{"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":"2013-09-21T08:55:57","modified_gmt":"2013-09-21T07:55:57","slug":"an-example-of-an-extreme-gauche-effect-fssf","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2013\/09\/21\/an-example-of-an-extreme-gauche-effect-fssf\/","title":{"rendered":"An example of an extreme gauche effect: FSSF."},"content":{"rendered":"

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[cite]10.6084\/m9.figshare.804328[\/cite] 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[cite]10.6084\/m9.figshare.805048[\/cite] 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.<\/figcaption><\/figure>\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[cite]10.6084\/m9.figshare.802815[\/cite] 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[cite]10.6084\/m9.figshare.804332[\/cite] 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","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":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11,13],"tags":[888,2459],"class_list":["post-11279","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","category-reaction-mechanism-2","tag-elf","tag-transition-state"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/11279","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=11279"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/11279\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=11279"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=11279"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=11279"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}