{"id":22996,"date":"2020-11-07T14:54:01","date_gmt":"2020-11-07T14:54:01","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=22996"},"modified":"2020-11-07T14:54:01","modified_gmt":"2020-11-07T14:54:01","slug":"an-interesting-aromatic-molecule-found-intitans-atmosphere-cyclopropylidene","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2020\/11\/07\/an-interesting-aromatic-molecule-found-intitans-atmosphere-cyclopropylidene\/","title":{"rendered":"An interesting aromatic molecule found in Titan&#8217;s atmosphere: Cyclopropenylidene"},"content":{"rendered":"<p>Cyclopropenylidene must be the smallest molecule to be aromatic due to \u03c0-electrons, with just three carbon atoms and two hydrogen atoms. It has now been detected in the atmosphere of Titan, one of Saturn&#8217;s moons[cite]10.3847\/1538-3881\/abb679[\/cite] and joins benzene, another aromatic molecule together with the protonated version of cyclopropenylidene, C<sub>3<\/sub>H<sub>3<\/sub><sup>+<\/sup> also found there.<\/p>\n<p><a href=\"https:\/\/rzepa.net\/blog\/wp-content\/uploads\/2020\/11\/cp.svg\"><img decoding=\"async\" class=\"aligncenter size-large wp-image-23009\" src=\"https:\/\/rzepa.net\/blog\/wp-content\/uploads\/2020\/11\/cp.svg\" alt=\"\" width=\"100\" \/><\/a><\/p>\n<p>The molecule has two \u03c0-electrons in the three membered ring and a carbene lone pair in the \u03c3-framework. As with the cyclopropenium cation (C<sub>3<\/sub>H<sub>3<\/sub><sup>+<\/sup>), these two electrons make it \u03c0-aromatic, as indicated by H\u00fcckel&#8217;s 4n+2 rule (n=0). I thought it might be fun to show the molecular orbitals containing these two pairs of electrons and then to show the result of a double excitation of the carbene lone pair into the \u03c0-system to make a anti-aromatic isomer with four \u03c0-electrons. This species is a whopping 209.3 kcal\/mol higher in free energy, made up of the double electronic excitation energy topped up by conversion of the stabilizing aromaticity into destabilizing anti-aromaticity. Because of this antiaromaticity, the excited state is in fact a second order saddle point, <a href=\"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=2973\" target=\"_blank\" rel=\"noopener noreferrer\">avoiding anti-aromaticity<\/a> by asymmetric distortion back down to the ground state and resymmetrisation.<\/p>\n<table border=\"1\">\n<tbody>\n<tr>\n<th colspan=\"2\">Ground state of Cyclopropenylidene<\/th>\n<\/tr>\n<tr>\n<td>\n<p><figure id=\"attachment_22976\" aria-describedby=\"caption-attachment-22976\" style=\"width: 250px\" class=\"wp-caption aligncenter\"><img decoding=\"async\" class=\"size-medium wp-image-22976\" src=\"https:\/\/rzepa.net\/blog\/wp-content\/uploads\/2020\/11\/cp2-10.jpg\" alt=\"\" width=\"250\" \/><figcaption id=\"caption-attachment-22976\" class=\"wp-caption-text\">Click to view 3D model of NBO 10<\/figcaption><\/figure><\/td>\n<td>\n<p><figure id=\"attachment_22975\" aria-describedby=\"caption-attachment-22975\" style=\"width: 250px\" class=\"wp-caption aligncenter\"><img decoding=\"async\" class=\"size-medium wp-image-22975\" src=\"https:\/\/rzepa.net\/blog\/wp-content\/uploads\/2020\/11\/cp2-8.jpg\" alt=\"\" width=\"250\" \/><figcaption id=\"caption-attachment-22975\" class=\"wp-caption-text\">Click to view 3D model of NBO 8<\/figcaption><\/figure><\/td>\n<\/tr>\n<tr>\n<th colspan=\"2\">Doubly excited anti-aromatic state of Cyclopropenylidene<\/th>\n<\/tr>\n<tr>\n<td>\n<p><figure id=\"attachment_22974\" aria-describedby=\"caption-attachment-22974\" style=\"width: 250px\" class=\"wp-caption aligncenter\"><img decoding=\"async\" class=\"size-medium wp-image-22974\" src=\"https:\/\/rzepa.net\/blog\/wp-content\/uploads\/2020\/11\/cp1-10.jpg\" alt=\"\" width=\"250\" \/><figcaption id=\"caption-attachment-22974\" class=\"wp-caption-text\">Click to view 3D model of NBO 10<\/figcaption><\/figure><\/td>\n<td>\n<p><figure id=\"attachment_22973\" aria-describedby=\"caption-attachment-22973\" style=\"width: 250px\" class=\"wp-caption aligncenter\"><img decoding=\"async\" class=\"size-large wp-image-22973\" src=\"https:\/\/rzepa.net\/blog\/wp-content\/uploads\/2020\/11\/cp1-8.jpg\" alt=\"\" width=\"250\" \/><figcaption id=\"caption-attachment-22973\" class=\"wp-caption-text\">Click to view 3D model of NBO 8<\/figcaption><\/figure><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>It might be a tiny molecule, but to chemists at least it is very interesting in a historical sense at least. Curiously, the astrophysicists describe it as a &#8220;complex molecule&#8221;!<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Cyclopropenylidene must be the smallest molecule to be aromatic due to \u03c0-electrons, with just three carbon atoms and two hydrogen atoms. It has now been detected in the atmosphere of Titan, one of Saturn&#8217;s moons[cite]10.3847\/1538-3881\/abb679[\/cite] and joins benzene, another aromatic molecule together with the protonated version of cyclopropenylidene, C3H3+ also found there. The molecule has [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11],"tags":[],"class_list":["post-22996","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/22996","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=22996"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/22996\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=22996"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=22996"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=22996"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}