{"id":9572,"date":"2013-02-21T10:51:22","date_gmt":"2013-02-21T10:51:22","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=9572"},"modified":"2013-02-21T10:51:22","modified_gmt":"2013-02-21T10:51:22","slug":"a-to-and-fro-of-electrons-operating-in-s-cis-esters","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=9572","title":{"rendered":"A to-and-fro of electrons operating in s-cis esters."},"content":{"rendered":"
\n

I conclude my exploration of conformational preferences by taking a look at esters. As before<\/a>, I start with a search definition, the ester being restricted to one bearing only sp3<\/sup> carbon centers.<\/p>\n

\"s-cis-ester-torsion-search\"<\/p>\n

The result of such a search is pretty clear-cut; they all exist in just one conformation, the s-cis<\/em><\/a>, in which a lone pair of electrons on the alkyl-oxygen is aligned quite precisely anti-periplanar with the axis of the C=O bond. This very narrow distribution suggests a relatively large energy preference for this orientation, and we need to seek its origins.<\/p>\n

\"s-cis-ester-torsion\"<\/p>\n

This arises from two electronic alignments. The first orients the in-plane alkyl oxygen lone pair (orange-purple below) anti-periplanar with the C=O \u03c3* empty orbital (red-blue; orange=red, blue=purple), an interaction mapping to 7.7 kcal\/mol in the NBO E(2) energy. The second reinforcement (not shown) aligns the (O=)C-Me donor bond with the antiperiplanar O-Me acceptor (5.3 kcal\/mol). These two interactions are weaker in the s-trans<\/em> ester<\/a>, which is 8.1 kcal\/mol higher in \u0394G298<\/sub> and for which the E(2) terms are respectively 3.0 and 0.6 kcal\/mol.\u00a0<\/p>\n

\"Click

Lp(alkyl-O)\/C=O \u03c3* Click for 3D.<\/p><\/div>\n

But wait, this interaction has electrons moving from the alkyl oxygen to the acyl oxygen (red arrows below) and apparently weakening the C=O bond in the process. But in an entirely different context, we learn that the C=O vibrational stretching wavenumber for an ester (1750 cm-1<\/sup>) is higher than that of a ketone (~1715 cm-1<\/sup>); the C=O is stronger rather than weaker in the ester. So now we have to move the \u03c3-electrons back again (green arrows below).\"s-cis-ester\"<\/p>\n

This strengthening of the C=O bond arises from the following overlap of the \u03c3-lone pair on the carbonyl oxygen with the alkyl-O-C\u00a0\u03c3* empty orbital, for which E(2) is 41.5 kcal\/mol, much larger than the previous effect. It however does NOT discriminate between the s-cis<\/em> and s-trans<\/em>\u00a0 conformations, since this interaction is almost the same in the latter (41.8). So we have a to-of-(red)-electrons<\/em> which promote the s-cis conformation, and rather stronger fro-of-(green)-electrons<\/em> which strengthen the C=O bond. But they do not cancel each-other; each has its own job to do!<\/p>\n

\"Click

Lp(acyl-O)\/C-O \u03c3* Click for 3D.<\/p><\/div>\n

There is one other overlap which may differentiate between s-cis<\/em> and s-trans<\/em>, but a rather less obvious one. That is the alkyl-O\u03c0<\/sub> donating to the acyl C=O\u03c0*<\/sub> which has E(2) 64.7 for the former and 59.4 kcal\/mol for the latter. It is not immediately apparent why this overlap should favour s-cis<\/em>. It is however the effect that induces a significant rotational barrier about the C-O bond (~12 kcal\/mol).<\/p>\n

\"Click

Lp (alkyl-O \u03c0)\/C=O \u03c0* Click for 3D.<\/p><\/div>\n

Here is the result of another search of the crystal database; \u00a0namely the C=O distance (DIST1) vs the \u00a0C-O distance (DIST2). You can see that the red hot spot (~1400 examples) is very isolated (the blue squares represent < 200 hits), and there seems to be no significant correlation between the two lengths and the structure.<\/p>\n<\/p>\n

\"s-cis-ester-distance\"
I will conclude with a brief discussion of the carbonyl lone pairs. There are two, and one of them has been shown above in the\u00a0Lp(acyl-O)\/C-O \u03c3* interaction. There is another, but it plays no role in the conformation, and is of quite a different character. Although a low-lying orbital, it is clearly non bonding; indeed might be slightly anti-bonding along the C=O axis. These two carbonyl lone pairs are quite different in character, since each performs a different role in the molecule.<\/p>\n

\"Click

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

So the conformational analysis of this simple little molecule reveals some interesting toos-and-fros in the electrons. I will deal with the issue of the carbonyl stretching frequencies in another post.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

I conclude my exploration of conformational preferences by taking a look at esters. As before, I start with a search definition, the ester being restricted to one bearing only sp3 carbon centers. The result of such a search is pretty clear-cut; they all exist in just one conformation, the s-cis, in which a lone pair […]<\/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,"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":[6],"tags":[108,147,1001,1009,1527],"class_list":["post-9572","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-conformational-analysis","tag-energy","tag-large-energy-preference","tag-search-definition","tag-tutorial-material"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-2uo","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9572","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=9572"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/9572\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=9572"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=9572"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=9572"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}