{"id":11165,"date":"2013-09-12T19:30:10","date_gmt":"2013-09-12T18:30:10","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=11165"},"modified":"2013-09-15T09:13:26","modified_gmt":"2013-09-15T08:13:26","slug":"the-dimer-of-sf2-small-is-beautiful-and-weird","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=11165","title":{"rendered":"The dimer of SF2: small is beautiful (and weird)."},"content":{"rendered":"
\n

Andy Extance at the Chemistry World blog<\/a> has picked up on a fascinating article[1]<\/a><\/span> on the dimer of SF2<\/sub>. This molecule has three F atoms on one S, and only one on the other; FSSF3<\/sub>. But all four S-F bonds are of different length. Lindquist and Dunning\u00a0claim that the minimum energy pathway to dissociation to two SF2<\/sub> molecules does not involve breaking either the longest<\/em> or the weakest<\/em> SF bond. This was too much for me to resist investigating further!<\/p>\n

\"Click

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

The computed \u03c9B97XD\/aug-cc-pVTZ geometry above[2]<\/a><\/span> reveals the extent of the difference in bond lengths. What Lindquist and Dunning showed is that the two apparently longest (and implied weakest) SF bonds do not have the lowest bond dissociation energies, and offered an explanation using their methodology of coupled bond dyads obtained from GVB theory. But as a “transition state” person myself, I could not help but note that no such were described in their article. So I set out to look for one.<\/p>\n

Firstly, I tried to find a transition state for [1,2] migration of the longest S-F bond (labelled 1.741\u00c5 above, axial at a trigonal bipyramidal S). Nope.\u2021<\/sup> The one labelled\u00a01.654\u00c5 seemed unlikely, so in desperation I tried the shortest S-F bond\u00a0(1.582\u00c5, equatorial at a trigonal bipyramidal S). Success! The computed IRC (\u03c9B97XD\/6-311G(d,p)[3]<\/a><\/span> shows a surprise. The shortest<\/em> S-F bond does indeed do a [1,2] migration, but it is accompanied in its travels by the longest<\/em> S-F bond which also moves more or less in synchrony, although its motion is far less.<\/p>\n

\"FSSF3\"<\/p>\n

\"FSSF3G\"<\/a>
\"FSSF3\"<\/p>\n

This transition state, it transpires is not on the pathway to dissociation, but is a double<\/a> [1,2] migration (in this it is related to the coarctate mechanism<\/a>, which involves two bonds breaking and two forming in a concurrent manner). It also has the feature associated with dyotropic rearrangements[4]<\/a><\/span> that the central\u00a0S-S bond is shorter at the transition state<\/a> ((1.937\u00c5) than for either reactant or product (2.073\u00c5).<\/p>\n

The barrier is > 60 kcal\/mol (reduced only slightly by solvation[5]<\/a><\/span>) which does rather tend to suggest that the formation of SF2<\/sub> almost certainly involves homolytic dissociation and hence radicals, rather than a closed shell route found above.<\/p>\n

Not relevant to the previous topic, but related to the post on ClF3<\/sub><\/a> is the ELF analysis; an attempt to identify where the electron basins (mono and disynaptic) are. The sulfur bearing the three fluorines has one lone “pair” (1.88e to be precise, ringed below), whilst the other S has two (2.25e each). The total integration for the 4-coordinate trigonal-bipyramidal S valence basins however (6.43e) clearly indicates no “octet expansion”.\u00a0<\/p>\n

\"S2F4-bas\"<\/p>\n

There are lots lots more to enjoy about this molecule, do go read the\u00a0Lindquist and Dunning article for a sense of this.<\/p>\n

