{"id":18632,"date":"2017-07-18T09:03:11","date_gmt":"2017-07-18T08:03:11","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=18632"},"modified":"2017-07-18T09:03:11","modified_gmt":"2017-07-18T08:03:11","slug":"dispersion-bonds-not-involving-just-hydrogen-can-it-work-for-f-h","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=18632","title":{"rendered":"Dispersion \u201cbonds\u201d not involving just hydrogen: can it work for F…H?"},"content":{"rendered":"
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The effects of loading up lots of dispersion attractions (between t-butyl groups) into a compact molecule has the interesting consequence of allowing two “non-bonded” hydrogen atoms to approach to ~1.5\u00c5 of each other, thus creating the appearance of a “bond” where one normally would not be found. Can such an effect be injected into other combinations of two atoms, say H and F? Here I briefly explore this notion.<\/p>\n

The system is a slightly modified version of the one[1]<\/a><\/span> already studied; R3<\/sub>C-F…H-CR3<\/sub> (R=3,5-bis-t-butylphenyl), and a B3LYP+D3BJ\/6-311G(d,p) calculation\u2021<\/sup> (with C3<\/sub>-symmetry imposed) shows (DOI: 10.14469\/hpc\/2734<\/a>)\u00a0the following, with the key atom pair distances shown below. Note the abnormally short F…H distance, and the relatively long C-F one.<\/p>\n

\"\"<\/p>\n

Note the casual phrase “C3<\/sub>-symmetry imposed”. This is a little “shortcut” one can try to use to shorten the calculation time. I should explain that on our computer system here, we are allowed a maximum of 72 hours per calculation. I already suspected that without the use of such symmetry the calculation would take longer and so used symmetry to “fit the calculation in” to this time slot. In the event it took 55 hours. There is a simple test however to see if this shortcut is justified; does the resulting molecule have 3N-6 real normal vibrational modes (i.e. ones with +ve force constants)? In fact this system fails this test; two of these modes have small negative force constants, corresponding to\u00a0\u03bd -11 cm-1<\/sup>. You might think this is small enough to perhaps attribute to e.g.<\/em> the use of too-small a basis set or some other computational imperfection. Actually, although\u00a0-11 cm-1<\/sup> is numerically small, the mass-weighting associated with the vibration is in effect the entire system (below) and hence this mode is indeed significant.<\/p>\n

\"\"<\/p>\n

So time to release the symmetry and when one does this an entirely different geometry emerges (DOI:\u00a010.14469\/hpc\/2736<\/a>) for which now all the 3N-6 normal modes have +ve force constants.<\/p>\n

\"\"<\/p>\n

The “non-bonded” F…H interaction is now considerably longer, although still ~0.5\u00c5 shorter than the sum of the van der Waals radii (~2.65\u00c5). This F…H non-bonded distance shows up as below in the Cambridge structure database (CSD) distribution. This suggests the shortest interaction is indeed ~2.1\u00c5. The string of isolated examples with shorter distances down to < 1\u00c5 are very likely all crystallographic artefacts or errors.<\/p>\n

\"\"<\/p>\n

So we may conclude that using the same system that was so successfully used to demonstrate the dispersion-induced ultra-short H…H distance cannot be modified to produce any such extreme effects in the F…H pair. \u00a0Perhaps indeed “dispersion bonds” will always be limited to \u00a0H…H pairs.<\/p>\n

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\u2021<\/sup> When this method is used for the original H…H system, it yields a H…H distance of 1.529\u00c5 for which all the normal vibrational modes are real; DOI: 10.14469\/hpc\/2739<\/a>).<\/p>\n

References<\/h2>\n
  1. S. R\u00f6sel, H. Quanz, C. Logemann, J. Becker, E. Mossou, L. Ca\u00f1adillas-Delgado, E. Caldeweyher, S. Grimme, and P.R. Schreiner, \"London Dispersion Enables the Shortest Intermolecular Hydrocarbon H\u00b7\u00b7\u00b7H Contact\", Journal of the American Chemical Society<\/i>, vol. 139, pp. 7428-7431, 2017. https:\/\/doi.org\/10.1021\/jacs.7b01879<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The effects of loading up lots of dispersion attractions (between t-butyl groups) into a compact molecule has the interesting consequence of allowing two “non-bonded” hydrogen atoms to approach to ~1.5\u00c5 of each other, thus creating the appearance of a “bond” where one normally would not be found. Can such an effect be injected into other […]<\/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":[1526],"class_list":["post-18632","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-interesting-chemistry"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-4Qw","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/18632","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=18632"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/18632\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=18632"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=18632"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=18632"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}