{"id":29429,"date":"2025-08-18T10:50:36","date_gmt":"2025-08-18T09:50:36","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29429"},"modified":"2025-08-23T08:58:55","modified_gmt":"2025-08-23T07:58:55","slug":"the-even-more-mysterious-n%e2%89%a1n-triple-bond-in-a-nitric-oxide-dimer","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29429","title":{"rendered":"The even more mysterious N\u2261N triple bond in a nitric oxide dimer."},"content":{"rendered":"
\n

Previously, I pondered about the strange N=N double bond in nitrosobenzene dimer[1]<\/a><\/span> as a follow up to commenting on the curly arrow mechanism of the dimerisation.[2]<\/a><\/span> By the same curly arrow method, one can produce the below, showing how the simpler nitric oxide radical could potentially dimerise to a species with a N\u2261N triple bond!†<\/sup>\u00a0This involves a total of six electrons entering the N-N region, and hence raises the question of whether these all move in a single concerted\/synchronous bond forming reaction, or whether they might go in (asynchronous) stages. Here are some calculations[3]<\/a><\/span>) which might shed some light on this aspect.<\/p>\n

\"\"<\/p>\n

The structure[4]<\/a><\/span>\u00a0of a nitric oxide dimer was shown in 1982 to have a very long (rather than short) N-N bond length of 2.237\u00c5 and a theoretical analysis[5]<\/a><\/span> showed it to be a weak complex with a very complex wavefunction showing multi-reference character. <\/p>\n

Firstly, an IRC-based reaction path (method: u\u03c9B97XD, scrf=(cpcm,solvent=water) guess=(mix,always) def2tzvpp to allow either an open shell biradical to form and also to encourage any ion pair formation). As you can see, the (total) energy goes up to a very \u00a0shallow transition state (with a tiny reverse barrier) to form a biradical \u00a0with <S2<\/sup>> 0.628. This species, as noted existing in a very shallow energy well, has an\u00a0N-N bond length of 1.725\u00c5.
\n\"\"<\/a><\/p>\n

\"\"<\/p>\n

The bonding for this species is complex (analysis for a later post), but the\u00a0calculated biradical spin density below shows the unpaired electrons are in the \u03c0-system (click on the image to get a 3D rotatable model).<\/p>\n

\"\"<\/p>\n

Further contraction of the\u00a0N-N length results in an IRC energy potential to a transition state with a N-N length 1.294\u00c5 across a further barrier of\u00a0~12 kcal\/mol (\u0394E; \u0394G 13.6 kcal\/mol). The overall barrier from two nitric oxide molecules is \u0394G 31.0 kcal\/mol\u00a0with the overall thermochemistry summarised in the table. Basically, this barrier is unsurmountable at normal temperatures and the reverse barrier of \u0394G 6.7 kcal\/mol ensures that the N\u2261N triple bonded species shown above is not likely stable and will not be observed experimentally.‡<\/sup> However this product is NOT a biradical but a normal closed shell singlet molecule.[6]<\/a><\/span><\/p>\n

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

\"\"<\/p>\n

So to answer my first question, the six electrons appear to move in two stages, firstly two electrons form a weak N-N bond and then a further four electrons contract this to a triple bond. Their motion is effectively concerted, but asynchronous.<\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n\n\n\n\n\n\n\n\n
Species<\/th>\n\u0394G<\/th>\n\u0394H<\/th>\n\u0394\u0394G<\/th>\n\u0394\u0394H<\/th>\nT.\u0394S<\/th>\nrNN<\/sub>, \u00c5<\/td>\n<S2<\/sup>><\/td>\nDOI<\/td>\n<\/tr>\n
2*Nitric oxide<\/td>\n-259.83494<\/td>\n-259.78839<\/td>\n0.0<\/td>\n0.0<\/td>\n29.2<\/td>\n\u221e<\/td>\n0.753<\/td>\n15472<\/a>[7]<\/a><\/span><\/td>\n<\/tr>\n
Singlet biradical<\/td>\n-259.80716<\/td>\n-259.77615<\/td>\n17.4<\/td>\n7.7<\/td>\n19.5<\/td>\n1.725<\/td>\n0.628<\/td>\n15476<\/a>[8]<\/a><\/span><\/td>\n<\/tr>\n
Triplet biradical<\/td>\n-259.80865<\/td>\n-259.77672<\/td>\n16.5<\/td>\n7.3<\/td>\n20.0<\/td>\n1.779<\/td>\n2.016<\/td>\n15475<\/a>[9]<\/a><\/span><\/td>\n<\/tr>\n
Singlet TS<\/td>\n-259.78550<\/td>\n-259.75579<\/td>\n31.0 (13.6)<\/td>\n20.5 (12.8)<\/td>\n18.6<\/td>\n1.294<\/td>\n0.000<\/td>\n15483<\/a>[3]<\/a><\/span><\/td>\n<\/tr>\n
Singlet N\u2261N dimer<\/td>\n-259.79614<\/td>\n-259.76693<\/td>\n24.3 \u00a0(7.8)<\/td>\n13.5<\/td>\n18.3<\/td>\n1.114<\/td>\n0.000<\/td>\n15467<\/a>[10]<\/a><\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Now for a NEDA energy decomposition analysis[11]<\/a><\/span><\/p>\n

