{"id":30746,"date":"2026-02-13T11:51:23","date_gmt":"2026-02-13T11:51:23","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746"},"modified":"2026-02-13T11:51:23","modified_gmt":"2026-02-13T11:51:23","slug":"quantum-crystallography-the-structure-and-c-c-bond-length-alternation-of-18-annulene","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746","title":{"rendered":"Quantum crystallography: The structure and C-C bond length alternation of [18]-annulene."},"content":{"rendered":"
\n

In my story about one of the molecules of the year, cyclo[48]carbon,[1]<\/a><\/span> I noted that the DFT method used in the literature to model the C-C bond length alternation around the ring (OX B3LYP30[2]<\/a><\/span>) had been re-calibrated against a remeasured crystal structure[3]<\/a><\/span> of C18<\/sub>H18<\/sub> or [18]-annulene (below) in order to reproduce the observed values for this molecule.<\/p>\n

\"\"<\/a>
\n[18]-annulene<\/strong><\/p>\n

A noteworthy aspect of this structure is the six hydrogen atoms pointing into the centre of the ring, which come into very close contact with each other. The conventional method of refining the crystal structure (which includes an assumption that the electron density surrounding the H and indeed other atoms is spherical) results in C-H distances which are too short by about 0.1\u00c5, which has the knock on effect that the H…H separations are now too long. The recent introduction of a refinement method (NoSpherA2) which uses DFT-calculated non-spherical atom electron density distributions rather than spherical ones has the effect of producing more sensible values for e.g. C-H distances[4]<\/a><\/span> and so by implication, results in much shorter inner H..H distances for [18]-annulene. The question now is: do these shorter H…H distances in turn have any effect on the C-C ring distances, and hence affect the alternation of these distances around the ring and the resulting outcome of the calibration process for the development of the OX B3LYP30 method.<\/p>\n

Method:<\/strong> We decided to re-refine the structure of [18]annulene (CCDC refcode ANULEN03[3]<\/a><\/span>) using modern quantum crystallography (NoSpherA2[5]<\/a><\/span>,[6]<\/a><\/span>). To do this, we used Def2-SVP as the basis set and wB97X-V for the method, with a multiplicity of 2 in the settings for the OLEX2 program.<\/p>\n

The published structure has the molecule sitting across a centre of symmetry so only half of it is unique, and it was found to be disordered with a second orientation of the complete molecule (effectively the macrocycle rotated in plane by ca. 30\u00b0) in a ca. 84:16 ratio. This caused trouble with the quantum crystallography refinements as allowing all of the hydrogen atoms to be positionally free (i.e. removing the AFIX commands) and anisotropic at the same time caused 6 of the 8 hydrogen atoms of the minor occupancy component to \u201cwander off\u201d into chemically nonsensical positions, and 4 of the major occupancy plus all 8 of the minor occupancy hydrogen atoms went non positive definite (one of the thermal ellipsoid radii refined to a negative length).<\/p>\n

However, we discovered that doing the refinement in stages allowed a more settled structure. Starting with the published structure and allowing all of the hydrogen atoms to be positionally free gave a nice stable result. Allowing the hydrogens to go anisotropic afterwards did result in 1 of the major occupancy and all 8 of the minor occupancy hydrogen atoms going non positive definite (n.p.d.), but the positions of the hydrogen atoms remained sensible. Subsequently reverting all 8 hydrogen atoms of the minor occupancy component to be isotropic resulted in a stable and sensible refinement where the sole non positive definite atom of the major occupancy component corrected itself into being normal (i.e.<\/i> no longer non positive definite). This is the re-refined version of the structure we used for further analysis below.[7]<\/a><\/span><\/p>\n

Analysis:<\/b> The closest H\u00b7\u00b7\u00b7H separations for the \u201cinner\u201d hydrogen atoms of the major occupancy orientation emerge as\u00a01.8276(9), 1.8791(9) and 1.9022(8) \u00c5\u2021<\/sup> (Figure 1, mean 1.870\u00c5) This compares to the values extracted from the published structure of 1.99252(4), 2.02490(3) and 2.05217(3), mean = 2.0232\u00c5 for the major occupancy orientation, a difference\u00a0of \u0394 -0.153\u00c5.
\n\"\"<\/p>\n

Figure 1.<\/strong><\/p>\n

A search for other close H…H contacts<\/strong>: A search of the crystal structure database for close intramolecular H…H distances of <1.9 \u00c5 (< 100K, R < 0.05, no errors, excluding H-C-H substructures) reveals the following distribution (Figure 2). Although examples of distances <1.9 \u00c5 are relatively sparse (95, February 2026),\u00a0they are not that unusual.\u00a0It is highly probable that all these examples were determined using the classical method of spherical atoms. It is to be expected that in the future, examples refined using non-spherical atoms will start appearing – and that one will be specifically able to search for such analyses.<\/p>\n

