{"id":19207,"date":"2017-12-17T09:53:39","date_gmt":"2017-12-17T09:53:39","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=19207"},"modified":"2018-02-25T08:47:49","modified_gmt":"2018-02-25T08:47:49","slug":"ammonide-an-alkalide-formed-from-ammonia-and-resembling-an-electride","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=19207","title":{"rendered":"Ammonide: an alkalide formed from ammonia and resembling an electride."},"content":{"rendered":"
\n

Alkalides are anionic alkali compounds containing e.g.<\/em>\u00a0sodide (Na–<\/sup>), kalide (K–<\/sup>), rubidide (Rb–<\/sup>) or caeside (Cs–<\/sup>). Around 90 examples can be found in the Cambridge structure database (see DOI: 10.14469\/hpc\/3453<\/a>\u00a0 for the search query and results). So what about the ammonium analogue, ammonide (NH4<\/sub>–<\/sup>)? A quick search of Scifinder drew a blank! So here I take a look at this intriguingly simple little molecule.\u2021<\/sup><\/p>\n

It can be formed by adding two electrons to the ammonium cation; NH4<\/sub>+<\/sup> + 2e\u00a0\u21a0 NH4<\/sub>–<\/sup>. One might be encouraged to do this since the LUMO (lowest unoccupied molecular orbital, below) of the ammonium cation has A1<\/sub>\u00a0symmetry and so can accept two electrons without the penalty of Jahn-Teller distortions. These electrons will apparently expand the valence electron “octet” around the nitrogen from 8 to 10; a hypervalent species then?\"\"<\/p>\n

So what are the (calculated) properties of NH4<\/sub>–<\/sup>? The energy of the now HOMO (highest occupied molecular orbital) at the \u03c9B97XD\/Def2-TZVPPD\/solvent=water level is -3.6eV, a respectable electron affinity (the additional electrons are said to be bound). More insight can be obtained from the NBO analysis, which produces localized versions of the molecular orbitals. There are four equivalent NBOs, one of which is shown below.<\/p>\n

\"\"<\/p>\n

Each is bonding along one H-N bond, mildly anti-bonding along the other three N-H bonds, but again bonding in the H-H regions! This matches the observations made earlier<\/a> that when more electrons are pumped into normally valent main group molecules, they will occupy the antibonding levels. This is accompanied by a reduction in the bond orders associated with the central atom. In this case, the N-H bond orders are reduced from 0.79 to 0.602 and the total bond index at the nitrogen is reduced from 3.16 to 2.408. The bond index at hydrogen is at first sight increased from 0.79 to a surprising 1.0003, but this is explained because the H-H bond orders are 0.1328 (three for each H), which brings the H index up to 1.0. The N-H vibration (A1<\/sub> symmetric) is 3466 cm-1<\/sup> for NH4<\/sub>+<\/sup>\u00a0 which is reduced to 2659 for\u00a0NH4<\/sub>–<\/sup>.\u2020<\/sup><\/p>\n

So it appears that adding two electrons to the ammonium cation induces H-H bonding! More insight can be obtained from an ELF analysis of the electron density basins.<\/p>\n

\"\"<\/p>\n

The above shows four attractors (as they are called) centered at the hydrogen nuclei, with 2.053e each (4*2.053 = 8.212e). The remaining ~2e are located in basins (green) centered at two different types of attractors. One is along the axis of each N-H bond and exo<\/em> to it (0.316e) and the other sits on top of any set of three hydrogens (0.103e), 1.68e in total. The value of the ELF function at the attractor is shown above. You should realize that ELF=1.0 corresponds to perfectly localized electrons (for which the kinetic energy density is zero) and ELF=0.5 would correspond to a free-electron gas. The ELF value of e.g.<\/em> 0.77 is getting close to an electron gas, and in fact corresponds to what we call an electride<\/strong>.
\n \"\"<\/p>\n

So, the nitrogen valence shell electron octet is not actually exceeded! The additional two electrons in ammonide sit beyond the nitrogen, in H-H regions. Whether or not it is a viable species for detection remains to be established, but even its computed bonding properties have proved interesting and it deserves to join the alkalide family.\u00a0<\/p>\n

Postscript<\/b><\/p>\n

Ammonide exists in a shallow well in the potential energy surface, shown below, with the dissociation to ammonia and hydride anion being exothermic.\"\"<\/a><\/p>\n

The intrinsic reaction coordinate shows one interesting feature at \u00a0IRC ~-1.1 which corresponds to repulsion between the hydride and the lone pair of the nitrogen atom resulting in inversion of configuration during the latter stages of the IRC.<\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

