{"id":21592,"date":"2019-12-09T13:04:22","date_gmt":"2019-12-09T13:04:22","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=21592"},"modified":"2019-12-21T07:32:50","modified_gmt":"2019-12-21T07:32:50","slug":"sign-inversions-in-optical-rotation-as-a-function-of-wavelength-ord-spectra","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21592","title":{"rendered":"Sign inversions in optical rotation as a function of wavelength (ORD spectra)"},"content":{"rendered":"
\n

I have been discussing some historical aspects of the absolute configuration of molecules and how it was connected to their optical rotations. The nomenclature for certain types of molecules such as sugars and less commonly amino acids includes the notation (+) to indicate that the specific optical rotation of the molecule has a positive (rather than a negative) value. What is rarely mentioned is the implicit wavelength at which the rotation is measured. Historically polarimeters operated at the so-called sodium Fraunhofer D-line (588.995nm, hence the name [\u03b1]D<\/sub>) and only much more recently at the mercury e-line (546.073nm). The former was used for uncoloured organic molecules, since it was realised early on that colour and optical rotation did not mix well. Here I take a closer look at this aspect by constructing the hypothetical molecule shown below.<\/p>\n

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

The rational behind this choice is that it is (a) based on indigo, which is deep blue in colour and (b) has a bridge of four methylene groups added to make it (axially) asymmetric. The calculated UV\/Vis spectrum (\u03c9B97XD\/Def2-SVP\/SCRF=water, FAIR DOI: 10.14469\/hpc\/6457<\/a>) is shown below and you can see the very intense absorption at 535nm (corresponding to a visually blue colour).<\/p>\n

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

The electronic circular dichroism version of this spectrum (simply the difference in absorbance between left and right polarised light instead of absorbance by unpolarised light) is shown below, and this form of chiroptical spectroscopy in large measure replaced the use of specific optical rotations as a means of assigning absolute configurations from the 1960s onwards. Note that the large peak at 535nm is replaced by a much smaller one (the Cotton effect) in the ECD spectrum.<\/p>\n

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

Now I show the original optical rotation as a function of wavelength in 10nm increments. At 589nm ([\u03b1]D<\/sub>)\u00a0it is negative (-1364\u00b0), but what on earth is going on at a wavelength of ~535nm, which as you can see above is the value of the first electronic excitation?<\/p>\n

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

An expansion in 0.2nm increments shows more clearly what is happening. The negative value suddently shoots down to -1,200,000\u00b0, frankly an absurd value, before discontinuously reversing sign to a positive value of 75,000\u00b0. At exactly the value of the electronic absorption it is zero. Most people seeing this happen would conclude that the mathematics derived from the solutions of the quantum mechanical equations is resulting in an unphysical discontinuity. It is in fact the result of the behaviour of the electric and magnetic dipole moment vectors, and it CAN be seen experimentally, albeit never in quite such extreme form![1]<\/a><\/span> The sign of the optical rotation CAN invert, but in this very strange manner whereby if it starts as negative, it first becomes infinitely negative before passing through zero and becoming infinitely positive and finally settling down to a normal positive value. The reason by the way why the “blip” in the ORD spectrum above is +ve, but -ve in the expansion below is “digital resolution”, with the top trace having too coarse a resolution to capture the detail. Now the reason why optical rotation measurement at 589nm becomes clear; it avoids any inversions caused by this effect for the majority of less-coloured molecules. However, if you do have a molecule that were to absorb at 589nm itself, the sodium D-line is the last wavelength you would want to use to measure its optical rotation!<\/p>\n

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

So the notation (+) or (-) used to describe the sign of the specific rotation of a chiral molecule might give the misleading impression that it is a characteristic of the molecule at all wavelengths used to measure it. The rotation can change sign an impressive number of times as the wavelength changes! Does anyone know of any coloured pharmaceutical drugs that are available as pure enantiomers? \u00a0It would be fun to repeat the above on such a molecule.<\/p>\n

