{"id":23712,"date":"2021-05-10T10:15:27","date_gmt":"2021-05-10T09:15:27","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=23712"},"modified":"2021-05-10T10:15:27","modified_gmt":"2021-05-10T09:15:27","slug":"what-does-a-double-%cf%83-bond-along-a-bond-axis-look-like","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=23712","title":{"rendered":"What does a double \u03c3-bond along a bond axis look like?"},"content":{"rendered":"
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Introductory chemistry will tell us that a triple bond between say two carbon atoms comprises just one bond of \u03c3-axial symmetry and two of π-symmetry. Increasingly mentioned nowadays is the possibility of a quadruple bond between carbon and either itself or a transition metal, as discussed in the previous post.<\/a> Such a bond comprises TWO bonds of \u03c3-axial symmetry. Since most people are unfamiliar with such double bonds and in particular with how that second \u03c3-bond sits with the first, I thought it would be interesting to show such an orbital. This one is a localised orbital 41<\/b>, selected from the previous post<\/a> for the molecule (PH3<\/sub>)2<\/sub>(CN)2<\/sub>Mo\u2a78C. <\/p>\n\n\n\n\n\n\n\n\n
NBO 41, threshold 0.040 au<\/th>\nNBO 41, threshold 0.018<\/th>\n<\/tr>\n
\"\"<\/td>\n\"\"<\/td>\n<\/tr>\n
NBO 41, threshold 0.016<\/th>\nNBO 41, threshold 0.014<\/th>\n<\/tr>\n
\"\"<\/td>\n\"\"<\/td>\n<\/tr>\n
NBO 41, threshold 0.012<\/th>\nNBO 41, threshold 0.007<\/th>\n<\/tr>\n
\"\"<\/td>\n\"\"<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The above shows how the orbital changes with the isosurface threshold. At high values, it looks very similar to the normal \u03c3-bond but as the threshold gradually decreases, a second “sheath” starts to surround the inner orbital until the latter is entirely enclosed. This orbital has a node not so much along the bond itself, but between the inner and outer layers of the bond, which is how the two σ-bonds are differentiated. This effect was first noted in 2016<\/a> in terms of the compound CH3<\/sub>F2-<\/sup>, in which an expanded carbon valence shell creates a second σ-bond.<\/p>\n

Certainly not a representation that has ever appeared in a text book I think! But perhaps one that chemists may have increasingly to become familiar with.<\/p>\n

\n

Appendix<\/b> Here are some superimposed orbitals to facilitate comparisons. Firstly orbital 41 (the higher energy σ-orbital) with orbital 22 (the lower energy σ-orbital). The first has yellow\/green for the two phases, the second has red\/blue.
\n\"\"<\/p>\n

Next, σ-orbital 22 (yellow\/green) with orbital 42 (red\/blue) surrounding it, revealing the avoided overlaps (Pauli repulsions) between the two by virtue of having orbital 42 unoccupied. <\/p>\n

\"\"<\/p>\n

Next, σ-orbital 41 (yellow\/green) with orbital 42 (red\/blue) surrounding it, revealing the reduced overlap between these two. <\/p>\n

\"\"<\/p>\n

Appendix 2<\/b> A “pure” form of the double-layered σ-bond can be seen with the diatomic molecule Ti2<\/sub>, contoured at 0.0225 au. The red phase is about to join in the middle.<\/p>\n

\"\"<\/p>\n

The electron density from this orbital is shown below and shows clearly the two layers of density comprising the σ-bond, with the outer layer at this isosurface value (0.00052 au) about to join up in the middle to complete the outer sheath. I have left it unjoined so that you can see “inside layer”, since translucency does not always get the message across.<\/p>\n

\"\"<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Introductory chemistry will tell us that a triple bond between say two carbon atoms comprises just one bond of \u03c3-axial symmetry and two of π-symmetry. Increasingly mentioned nowadays is the possibility of a quadruple bond between carbon and either itself or a transition metal, as discussed in the previous post. Such a bond comprises TWO […]<\/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-23712","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-6as","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/23712","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=23712"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/23712\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=23712"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=23712"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=23712"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}