\"\"<\/p>\n

Figure 2.<\/strong><\/p>\n

We focussed on just one of the entries in the figure: DOCKEO<\/strong>[8]<\/a><\/span>
\n,[9]<\/a><\/span> (which is a masked [14]annulene) being an example (see Figure 3) of a compound having an even shorter apparent H…H contact of ~1.65\u00c5 (after C-H distance correction). This was also subjected to NoSpherA2[5]<\/a><\/span>,[6]<\/a><\/span> analysis. The structure is in a chiral space group P212121 with no firm indication of the correct enantiomer (the Flack of 0.3(4) is very indeterminate with a large error that encompasses the whole range). It was initially refined as a 2-component racemic twin (using TWIN\/BASF) to no real effect. Although this is the standard approach when the Flack is far from zero, it was not really surprising that it had no effect, given the large error (\u03c3 = 0.4). Next it was noticed that the original authors had not modelled some evident disorder in one of the CF3<\/sub> groups. Since the fluorine thermal parameters were reasonable, it is understandable to ignore it, but the largest residual electron density peaks were around this group in obvious disorder positions and with the extra precision desired in quantum crystallography refinements, it was best to model this. A quick rough and ready approach was adopted, one not to be used in a structure of “publication quality”, but enough to “soak up” the electron density. Next, NoSpherA2 was used in a refinement that relaxed the H atom positions (no AFIXes). This worked sensibly, though it had fairly little effect on the R-factor. However, refining the hydrogen atoms anisotropically went poorly; of the 15 hydrogen atoms, 5 went n.p.d, another 5 went nearly n.p.d, and only 2 of them could be described as approaching reasonable. Ultimately, handling the hydrogen atoms was done isotropically. Finally, adding an extinction parameter caused a final 0.2% drop in the R-factor and the H…H distance of closest approach emerged as 1.600\u00c5 (Figure 3).<\/p>\n

\"\"<\/p>\n

Figure 3.<\/p>\n

It is worth noting that this distance is not what it might seem. Thus the calculated DFT H-H distance (using r2<\/sup>SCAN-3c) is 1.8975\u00c5, or \u03940.2975\u00c5. It corresponds to a calculated double minimum potential energy well. However, a Cs<\/sub>-symmetric form with the hydrogen located at the centre of this double well turns out to be a transition state with a shorter H…H separation of 1.7393\u00c5. The imaginary calculated transition mode of \u03bdi<\/sub> 61 cm-1<\/sup> is associated with a tiny free energy barrier of ~0.03 kcal\/mol, well below a quantum of vibrational energy and hence the observed hydrogen will in fact correspond to that of a single<\/strong> minimum potential well. The lesson learnt from this analysis is that measured distances (for a single potential well) and calculated distances (for a double potential well) may not always correspond and care must be taken in interpreting such distances.<\/p>\n

C-C Distances in [18]-annulene<\/strong>. The nine\u2021<\/sup> unique pairs of C-C distances in the measured structure of [18]annulene derive from Figure 4 and the atom numbering shown there.<\/p>\n

\"\"<\/p>\n

Figure 4.<\/strong><\/p>\n

Table 1. Crystallographic C-C bond lengths and \u0394 <\/tt>differences for [18]-annulene, \u00c5<\/b><\/p>\n