References<\/h2>\n
  1. M.S. Andrade, V.S. Silva, A.M. Louren\u00e7o, A.M. Lobo, and H.S. Rzepa, \"Chiroptical Properties of Streptorubin B: The Synergy Between Theory and Experiment\", Chirality<\/i>, vol. 27, pp. 745-751, 2015. https:\/\/doi.org\/10.1002\/chir.22486<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    I have been discussing some historical aspects of the absolute configuration of molecules and how it was connected to their optical rotations. The nomenclature for certain types of molecules such as sugars and less commonly amino acids includes the notation (+) to indicate that the specific optical rotation of the molecule has a positive (rather […]<\/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":[2644,4],"tags":[],"ppma_author":[2661],"class_list":["post-21592","post","type-post","status-publish","format-standard","hentry","category-chiroptics","category-interesting-chemistry"],"yoast_head":"\nSign inversions in optical rotation as a function of wavelength (ORD spectra) - 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=21592\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Sign inversions in optical rotation as a function of wavelength (ORD spectra) - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"I have been discussing some historical aspects of the absolute configuration of molecules and how it was connected to their optical rotations. The nomenclature for certain types of molecules such as sugars and less commonly amino acids includes the notation (+) to indicate that the specific optical rotation of the molecule has a positive (rather […]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21592\" \/>\n<meta property=\"og:site_name\" content=\"Henry Rzepa's Blog\" \/>\n<meta property=\"article:published_time\" content=\"2019-12-09T13:04:22+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2019-12-21T07:32:50+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2019\/12\/indigo.svg\" \/>\n<meta name=\"author\" content=\"Henry Rzepa\" \/>\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=\"3 minutes\" \/>\n<!-- \/ Yoast SEO plugin. -->","yoast_head_json":{"title":"Sign inversions in optical rotation as a function of wavelength (ORD spectra) - 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=21592","og_locale":"en_GB","og_type":"article","og_title":"Sign inversions in optical rotation as a function of wavelength (ORD spectra) - Henry Rzepa's Blog","og_description":"I have been discussing some historical aspects of the absolute configuration of molecules and how it was connected to their optical rotations. 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He had identified as essential knowing the relative orientation (the term conformation was not yet in common use) of the two methyl groups (the modern terms\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\/2019\/12\/glyceraldehyde-rotations-1024x530.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":21694,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21694","url_meta":{"origin":21592,"position":1},"title":"L-Malic acid: An exercise in conformational analysis impacting upon optical rotatory dispersion (ORD).","author":"Henry Rzepa","date":"December 20, 2019","format":false,"excerpt":"My momentum of describing early attempts to use optical rotation to correlate absolute configuration of small molecules such as glyceraldehyde and lactic acid with their optical rotations has carried me to L-Malic acid (below labelled as (S)-Malic acid). The measured optical rotatory dispersion curve at low wavelengths is shown below\u2026","rel":"","context":"In "Chiroptics"","block_context":{"text":"Chiroptics","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2644"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":399,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=399","url_meta":{"origin":21592,"position":2},"title":"The Chirality of Lemniscular Octaphyrins","author":"Henry Rzepa","date":"April 28, 2009","format":false,"excerpt":"In the previous post, \u00a0it was noted that \u00a0M\u00f6bius annulenes are intrinsically chiral, and should therefore in principle be capable of resolution into enantiomers. The synthesis of such an annulene by Herges and co-workers was a racemic one; no attempt was reported at any resolution into such enantiomers. Here theory\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 34-Octaphyrin. Click to see molecule","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2009\/04\/qarfar.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":6455,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=6455","url_meta":{"origin":21592,"position":3},"title":"A golden age for (computational) spectroscopy.","author":"Henry Rzepa","date":"April 2, 2012","format":false,"excerpt":"I mentioned in my last post an unjustly neglected paper from that golden age of 1951-1953 by Kirkwood and co. They had shown that Fischer's famous guess for the absolute configurations of organic chiral molecules was correct. The two molecules used to infer this are shown below. Using the theory\u2026","rel":"","context":"In "Chiroptics"","block_context":{"text":"Chiroptics","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2644"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":21720,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21720","url_meta":{"origin":21592,"position":4},"title":"Molecules of the year \u2013 2019: twisty tetracene.","author":"Henry Rzepa","date":"December 22, 2019","format":false,"excerpt":"All of the molecules in this year's C&EN list are fascinating in their very different ways. Here I take a look at the twisty tetracene (dodecaphenyltetracene) which is indeed very very twisty. Unfortunately, the authors point that the twisty-ness does not lead to a stable helical configuration at room temperatures\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\/12\/tetracene-1024x726.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":21546,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21546","url_meta":{"origin":21592,"position":5},"title":"The (+) in D-(+)-glyceraldehyde means it has a positive optical rotation? Wrong!","author":"Henry Rzepa","date":"December 6, 2019","format":false,"excerpt":"Text books often show the following diagram, famously consolidated over many years by Emil Fischer from 1891 onwards. At the top sits D-(+)-glyceraldehyde, to which all the monosaccharides below are connected by painstaking chemical transformations. In this notation, D (for all these structures) indicates the absolute configuration of the series,\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\/2019\/12\/glyceraldehyde-concentrations-1024x481.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\/21592","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=21592"}],"version-history":[{"count":14,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21592\/revisions"}],"predecessor-version":[{"id":21615,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/21592\/revisions\/21615"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=21592"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=21592"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=21592"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=21592"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}