From chemistry-request \\at// www.ccl.net Tue Aug 11 13:32:27 1998 Received: from alchemy.chem.utoronto.ca (alchemy.chem.utoronto.ca [142.150.224.224]) by www.ccl.net (8.8.3/8.8.6/OSC/CCL 1.0) with ESMTP id NAA22811 Tue, 11 Aug 1998 13:32:26 -0400 (EDT) Received: (from elewars |-at-| localhost) by alchemy.chem.utoronto.ca (8.7.4/8.7.3) id NAA26595 for chemistry |-at-| www.ccl.net; Tue, 11 Aug 1998 13:32:26 -0400 (EDT) Date: Tue, 11 Aug 1998 13:32:26 -0400 (EDT) From: "E. Lewars" Message-Id: <199808111732.NAA26595 -A_T- alchemy.chem.utoronto.ca> To: chemistry%!at!%www.ccl.net Subject: NEG ACTIV E: SUMMARY Tuesday 1998 August 11 Hello, Thanks to all who replied to my question about negative activation energies: P-O Norby, A. Korkin, S. Abrash, M. Hohmann, J. Vill'ai Fre, l. Burke, A. Shusterman and J. Perry (I hope I haven't missed any one). The question was: Sometimes one gets a _negative_ value for an activation E. For example O O \\ \\... C---O C---O | | -----> . . transition state --> CO2 + N2 N===N . . ..... N====N -297.56194 h -297.56281 h activ E = -0.00087 h = -2.28 kJ/mol G2(MP2) energies Sometimes one gets a negative value with and without the ZPE corection. Is there any special theoretical significance to such "impossible" activation E's? Or can one just say that (since not much below zero) they are compatible with a low positive barrier? Thanks E. Lewars ============== I now see that -ve activation E's can easily be obtained if the barrier is low; they are essentially just errors caused by errors in the ZPE, or, in the case of single-point calcs on a structure optimized at a lower level (which is what G2-type calcs are), errors can arise from the fact that the lower-level statonary points will not be exactly the max and min on the higher-level PES. . . . . .x . x . . | . | .. . | . .~. | -ve ~ . | . ./|\ . | /|\ . \|/ . .| . \|/ | . ----------- x . x | +ve . . |. . | . . ________|_. . | ~ . ~ . I think this is the cause in this case; as others pointed out (SUMMARY BELOW), there can be other causes of neg activ E's, like precomplexation (van der Waals complexes) and etc (J Chem Phys, 1997, 107, 7266), and when the Arrhenius A factor dominates the exponential factor (deltaE ~- 0). ------------ SUMMARY OF RESPONSES: 3-AUG-1998 17:36:45.52 >From: IN%"peon-: at :-medchem.dfh.dk" "Per-Ola Norrby" To: IN%"elewars#* at *#alchemy.chem.utoronto.ca" "E. Lewars" Subj: CCL:NEGATIVE ACTIVATION E's Well, in this case, it just means you use an improper method... The G2 methods are extrapolation methods that depend on the geometry being well represented by a low level method. In this case, you are using HF/6-31G* to locate the TS geometry, I guess this is the source of your problem. If you use a method where you calculate the energy at the same level as your geometry, this type of problem should disappear (there are other possible sources of apparent negative TS energies, like precomplexing, but that should not apply in your case). For some good possible choices for geometry optimization levels, I'd suggest MP2/6-311G**, or B3LYP/6-31G* (depending on your computational resources and/or theological leaning). The G2 methods were developed to calculate VERY accurate heats of formation, but in general, I wouldn't recommend them for TS calculations (just a personal opinion, I'd like to hear if there is a concensus on this). Regards, Per-Ola Norrby ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Per-Ola Norrby, peon -A_T- medchem.dfh.dk Royal Danish School of Pharmacy, Dept. of Med. Chem. Universitetsparken 2, DK-2100 Copenhagen, Denmark Tel: +45-35376777-506, +45-35370850, fax +45-35372209 ============= >From: IN%"r40757 |-at-| email.mot.com" "Anatoli Korkin" To: IN%"elewars |-at-| alchemy.chem.utoronto.ca" "E. Lewars", IN%"chemistry |-at-| infome ister.ccl.net" "chemistry;at;www.ccl.net", IN%"bartlett;at;qtp.ufl.edu" "Rod Bartlett" Subj: CCL:NEGATIVE ACTIVATION E's Negative activition energies you may get in two cases (may be someone knows more): 1) There is an intermediate complex between reagents and a TS. In this case everything is "normal". Some SN2 gas phase reactions have "negative" activitations energies. This led first to a suspecion among experimentalists, who studied these reactions in solutions, that theory was wrong, but later this result has been confirmed for gas phase reactions. 2) For very low barriers you may get negative activation energies, if you do (higher level) single point calculations using geometries optimized at another (lower correlation) approach. If you optimize a TS and reagents at the same level and there is no an intemediate mimimum on a reaction pathway you should not get your electronic energy for TS lower than that one for reagents. Of course, zero point energy correction may change the sign of the barrier, if it is very small (e.g. 1 kcal/mol or lower). Anyway at such low barriers a conventional transition state theory is hardly applicable, and a dynamical treatment is required. Regards, Anatoli P.S. We have a paper on CN2O2 species including the complex below. An activation energy we got at CCSD(T)/TZP//MBPT(3)/6-31G* was 0.5 kcal/mol before and -0.3 kcal/mol after ZPE correction. ----------- ****************************************************************** Anatoli A. Korkin, Ph.D Predictive Engineering Lab Computational Chemist Semiconductor Products Sector Motorola Inc., MD M360 Tel: (602) 655-3171 2200 W. Broadway Road Fax: (602) 655-5013 Mesa AZ 85202 E.mail: r40757*- at -*email.mot.com ******************************************************************* X-Sender: sabrash-: at :-facstaff.richmond.edu Subject: Re: CCL:NEGATIVE