Original refinement[3]<\/a><\/span>\r\n                                   old \u0394\r\nC2 C3 1.403(2)   C2 C1 1.3883(17)  0.0147\r\nC3 C4 1.3913(14) C2 C3 1.403(2)    0.0117\r\nC5 C4 1.3926(15) C3 C4 1.3913(14)  0.0013\r\nC5 C6 1.4056(18) C5 C4 1.3926(15)  0.0130\r\nC7 C6 1.3870(15) C6 C5 1.4056(18)  0.0186\r\nC8 C7 1.3927(15) C7 C6 1.3870(15)  0.0057\r\nC8 C9 1.4032(19) C8 C7 1.3927(15)  0.0105\r\nC9 C1 1.3897(15) C8 C9 1.4032(19)  0.0135\r\nC2 C1 1.3883(17) C9 C1 1.3897(15)  0.0014\r\n                              Mean 0.0100\u00c5<\/strong>\r\nNoSpherA2 refinement                New \u0394     old \u0394\r\nC2 C3 1.4036(12)  C2 C1 1.3948(11)  0.0088   0.0147\r\nC3 C4 1.3954(9)   C2 C3 1.4036(12)  0.0082   0.0117\r\nC5 C4 1.3939(10)  C3 C4 1.3954(9)   0.0015   0.0013\r\nC5 C6 1.4064(11)  C5 C4 1.3939(10)  0.0125   0.0130\r\nC7 C6 1.3928(10)  C5 C6 1.4064(11)  0.0136   0.0186\r\nC8 C7 1.3938(10)  C7 C6 1.3928(10)  0.0010   0.0057\r\nC8 C9 1.4126(12)  C8 C7 1.3938(10)  0.0188   0.0105\r\nC9 C1 1.3846(10)  C8 C9 1.4126(12)  0.0280   0.0135\r\nC2 C1 1.3948(11)  C9 C1 1.3846(10)  0.0102   0.0014\r\n                               Mean 0.0114\u00c5  0.0100\u00c5<\/strong><\/tt>\r\n<\/pre>\n
Shown below is the atom numbering used in the r2<\/sup>-SCAN-3C DFT geometry optimisation[10]<\/a><\/span> (Figure 5).<\/p>\n
\"\"<\/p>\n
Figure 5<\/strong><\/p>\n
Table 2. Computed r2<\/sup>-SCAN-3c C-C bond lengths and \u0394 <\/tt>differences for [18]-annulene, \u00c5<\/b><\/p>\n
                                 \u0394\r\n1 5   1.40751  5 11  1.39010  0.01741\r\n5 11  1.39010 11 3   1.39020  0.00010\r\n11 3  1.39020  3 15  1.40755  0.01735\r\n3 15  1.40755 15 13  1.39017  0.01738\r\n15 13 1.39017 13 7   1.38998  0.00019\r\n13 7  1.38998  7 9   1.40748  0.01750\r\n7 9   1.40748  9 35  1.39009  0.01739\r\n9 35  1.39009 35 19  1.39009  0.00000\r\n35 19 1.39009 19 23  1.40752  0.01743\r\n                         Mean 0.0116<\/strong><\/tt>\r\n<\/pre>\n
Conclusions<\/strong>: We set out to study the extent to which the C-C distances in the [18]-annulene molecule, as used to calibrate a modified DFT method[1]<\/a><\/span> could be affected by steric compressions in the centre of the ring caused by close approaches of the inward pointing hydrogens. The NoSpherA2 method of crystal structure refinement results in a slight increase in the C-C bond length alternation around the ring, from 0.0100 to 0.0114<\/strong>\u00c5, but \u00a0given that this analysis is quick and easy to perform, there is no reason not to use it as a standard method for structures used for calibration purposes. The newly re-refined bond alternating distance compares with 0.0116<\/strong>\u00c5 calculated using the r2<\/sup>-SCAN-3c DFT procedure and 0.0112<\/strong>\u00c5 calculated using the original literature[1]<\/a><\/span> OX B3LYP30 method which had been calibrated against this distance. Both the DFT methods are thus seen to perform very well against the measured bond length alternation. Clearly however there is a need to undertake more such studies for a clearer understanding of the performance of DFT methods in this area.<\/p>\n
\n
\u2021<\/sup>As the macrocycle sits across a centre of symmetry there are only 3 unique H…H distances and 9 unique C-C differences.<\/p>\n
References<\/h2>\n    
  1. Y. Gao, P. Gupta, I. Ron\u010devi\u0107, C. Mycroft, P.J. Gates, A.W. Parker, and H.L. Anderson, \"Solution-phase stabilization of a cyclocarbon by catenane formation\", Science<\/i>, vol. 389, pp. 708-710, 2025. https:\/\/doi.org\/10.1126\/science.ady6054<\/a>\n\n<\/li>\n
  2. M. Vitek, J. Deng, H.L. Anderson, and I. Ron\u010devi\u0107, \"Global Aromatic Ring Currents in Neutral Porphyrin Nanobelts\", ACS Nano<\/i>, vol. 19, pp. 1405-1411, 2024. https:\/\/doi.org\/10.1021\/acsnano.4c14100<\/a>\n\n<\/li>\n
  3. Stawski, Wojciech., Zhu, Yikun., Ron\u010devi\u0107, Igor., Wei, Zheng., Petrukhina, Marina A.., and Anderson, Harry L.., \"CCDC 2293565: Experimental Crystal Structure Determination\", 2024. https:\/\/doi.org\/10.5517\/ccdc.csd.cc2gzmz2<\/a>\n\n<\/li>\n
  4. H. Rzepa, \"Crystallography meets DFT Quantum modelling.\", 2025. https:\/\/doi.org\/10.59350\/5dy8w-0zs92<\/a>\n\n<\/li>\n
  5. O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, and H. Puschmann, \"<i>OLEX2<\/i>: a complete structure solution, refinement and analysis program\", Journal of Applied Crystallography<\/i>, vol. 42, pp. 339-341, 2009. https:\/\/doi.org\/10.1107\/s0021889808042726<\/a>\n\n<\/li>\n
  6. H. Rzepa, \"Quantum crystallography: The structure of [18]-annulene.\", 2026. https:\/\/doi.org\/10.14469\/hpc\/15681<\/a>\n\n<\/li>\n
  7. A. Yoshii, K. Ikemoto, T. Izumi, H. Taka, H. Kita, S. Sato, and H. Isobe, \"Periphery Design of Macrocyclic Materials for Organic Light-Emitting Devices with a Blue Phosphorescent Emitter\", Organic Letters<\/i>, vol. 21, pp. 2759-2762, 2019. https:\/\/doi.org\/10.1021\/acs.orglett.9b00717<\/a>\n\n<\/li>\n
  8. A. Yoshii, K. Ikemoto, T. Izumi, H. Taka, H. Kita, S. Sato, and H. Isobe, \"CCDC 1898512: Experimental Crystal Structure Determination\", 2019. https:\/\/doi.org\/10.5517\/ccdc.csd.cc21qkbp<\/a>\n\n<\/li>\n
  9. H. Rzepa, \"Molecules of the year 2025: Cyclo[48]carbon \u2013 bond alternation and Raman Activity Spectrum. ORCA 6.1 calculations\", 2025. https:\/\/doi.org\/10.14469\/hpc\/15615<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"
