{"id":3392,"date":"2011-01-07T10:55:36","date_gmt":"2011-01-07T09:55:36","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3392"},"modified":"2011-11-28T13:18:03","modified_gmt":"2011-11-28T13:18:03","slug":"do-electrons-prefer-to-move-in-packs-of-4-6-or-8-during-proton-exchange-in-a-calixarene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3392","title":{"rendered":"Do electrons prefer to move in packs of 4, 6 or 8 during proton exchange in a calixarene?"},"content":{"rendered":"
\n

This story starts with a calixarene, a molecule (suitably adorned with substituents) frequently used as a host to entrap a guest and perchance make the guest do something interesting. Such a calixarene was at the heart of a recent story<\/a> where an attempt was made to induce it to capture cyclobutadiene in its cavity.<\/p>\n

\"\"<\/a>

The basic skeleton of a calixarene<\/p><\/div>\n

At the base of the calixarene are four hydroxyl groups, arranged in either a left or right handed manner. The molecule, in other words is chiral (C4<\/sub> symmetry to be precise). As a chiral molecule, it might trap left and right-handed guests in a slightly different manner (forming two possible diastereomeric host-guest complexes). \u00a0As it happens, the guest in the cyclobutadiene story was just such a chiral molecule. But an essential question to ask is what the barrier to enantiomerization of such a calixarene might be? \u00a0One can envisage several ways of accomplishing such a conversion.<\/p>\n

\"\"<\/a>

Enantiomerization pathways for a chiral calixarene<\/p><\/div>\n

All four hydrogens can be moved in a single step, one might move two at a time in two steps, or one might move one at a time in four steps. These processes would involve respectively 8, 6 or 4 electrons<\/a> in each step. There is a fundamental difference between the first pathway and the last two; \u00a0the latter involve \u00a0ionic intermediates (zwitterions) whereas the first is neutral. As such one might imagine the process would depend on the ability of the solvent to stabilize any such zwitterion.<\/p>\n

Let us start with a gas phase model (\u03c9B97XD\/6-311G(d,p)), and a transition state<\/a> with one negative force constant is indeed found with \u00a0C4v<\/sub> symmetry. The free energy barrier \u0394G\u2020<\/sup> for the process is 14.0 kcal\/mol, which means the reaction will occur rapidly, even at lower temperatures of \u00a0~200K. A pack size of 8 seems preferred for this model. This is hardly a surprise since the formation of ionic intermediates would not be expected. One might however speculate thus. In the schematic above, n=1 and one might be tempted to ask if higher values of n (lets say \u00a0n=2, a pack size of 10, or n=3, a pack size of 12, etc ) might exhibit similar behaviour. Is there any limit to the ring\/pack size for this type of proton exchange?<\/p>\n

\"\"

Transition state for enantiomerization of a calixarene in the gas phase. Click for 3D.<\/p><\/div>What about in solution? Well, let us apply the mildest of solvents, benzene as a so-called continuum field. This has a very low dielectric (2.3) and you might imagine it would have hardly any effect. Well click on the below. The C4v<\/sub> geometry now has three computed<\/a> negative force constants; the two additional ones are shown below (they are degenerate with a wavenumber of 101i cm-1<\/sup>).<\/p>\n\n\n\n
\"\"

C4v symmetric geometry for calixarene in benzene solvent, with three negative force constants. Click for animation of E mode.<\/p><\/div><\/td>\n

\"\"

C4v geometry for calixarene in benzene. Click for animation of second E mode.<\/p><\/div><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Each of these additional two negative force constants shows a displacement heading towards the zwitterion shown in the scheme below. As one increases the polarity of the solvent, so the force constant becomes more negative. Thus for dichloromethane<\/a>, it is now 322i cm-1<\/sup> and with water<\/a> it is 376i cm-1<\/sup><\/p>\n

So now the question is what happens when either of the two additional negative force constants is followed downhill? Will it form a true zwitterion (which would have Cs<\/sub> symmetry), in which case it would be (two) 6 electron processes to enantiomerize the calixarene instead of one 8 electron one.<\/p>\n

\"\"

True transition state for proton exchange in solution phase calixarene<\/p><\/div>In fact this geometry of Cs <\/sub>symmetry, which does resemble the zwitterion shown in the scheme above, is NOT<\/strong> a minimum but a true transition state itself (the free energy barrier hardly changed from the value for the gas phase). So the answer seems to be that a calixarene enantiomerizes via<\/em> transition state<\/em> not of C4v<\/sub> but of Cs<\/sub> symmetry, and which resembles a zwitterion but is not actually one. The 8-pack<\/em>of electrons was tempted to take a short rest-break on their way to shifting the four protons, but in the end did it in a single journey! So we have an unusual zwitterionic but nevertheless concerted transition state for the process.<\/p>\n

