic molecule? - 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=26009\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Tunable aromaticity? An unrecognized new aromatic molecule? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Some time ago in 2010, I showed a chemical problem I used to set during university entrance interviews. It was all about pattern recognition and how one can develop a hypothesis based on this. 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