ACTIVATION E's Dear Professor Lewars, I cannot give you the answer for the theoretical case, but can explain t he significance for experimentally determined negative activation energies. The rate constant for a reaction in the Arrhenius model is given by k = A(T) exp (-Ea/RT). Because the activation energy temperature dependence typically swamps that due to the preexponential factor, typical Arrhenius analyses ignore the temperature dependence of the A term. However when Ea is for all practical purposes zero, then the temperature dependence is that of the A factor which tends to increase with decreasing temperature. This therefore appears as a negative activation energy. Such reactions are better fit by the equation k = AT^m exp(-E0/RT), where E0 is equivalent to the zero k activation energy. Best regards, Sam Abrash >============== Sam Abrash Associate Professor Department of Chemistry University of Richmond Richmond, VA 23174 (804) 289-8248 Fax: (804)289-8482 sabrash&$at$&richmond.edu "I believe in the open mind, but not so open your brain falls out." ================== Subject: neg. E act To: elewars ^at^ alchemy.chem.utoronto.ca Date: Mon, 3 Aug 1998 15:10:42 -0500 (EDT) X-Mailer: ELM [version 2.4 PL20] Hi! Negative activation energies can result - from ZPE corrections or - if you derive your relative energies from single point calculations and the geometries used are not good enough (i.e. the geometries at the level of opt and at the level of single points deviate considerably) If you get a "neg. activation energy" at the level of geometry opt., something is wrong. I suggest to look at the vector of the imaginary frequency first. If it seems to be ok check the transition state with an IRC (intrinsic reaction coordinate) calculation. Good luck, Matthias M. Hohmann ============ Jordi Villa i Freixa Department of Chemistry University of Southern California Los Angeles, CA, USA, 90089-1062 Tlf: 1-(213)-740 7671 Fax: 1-(213)-740 2701 villa-!at!-mutant.usc.edu http://t1.chem.umn.edu/~villa -What you call "activation energy" is, in fact, an energy barrier. Activation energy is a kinetic concept that comes from the Arrhenius equation. -Probably the small negative value you obtain is due to the G2 method, where you are not exploring exactly a concrete potential energy surface, but adding the contributions of several corrections to the initial energy surface. -The example you are posting is really interesting because of the low barrier (in your case is negative, but I guess that using only the non-G2-corrected surface the barrier would be positive or zero). This means that considering the barrier to be the given value could lend you to an error when trying to understand kinetic experimental results, for example. The location of only the saddle point on this reaction will not be enough to understand the dynamics of the system, and a treatment with variational transition state theory could be necessary. If you want more information about the dynamic treatment of this kind of barrierless or almost barrierless reactions take a look to: J. Chem. Phys. 1997, 107, 7266 J. Am. Chem. Soc. 1998, 120, 5559. I hope this will help Yours Jordi =========== Aug 3 Luke A. Burke (31) impos tsCommand: Read MessageMessage 5/17 from Luke A. Burke Aug 3 '98 at 6:59 pm Date: Mon, 3 Aug 1998 18:59:03 -0400 X-Mailer: Z-Mail (3.2.3 08feb96 MediaMail) To: elewars*- at -*alchemy.chem.utoronto.ca Subject: impos ts Mime-Version: 1.0 ............... If your structure is indeed a ts, then there must be a minimum and another ts between your drawn structures. You did not give the geometry of the ts but I suspect that there is the involvement of van der Waals complexes. BTW, in the history of ts oddities, I found a reaction (rotation) where the Ea >0, but Ha<0 and Ga<0. Salut, de koste en de wind van achter, en de steenweg tot huis toe (plat Leuvense) Luke -- Luke Anthony Burke tel:609-225-6158 (-6142) Professor and Chair, fax:609-225-6506 Rutgers University burke {*at*} camden.rutgers.edu Camden, NJ 08102, USA http://camchem.rutgers.edu/~burke ============== Alan Shusterman Date: 03 Aug 98 13:42:35 PDT Subject: Re: CCL:NEGATIVE ACTIVATION E's I noticed that your negative barrier results from comparison of = G2(MP2) energies. I'm not very familiar with the G2 model, but I = was wondering what model was used to identify the reactant minimum = and transition state, and did this model give a negative or = positive barrier? Alan ---------------- Alan Shusterman Department of Chemistry Reed College Portland, OR www.reed.edu/~alan ============================== Aug 3 Jason K. Perry (31) Re: CCL:NEGATIVE ACTIVATION E'sCommand: Read MessageMessage 8/17 from Jason K. Perry Aug 3 '98 at 1:53 pm Date: Mon, 3 Aug 1998 13:53:36 -0700 (PDT) To: "E. Lewars" Subject: Re: CCL:NEGATIVE ACTIVATION E's > Sometimes one gets a negative value with and without the ZPE corection. The reason you're seeing negative activation energies is because with the G2 method you are calculating the geometry at one level and reporting an energy at a different level. At the level that the geometry was determined it is most certainly the case that the activation barrier is positive. For example, if I calculate geometries for reactant, product and transition state at the HF/6-31G** level, the transition state will be correct for that level and show a positive barrier. If I then do single point B3LYP/6-31G** calculations using those HF/6-31G** geometries the transition state will not be correct for the new level, and thus there is no guarantee that the barrier will be positive. Jason Perry First Principles Research, Inc. ================