{"id":1663,"date":"2010-02-14T13:03:17","date_gmt":"2010-02-14T12:03:17","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1663"},"modified":"2011-11-29T15:13:03","modified_gmt":"2011-11-29T15:13:03","slug":"conformational-analysis-of-cyclotriborazane","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1663","title":{"rendered":"Conformational analysis of cyclotriborazane"},"content":{"rendered":"
\n

In an earlier post<\/a>, I re-visited the conformational analysis of cyclohexane by looking at the vibrations of the entirely planar form (of D6h<\/sub> symmetry). The method also gave interesting results for the larger cyclo-octane ring. How about a larger leap into the unknown?<\/p>\n

Let us proceed as follows. One fun game to play in chemistry is to invoke iso-electronic<\/em> substitutions. In this case, we can subtitute a nitrogen and a boron atom for a pair of carbons. Thrice invoked, it leads to a molecule known as cyclotriborazane.<\/p>\n

\"\"<\/a>

Cyclotriborazane.<\/p><\/div>\n

This species is in fact very well known, and a crystal-structure was determined some time ago (DOI: 10.1021\/ja00786a022<\/a>). It is worth considering some of its properties.<\/p>\n

    \n
  1. The species is crystalline, and sublimes rather than melts. Contrast this with the iso-electronic cyclohexane, which melts at around 6C (itself a surprisingly high value).<\/li>\n
  2. The parent H3<\/sub>BNH3<\/sub> also has a very high melting point of > 100C, which is attributed to an extensive array of so-called dihydrogen<\/strong> bonds in the crystal lattice, in which a positively charged hydrogen deriving from an NH is attracted to a negatively charged hydrogen deriving from a BH. Such dihydrogen<\/strong> bonds have been shown to be quite strong due to this electrostatic interaction, and are responsible for the extraordinary elevation of the melting point compared to the iso-electronic ethane.<\/li>\n
  3. The chair form of cyclotriborazane aligns the three hydrogens shown in either blue or red in the axial positions. The three red hydrogens might be expected to be all negatively charged, and the three blue ones positively charged. So in the chair conformation, might we expected the electrostatic repulsions between either the blue or the red hydrogens to destabilize these axial positions, and hence perhaps even destabilize the chair conformation itself?<\/li>\n<\/ol>\n

    The crystal structure however shows clearly that the chair is still the favoured conformation. Equally intriguing, one might expect the three blue hydrogens to stack up to attract electrostatically to the three red hydrogens. But you can see from the crystal packing if you activate the model below that this does not happen!<\/p>\n

    \"\"

    Cyclotriborazane Crystal structure. Click for 3D<\/p><\/div>What of the vibrational analysis, conducted as it was for cyclohexane itself (DOI: 10042\/to-4170<\/a>). Well, just as before, for the planar geometry, three imaginary modes are calculated (A2<\/sub>“, E”) and just as before, they distort the geometry in the direction of a chair (Cs<\/sub> symmetry), a C2<\/sub>-disymmetric twist boat (with a predicted optical rotation of\u00a0-54\u00b0) and a boat respectively (the latter, as before, being a transition state connecting the two C2<\/sub>-enantiomers).<\/p>\n

    \"\"

    Planar cyclotriborazane distorting to chair.<\/p><\/div>But here we get a surprise! According to the B3LYP\/6-311G(d,p) model, the final resting energy for the chair is almost the same (indeed 0.2 kcal\/mol higher in free energy) as the twist-boat. Perhaps that blue\/red repulsion did have an effect after all! If you look at the calculated structure, you can indeed see that the blue\/red hydrogens are splayed-out, avoiding each other!<\/p>\n

    \"\"

