{"id":17662,"date":"2017-03-12T07:53:01","date_gmt":"2017-03-12T07:53:01","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=17662"},"modified":"2017-03-17T07:24:03","modified_gmt":"2017-03-17T07:24:03","slug":"expanding-on-the-curious-connection-between-the-norbornyl-cation-and-small-ring-aromatics","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17662","title":{"rendered":"Expanding on the curious connection between the norbornyl cation and small-ring aromatics."},"content":{"rendered":"
\n

This is another of those posts that has morphed from an earlier one noting\u00a0the death of the great chemist George Olah<\/a>. The discussion about the norbornyl cation concentrated on whether this species existed in a single minimum symmetric energy well (the non-classical Winstein\/Olah proposal) or a double minimum well connected by a symmetric transition state (the classical Brown proposal). In a comment on the post, I added other examples in chemistry of single\/double minima, mapped here to non-classical\/classical structures. I now expand on the examples related to small aromatic or anti-aromatic rings<\/span>.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n
Examples of symmetric energy potentials<\/th>\n<\/tr>\n
System<\/th>\nClassical with 1 imaginary normal mode<\/th>\nNon-classical with 0 imaginary modes<\/th>\n<\/tr>\n
Norbornyl cation<\/td>\nTS for [1,2] sigmatropic<\/td>\nMinimum, this post<\/td>\n<\/tr>\n
Singlet [6], [10]; 4n+2 annulenes<\/span><\/td>\n–<\/span><\/td>\nMinimum with <\/a>Kekul\u00e9<\/a>\u00a0vibration<\/a><\/span><\/td>\n<\/tr>\n
Singlet [4], [8]; 4n annulenes<\/span><\/td>\nTS for bond shift, 1 imaginary normal mode<\/a><\/span><\/td>\n–<\/span><\/td>\n<\/tr>\n
Triplet [4], [8]; 4n annulenes<\/span><\/td>\n–<\/span><\/td>\nMinimum, with Kekul\u00e9<\/a>\u00a0vibration (?<\/strong>)<\/span><\/td>\n<\/tr>\n
Semibullvalenes<\/td>\nTS for [3,3] sigmatropic<\/td>\nMinimum<\/a><\/td>\n<\/tr>\n
Strong Hydrogen bonds<\/td>\nTS for proton transfer<\/td>\nMinimum<\/a><\/td>\n<\/tr>\n
SN<\/sub>2 substitutions<\/td>\nTS for substitution (C)<\/td>\nMinimum (Si)<\/a><\/td>\n<\/tr>\n
Jahn-Teller distortions<\/td>\nDynamic Jahn-Teller effects<\/a><\/td>\nNo Jahn-Teller distortions<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

In the table above, you might notice a (?<\/span><\/strong>) associated with the entry for (aromatic) triplet state 4n\u00a0<\/strong>annulenes. Here I expand the ?<\/span><\/strong> by considering triplet cyclobutadiene and triplet cyclo-octatetraene (\u03c9B97XD\/Def2-TZVPP, 10.14469\/hpc\/2241<\/a> and\u00a010.14469\/hpc\/2242<\/a> respectively). Each has a normal vibrational mode shown animated below, which oscillates between the two Kekul\u00e9<\/a>\u00a0representations of the molecule with wavenumbers of 1397 and 1744 cm-1<\/sup> respectively. These Kekul\u00e9<\/a>\u00a0modes are both real, which indicates that the symmetric species (D4h<\/sub> and D8h\u00a0<\/sub>symmetry) is in each case the equilibrium minimum energy position (rCC<\/sub> 1.431 and 1.395\u00c5). For comparison the aromatic singlet state 4n+2 annulene benzene (rCC<\/sub> 1.387\u00c5) has the value 1339 cm-1<\/sup>. Notice that both the triplet state wavenumbers are elevated compared to singlet benzene, because in each case the triplet ring \u03c0-bond orders are lower, thus decreasing the natural tendency of the\u00a0\u03c0-system to desymmetrise the ring.<\/a>[1]<\/a><\/span><\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

To complete the theme, I will look at singlet cyclobutadiene. According to the table above, the symmetric form should be a transition state (TS) for bond shifting, with one imaginary normal mode. To calculate this mode, one has to use a method that correctly reflects the symmetry, in this case a CASSCF(4,4)\/6-311G(d,p) wavefunction (DOI: 10.14469\/hpc\/2244<\/a>). The mode (rCC<\/sub>\u00a01.444\u00c5) shown below has a wavenumber of 1477i<\/em> cm-1<\/sup>;\u00a0<\/sup>its vectors of course resemble those of the triplet mode, but its force constant is now negative rather than positive!<\/p>\n

