{"id":31727,"date":"2026-07-18T07:36:22","date_gmt":"2026-07-18T06:36:22","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31727"},"modified":"2026-07-18T08:45:40","modified_gmt":"2026-07-18T07:45:40","slug":"how-long-can-an-n-n-bond-get","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=31727","title":{"rendered":"How long can an N-N bond get?"},"content":{"rendered":"
\n

(Some) chemists have a strange fascination with bonds between two specified atoms – more exactly how short or how long can such a bond get? I asked a slight different question[1]<\/a><\/span> of a molecule known as nitrosobenzene dimer, noting that both nitrogens were both connected to each other and carried a (formal) positive charge; one might naively imagine that the coulomb effects between two positive atoms might result in a repulsion which would greatly lengthen the bond between them (it does not, but it does weaken it). I moved from this to asking how many examples of such molecules there might be, and whether any exhibited unusual bond lengths. After a search of\u00a0the\u00a0CSD, one (“JEGRAS”) caught my interest, shown in blue below[2]<\/a><\/span>,[3]<\/a><\/span> and exhibiting a crystallographic N-N distance of 1.695<\/strong>\u00c5 (to answer the question posed in the title above).
\n\"\"<\/a><\/p>\n

Now this is a long N-N bond, probably the longest such bond ever found! And the Lewis valence bond structure has a remarkable up to three centres containing groups with ionic charge separation.\u00a0The one shown in blue is how it is represented in the literature[2]<\/a><\/span>, but many others can be devised. The list above is by no means comprehensive, some with a double bond between the two nitrogens and others with no bond there! Two are aromatic (red) and one has little charge separation (green).\u00a0Some have three centres of charge separation, others with only one. This might be an interesting problem to set students in a tutorial – how many Lewis structures can they devise in a set time?<\/p>\n

The first task is to see if this effect might be replicated using DFT theory. Here are some models (again by no means comprehensive). It seems that adding a DCM solvation field\u00a0(to mimic the polar environment of a crystal)\u00a0does make a difference.<\/p>\n

Expt<\/p>\n\n\n\n\n\n\n\n\n\n
Method<\/th>\nN-N length<\/th>\nDOI<\/th>\n<\/tr>\n
Expt<\/td>\n1.695<\/strong><\/td>\n[3]<\/a><\/span><\/td>\n<\/tr>\n
MN15L\/Def2-TZVPP<\/td>\n1.732<\/td>\n[4]<\/a><\/span><\/td>\n<\/tr>\n
MN15L\/Def2-TZVPP + water solvation<\/td>\n1.678<\/td>\n[5]<\/a><\/span><\/td>\n<\/tr>\n
MN15L\/Def2-TZVPP + DCM solvation<\/td>\n1.686<\/td>\n[6]<\/a><\/span><\/td>\n<\/tr>\n
MN15L\/Def2-QZVPP<\/td>\n1.728<\/td>\n[7]<\/a><\/span><\/td>\n<\/tr>\n
r2<\/sup>-SCAN-3c\/Def2-mTZVPP + DCM<\/td>\n1.787<\/td>\n–<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The calculated (MN15L\/Def2-TZVPP + DCM solvation[8]<\/a><\/span>) Wiberg bond index of the N-N bond is 0.5594, an unusually low value for a \u03c3-bond. The corresponding NBO localised orbital however has a normal electron occupancy (1.9507e).<\/p>\n

