{"id":4196,"date":"2011-05-29T09:11:49","date_gmt":"2011-05-29T09:11:49","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=4196"},"modified":"2011-06-30T14:22:14","modified_gmt":"2011-06-30T14:22:14","slug":"the-inner-secrets-of-an-ion-pair-isobornyl-chloride-rearrangements","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196","title":{"rendered":"The inner secrets of an ion-pair: Isobornyl chloride rearrangements."},"content":{"rendered":"
\n

Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led\u00a0to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a chloride anion<\/strong> is partnered by a carbocation<\/strong> long ago entered the standard textbooks (see DOI\u00a010.1021\/ed800058c<\/a> and 10.1021\/jo100920e<\/a> for background reading). But the intimate secrets of such ion-pairs are still perhaps not fully recognised. Here, to tease some of them them out, I use the NCI method, which has been the subject of several recent posts.<\/p>\n

\"\"

NCI analysis of the iod-pair transition state for methyl migration in isobornyl chloride. Click for 3D.<\/p><\/div>To remind, the colour coding of the NCI surface is blue=strongly attractive<\/span>, red=strongly repulsive<\/span>, green=weakly attractive<\/span>, yellow=weakly repulsive<\/span>. Shown above is the ion-pair transition state<\/a> for [1,2]methyl migration. Note how the hydrogen bonds between the chloride anion and the water molecules are clearly blue. Only slightly weaker (with a turquoise tint) is a pair of hydrogen bonds between the oxygen atoms and H-C bonds in the isobornyl cation. Such C-H…O bonding in ion-pairs seems to be particularly important. There are other blue regions, between an H…H pair, and a C-H bond and the carbon of the migrating methyl group. Also noteworthy is that many atom pairs have multi-coloured NCI regions, suggesting the interaction is not homogenous, and can be both attractive AND repulsive between any pair of atoms.<\/p>\n

The NCI plot below shows the competing 1,6-hydride shift in isobornyl chloride, again involving an ion-pair transition state.<\/p>\n

\"\"

NCI surfaces for the 1,6 hydride migration transition state in isobornyl chloride. Click for 3D.<\/p><\/div>Notice in this example how the migrating hydrogen supports an attractive hydrogen bond to the chloride anion (ostensibly between a hydride atom and an anionic chloride?), and again how there are a number of blue<\/strong> regions elsewhere.<\/p>\n

Modelling is increasingly focusing on these weaker interactions, that probably mediate much (stereo)selectivity in organic reactions. How long before such approaches themselves enter the text-books?<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led\u00a0to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a chloride anion is partnered by a carbocation long ago entered the standard textbooks (see DOI\u00a010.1021\/ed800058c and 10.1021\/jo100920e for […]<\/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":[543,195,538,249,539,572],"ppma_author":[2661],"class_list":["post-4196","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-chemical-reactivity","tag-ion-pair","tag-isobornyl","tag-julia-contreras-garcia","tag-nnon-covalent-interactions","tag-watoc11"],"yoast_head":"\nThe inner secrets of an ion-pair: Isobornyl chloride rearrangements. - 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=4196\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The inner secrets of an ion-pair: Isobornyl chloride rearrangements. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led\u00a0to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a chloride anion is partnered by a carbocation long ago entered the standard textbooks (see DOI\u00a010.1021\/ed800058c and 10.1021\/jo100920e for […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2011-05-29T09:11:49+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2011-06-30T14:22:14+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/05\/isobornyl.jpg\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"2 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"The inner secrets of an ion-pair: Isobornyl chloride rearrangements. - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196","og_locale":"en_GB","og_type":"article","og_title":"The inner secrets of an ion-pair: Isobornyl chloride rearrangements. - Henry Rzepa's Blog","og_description":"Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led\u00a0to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a chloride anion is partnered by a carbocation long ago entered the standard textbooks (see DOI\u00a010.1021\/ed800058c and 10.1021\/jo100920e for […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196","og_site_name":"Henry Rzepa's Blog","article_published_time":"2011-05-29T09:11:49+00:00","article_modified_time":"2011-06-30T14:22:14+00:00","og_image":[{"url":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/05\/isobornyl.jpg","type":"","width":"","height":""}],"author":"Henry Rzepa","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"2 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"The inner secrets of an ion-pair: Isobornyl chloride rearrangements.","datePublished":"2011-05-29T09:11:49+00:00","dateModified":"2011-06-30T14:22:14+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196"},"wordCount":366,"commentCount":0,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/05\/isobornyl.jpg","keywords":["chemical reactivity","ion pair","isobornyl","Julia Contreras-Garcia","nnon-covalent-interactions","watoc11"],"articleSection":["Interesting chemistry"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4196","name":"The inner secrets of an ion-pair: Isobornyl chloride rearrangements. - 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Meerwein himself suggested (in effect,\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\/01\/h-shift.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":10743,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=10743","url_meta":{"origin":4196,"position":1},"title":"Mechanism of the Boekelheide rearrangement","author":"Henry Rzepa","date":"June 26, 2013","format":false,"excerpt":"A reader asked me about the mechanism of\u00a0the reaction of 2-picoline N-oxide with acetic anhydride to give 2-acetoxymethylpyridine (the\u00a0Boekelheide Rearrangement). He wrote \"\u00a0I don't understand why the system should prefer to go via fragmentation-recombination (... the evidence being that\u00a0oxygen labelling shows scrambling)\u00a0when there is an easy concerted pathway available (...\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":"Boek1","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/06\/Boek1.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":19983,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19983","url_meta":{"origin":4196,"position":2},"title":"Organocatalytic cyclopropanation of an enal: (computational) mechanistic understanding.","author":"Henry Rzepa","date":"August 25, 2018","format":false,"excerpt":"Symbiosis between computation and experiment is increasingly evident in pedagogic journals such as J. Chemical Education. Thus an example of original laboratory experiments, that later became twinned with a computational counterpart. So when I spotted this recent lab experiment I felt another twinning approaching. The reaction under consideration is that\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":13033,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=13033","url_meta":{"origin":4196,"position":3},"title":"The solvation of ion pairs.","author":"Henry Rzepa","date":"November 6, 2014","format":false,"excerpt":"Solvolytic mechanisms are amongst the oldest studied, but reproducing their characteristics using computational methods has been a challenging business. This post was inspired by reading Steve Bachrach's post, itself alluding to this aspect in the title \"Computationally handling ion pairs\". 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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":8961,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8961","url_meta":{"origin":4196,"position":5},"title":"The mechanism of the Benzidine rearrangement.","author":"Henry Rzepa","date":"January 6, 2013","format":false,"excerpt":"The benzidine rearrangement is claimed to be an example of the quite rare\u00a0[5,5] sigmatropic migration, which is a ten-electron homologation of the very common [3,3] sigmatropic reaction (e.g. the Cope or Claisen). Some benzidine rearrangements are indeed thought to go through the [3,3] route. The topic has been reviewed here.\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":"NCI surface. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/benzidinenci.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","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4196","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=4196"}],"version-history":[{"count":0,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4196\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4196"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4196"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4196"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=4196"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}