    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 reproduce the observed values for […]<\/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_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":[1745,4],"tags":[],"ppma_author":[2661,2662],"class_list":["post-30746","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","category-interesting-chemistry"],"yoast_head":"\nQuantum crystallography: The structure and C-C bond length alternation of [18]-annulene. - 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=30746\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Quantum crystallography: The structure and C-C bond length alternation of [18]-annulene. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"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 reproduce the observed values for […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2026-02-13T11:51:23+00:00\" \/>\n<meta name=\"author\" content=\"Henry Rzepa, Andrew White\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Henry Rzepa\" \/>\n\t<meta name=\"twitter:label2\" content=\"Estimated reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"8 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Quantum crystallography: The structure and C-C bond length alternation of [18]-annulene. - Henry Rzepa's Blog","robots":{"index":"index","follow":"follow","max-snippet":"max-snippet:-1","max-image-preview":"max-image-preview:large","max-video-preview":"max-video-preview:-1"},"canonical":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746","og_locale":"en_GB","og_type":"article","og_title":"Quantum crystallography: The structure and C-C bond length alternation of [18]-annulene. - Henry Rzepa's Blog","og_description":"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 reproduce the observed values for […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746","og_site_name":"Henry Rzepa's Blog","article_published_time":"2026-02-13T11:51:23+00:00","author":"Henry Rzepa, Andrew White","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"8 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Quantum crystallography: The structure and C-C bond length alternation of [18]-annulene.","datePublished":"2026-02-13T11:51:23+00:00","mainEntityOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746"},"wordCount":1455,"commentCount":0,"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746#primaryimage"},"thumbnailUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2026\/02\/C18H18.svg","articleSection":["crystal_structure_mining","Interesting chemistry"],"inLanguage":"en-GB","potentialAction":[{"@type":"CommentAction","name":"Comment","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746#respond"]}]},{"@type":"WebPage","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746","name":"Quantum crystallography: The structure and C-C bond length alternation of [18]-annulene. - Henry Rzepa's Blog","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#website"},"primaryImageOfPage":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746#primaryimage"},"image":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746#primaryimage"},"thumbnailUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2026\/02\/C18H18.svg","datePublished":"2026-02-13T11:51:23+00:00","author":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"breadcrumb":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746#breadcrumb"},"inLanguage":"en-GB","potentialAction":[{"@type":"ReadAction","target":["https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746"]}]},{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746#primaryimage","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2026\/02\/C18H18.svg","contentUrl":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2026\/02\/C18H18.svg"},{"@type":"BreadcrumbList","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30746#breadcrumb","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog"},{"@type":"ListItem","position":2,"name":"Quantum crystallography: The structure and C-C bond length alternation of [18]-annulene."}]},{"@type":"WebSite","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#website","url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/","name":"Henry Rzepa's Blog","description":"Chemistry with a twist","potentialAction":[{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?s={search_term_string}"},"query-input":{"@type":"PropertyValueSpecification","valueRequired":true,"valueName":"search_term_string"}}],"inLanguage":"en-GB"},{"@type":"Person","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281","name":"Henry Rzepa","image":{"@type":"ImageObject","inLanguage":"en-GB","@id":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g370be3a7397865e4fd161aefeb0a5a85","url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","contentUrl":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","caption":"Henry Rzepa"},"description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London.","sameAs":["https:\/\/orcid.org\/0000-0002-8635-8390"],"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?author=1"}]}},"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/pDef7-7ZU","jetpack-related-posts":[{"id":9512,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9512","url_meta":{"origin":30746,"position":0},"title":"Helically conjugated molecules. A follow-up to [144]-annulene.","author":"Henry Rzepa","date":"February 12, 2013","format":false,"excerpt":"An extensive discussion developed regarding my post on a fascinating helical [144]-annulene. Topics included the nature of the ring current sustained by the\u00a0\u03c0-electrons and in particular the bond-length alternation around the periphery and whether this should alter if the electron count were to be changed to that of a 4n+2\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":"SELQUW. Click for 3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/02\/SELQUW.