\"\"<\/p>\n

So this unusual N-N bond can be added to the list of “longest ever” bonds. However, unlike e.g.<\/i> ultra long C-C single bonds, which tend to be dominated by steric repulsions, this one achieves its status purely electronically.<\/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. Q. Zhang, C. He, and S. Pang, \"Synthesis of heterocyclic (triazole, furoxan, furazan) fused pyridazine di-\n <i>N<\/i>\n -oxides\n <i>via<\/i>\n hypervalent iodine oxidation\", New Journal of Chemistry<\/i>, vol. 46, pp. 14324-14327, 2022. https:\/\/doi.org\/10.1039\/d2nj02908a<\/a>\n\n<\/li>\n
  3. Zhang, Qi., He, Chunlin., and Pang, Siping., \"CCDC 2175700: Experimental Crystal Structure Determination\", 2022. https:\/\/doi.org\/10.5517\/ccdc.csd.cc2c0zw8<\/a>\n\n<\/li>\n
  4. H. Rzepa, \"JEGRAS N-N long bond\", 2026. https:\/\/doi.org\/10.5281\/zenodo.21218277<\/a>\n\n<\/li>\n
  5. H. Rzepa, \"JEGRAS N-N long bond in water Def2-QZVPP\", 2026. https:\/\/doi.org\/10.5281\/zenodo.21416121<\/a>\n\n<\/li>\n
  6. H. Rzepa, \"JEGRAS N-N long bond in DCM\", 2026. https:\/\/doi.org\/10.5281\/zenodo.21424196<\/a>\n\n<\/li>\n
  7. H. Rzepa, \"JEGRAS N-N long bond in water\", 2026. https:\/\/doi.org\/10.5281\/zenodo.21416139<\/a>\n\n<\/li>\n
  8. H. Rzepa, \"JEGRAS N-N long bond in DCM NBO7\", 2026. https:\/\/doi.org\/10.5281\/zenodo.21425098<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    (Some) chemists have a strange fascination with bonds between two specified atoms – more exactly how short or how long can such a bond get? I asked a slight different question of a molecule known as nitrosobenzene dimer, noting that both nitrogens were both connected to each other and carried a (formal) positive charge; one […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_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},"jetpack_post_was_ever_published":false},"categories":[4],"tags":[],"ppma_author":[2661],"class_list":["post-31727","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry"],"yoast_head":"\nHow long can an N-N bond get? - 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=31727\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How long can an N-N bond get? - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"(Some) chemists have a strange fascination with bonds between two specified atoms – more exactly how short or how long can such a bond get? 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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":9973,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9973","url_meta":{"origin":31727,"position":1},"title":"A (very) short history of shared-electron bonds.","author":"Henry Rzepa","date":"March 26, 2013","format":false,"excerpt":"The concept of a shared electron bond and its property of an order is almost 100 years old in modern form, when G. N. Lewis suggested a model for single and double bonds that involved sharing either 2 or 4 electrons between a pair of atoms. 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Lewis in 1916, chemists have been fascinated by the related concept of a bond order (the number of such bonds that two atoms can participate in, however a bond is defined) and pushing it ever higher\u2026","rel":"","context":"In "Hypervalency"","block_context":{"text":"Hypervalency","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=7"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/02\/Be2Li6_mo13-150x150.png?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":27788,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=27788","url_meta":{"origin":31727,"position":3},"title":"A carbon-carbon one-electron bond! Or a weak carbon-carbon interaction?","author":"Henry Rzepa","date":"October 1, 2024","format":false,"excerpt":"More than 100 years ago, before the quantum mechanical treatment of molecules had been formulated, G. N. Lewis proposed a simple model for chemical bonding that is still taught today. This is the idea of the three categories of bond we know as single, double and triple, comprising respectively two,\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":15635,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15635","url_meta":{"origin":31727,"position":4},"title":"Bond stretch isomerism. Did this idea first surface 100 years ago?","author":"Henry Rzepa","date":"February 9, 2016","format":false,"excerpt":"The phenomenon of bond stretch isomerism, two isomers of a compound differing predominantly in just one bond length, is one of those chemical concepts that wax and occasionally\u00a0wane. Here I explore such isomerism for the elements Ge, Sn and Pb. In one earlier post, I noted a form of\u00a0bond stretch\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"lewis1","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/02\/lewis1-1-1024x421.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":30746,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746","url_meta":{"origin":31727,"position":5},"title":"Quantum crystallography: The structure and C-C bond length alternation of [18]-annulene.","author":"Henry Rzepa","date":"February 13, 2026","format":false,"excerpt":"In my story about one of the molecules of the year, cyclo[48]carbon, I noted that the DFT method used in the literature to model the C-C bond length alternation around the ring (OX B3LYP30) had been re-calibrated against a remeasured crystal structure of C18H18 or [18]-annulene (below) in order to\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"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","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/31727","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=31727"}],"version-history":[{"count":38,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/31727\/revisions"}],"predecessor-version":[{"id":31774,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/31727\/revisions\/31774"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=31727"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=31727"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=31727"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=31727"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}