\"\"<\/p>\n

At first sight any connection between the property of the norbornyl cation at the core of the controversies all those decades ago and aromatic\/antiaromatic rings might seem tenuous. But in the end many aspects of chemistry boil down to symmetries and from there to\u00a0\u00c9variste Galois<\/a>, who started the ball rolling.<\/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<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    This is another of those posts that has morphed from an earlier one noting\u00a0the death of the great chemist George Olah. The discussion about the norbornyl cation concentrated on whether this species existed in a single minimum symmetric energy well (the non-classical Winstein\/Olah proposal) or a double minimum well connected by a symmetric transition state […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":true,"_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},"jetpack_post_was_ever_published":false},"categories":[4],"tags":[561,16,2060,2055,1395,2057,2048,2052,2061,356,2059,2054,1442,2056,2058],"ppma_author":[2661],"class_list":["post-17662","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-antiaromaticity","tag-aromaticity","tag-evariste-galois","tag-carbocation","tag-chemistry","tag-equilibrium-minimum-energy-position","tag-george-andrew-olah","tag-george-olah","tag-great-chemist","tag-jahn-teller","tag-minimum-symmetric-energy","tag-olah","tag-physical-organic-chemistry","tag-saul-winstein","tag-symmetric-energy-potentials"],"yoast_head":"\nExpanding on the curious connection between the norbornyl cation and small-ring aromatics. - 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=17662\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Expanding on the curious connection between the norbornyl cation and small-ring aromatics. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"This is another of those posts that has morphed from an earlier one noting\u00a0the death of the great chemist George Olah. 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He was part of the generation of scientists in the post-war 1950s who had access to chemical instrumentation that truly revolutionised chemistry. In particular he showed how the then newly available NMR spectroscopy illuminated structures of cations in solvents such \"Magic acid\". 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":17692,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17692","url_meta":{"origin":17662,"position":1},"title":"What is the (calculated) structure of a norbornyl cation anion-pair in water?","author":"Henry Rzepa","date":"April 1, 2017","format":false,"excerpt":"In a comment appended to an earlier post, I mused about the magnitude of the force constant relating to the interconversion between a classical and a non-classical structure for the norbornyl cation. Most calculations indicate the force constant for an \"isolated\" symmetrical cation\u00a0is +ve, which means it is a true\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/b3lypd3bj.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":17805,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17805","url_meta":{"origin":17662,"position":2},"title":"Silyl cations?","author":"Henry Rzepa","date":"March 23, 2017","format":false,"excerpt":"It is not only the non-classical norbornyl cation that has proved controversial in the past. A colleague mentioned at lunch (thanks Paul!) that tri-coordinate group 14 cations such as R3Si+ have also had an interesting history. Here I take a brief look at some of these systems. Their initial characterisations,\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/164-1024x748.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4837,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4837","url_meta":{"origin":17662,"position":3},"title":"Anatomy of a simple reaction: the hydration of an alkene.","author":"Henry Rzepa","date":"September 4, 2011","format":false,"excerpt":"The hydration of an alkene by an acid is one of those fundamental reactions, taught early on in most chemistry courses. What can quantum mechanics teach us about the mechanism of the reaction? The diagram below shows us the IRC, or intrinsic reaction coordinate for the process (for definitions, see\u2026","rel":"","context":"In \"alkene\"","block_context":{"text":"alkene","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=alkene"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/09\/hydration.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":17702,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17702","url_meta":{"origin":17662,"position":4},"title":"How does methane invert (its configuration)?","author":"Henry Rzepa","date":"March 16, 2017","format":false,"excerpt":"This is a spin-off from the table I constructed here for further chemical examples of the classical\/non-classical norbornyl cation conundrum. One possible entry would include the transition state for inversion of methane via a square planar geometry as compared with e.g. NiH4 for which the square planar motif is its\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":4893,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4893","url_meta":{"origin":17662,"position":5},"title":"Some fun with no-go areas of chemistry: cyclobutadiene.","author":"Henry Rzepa","date":"September 18, 2011","format":false,"excerpt":"Organic chemistry has some no-go areas, where few molecules dare venture. One of them is described by a concept known as anti-aromaticity. Whereas aromatic molecules are favoured species, their anti-equivalent is avoided. I previously illustrated this (H\u00fcckel rule) with cyclopropenium anion. Now I take a look at cyclobutadiene, for which\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\/2011\/09\/cbdzw.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\/17662","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=17662"}],"version-history":[{"count":20,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/17662\/revisions"}],"predecessor-version":[{"id":17691,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/17662\/revisions\/17691"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=17662"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=17662"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=17662"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=17662"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}