Electrical (ES+POL+SE) :  -9414.608\r\n   Charge Transfer (CT) :  -1363.597\r\n       Core (XC+DEF-SE) :  10782.725                      \r\n  Total Interaction (E) :      4.520 kcal\/mol.<\/tt><\/pre>\n
Normally NEDA total interaction energies are -ve, but this one is positive! So the triple bond dissociation energy is not merely small, but actually negative. That is a weak triple bond and as the title implies, a very mysterious bond. In some aspects however it is conventional. Thus calculated rNN<\/sub> 1.114\u00c5 and \u03bdNN<\/sub> 2604 cm-1<\/sup>.\u00a0However partial occupancies of NBO antibonding BD*<\/tt> orbitals results in a calculated Wiberg bond order of only 1.01; there is still a great deal of mystery left about this species! Probably what is fairly certain is that the closed shell single-reference wavefunction used here is not appropriate for a full explanation and more complex multi-reference procedures would have to be used to get a more complete picture of this strange non-existing‡<\/sup> little molecule. It may even be that such procedures remove the small reverse barrier noted above, thus preventing the molecule from even existing in an energy well.<\/p>\n
\n
†<\/sup>This species does not appear to have been previously discussed or suggested, according to SciFinder\/CAS.
\n‡<\/sup>Might it exist at very high pressures in water?<\/p>\n
\n
To find all blog posts authored here, along with their  DOIs,  try https:\/\/rogue-scholar.org\/search?q=orcid:0000-0002-8635-8390&sort=newest<\/a><\/p>\n
References<\/h2>\n    
  1. H. Rzepa, \"The mysterious N=N double bond in nitrosobenzene dimer.\", 2025. https:\/\/doi.org\/10.59350\/rzepa.29383<\/a>\n\n<\/li>\n
  2. H. Rzepa, \"Mechanism of the dimerisation of Nitrosobenzene.\", 2025. https:\/\/doi.org\/10.59350\/rzepa.28849<\/a>\n\n<\/li>\n
  3. H. Rzepa, \"N2O2 as strong dimer TS as biradical cis, G = -259.785500\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15483<\/a>\n\n<\/li>\n
  4. S.G. Kukolich, \"The structure of the nitric oxide dimer\", Journal of the American Chemical Society<\/i>, vol. 104, pp. 4715-4716, 1982. https:\/\/doi.org\/10.1021\/ja00381a052<\/a>\n\n<\/li>\n
  5. N. Taguchi, Y. Mochizuki, T. Ishikawa, and K. Tanaka, \"Multi-reference calculations of nitric oxide dimer\", Chemical Physics Letters<\/i>, vol. 451, pp. 31-36, 2008. https:\/\/doi.org\/10.1016\/j.cplett.2007.11.084<\/a>\n\n<\/li>\n
  6. H. Rzepa, \"N2O2 as strong dimer? G = -259.796140, STABLE wavefunction!\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15474<\/a>\n\n<\/li>\n
  7. H. Rzepa, \"Nitric oxide monomer, G = -129.917471 *2 = -259.834942\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15472<\/a>\n\n<\/li>\n
  8. H. Rzepa, \"N2O2 as strong dimer singlet trans biradical state G = -259.807165\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15476<\/a>\n\n<\/li>\n
  9. H. Rzepa, \"N2O2 as strong dimer triplet state G = -259.808649 DG 16.5\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15475<\/a>\n\n<\/li>\n
  10. H. Rzepa, \"N2O2 as strong dimer? bent G = -259.796140\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15467<\/a>\n\n<\/li>\n
  11. E.D. Glendening, and A. Streitwieser, \"Natural energy decomposition analysis: An energy partitioning procedure for molecular interactions with application to weak hydrogen bonding, strong ionic, and moderate donor\u2013acceptor interactions\", The Journal of Chemical Physics<\/i>, vol. 100, pp. 2900-2909, 1994. https:\/\/doi.org\/10.1063\/1.466432<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"
    Previously, I pondered about the strange N=N double bond in nitrosobenzene dimer as a follow up to commenting on the curly arrow mechanism of the dimerisation. By the same curly arrow method, one can produce the below, showing how the simpler nitric oxide radical could potentially dimerise to a species with a N\u2261N triple bond!†\u00a0This […]<\/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,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"federated","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":[4],"tags":[],"ppma_author":[2661],"class_list":["post-29429","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry"],"yoast_head":"\nThe even more mysterious N\u2261N triple bond in a nitric oxide dimer. - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29429\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The even more mysterious N\u2261N triple bond in a nitric oxide dimer. