{"id":11279,"date":"2013-09-21T08:55:57","date_gmt":"2013-09-21T07:55:57","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=11279"},"modified":"2013-09-22T16:07:56","modified_gmt":"2013-09-22T15:07:56","slug":"an-example-of-an-extreme-gauche-effect-fssf","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11279","title":{"rendered":"An example of an extreme gauche effect: FSSF."},"content":{"rendered":"
\n

The best known example of the gauche effect<\/em> is 1,2-difluoroethane<\/a>, which exhibits a relatively small preference of ~0.5 kcal\/mol for this conformer over the anti<\/em> orientation, which is also a minimum. But FSSF, which I discussed in the previous post<\/a>, beats this hands down! It also, by the way, must surely be the smallest molecule (only four atoms) which could be theoretically resolved into two enantiomers (possibly at say 273K?).\u2021<\/sup><\/a><\/p>\n

\"FSSF-360\"\"FSSF-360g\"<\/p>\n

From this optimised scan[1]<\/a><\/span> of the F-S-S-F torsion angle, you can see two striking differences<\/p>\n

    \n
  1. Only the gauche<\/em> form is stable. The anti form is in fact a transition state[2]<\/a><\/span> for enantiomerisation of the two chiral C2<\/sub>-disymmetric gauche<\/em> forms.<\/li>\n
  2. The difference in free energy between the gauche form and the anti is 25.3 kcal\/mol, compared with which the 0.5 kcal\/mol for 1,2-difluoroethane looks puny indeed.<\/li>\n
  3. The effect arises, as with difluoroethane, from overlap of the filled p-lone pair on one sulfur, with the accepting S-F \u03c3* orbital.
    \n
    \"FSSF-NBO\"

    Click for 3D.<\/p><\/div>\n<\/li>\n

  4. This orbital overlap results in an NBO E(2) interaction energy of 39 kcal\/mol. This compares with 5.6 kcal\/mol for the equivalent C-H\/C-F* term for difluoroethane, and it is of course larger because an S lone pair is a far better donor than a C-H bond. It is also far greater than the anomeric effect, which normally weighs in at about 16 kcal\/mol.<\/li>\n
  5. There is of course an alternative (and perhaps more unusual) transition state[3]<\/a><\/span> for interconverting the two enantiomers of F-S-S-F which I described previously<\/a> for F-S-S-Cl as involving a [1,2] migration of F. It however is 23.5 kcal\/mol higher in energy than this pure bond rotation. Whereas the [1,2] F migration contracts the S-S bond at the transition state, the bond rotation lengthens it (from 1.922 to 2.142\u00c5).\u00a0 This arises because the partial double bond character for the S-S bond is destroyed by rotation. The challenge then is whether one can find a 4-atom system where enantiomerisation proceeds by a lower energy continuously-chiral [1,2] migratory pathway rather than just by a simple bond rotation.<\/li>\n
  6. An alternative visualisation of the electronic effects resulting in an extreme gauche<\/em> effect can be seen from the ELF analysis[4]<\/a><\/span> of the lone pair basins;\"FSSF-ELF\"
    The two basins ringed in blue (2.25e) are each aligned at an angle of 167\u00b0 to the axis of the antiperiplanar S-F bond. The knock-on effect of this is that the two lone pairs on each sulfur themselves subtend an unusual angle of 145\u00b0 at the common sulfur, almost diaxial in fact.<\/li>\n<\/ol>\n

    I again marvel at how just four atoms and just two elements, can teach us so much chemistry!<\/p>\n

