{"id":21346,"date":"2019-10-03T10:04:07","date_gmt":"2019-10-03T09:04:07","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=21346"},"modified":"2019-10-03T10:04:07","modified_gmt":"2019-10-03T09:04:07","slug":"bond-length-alternation-bla-in-large-conjugated-rings-an-update","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=21346","title":{"rendered":"Bond length alternation (BLA) in large conjugated rings: an (anti-aromatic) update."},"content":{"rendered":"
\n

In the previous post<\/a>, I looked at a class of molecule known as hexaphyrins, inspecting bond length alternation (BLA) at the so-called meso<\/em> position, the carbon atom joining two pyrrole rings. A search of the difference in bond lengths at this position had shown two significant clusters of crystal structures.
\n\"\"<\/a>Molecules in the bottom left of this diagram shows little or no bond length alternation. The right middle shows another cluster with more extreme (and unequal) bond length alternation. I have selected one molecule from this cluster, EGIJEK and it differs from EGIHUY in having four NH units in the ring, whereas the latter has only two.<\/p>\n

\"\"<\/a>

EGIJEK<\/p><\/div>

\"\"<\/a>

EGIHUY<\/p><\/div><\/p>\n

The effect this has is illustrated below. A pyrrole with an NH group contributes two\u00a0\u03c0-electrons to the conjugated periphery whereas a pyrrole with just an N contributes only one. Thus the four NH rings in EGIJEK contribute overall two more \u03c0-electrons to the periphery than EGIHUY with just two NH rings. This increases the \u03c0-electron count from 26 to 28, driving EGIJEK from a 4n+2 into a 4n \u03c0-electron count (n=7) and making it formally anti-aromatic. This in turn induces strong bond length alternation (as for example in cyclobutadiene).<\/p>\n

\"\"<\/a><\/p>\n

Here is a reprise of the bond length table shown in the previous post, but for\u00a0EGIJEK.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Meso distances, \u00c5<\/th>\nabs(\u0394r)<\/th>\n<\/tr>\n
EGIJEK crystal, Ci<\/sub> symmetry, DOI: 10.5517\/ccrts2d<\/a><\/td>\n<\/tr>\n
1,43657<\/td>\n1.37034<\/td>\n0.06623<\/td>\n<\/tr>\n
1.35661<\/td>\n1.44895<\/td>\n0.09234<\/td>\n<\/tr>\n
1.42356<\/td>\n1.37287<\/td>\n0.05069<\/td>\n<\/tr>\n
B3LYP+GD3BJ\/Def2-SVPP (FAIR DOI: 10.14469\/hpc\/6194<\/a>)<\/td>\n<\/tr>\n
1.44019<\/td>\n1.38259<\/td>\n0.0576<\/td>\n<\/tr>\n
1.37732<\/td>\n1.44481<\/td>\n0.06749<\/td>\n<\/tr>\n
1.43261<\/td>\n1.38568<\/td>\n0.04693<\/td>\n<\/tr>\n
\u03c9B97XD\/Def2-SVPP (FAIR DOI: 10.14469\/hpc\/6194<\/a>\u00a0)<\/td>\n<\/tr>\n
1.44575<\/td>\n1.37537<\/td>\n0.07038<\/td>\n<\/tr>\n
1.36214<\/td>\n1.45508<\/td>\n0.09294<\/td>\n<\/tr>\n
1.44221<\/td>\n1.36958<\/td>\n0.07263<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Of the two functionals used in the calculations, the \u03c9B97XD form slightly over-estimates the BLA, with the B3LYP slightly under-estimating it. This seems to tally with the earlier observations<\/a> made for cyclo[n]carbons.<\/p>\n

Perhaps these sorts of molecules might form useful reality checks on calculating bond length alternations in large ring cyclic conjugated molecules.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

In the previous post, I looked at a class of molecule known as hexaphyrins, inspecting bond length alternation (BLA) at the so-called meso position, the carbon atom joining two pyrrole rings. A search of the difference in bond lengths at this position had shown two significant clusters of crystal structures. Molecules in the bottom left […]<\/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-21346","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-5yi","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21346","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=21346"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21346\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=21346"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=21346"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=21346"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}