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Previously, I pondered about the strange N=N double bond in nitrosobenzene dimer as a follow up to commenting on the curly arrow mechanism of the dimerisation. 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not the known weak dimer but a higher energy form with a \"triple\" N\u2261N bond. This valence bond isomer of the weak dimer was some 24 kcal\/mol higher in free energy than the\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":29665,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29665","url_meta":{"origin":29429,"position":1},"title":"Alternative reactions of the N\u2261N “triple bond” in a nitric oxide dimer: forming the trimer N3O3.","author":"Henry Rzepa","date":"September 3, 2025","format":false,"excerpt":"In the previous post I mooted the possibility that a high energy form of the dimer of nitric oxide 1 might nonetheless be able to be detected using suitable traps (such as hydrogenation or cycloaddition). However, an interesting alternative is that this species could be trapped by nitric oxide itself.\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":30890,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30890","url_meta":{"origin":29429,"position":2},"title":"Valence bond representations with +ve charges on adjacent atoms? An odd titanium complex analysed.","author":"Henry Rzepa","date":"March 8, 2026","format":false,"excerpt":"A few posts back, I contemplated the curly arrows appropriate for the formation of nitrosobenzene dimer from nitrosobenzene, and commented on the odd nature of the N=N double bond formed in this process.. Odd, because the valence bond representation of this dimer (1 below) has two formally positive adjacent nitrogen\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":29383,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29383","url_meta":{"origin":29429,"position":3},"title":"The mysterious N=N double bond in nitrosobenzene dimer.","author":"Henry Rzepa","date":"August 11, 2025","format":false,"excerpt":"In an earlier blog, I discussed the curly arrows associated with the known dimerisation of nitrosobenzene, and how the\u00a0N=N double bond (shown in red below) forms in a single concerted process. One of the properties of this molecule is that the equilibrium between the monomer and dimer can be detected,\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":28849,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=28849","url_meta":{"origin":29429,"position":4},"title":"Mechanism of the dimerisation of Nitrosobenzene.","author":"Henry Rzepa","date":"June 14, 2025","format":false,"excerpt":"I am in the process of revising my annual lecture to first year university students on the topic of \"curly arrows\". I like to start my story in 1924, when Robert Robinson published the very first example as an illustration of why nitrosobenzene undergoes electrophilic bromination in the para position\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/06\/trans.gif?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":29410,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29410","url_meta":{"origin":29429,"position":5},"title":"Energy decomposition analysis of hindered alkenes: Tetra t-butylethene and others.","author":"Henry Rzepa","date":"August 13, 2025","format":false,"excerpt":"In the previous post, I introduced the N=N double bond in nitrosobenzene dimer, arguing that even though it was a formal double bond, its bond dissociation energy made it nonetheless a very weak double bond! This was backed up by a technique known as energy decomposition analysis or EDA. Here\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/29429","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=29429"}],"version-history":[{"count":77,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/29429\/revisions"}],"predecessor-version":[{"id":29623,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/29429\/revisions\/29623"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=29429"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=29429"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=29429"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=29429"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}