{"id":1887,"date":"2010-04-06T12:36:51","date_gmt":"2010-04-06T11:36:51","guid":{"rendered":"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1887"},"modified":"2026-06-17T16:51:33","modified_gmt":"2026-06-17T15:51:33","slug":"the-conformation-of-12-difluoroethane","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=1887","title":{"rendered":"The conformation of 1,2-difluoroethane"},"content":{"rendered":"<div class=\"kcite-section\" kcite-section-id=\"1887\">\n<p>Here I offer another spin-off from writing a lecture course on conformational analysis. This is the famous example of why <strong>1,2-difluoroethane<\/strong> adopts a <em>gauche<\/em> rather than <em>antiperiplanar<\/em> conformation.<br \/>\n<div id=\"attachment_1888\" style=\"width: 410px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/04\/difluoroethane.jpg\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1888\" class=\"size-full wp-image-1888\" title=\"difluoroethane\" alt=\"\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/04\/difluoroethane.jpg\" width=\"400\" height=\"158\" srcset=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/04\/difluoroethane.jpg 952w, https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/04\/difluoroethane-300x118.jpg 300w\" sizes=\"auto, (max-width: 400px) 100vw, 400px\" \/><\/a><p id=\"caption-attachment-1888\" class=\"wp-caption-text\">The gauche and antiperiplanar conformations of 1,2-difluoroethane<\/p><\/div><\/p>\n<p>One major contribution to the greater stability of the gauche is the stereoelectronic interactions, and this is best probed using the NBO (Natural Bond Orbital) approach of Weinhold<span id=\"cite_ITEM-1887-0\" name=\"citation\"><a href=\"#ITEM-1887-0\">[1]<\/a><\/span>. The process is approximately described as first reducing the wavefunction down to a set of orbitals which have been localized (using appropriate algorithms) down to two or one centres (corresponding to two-centre covalent bonds, or one-centre electron lone pairs). Perturbation theory is then used to evaluate the interaction energy between any filled and any empty combination. For the molecule above,<strong> six<\/strong> such combinations are inspected, involving any one of the six filled C-H or C-F \u03c3-orbitals, and the best-overlapping \u03c3* orbital which turns out to be located on the C-H or C-F bond anti-periplanar to the filled orbital.<\/p>\n<table border=\"0\">\n<tbody>\n<tr>\n<td>\n<div id=\"attachment_1891\" style=\"width: 210px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1891\" class=\"size-full wp-image-1891\" title=\"fluoroethane1\" onclick=\"jmolInitialize('..\/Jmol\/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2010\/04\/11.jvxl;isosurface  &quot;&quot; color yellow green translucent;isosurface append wp-content\/uploads\/2010\/04\/18.jvxl translucent;set fontscaling TRUE; font label 18;select atomno=3;color black;label %A C-H \u03c3 orbital;');\" alt=\"\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/04\/fluoroethane1.jpg\" width=\"200\" height=\"207\" \/><p id=\"caption-attachment-1891\" class=\"wp-caption-text\">Filled C-H NBO orbital. Click for 3D to superimpose empty C-F anti bonding orbital.<\/p><\/div>\n<\/td>\n<td>\n<div id=\"attachment_1892\" style=\"width: 210px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1892\" class=\"size-full wp-image-1892\" title=\"fluoroethane2\" onclick=\"jmolInitialize('..\/Jmol\/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content\/uploads\/2010\/04\/18.jvxl;isosurface &quot;&quot; translucent;set fontscaling TRUE; font label 18;select atomno=7;color black;label %A C-F \u03c3* orbital;');\" alt=\"\" src=\"http:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2010\/04\/fluoroethane2.jpg\" width=\"200\" height=\"245\" \/><p id=\"caption-attachment-1892\" class=\"wp-caption-text\">Empty C-F antibonding NBO orbital. Click for 3D<\/p><\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>A filled C-H orbital is shown above on the left, accompanied by an empty C-F \u03c3* orbital on the right which is anti-periplanar to the first. This alignment allows the phases of the two orbitals to overlap maximally (blue-blue on the top, red-red beneath).<\/p>\n<p>The interaction energy between this pair is determined not only by the efficacy of the overlap, but by the energy gap between the two. The smaller the gap, the better the interaction energy (referred to as <strong>E2<\/strong>, in kcal\/mol). For the gauche conformation, the six pairs of orbitals have the following interaction energies; two \u03c3<sub>C-H<\/sub>\/\u03c3*<sub>C-F<\/sub> interactions (illustrated above), 4.9; two \u03c3<sub>C-H<\/sub>\/\u03c3*<sub>C-H <\/sub>2.6 and two \u03c3<sub>C-F<\/sub>\/\u03c3*<sub>C-H<\/sub> 0.8 kcal\/mol. For the anti-periplanar conformation, the terms are four \u03c3<sub>C-H<\/sub>\/\u03c3*<sub>C-H<\/sub> 2.5 and two \u03c3<sub>C-F<\/sub>\/\u03c3*<sub>C-F <\/sub>1.8 kcal\/mol. The two totals (16.6 vs 13.6) indicate that gauche is stabilized more by such interactions.<\/p>\n<p>There is of course a bit more to this story, but I have documented the above here, since I can include an explicit (and rotatable) illustration of the orbitals involved (which \u00a0I have not seen elsewhere). If you want a recipe for generating these orbitals, <a href=\"http:\/\/www.ch.ic.ac.uk\/local\/organic\/conf\/c1_nbo.html\" target=\"_blank\" rel=\"noopener\">go here<\/a>.