\n

\u2021<\/sup>FAIR data collection; 10.14469\/hpc\/3455<\/a>. \u2020<\/sup>Perhaps unsurprisingly, these values are somewhat basis set dependent. Thus a \u03c9B97XD\/Def2-Q<\/strong><\/span>ZVPPD\/Water calculation gives the following values: bond index at N, 1.998, N-H bond index, 0.4995, H-H bond index 0.1689, H bond index 1.0062, total Rydberg population,\u00a00.2738, \u03bd(A1<\/sub>) 2686 cm-1<\/sup>. The ELF basins are H, 2.039, exo<\/em>-basins 0.282 and 0.141 (total 1.692). The improved basis set better describes the diffuse regions beyond the N-H bonds.\"\"<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Alkalides are anionic alkali compounds containing e.g.\u00a0sodide (Na–), kalide (K–), rubidide (Rb–) or caeside (Cs–). Around 90 examples can be found in the Cambridge structure database (see DOI: 10.14469\/hpc\/3453\u00a0 for the search query and results). So what about the ammonium analogue, ammonide (NH4–)? A quick search of Scifinder drew a blank! So here I take […]<\/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":"","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":[7],"tags":[2375,1712,1830,2329,1395,2376,24,2377,356,253,1431,1885,142,734],"ppma_author":[2661],"class_list":["post-19207","post","type-post","status-publish","format-standard","hentry","category-hypervalency","tag-alkalide","tag-ammonium","tag-anions","tag-atomic-physics","tag-chemistry","tag-electron-gas","tag-energy","tag-free-electron-gas","tag-jahn-teller","tag-kinetic-energy-density","tag-matter","tag-nitrogen","tag-potential-energy-surface","tag-search-query"],"yoast_head":"\nAmmonide: an alkalide formed from ammonia and resembling an electride. - 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=19207\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Ammonide: an alkalide formed from ammonia and resembling an electride. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Alkalides are anionic alkali compounds containing e.g.\u00a0sodide (Na–), kalide (K–), rubidide (Rb–) or caeside (Cs–). 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Here I am asking how many water molecules are required to form the ionic ammonium hydroxide from ammonia and water. As Wikipedia will inform you, \"it is actually impossible to isolate samples of\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":16402,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16402","url_meta":{"origin":19207,"position":1},"title":"The mechanism of silylether deprotection using a tetra-alkyl ammonium fluoride.","author":"Henry Rzepa","date":"May 25, 2016","format":false,"excerpt":"The substitution of a nucleofuge (a good leaving group) by a nucleophile at a carbon centre\u00a0occurs with inversion\u00a0of configuration at the carbon, the mechanism being known by\u00a0the term\u00a0SN2\u00a0(a story I have also told\u00a0in this post). Such displacement at silicon famously proceeds by a quite different mechanism, which\u00a0I here quantify with\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":"","width":0,"height":0},"classes":[]},{"id":16031,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16031","url_meta":{"origin":19207,"position":2},"title":"Ways to encourage water to protonate an amine: superbasing.","author":"Henry Rzepa","date":"April 8, 2016","format":false,"excerpt":"Previously, I looked at\u00a0models of how\u00a0ammonia could be protonated by water to form ammonium hydroxide. The energetic outcome of my\u00a0model matched the known equilbrium in water as favouring the unprotonated form (pKb ~4.75). I add here two amines for which\u00a0R=Me3Si and R=CN. The idea is that the first will assist\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":17579,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17579","url_meta":{"origin":19207,"position":3},"title":"Ammonium tetraphenylborate and the mystery of its \u03c0-facial hydrogen bonding.","author":"Henry Rzepa","date":"March 10, 2017","format":false,"excerpt":"A few years back, I did a post about the Pirkle reagent and the unusual \u03c0-facial hydrogen bonding structure it exhibits. For the Pirkle reagent, this bonding manifests as a close contact between the acidic OH hydrogen and the edge of a phenyl ring; the hydrogen bond is off-centre from\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/03\/142-1024x770.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":13645,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=13645","url_meta":{"origin":19207,"position":4},"title":"A 5-high straight flush of water-ionised acids?","author":"Henry Rzepa","date":"March 17, 2015","format":false,"excerpt":"I do not play poker,\u2021 and so I had to look up a 5-4-3-2-1(A), which Wikipedia informs me is a 5-high straight flush, also apparently known as a steel wheel. In previous posts \u00a0I have suggested acids which can be ionised by (probably)\u00a05, 4, 3 or \u00a01 discrete water molecules\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":22153,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22153","url_meta":{"origin":19207,"position":5},"title":"Choreographing a chemical ballet: a story of the mechanism of 1,4-Michael addition.","author":"Henry Rzepa","date":"April 13, 2020","format":false,"excerpt":"A reaction can be thought of as molecular dancers performing moves. A choreographer is needed to organise the performance into the ballet that is a reaction mechanism. Here I explore another facet of the Michael addition of a nucleophile to a conjugated carbonyl compound. The performers this time are p-toluene\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2020\/04\/SC.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"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\/19207","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=19207"}],"version-history":[{"count":24,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/19207\/revisions"}],"predecessor-version":[{"id":19242,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/19207\/revisions\/19242"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=19207"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=19207"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=19207"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=19207"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}