{"id":21009,"date":"2019-06-02T13:52:10","date_gmt":"2019-06-02T12:52:10","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=21009"},"modified":"2019-06-02T13:52:10","modified_gmt":"2019-06-02T12:52:10","slug":"diatomics-with-eight-valence-electrons-formation-by-radioactive-decay","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=21009","title":{"rendered":"Diatomics with eight valence-electrons: formation by radioactive decay."},"content":{"rendered":"
\n

This is a follow up to my earlier post<\/a> about C\u2a78N+<\/sup>, itself inspired by this ChemRxiv pre-print[1]<\/a><\/span> which describes a chemical synthesis of singlet biradicaloid C2<\/sub> and its proposed\u00a0identification as such by chemical trapping.<\/p>\n

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

First row diatomics based on the iso-electronic principle of eight valence electrons include both C\u2a78N+<\/sup> and C\u2a78C, as well as species such as B\u2a78N, C\u2a78O2+<\/sup>\u00a0and even the unlikely N\u2a78O3+<\/sup>. The diatomic bond is represented here by \u2a78<\/span> which carries the message of six electrons pairing to form a conventional triple bond and the remaining two valence electrons more weakly spin-pairing to form overall a singlet biradicaloid species with a quadruple bond. The “BDE” (bond dissociation energy) of the 4th pair is around 20 kcal\/mol for C\u2a78C,[2]<\/a><\/span> \u00a0which arguably entitles it to be called a weak bond.[3]<\/a><\/span><\/p>\n

Here I am going to explore –<\/sup>B\u2a78N+<\/sup>\u00a0and isoelectronic C\u2a78C via<\/em>\u00a0formation by radioactive decay of tritium into helium (Table, FAIR data DOI: 10.14469\/hpc\/5691<\/a>).<\/p>\n\n\n\n\n\n\n
Entry<\/th>\nsystem<\/th>\n\u0394\u0394G<\/th>\n\u0394\u0394H<\/th>\n<\/tr>\n
1<\/td>\n[Li-C\u2261C-T\u00a0<\/sup>\u2192] Li-C\u2261C-He+<\/sup>\u00a0+ e \u2192 Li+<\/sup>\u00a0+ C\u2a78C + He\u00a0<\/strong><\/td>\n-44.9<\/td>\n-27.6<\/td>\n<\/tr>\n
2<\/td>\n[(-)C\u2261C-T\u00a0<\/sup>\u2192] (-)C\u2261C-He+<\/sup>\u00a0+ e \u2192 C\u2a78C + He\u00a0<\/strong><\/td>\n-42.2<\/td>\n-31.9<\/td>\n<\/tr>\n
3<\/td>\n[Li-N\u2261B-T\u00a0<\/sup>\u2192] Li-N\u2261B-He+<\/sup>\u00a0+ e \u2192 Li+<\/sup>\u00a0+ –<\/sup>B\u2a78N+<\/sup> + He\u00a0<\/strong><\/td>\n-9.0<\/td>\n+2.9<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The thermochemistry includes a significant contribution from entropy, which favours the reaction. At its simplest, this involves the replacement of a X-He (X=C,B) bond by the 4th C\u2a78X bond. The BDEs (bond dissociation energies) of the X-He bond are very small (< 20 kcal\/mol) and hence the reaction is driven largely by the enthalpy of forming the final C\u2a78X bond, together with entropy increase. Contrast this with the reaction reported above involving cleavage of a C–<\/span>IPh bond,[1]<\/a><\/span> where the C–<\/span>I BDE is\u00a0larger<\/a> (~70-80 kcal\/mol; > 20 kcal\/mol). This makes the reported trapping of C2<\/sub> from this reaction all the more intriguing.<\/p>\n

References<\/h2>\n
  1. K. Miyamoto, S. Narita, Y. Masumoto, T. Hashishin, M. Kimura, M. Ochiai, and M. Uchiyama, \"Room-Temperature Chemical Synthesis of C2\", 2019. https:\/\/doi.org\/10.26434\/chemrxiv.8009633.v1<\/a>\n\n<\/li>\n
  2. D. Danovich, P.C. Hiberty, W. Wu, H.S. Rzepa, and S. Shaik, \"The Nature of the Fourth Bond in the Ground State of C<sub>2<\/sub>: The Quadruple Bond Conundrum\", Chemistry \u2013 A European Journal<\/i>, vol. 20, pp. 6220-6232, 2014. https:\/\/doi.org\/10.1002\/chem.201400356<\/a>\n\n<\/li>\n
  3. S. Shaik, D. Danovich, B. Braida, and P.C. Hiberty, \"The Quadruple Bonding in C<sub>2<\/sub> Reproduces the Properties of the Molecule\", Chemistry \u2013 A European Journal<\/i>, vol. 22, pp. 4116-4128, 2016. https:\/\/doi.org\/10.1002\/chem.201600011<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    This is a follow up to my earlier post about C\u2a78N+, itself inspired by this ChemRxiv pre-print which describes a chemical synthesis of singlet biradicaloid C2 and its proposed\u00a0identification as such by chemical trapping. First row diatomics based on the iso-electronic principle of eight valence electrons include both C\u2a78N+ and C\u2a78C, as well as species […]<\/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-21009","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-5sR","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21009","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=21009"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21009\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=21009"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=21009"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=21009"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}