<\/p>\n<hr \/>\n<h4>Acknowledgments<\/h4>\n<p>This post has been cross-posted in PDF format at <span id=\"cite_ITEM-1887-1\" name=\"citation\"><a href=\"#ITEM-1887-1\">[2]<\/a><\/span><\/p>\n<h2>References<\/h2>\n    <ol class=\"kcite-bibliography csl-bib-body\"><li id=\"ITEM-1887-0\">T.K. Brunck, and F. Weinhold, \"Quantum-mechanical studies on the origin of barriers to internal rotation about single bonds\", <i>Journal of the American Chemical Society<\/i>, vol. 101, pp. 1700-1709, 1979. <a href=\"https:\/\/doi.org\/10.1021\/ja00501a009\">https:\/\/doi.org\/10.1021\/ja00501a009<\/a>\n\n<\/li>\n<li id=\"ITEM-1887-1\">H. Rzepa, \"The conformation of 1,2-difluoroethane\", 2015. <a href=\"https:\/\/doi.org\/10.15200\/winn.143125.51716\">https:\/\/doi.org\/10.15200\/winn.143125.51716<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> <!-- kcite-section 1887 -->","protected":false},"excerpt":{"rendered":"<p>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 (Natural Bond Orbital) approach of [&hellip;]<\/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_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"federated","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":[211,17,212,210,41,2648,213],"ppma_author":[2661],"class_list":["post-1887","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-appropriate-algorithms","tag-conformational-analysis","tag-energy-gap","tag-here-i","tag-interaction-energy","tag-interesting-chemistry","tag-spin-off"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.9 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>The conformation of 1,2-difluoroethane - Henry Rzepa&#039;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=1887\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The conformation of 1,2-difluoroethane - Henry Rzepa&#039;s Blog\" \/>\n<meta property=\"og:description\" content=\"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. 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But FSSF, which I discussed in the previous post, beats this hands down! It also, by the way, must\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"FSSF-ELF","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/09\/FSSF-ELF.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6205,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6205","url_meta":{"origin":1887,"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 &quot;Interesting chemistry&quot;","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":8898,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=8898","url_meta":{"origin":1887,"position":2},"title":"The gauche effect: seeking evidence by a survey of crystal structures.","author":"Henry Rzepa","date":"January 4, 2013","format":false,"excerpt":"I previously blogged about anomeric effects involving\u00a0\u03c0 electrons as donors, and my post on the conformation of 1,2-difluorethane\u00a0turned out one of the most popular. 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 &quot;crystal_structure_mining&quot;","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":9841,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9841","url_meta":{"origin":1887,"position":3},"title":"The mysterious (aromatic) structure of n-Butyl lithium.","author":"Henry Rzepa","date":"March 17, 2013","format":false,"excerpt":"n-Butyl lithium is hexameric in the solid state and in cyclohexane solutions. Why? Here I try to find out some of its secrets. The crystal structure reveals the following points of interest: Six lithium atoms form a cluster with triangular faces. An off-centre carbanion caps a triangular lithium face. Four\u2026","rel":"","context":"In &quot;Interesting chemistry&quot;","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"SUHBEC. CLICK FOR 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/03\/SUHBEC.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6279,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6279","url_meta":{"origin":1887,"position":4},"title":"E2 elimination vs ring contraction: anti-periplanarity in action.","author":"Henry Rzepa","date":"February 20, 2012","format":false,"excerpt":"The anti-periplanar principle permeates organic reactivity. Here I pick up on an example of the antiperiplanar E2 elimination (below, blue) by comparing it to a competing reaction involving a [1,2] antiperiplanar migration (red). The relative rates of these two processes will depend on several factors such as the ability of\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":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/02\/Ring.svg","width":350,"height":200},"classes":[]},{"id":9459,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9459","url_meta":{"origin":1887,"position":5},"title":"The conformational preference of s-cis amides.","author":"Henry Rzepa","date":"February 10, 2013","format":false,"excerpt":"Amides with an H-N group are a component of the peptide linkage\u00a0(O=C-NH). Here I ask what the conformation (it could also be called a configuration) about the C-N bond is. A search of the following type can be defined: The dihedral shown is for H-N-C=O (but this is equivalent to\u2026","rel":"","context":"In &quot;crystal_structure_mining&quot;","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"cis-amide","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/02\/cis-amide.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\/1887","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=1887"}],"version-history":[{"count":10,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1887\/revisions"}],"predecessor-version":[{"id":31691,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1887\/revisions\/31691"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1887"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1887"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1887"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=1887"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}