    Calculated geometry of the chair form of cyclotriborazane<\/p><\/div>This is one of those molecules where one might have expected surprises. In the end, it is surprising at how similar cyclotriborazane is to its iso-electronic cousin cyclohexane.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    In an earlier post, I re-visited the conformational analysis of cyclohexane by looking at the vibrations of the entirely planar form (of D6h symmetry). The method also gave interesting results for the larger cyclo-octane ring. How about a larger leap into the unknown? Let us proceed as follows. One fun game to play in chemistry […]<\/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":[48,17,184,40,2648],"ppma_author":[2661],"class_list":["post-1663","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-chair","tag-conformational-analysis","tag-final-resting-energy","tag-free-energy","tag-interesting-chemistry"],"yoast_head":"\nConformational analysis of cyclotriborazane - 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=1663\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Conformational analysis of cyclotriborazane - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"In an earlier post, I re-visited the conformational analysis of cyclohexane by looking at the vibrations of the entirely planar form (of D6h symmetry). The method also gave interesting results for the larger cyclo-octane ring. How about a larger leap into the unknown? Let us proceed as follows. One fun game to play in chemistry […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1663\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2010-02-14T12:03:17+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2011-11-29T15:13:03+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/02\/B3N3H12.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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Conformational analysis of cyclotriborazane - 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=1663","og_locale":"en_GB","og_type":"article","og_title":"Conformational analysis of cyclotriborazane - Henry Rzepa's Blog","og_description":"In an earlier post, I re-visited the conformational analysis of cyclohexane by looking at the vibrations of the entirely planar form (of D6h symmetry). 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At ~neutral pH, compound 13 hydrolyses with a half life of 21 minutes, whereas 14 takes 840 minutes. Understanding this difference in reactivity may allow us to understand why some enzymes can catalyze the hydrolysis\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":"Models for peptide cleavage.","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/amide-cleavage.png?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6205,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6205","url_meta":{"origin":1663,"position":1},"title":"An orbital analysis of the stereochemistry of the E2 elimination reaction","author":"Henry Rzepa","date":"February 4, 2012","format":false,"excerpt":"The so-called E2 elimination mechanism is another one of those mainstays of organic chemistry. It is important because it introduces the principle that anti-periplanarity of the reacting atoms is favoured over other orientations such as the syn-periplanar form; Barton used this principle to great effect in developing the theory of\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":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/02\/E2.svg","width":350,"height":200},"classes":[]},{"id":9090,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9090","url_meta":{"origin":1663,"position":2},"title":"A conflation of concepts: Conformation and pericyclic.","author":"Henry Rzepa","date":"January 10, 2013","format":false,"excerpt":"This is an interesting result I got when studying the [1,4] sigmatropic rearrangement of\u00a0heptamethylbicyclo-[3.1.0]hexenyl cations. It fits into the last lecture of a series on pericyclic mechanisms, and just before the first lecture on conformational analysis. This is how they join. The experiment it relates to may well be a\u2026","rel":"","context":"In \"conformational analysis\"","block_context":{"text":"conformational analysis","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=conformational-analysis"},"img":{"alt_text":"Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/14me.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":8850,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8850","url_meta":{"origin":1663,"position":3},"title":"Sharpless epoxidation, enantioselectivity and conformational analysis.","author":"Henry Rzepa","date":"January 3, 2013","format":false,"excerpt":"I return to this reaction one more time. Trying to explain why it is enantioselective for the epoxide product poses peculiar difficulties. Most of the substituents can adopt one of several conformations, and some exploration of this conformational space is needed. Amongst the conformational possibilities are the two rotations shown\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":1856,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1856","url_meta":{"origin":1663,"position":4},"title":"Conformational analysis of biphenyls: an upside-down view","author":"Henry Rzepa","date":"April 2, 2010","format":false,"excerpt":"One of the (not a few) pleasures of working in a university is the occasional opportunity that arises to give a new lecture course to students. New is not quite the correct word, since the topic I have acquired is Conformational analysis. The original course at Imperial College was delivered\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\/04\/biphenyl-1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":4038,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=4038","url_meta":{"origin":1663,"position":5},"title":"Updating a worked problem in conformational analysis. Part 2: an answer.","author":"Henry Rzepa","date":"May 17, 2011","format":false,"excerpt":"The previous post set out a problem in conformational analysis. Here is my take, which includes an NCI (non-covalent interaction) display as discussed in another post. The lowest energies of the four diastereomers A-D, each in two conformations (1\/2) were calculated at the \u03c9B97D\/6-311G(d,p)\/SCRF=ethanol level, and are shown here relative\u2026","rel":"","context":"In \"conformational analysis\"","block_context":{"text":"conformational analysis","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?tag=conformational-analysis"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/05\/problem_sheet_answer42.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\/1663","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=1663"}],"version-history":[{"count":1,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1663\/revisions"}],"predecessor-version":[{"id":5757,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1663\/revisions\/5757"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1663"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1663"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1663"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=1663"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}