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":30276,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30276","url_meta":{"origin":30746,"position":1},"title":"Molecules of the year 2025: Cyclo[48]carbon and others – the onset of bond alternation and the Raman Activity Spectrum.","author":"Henry Rzepa","date":"December 29, 2025","format":false,"excerpt":"The annual \"Molecules of the Year\" selections are available for the year 2025. A theme was elemental allotropes and one such was carbon in the form of C48\u00a0stabilised by formation of a catenane C48.M3 (M = red ligand below) - it was not possible however to crystallise C48.M3. When \"unmasked\"\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":275,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=275","url_meta":{"origin":30746,"position":2},"title":"A molecule with an identity crisis: Aromatic or anti-aromatic?","author":"Henry Rzepa","date":"April 13, 2009","format":false,"excerpt":"In 1988, Wilke reported molecule 1 It was a highly unexpected outcome of a nickel-catalyzed reaction and was described as a 24-annulene with an unusual 3D shape. Little attention has been paid to this molecule since its original report, but the focus has now returned! The reason is that a\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":"A [24] annulene. Click on image for model.","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/gaytab.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":24503,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=24503","url_meta":{"origin":30746,"position":3},"title":"Molecule of the year 2021: Infinitene.","author":"Henry Rzepa","date":"December 16, 2021","format":false,"excerpt":"The annual \"molecule of the year\" results for 2021 are now available ... and the winner is Infinitene., This is a benzocirculene in the form of a figure eight loop (the infinity symbol), a shape which is also called a lemniscate after the mathematical (2D) function due to Bernoulli. The\u2026","rel":"","context":"In "Chiroptics"","block_context":{"text":"Chiroptics","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2644"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/12\/infinitene.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":9322,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=9322","url_meta":{"origin":30746,"position":4},"title":"Anapolar ring currents: a [144]-Annulene.","author":"Henry Rzepa","date":"February 1, 2013","format":false,"excerpt":"This is a recently published (hypothetical) molecule which has such unusual properties that I cannot resist sharing it with you. It is an annulene with 144 all-cis CH groups, being a (very) much larger cousin of (also hypothetical) systems mooted in 2009,. One fascinating novel aspect of Berger's work is\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":"A 144-carbon annulene. Click for  3D.","src":"https:\/\/i0.wp.com\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2013\/02\/C144.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":21176,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21176","url_meta":{"origin":30746,"position":5},"title":"Cyclo[18]carbon: The Kekul\u00e9 vibration calculated and hence a mystery!","author":"Henry Rzepa","date":"August 30, 2019","format":false,"excerpt":"I have discussed the vibration in benzene known as the Kekul\u00e9 mode in other posts, the first of which was all of ten years ago. It is a stretching mode that lengthens three of the bonds in benzene (a [6]-annulene) and shortens the other three, thus leading to a cyclohexatriene\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":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2019\/08\/C18.gif?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":""},{"term_id":2662,"user_id":8,"is_guest":0,"slug":"andrew-white","display_name":"Andrew White","avatar_url":{"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/03\/AJPW-photo.jpg","url2x":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2025\/03\/AJPW-photo.jpg"},"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\/30746","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=30746"}],"version-history":[{"count":91,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/30746\/revisions"}],"predecessor-version":[{"id":30860,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/30746\/revisions\/30860"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=30746"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=30746"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=30746"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=30746"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}