{"id":9894,"date":"2013-03-21T13:12:48","date_gmt":"2013-03-21T13:12:48","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=9894"},"modified":"2013-03-21T13:12:48","modified_gmt":"2013-03-21T13:12:48","slug":"to-be-cyclobutadiene-or-not-to-be-that-is-the-question-you-decide","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2013\/03\/21\/to-be-cyclobutadiene-or-not-to-be-that-is-the-question-you-decide\/","title":{"rendered":"To be cyclobutadiene, or not to be, that is the question? You decide."},"content":{"rendered":"

A quartet of articles has recently appeared on the topic of cyclobutadiene.[cite]10.1002\/chem.201102942[\/cite],[cite]10.1002\/chem.201103017[\/cite],[cite]10.1002\/chem.201203234[\/cite],[cite]10.1002\/chem.201203235[\/cite]. You will find a great deal discussed there, but I can boil it down to this essence. Do the following coordinates (obtained from a (disordered) previously<\/a> published[cite]10.1126\/science.1188002[\/cite] x-ray refinement) correspond to a van der Waals complex<\/em><\/strong> of 1,3-dimethyl cyclobutadiene and carbon dioxide, or do they instead represent a covalent interaction<\/strong><\/em> between these two components resulting in a compound with the chemical name 2-oxabicyclo[2.2.0]hex-5-en-3-one (i.e.<\/em> not a cyclobutadiene)?<\/p>\n

\"Click
Click for 3D. The unconnected atoms are the result of disordered partial occupancy.<\/figcaption><\/figure>\n

The two bonds to concentrate on are shown in gold; a O…C pair with a distance of 1.61\u00c5 as obtained from the x-ray refinement and a C…C pair with a distance of 1.5\u00c5 (and if you want to go further, the O=C=O bond angle). I list below values obtained from the wonderful Webelements<\/a> site. Using these values, this makes a van der Waals O…C contact 3.22\u00c5 and a C…C contact 3.40\u00c5 and covalent values of respectively 1.38\u00c5 and 1.5\u00c5.<\/p>\n\n\n\n\n\n
Element<\/td>\nCovalent radius, \u00c5<\/td>\nvan der Waals radius<\/td>\n<\/tr>\n
C<\/td>\n0.75<\/td>\n1.70<\/td>\n<\/tr>\n
O<\/td>\n0.63<\/td>\n1.52<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

According to chemistry convention, we classify the interaction between a pair of atoms according to which category best fits the observed distance. So this should allow you to decide if the molecule is a van der Waals complex of 1,3-dimethyl cyclobutadiene and carbon dioxide or the covalent system 2-oxabicyclo[2.2.0]hex-5-en-3-one.\u00a0<\/p>\n

Oh, if the observed\u00a0O…C pair with a distance of 1.61\u00c5 does not seem to perfectly fit either category above, one of the quartet of articles above[cite]10.1002\/chem.201102942[\/cite] offers the explanation of an unusual\u00a0\u03c0-anomeric effect lengthening the C…O bond in 2-oxabicyclo[2.2.0]hex-5-en-3-one\u00a0slightly beyond the standard covalent distance. Of course, if the system were to be a van der Waals complex, that explanation cannot apply.<\/p>\n","protected":false},"excerpt":{"rendered":"

A quartet of articles has recently appeared on the topic of cyclobutadiene.[cite]10.1002\/chem.201102942[\/cite],[cite]10.1002\/chem.201103017[\/cite],[cite]10.1002\/chem.201203234[\/cite],[cite]10.1002\/chem.201203235[\/cite]. You will find a great deal discussed there, but I can boil it down to this essence. Do the following coordinates (obtained from a (disordered) previously published[cite]10.1126\/science.1188002[\/cite] x-ray refinement) correspond to a van der Waals complex of 1,3-dimethyl cyclobutadiene and carbon dioxide, or […]<\/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":[696,709,2556,2627],"class_list":["post-9894","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-crystallography","tag-cyclobutadiene","tag-waals-complex","tag-x-ray"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/9894","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=9894"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/9894\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=9894"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=9894"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=9894"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}