\n

\u2021<\/sup>But I did find it eventually<\/a>.<\/p>\n

References<\/h2>\n
  1. B.A. Lindquist, and T.H. Dunning, \"Bonding in FSSF<sub>3<\/sub>: Breakdown in Bond Length-Strength Correlations and Implications for SF<sub>2<\/sub> Dimerization\", The Journal of Physical Chemistry Letters<\/i>, vol. 4, pp. 3139-3143, 2013. https:\/\/doi.org\/10.1021\/jz401578h<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"Gaussian Job Archive for F4S2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.791578<\/a>\n\n<\/li>\n
  3. H.S. Rzepa, \"Gaussian Job Archive for F4S2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.797514<\/a>\n\n<\/li>\n
  4. D. Christopher Braddock, D. Roy, D. Lenoir, E. Moore, H.S. Rzepa, J.I. Wu, and P. von Ragu\u00e9 Schleyer, \"Verification of stereospecific dyotropic racemisation of enantiopure d and l-1,2-dibromo-1,2-diphenylethane in non-polar media\", Chemical Communications<\/i>, vol. 48, pp. 8943, 2012. https:\/\/doi.org\/10.1039\/c2cc33676f<\/a>\n\n<\/li>\n
  5. H.S. Rzepa, \"Gaussian Job Archive for F4S2\", 2013. https:\/\/doi.org\/10.6084\/m9.figshare.798824<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Andy Extance at the Chemistry World blog has picked up on a fascinating article on the dimer of SF2. This molecule has three F atoms on one S, and only one on the other; FSSF3. But all four S-F bonds are of different length. Lindquist and Dunning\u00a0claim that the minimum energy pathway to dissociation to […]<\/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":[1128,1129],"ppma_author":[2661],"class_list":["post-11165","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","category-reaction-mechanism-2","tag-andy-extance","tag-minimum-energy-pathway"],"yoast_head":"\nThe dimer of SF2: small is beautiful (and weird). - 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=11165\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The dimer of SF2: small is beautiful (and weird). - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Andy Extance at the Chemistry World blog has picked up on a fascinating article on the dimer of SF2. 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Let me explain the connection, but also to follow-up further\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":"FSSCl","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/09\/FSSCl.gif?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":29383,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29383","url_meta":{"origin":11165,"position":1},"title":"The mysterious N=N double bond in nitrosobenzene dimer.","author":"Henry Rzepa","date":"August 11, 2025","format":false,"excerpt":"In an earlier blog, I discussed the curly arrows associated with the known dimerisation of nitrosobenzene, and how the\u00a0N=N double bond (shown in red below) forms in a single concerted process. One of the properties of this molecule is that the equilibrium between the monomer and dimer can be detected,\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":29626,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29626","url_meta":{"origin":11165,"position":2},"title":"Hydrogenating the even more mysterious N\u2261N triple bond in a nitric oxide dimer.","author":"Henry Rzepa","date":"August 25, 2025","format":false,"excerpt":"Previously I looked at some of the properties of the mysterious dimer of nitric oxide \u00a01 - not the known weak dimer but a higher energy form with a \"triple\" N\u2261N bond. This valence bond isomer of the weak dimer was some 24 kcal\/mol higher in free energy than the\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":29665,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29665","url_meta":{"origin":11165,"position":3},"title":"Alternative reactions of the N\u2261N “triple bond” in a nitric oxide dimer: forming the trimer N3O3.","author":"Henry Rzepa","date":"September 3, 2025","format":false,"excerpt":"In the previous post I mooted the possibility that a high energy form of the dimer of nitric oxide 1 might nonetheless be able to be detected using suitable traps (such as hydrogenation or cycloaddition). However, an interesting alternative is that this species could be trapped by nitric oxide itself.\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":31140,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31140","url_meta":{"origin":11165,"position":4},"title":"The fast disappearance of hydroxycarbene through hydrogen tunnelling – or is it really tunnelling?","author":"Henry Rzepa","date":"April 11, 2026","format":false,"excerpt":"In 2008, the previously elusive hydroxycarbene, H-C-OH was finally reported as having been captured by matrix isolation, accompanied by the observation that \"we unexpectedly find that H\u2013C\u2013OH rearranges to formaldehyde with a half-life of only 2h at 11K by pure hydrogen tunnelling through a large energy barrier in excess 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":"","width":0,"height":0},"classes":[]},{"id":18555,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=18555","url_meta":{"origin":11165,"position":5},"title":"Dispersion “bonds”: a new example with an ultra-short H…H distance.","author":"Henry Rzepa","date":"June 26, 2017","format":false,"excerpt":"About 18 months ago, there was much discussion\u00a0on this blog about a system reported by Bob Pascal and co-workers containing a short H...H contact of ~1.5\u00c5. In this system, the hydrogens were both attached to Si as Si-H...H-Si and compressed together by rings. Now a new report and\u00a0commented upon by\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\/2017\/06\/201-1024x987.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\/11165","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=11165"}],"version-history":[{"count":32,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11165\/revisions"}],"predecessor-version":[{"id":11234,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/11165\/revisions\/11234"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=11165"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=11165"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=11165"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=11165"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}