    \"Enhanced<\/a><\/div>\n

    References<\/h2>\n
    1. H.S. Rzepa, \"Gaussian Job Archive for F2S2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.804328<\/a>\n\n<\/li>\n
    2. H.S. Rzepa, \"Gaussian Job Archive for F2S2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.805048<\/a>\n\n<\/li>\n
    3. H.S. Rzepa, \"Gaussian Job Archive for F2S2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.802815<\/a>\n\n<\/li>\n
    4. H.S. Rzepa, \"Gaussian Job Archive for F2S2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.804332<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      The best known example of the gauche effect is 1,2-difluoroethane, which exhibits a relatively small preference of ~0.5 kcal\/mol for this conformer over the anti orientation, which is also a minimum. But FSSF, which I discussed in the previous post, beats this hands down! It also, by the way, must surely be the smallest molecule […]<\/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":[4,1086],"tags":[11,1136],"ppma_author":[2661],"class_list":["post-11279","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","category-reaction-mechanism-2","tag-elf","tag-transition-state"],"yoast_head":"\nAn example of an extreme gauche effect: FSSF. - 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=11279\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"An example of an extreme gauche effect: FSSF. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The best known example of the gauche effect is 1,2-difluoroethane, which exhibits a relatively small preference of ~0.5 kcal\/mol for this conformer over the anti orientation, which is also a minimum. But FSSF, which I discussed in the previous post, beats this hands down! It also, by the way, must surely be the smallest molecule […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11279\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2013-09-21T07:55:57+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2013-09-22T15:07:56+00:00\" \/>\n<meta property=\"og:image\" content=\"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/09\/FSSF-360.svg\" \/>\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":"An example of an extreme gauche effect: FSSF. - 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=11279","og_locale":"en_GB","og_type":"article","og_title":"An example of an extreme gauche effect: FSSF. - Henry Rzepa's Blog","og_description":"The best known example of the gauche effect is 1,2-difluoroethane, which exhibits a relatively small preference of ~0.5 kcal\/mol for this conformer over the anti orientation, which is also a minimum. But FSSF, which I discussed in the previous post, beats this hands down! It also, by the way, must surely be the smallest molecule […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11279","og_site_name":"Henry Rzepa's Blog","article_published_time":"2013-09-21T07:55:57+00:00","article_modified_time":"2013-09-22T15:07:56+00:00","og_image":[{"url":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/09\/FSSF-360.svg","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=11279#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11279"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"An example of an extreme gauche effect: FSSF.","datePublished":"2013-09-21T07:55:57+00:00","dateModified":"2013-09-22T15:07:56+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11279"},"wordCount":455,"commentCount":2,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11279#primaryimage"},"thumbnailUrl":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/09\/FSSF-360.svg","keywords":["ELF","Transition state"],"articleSection":["Interesting chemistry","reaction mechanism"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11279#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11279","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11279","name":"An example of an extreme gauche effect: FSSF. - 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Here I thought I would present the results of searching the Cambridge crystal database for examples of the gauche effect. The basic search is defined below\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":"CCDC-search","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/01\/CCDC-search.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":9360,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9360","url_meta":{"origin":11279,"position":1},"title":"\u03c3-\u03c0-Conjugation: seeking evidence by a survey of crystal structures.","author":"Henry Rzepa","date":"February 3, 2013","format":false,"excerpt":"The electronic interaction between a single bond and an adjacent double bond is often called \u03c3-\u03c0-conjugation (an older term for this is hyperconjugation), and the effect is often used to e.g. explain why more highly substituted carbocations are more stable than less substituted ones. This conjugation is more subtle in\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":"C-H\/alkene interaction. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/02\/cis-butene-orbitals.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":12276,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12276","url_meta":{"origin":11279,"position":2},"title":"Artemisinin: are stereo-electronics at the core of its (re)activity?","author":"Henry Rzepa","date":"April 13, 2014","format":false,"excerpt":"Around 100 tons of the potent antimalarial artemisinin is produced annually; a remarkable quantity given its very unusual and fragile looking molecular structure (below). When I looked at this, I was immediately struck by a thought: surely this is a classic molecule for analyzing stereoelectronic effects (anomeric and gauche). Here\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":"artemisinin1","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2014\/04\/artemisinin1.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6455,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6455","url_meta":{"origin":11279,"position":3},"title":"A golden age for (computational) spectroscopy.","author":"Henry Rzepa","date":"April 2, 2012","format":false,"excerpt":"I mentioned in my last post an unjustly neglected paper from that golden age of 1951-1953 by Kirkwood and co. They had shown that Fischer's famous guess for the absolute configurations of organic chiral molecules was correct. The two molecules used to infer this are shown below. Using the theory\u2026","rel":"","context":"In "Chiroptics"","block_context":{"text":"Chiroptics","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2644"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":5087,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=5087","url_meta":{"origin":11279,"position":4},"title":"cis-Butene: a reaction coordinate dissected and methyl flags.","author":"Henry Rzepa","date":"October 12, 2011","format":false,"excerpt":"In two previous posts, I have looked at why cis-butene adopts conformation (a) rather than (b). I suggested it boiled down to electronic interactions between the methyl groups and the central alkene resulting in the formation of a H...H \"topological\" bond, rather than attraction between the H...H region to form\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2011\/10\/cis-butene2.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1887,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1887","url_meta":{"origin":11279,"position":5},"title":"The conformation of 1,2-difluoroethane","author":"Henry Rzepa","date":"April 6, 2010","format":false,"excerpt":"Here I offer another spin-off from writing a lecture course on conformational analysis. This is the famous example of why 1,2-difluoroethane adopts a gauche rather than antiperiplanar conformation. One major contribution to the greater stability of the gauche is the stereoelectronic interactions, and this is best probed using the NBO\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\/difluoroethane.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\/11279","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=11279"}],"version-history":[{"count":20,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11279\/revisions"}],"predecessor-version":[{"id":11312,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11279\/revisions\/11312"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=11279"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=11279"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=11279"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=11279"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}