{"id":29626,"date":"2025-08-25T17:02:41","date_gmt":"2025-08-25T16:02:41","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29626"},"modified":"2025-08-27T10:31:52","modified_gmt":"2025-08-27T09:31:52","slug":"hydrogenating-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=29626","title":{"rendered":"Hydrogenating the even more mysterious N\u2261N triple bond in a nitric oxide dimer."},"content":{"rendered":"
\n

Previously[1]<\/a><\/span> I looked at some of the properties of the mysterious dimer of nitric oxide \u00a01<\/strong> – not the known weak dimer<\/a> 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 two nitric oxide molecules it would be formed from. An energy decomposition analysis\u00a0(NEDA) of 1<\/b> revealed an interaction energy[2]<\/a><\/span> of +4.5 kcal\/mol for the two radical fragments, compared to eg -27 kcal\/mol for the equivalent analysis of the N=N double bond in nitrosobenzene dimer[3]<\/a><\/span> So here I take a look at another property of N\u2261N bonds via<\/em> their hydrogenation energy (Scheme), mindful that the dinitrogen molecule requires forcing conditions to hydrogenate, in part because of the unfavourable entropy terms (See Wiki<\/a> and also here<\/a>\u2021<\/sup> for a calculation of \u0394G298<\/sub>).<\/p>\n

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

Calculations at the \u03c9B97XD\/Def2-TZVPP\/SCRF=water level[4]<\/a><\/span> that whilst hydrogenation of the triple bond in N2<\/sub> is strongly endo-energic, the same process for molecule 1<\/strong> is exo-energic (\u0394\u0394G -26.32<\/strong> kcal\/mol). The direct product is a zwitterion, but presumed rapid proton transfer to a neutral form 2<\/strong> increases exo-energicity. Whilst the second hydrogenation step \u00a0of N2<\/sub> is \u00a0exo-energic, the equivalent second step for 1 <\/strong>to \u00a0give 3<\/strong> is now mildly endo-energic. Overall however, the thermodynamic energies of these two types of triple bond hydrogenation could not be more different.<\/p>\n

So forming a N\u2261N triple bond by forcing two nitric oxide molecules to dimerise (using high pressure) in water produces a system where hydrogenation of that “difficult” N\u2261N bond is made very much easier thermodynamically. Time for an experiment?\u2665<\/sup><\/p>\n

\n

\u2021<\/sup>This site reports a gas phase experimental value for \u0394G -8.1 kcal\/mol at 298K for this equilibrium, although the pressure is not given. The calculated value shown in the scheme above (-20.1 kcal\/mol) \u00a0is for 298K and 1 atm for a model using water as solvent – which might be expected to differentially solvate the product ammonia and hence promote the reaction. In the limit of low pressure (0.0001M)[5]<\/a><\/span> this reduces to -13.0 kcal\/mol, increases to -26.6 kcal\/mol at 10M and becomes -14.3 kcal\/mol at 10M\/800K, illustrating how higher pressures make the reaction more exo-energic and higher temperatures less exo-energic. This was of course the problem solved in the Haber process of finding the sweet spot between pressure and temperature.<\/p>\n

\u2665<\/sup>Perhaps not, given the report that at high pressures, nitric oxide can become explosive.[6]<\/a><\/span><\/p>\n

\n

References<\/h2>\n
  1. H. Rzepa, \"The even more mysterious N\u2261N triple bond in a nitric oxide dimer.\", 2025. https:\/\/doi.org\/10.59350\/rzepa.29429<\/a>\n\n<\/li>\n
  2. H. Rzepa, \"N2O2 as strong dimer? bent NEDA 0 1 0 2 0 -2 Total Interaction (E) : 4.520 Wiberg NN bond index 1.0072 NN stretch 2604 cm-1\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15468<\/a>\n\n<\/li>\n
  3. H. Rzepa, \"Nitrosobenzene dimer NEDA=2, 0,1 0,1 0,1 Total Interaction (E) : -27.564\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15444<\/a>\n\n<\/li>\n
  4. H. Rzepa, \"Hydrogenating the even more mysterious N\u2261N triple bond in a nitric oxide dimer.\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15516<\/a>\n\n<\/li>\n
  5. G. Luchini, J.V. Alegre-Requena, I. Funes-Ardoiz, and R.S. Paton, \"GoodVibes: automated thermochemistry for heterogeneous computational chemistry data\", F1000Research<\/i>, vol. 9, pp. 291, 2020. https:\/\/doi.org\/10.12688\/f1000research.22758.1<\/a>\n\n<\/li>\n
  6. T. Melia, \"Decomposition of nitric oxide at elevated pressures\", Journal of Inorganic and Nuclear Chemistry<\/i>, vol. 27, pp. 95-98, 1965. https:\/\/doi.org\/10.1016\/0022-1902(65)80196-8<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Previously I looked at some of the properties of the mysterious dimer of nitric oxide \u00a01 – 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 two nitric oxide molecules it […]<\/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-29626","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry"],"yoast_head":"\nHydrogenating the 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=29626\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Hydrogenating 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 looked at some of the properties of the mysterious dimer of nitric oxide \u00a01 – not the known weak dimer but a higher energy form with a “triple” N\u2261N bond. 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By the same curly arrow method, one can produce the below, showing how the simpler nitric oxide radical could potentially dimerise to a species\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":29626,"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":29626,"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":29626,"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":29410,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=29410","url_meta":{"origin":29626,"position":4},"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. 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