{"id":21846,"date":"2020-01-02T10:05:36","date_gmt":"2020-01-02T10:05:36","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=21846"},"modified":"2020-01-02T10:05:36","modified_gmt":"2020-01-02T10:05:36","slug":"comment-on-resolving-the-quadruple-bonding-conundrum-in-c2-using-insights-derived-from-excited-state-potential-energy-surfaces-the-7%cf%83-heptet-excited-states-for-related-molecules","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=21846","title":{"rendered":"Comment on “Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces”: The 7\u03a3 heptet excited states for related molecules."},"content":{"rendered":"
\n

I noted in an earlier blog<\/a>, a potential (if difficult) experimental test of the properties of the singlet state of dicarbon, C2<\/sub>. Now, just a few days ago, a ChemRxiv article has been published suggesting another (probably much more realistic) test.[1]<\/a><\/span>\u00a0This looks at the so-called 7<\/sup>\u03a3 open shell state of the molecule where three electrons from one \u03c3 and two \u03c0 orbitals are excited into the corresponding \u03c3*<\/sup> and \u03c0*<\/sup> unoccupied orbitals.\u00a0The argument is presented that these states are not dissociative, showing a deep minimum and hence a latent quadruple bonding nature<\/i><\/span>. They also note that the isoelectronic BN molecule IS dissociative.\u2020<\/sup> Thus to quote: “Hence, the proof of existence of a minimum in the 7<\/sup>\u03a3u+<\/sub> for C2<\/sub> and the absence of such a minimum in the equivalent case for BN is likely to corroborate our findings on quadruple bonding in these two cases.<\/span>“<\/i><\/p>\n

Although a PES (potential energy surface) is shown for 7<\/sup>\u03a3 C2<\/sub>, no vibrational wavenumber is reported. So in the spirit of a commentary on this pre-print, I have calculated these values (CCSD(T)\/Def2-TZVPP) for the molecules noted in the article and a few other isoelectronic species. The results are collected at\u00a0DOI: 10.14469\/hpc\/6599<\/a><\/p>\n\n\n\n\n\n
Property<\/th>\nBeC2-<\/sup><\/th>\nBB2-<\/sup><\/th>\nBC1-<\/sup><\/th>\nBN<\/th>\nCC<\/th>\nCN1+<\/sup><\/th>\nNN2+<\/sup><\/th>\nBeO<\/th>\nBO1+<\/sup><\/th>\nLiF<\/th>\n<\/tr>\n
Bond length,
\n\u00c5<\/td>\n		2.281<\/td>\n1.858<\/td>\n1.768<\/td>\n3.216<\/td>\n1.585<\/td>\n1.638<\/td>\n1.523<\/td>\n2.602<\/td>\n200.7<\/td>\n2.179<\/td>\n<\/tr>\n
Bond stretch,
\ncm-1<\/sup><\/td>\n		182<\/td>\n638<\/td>\n699<\/td>\n48<\/td>\n1055<\/td>\n759<\/td>\n934<\/td>\n363<\/td>\n0.4<\/td>\n626\u2021<\/sup><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The two singly occupied \u03c3-orbitals are shown below and are in part responsible for the non-dissociative behaviour. \"\"<\/a> \"\"<\/a><\/p>\n

The results using this method (the article reports many more methods), reveal that C2<\/sub> does indeed exist in a minimum for the heptet state, whilst the isoelectronic BN is only very weakly bound, in effect dissociative. Another dissociative isoelectronic is BO1+<\/sup>, whilst N2<\/sub>2+<\/sup> despite the coulombic repulsions of a double positive charge, still manages to be bound with a relatively short bond length. A further neutral diatomic BeO is also clearly non-dissociative.<\/p>\n

Lets hope a spectroscopist somewhere tries to take a look at these heptet excited states of such molecules so that further experimental insight can be cast on this fascinating problem.<\/p>\n

\n

\u2021<\/sup>The orbital occupancy of this species is different from the others. \u2020<\/sup>As are the neutral ten valence electron N\u2261N (confirmed here<\/a>) and HC\u2261CH.<\/p>\n

References<\/h2>\n
  1. I. Bhattacharjee, D. Ghosh, and A. Paul, \"Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces: A Molecular Orbital Perspective\", 2019. https:\/\/doi.org\/10.26434\/chemrxiv.11446224.v1<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    I noted in an earlier blog, a potential (if difficult) experimental test of the properties of the singlet state of dicarbon, C2. Now, just a few days ago, a ChemRxiv article has been published suggesting another (probably much more realistic) test.\u00a0This looks at the so-called 7\u03a3 open shell state of the molecule where three electrons […]<\/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-21846","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-5Gm","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21846","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=21846"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21846\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=21846"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=21846"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=21846"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}