Still unresolved is whether such cyclic transfer of four protons between four oxygen atoms continues to be concerted for larger rings, or whether the system is finally tempted to break up the transfer by resting with one or more discrete intermediates along the way. I finally note that in the calixarene reported<\/a> which catalysed the thoughts above, the four oxygens are capped with a guanidinium cation sitting just above them, and this too may have an interesting effect on the proton transfer process.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

This story starts with a calixarene, a molecule (suitably adorned with substituents) frequently used as a host to entrap a guest and perchance make the guest do something interesting. Such a calixarene was at the heart of a recent story where an attempt was made to induce it to capture cyclobutadiene in its cavity. At […]<\/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":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},"jetpack_post_was_ever_published":false},"categories":[4],"tags":[152,396,399,431,206,145,22,2650,397,572,398],"ppma_author":[2661],"class_list":["post-3392","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-animation","tag-calixarene","tag-chiral","tag-dielectric","tag-free-energy-barrier","tag-gas-phase","tag-gas-phase-model","tag-pericyclic","tag-proton-transfer","tag-watoc11","tag-zwitterionic"],"yoast_head":"\nDo electrons prefer to move in packs of 4, 6 or 8 during proton exchange in a calixarene? - 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=3392\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Do electrons prefer to move in packs of 4, 6 or 8 during proton exchange in a calixarene? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"This story starts with a calixarene, a molecule (suitably adorned with substituents) frequently used as a host to entrap a guest and perchance make the guest do something interesting. 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Other journals (e.g.\u00a0Nature Chemistry ran the article as a research highlight (where the purpose\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\/11\/cbd.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4751,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4751","url_meta":{"origin":3392,"position":1},"title":"Computational “reality checks” for mechanistic speculations.","author":"Henry Rzepa","date":"September 1, 2011","format":false,"excerpt":"I have mentioned Lewis a number of times in these posts; his suggestion of the shared electron covalent bond still underpins much chemical thinking. Take for example mechanistic speculations on the course of a reaction, a very common indulgence in almost all articles reporting such, and largely based on informed\u00a0\u00a0arrow\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\/nsch001.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":2355,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2355","url_meta":{"origin":3392,"position":2},"title":"Reactions in supramolecular cavities – trapping a cyclobutadiene: ! or ?","author":"Henry Rzepa","date":"August 8, 2010","format":false,"excerpt":"Cavities promote reactions, and they can also trap the products of reactions. Such (supramolecular) chemistry is used to provide models for how enzymes work, but it also allows un-natural reactions to be undertaken. A famous example is the preparation of P4 (see blog post here), an otherwise highly reactive species\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\/08\/scheme.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":3723,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=3723","url_meta":{"origin":3392,"position":3},"title":"Chemicalizing a blog.","author":"Henry Rzepa","date":"March 30, 2011","format":false,"excerpt":"I am at the ACS meeting, attending a session on chemistry and the Internet. This post was inspired by Chemicalize, a service offered by ChemAxon, which scans a post like this one, and identifies molecules named. I had previously used generic post taggers, which frankly did not work well in\u2026","rel":"","context":"In "Chemical IT"","block_context":{"text":"Chemical IT","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":2469,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=2469","url_meta":{"origin":3392,"position":4},"title":"Solid carbon dioxide: hexacoordinate carbon?","author":"Henry Rzepa","date":"September 17, 2010","format":false,"excerpt":"Carbon dioxide is much in the news, not least because its atmospheric concentration is on the increase. How to sequester it and save the planet is a hot topic. Here I ponder its solid state structure, as a hint to its possible reactivity, and hence perhaps for clues as to\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/09\/CO2-solid.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8337,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8337","url_meta":{"origin":3392,"position":5},"title":"A chiral molecular wire.","author":"Henry Rzepa","date":"November 20, 2012","format":false,"excerpt":"More than 60 million molecules are known, and many are fascinating. But beauty is in the eye of the beholder. Thus it was that I came across the attached molecule. It struck me immediately as, well, beautiful! This is one that comes to life in 3D and I strongly urge\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\/2012\/11\/GOCTOH.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\/3392","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=3392"}],"version-history":[{"count":1,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3392\/revisions"}],"predecessor-version":[{"id":5683,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3392\/revisions\/5683"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3392"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3392"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3392"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=3392"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}