{"90311":{"#nid":"90311","#data":{"type":"event","title":"(10-0907) Prof. Scott Kable, University of Sydney, Australia","body":[{"value":"\u003Cp\u003EProf. Scott Kable, University of Sydney, Australia\n\u003C\/p\u003E\n\u003Cp\u003EReactions that don\u00e2\u0080\u0099t follow the transition state path\n\u003C\/p\u003E\n\u003Cp\u003EAACP Seminar Series\n\u003C\/p\u003E\n\u003Cp\u003EEvery chemical reaction has its own unique characteristics \u00e2\u0080\u0022 the identity of the chemical products is an obvious fingerprint, but the rate of the reaction and the energy deposited in the products is also characteristic of the mechanism.  The notion of a \u00e2\u0080\u009ctransition state (TS)\u00e2\u0080\u009d, proposed in the 1930\u00e2\u0080\u0099s, is a very powerful concept in helping us to understand and predict reaction mechanisms.  The TS geometry represents the chemical (nuclear + electronic) structure at the maximum of the lowest energy pathway between reactant and product.  Transition state theory (TST) proposes that all reaction trajectories pass through or near this configuration and that the energy and entropy of the TS, compared to the reactants, determines the rate of reaction.  The potential energy surface (PES) after the TS determines how the energy is deposited into the products.\n\u003C\/p\u003E\n\u003Cp\u003EIn this seminar I shall explain how detailed analysis of the energy disposed in the products reveals information about the reaction mechanism.  I will illustrate two unusual mechanisms discovered recently.  \u00e2\u0080\u009cRoaming\u00e2\u0080\u009d, first reported in 2004 in the photodissociation of H\u003Csub\u003E2\u003C\/sub\u003ECO [1], seems not to have a transition state \u00e2\u0080\u0022 at least not one that TST can accommodate.  It involves a long range excursion of a H-atom in the van der Waals region of the HCO fragment, leading to self-abstraction of the other H-atom.  Roaming has also been well-characterised in CH\u003Csub\u003E3\u003C\/sub\u003ECHO, where the methyl group \u00e2\u0080\u009croams\u00e2\u0080\u009d about the HCO core, abstracting H to produce CH\u003Csub\u003E4\u003C\/sub\u003E + CO. [2]  In the intervening 6 years, at least a dozen other systems have shown roaming mechanisms, however the theoretical calculation of the kinetics of roaming remains a challenge.\n\u003C\/p\u003E\n\u003Cp\u003EThe other unusual mechanism involves H\/D exchange in a unimolecular reaction.  Photo-dissociation of CD\u003Csub\u003E3\u003C\/sub\u003ECHO or CH\u003Csub\u003E3\u003C\/sub\u003ECDO produces about 10% H\/D exchange.  Conventional TST underestimates the branching ratio of the exchange mechanism by more than two orders of magnitude.  We propose a very unusual, high dimensional TS (ie not a true first order TS) to be responsible and that the very high entropy of this structure more than compensates for its higher energy.  All ketones, aldehydes and alkenes should have a structure similar to the one proposed for acetaldehyde and this may lead to unimolecular H\/D exchange in many more chemical systems.\n\u003C\/p\u003E\n\u003Cp\u003E[1] Signatures of H2CO photodissociation from two electronic states, H.M. Yin, S. H. Kable, X. Zhang and J.M. Bowman, Science, 311, 1443 (2006).\u003Cbr \/\u003E\n[2] Roaming dynamics:  the dominant pathway to molecular products in acetaldehyde photodissociation, B.R. Heazlewood, M.J.T. Jordan, S.H. Kable, T.M. Selby, D.L. Osborn, B.C. Shepler, B.J. Braams, J.M. Bowman, Proc. Nat. Acad. Sci., USA, 105, 12719 (2008)\n\u003C\/p\u003E\n\u003Cp\u003EFor more information contact \u003Ca href=\u0022mailto:christine.payne@chemistry.gatech.edu\u0022\u003EProf. Christine Payne\u003C\/a\u003E (404-385-3125).\u003C\/p\u003E","summary":null,"format":"limited_html"}],"field_subtitle":"","field_summary":[{"value":"Prof. Scott Kable, University of Sydney, Australia\n\nReactions that don\u00e2\u0080\u0099t follow the transition state path\n\nAACP Seminar Series","format":"limited_html"}],"field_summary_sentence":[{"value":"Prof. Scott Kable, University of Sydney, Australia"}],"uid":"27275","created_gmt":"2010-07-16 00:00:00","changed_gmt":"2016-10-08 01:47:09","author":"Shirley Tomes","boilerplate_text":"","field_publication":"","field_article_url":"","field_event_time":{"event_time_start":"2010-09-07T20:30:00-04:00","event_time_end":"2010-09-07T22:00:00-04:00","event_time_end_last":"2010-09-07T22:00:00-04:00","gmt_time_start":"2010-09-08 00:30:00","gmt_time_end":"2010-09-08 02:00:00","gmt_time_end_last":"2010-09-08 02:00:00","rrule":null,"timezone":"America\/New_York"},"extras":[],"related_links":[{"url":"http:\/\/www.chem.usyd.edu.au\/research\/kable.html","title":"Prof. Scott Kable, University of Sydney, Australia"}],"groups":[{"id":"85951","name":"School of Chemistry and Biochemistry"}],"categories":[],"keywords":[{"id":"89","name":"chemistry"}],"core_research_areas":[],"news_room_topics":[],"event_categories":[{"id":"1791","name":"Student sponsored"}],"invited_audience":[],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[{"value":"\u003Cstrong\u003EShirley Tomes\u003C\/strong\u003E\u003Cbr \/\u003EChemistry \u0026amp; Biochemistry\u003Cbr \/\u003E\u003Ca href=\u0022http:\/\/www.gatech.edu\/contact\/index.html?id=st81\u0022\u003EContact Shirley Tomes\u003C\/a\u003E\u003Cbr \/\u003E\u003Cstrong\u003E404-894-0591\u003C\/strong\u003E","format":"limited_html"}],"email":[],"slides":[],"orientation":[],"userdata":""}}}