From chemistry-request@server.ccl.net Fri Jul 2 12:55:52 1999 Received: from alchemy.chem.utoronto.ca (alchemy.chem.utoronto.ca [142.150.224.224]) by server.ccl.net (8.8.7/8.8.7) with ESMTP id MAA25953 for ; Fri, 2 Jul 1999 12:55:50 -0400 Received: (from elewars@localhost) by alchemy.chem.utoronto.ca (8.9.3/8.9.3) id MAA28047 for chemistry@ccl.net; Fri, 2 Jul 1999 12:51:43 -0400 (EDT) Date: Fri, 2 Jul 1999 12:51:43 -0400 (EDT) From: "E. Lewars" Message-Id: <199907021651.MAA28047@alchemy.chem.utoronto.ca> To: chemistry@ccl.net Subject: ACTIVATION E AND STABILITY: RESPONSES 1999 July 2 REPLIES TO QUESTION ON ACTIVATION ENERGY AND STABILITY Thanks very much to all (below) who responded to my question (below) E. Lewars ============== QUESTION: 1999 June 24 Hello, Does anyone know have some *experimental facts* for the relationship between ab initio (or DFT, or semiempirical) calculated activation enewrgies and stability? I mean simply the transition state energy minus the reactant energy, for formally unimolecular reactions, e.g. isomerizations, or decompositions. How high does the calculated barrier have to be for the compound to be isolable at room temperature, for example, from actual experience in the lab? Thanks E. Lewars ====================== ANSWERS: #1 Date: Thu, 24 Jun 1999 17:47:36 -0400 (EDT) To: "E. Lewars" Subject: Re: CCL:CALC ACTIVATION E AND STABILITY Hi, As in fact, I am modeling some system ivolving the isomerization and reactions. Depending on the syetems you are interested, the barrier varies from about 2 Kcal/mol to several decads Kcal/mol like 30 or 50. Of course this also depending on the methods you use. Usually HF overestimates the barrier(UMP2 for open shell too). While DFT (in general) underestimates the barrier ( however depending on what kind of DFTs you chose, hybrid DFT behave better than GGA and LSDA in many cases). There are lots of works about this topic, you may want to check in Chem. Phys. Lett., J. Phys. Chem. or J. Chem. Phys. or another European journals. Regards, Wei Quan Tian Univ. of Guelph > ====================== Date: Thu, 24 Jun 1999 17:49:08 -0400 (EDT) X-Sender: wtian@ccshst01 Another point for stability. DFT is better than HF for stability. Wei Quan Tian =============== #2 Date: Fri, 25 Jun 1999 01:31:39 +0200 (MET DST) To: "E. Lewars" Subject: Re: CCL:CALC ACTIVATION E AND STABILITY Dear Mr Lewars, I am very much interested in the answers you will to this question. Would summarize the responds you get or would it be possible to forward those mails which are adressed directly to you and not to CCl. Thank you very much for your efforts, Alexander ******************************************************************************* ("`-/")_.-'"``-._ Alexander Wittkopp (. . `) -._ )-;-,_() Institut fuer Organische Chemie (v_,)' _ )`-.\ ``- Georg-August Universitaet _;- _,-_/ / ((,' Tammannstrasse 2 ((,.-' ((,/ 37077 Goettingen Email: awittko@gwdg.de URL: http://www.gwdg.de/~awittko Germany Phone: +49-(0)551-393290 Fax: +49-(0)551-399475 "eine ungerade Zahl, am besten prim hat immer guenstige Auswirkungen auf cache-trashing." T. Nau ******************************************************************************* #3 Jun 25 Dr. Peter Burger (61) Re: CCL:CALC ACTIVATION E AND STABICommand: Read MessageMessage 5/5 from Dr. Peter Burger Jun 25 '99 at 9:37 am Subject: Re: CCL:CALC ACTIVATION E AND STABILITY To: elewars@alchemy.chem.utoronto.ca (E. Lewars) Hi, Concerning activation energies from DFT etc: simply the transition state energy minus the reactant energy - that's exactly, how it's done - entropic factors, however are not taken care of this way, but you can do that as well from a force calculation. We have one or two examples, where there was a good match between the experimental data and theory. Experimentally at RT, 25 kcal/mol is usually what one considers OK and gives you a half life of one day (unimolecular rxn). However, if required, experimental chemist can go down to the 15+ kcal/mol range, to isolate compounds, below this temperature things are getting more nasty, but it is possible, too, by instant freezing techniques and assuming that the compound is stable in the solid state. So just assume that you need in the range of some minutes to an hour to handle things and then use the Eyring equation to calculate the barrier at a given temperature! Clearly, I am referring here to preparative techniques rather than matrix isolation or gasphase studies. Best wishes Peter ------------------------------------------------------- Peter Burger University of Zuerich ============