Summary of activation free energy

From: Atte Sillanpaa <asillanp' at \`mail.student.oulu.fi>
Subject: Summary of activation free energy
Date: Thu, 23 Mar 2000 19:24:14 +0200
First of all thanks for everyone who replied. From the responses I gather
 that searching for a free energy maximum based on potential energy surface
 even if there is no extremum on the PES is a sound approach. The program
 POLYRATE, which Jordi suggested, gives possibility to use more
 sophisticated theories to calculate rate constants.
 As another alternative Cory Pye suggested the thermodynamic integration
 scheme from constrained molecular dynamics.
 The responses (slightly edited to save space) were:
 --------8<-------------------------
 From: Cory Pye <cpye' at \`crux.stmarys.ca>
 On Wed, 15 Mar 2000, Atte Sillanpaa wrote:
 > Hello CCLers,
 > I'm trying to calculate the free energy of activation for a process where
 > there is no minimum on the potential energy surface (PES). The (ethene
 > polymerisation termination) reaction begins with
 > a beta elimination from an alkyl group to the metal center. This forms a
 > fairly unstable metal hydride and a hapto2-alkene. The PES for scanning
 > the alkene away from the metal increases monotonically approaching the
 > separated products value. The free energy of separated products is readily
 > calculated, and is naturally much lower relative to PE. However, as the
 > alkene recedes from the metal it gradually gains the translational and
 > rotational degrees of freedom and its free energy drops, but there is a
 > maximum at some point.
 >
 > The question is that if I calculate the free energy with the harmonic
 > approximation at some (ambigious) constrained geometry (distance from the
 > metal) is it of any approximate value for the free energy of activation?
 If you use this approach, you would probably have to select some "reaction
 coordinate" and calculate the free energy at some selected points along
 your
 coordinate, say every 0.5 angstroms, to roughly locate your Free energy
 transition structure. Then you could refine the region near the maximum.
 >
 > Is there some other method for calculating the free energy of activation
 > in this kind of PES?
 >
 Another way is to calculate the free energy using constrained molecular
 dynamics with a DFT energy. Tom Ziegler's group at the U of Calgary has done
 these  types of simulations (I was a post-doc there with him on a different
 project). Check out:
 1) JACS            118  4434-4441  (1996) - Cp2Zr(C2H5)+   + C2H4
 2) JACS            118 13021-13030 (1996) - (CpSiH2NH)TiR+ + C2H4 I.
 3) Organometallics 16   3454-3468  (1997) - ""                    II.
 4) JACS            119  6177-6186  (1997) - Ni(II)-diimine + C2H4
 5) JPC B           101  7877-7880  (1997) - NI(II)-diimine + C2H4
 6) JACS            120  2174-2175  (1998) - Ti(IV)-H2
 7) "Transition State Modelling for Catalysis", D. G. Truhlar and K.
 Morokuma,
    (eds), ACS Symp. Ser. 721, ACS, Washington, D.C. (1999), p173-186
 8) Catalysis Today 50   479-500    (1999) - overview
    *************    !  Dr. Cory C. Pye
  *****************  !  Assistant Professor
 ***   **    **  **  !  Theoretical and Computational Chemistry
 **   *  ****        !  Department of Chemistry, Saint Mary's University
 **      *  *        !  923 Robie Street, Halifax, NS B3H 3C3
 **      *  *        !  cpye' at \`crux.stmarys.ca   http://husky1.stmarys.ca/cpye
 ***     *  *    **  !  Ph: (902)-420-5654  FAX:(902)-496-8104
  *****************  !
    *************    !  Les Hartree-Focks (Apologies to Montreal Canadien Fans)
 ----8<-----------------------
 From: Jordi =?iso-8859-1?Q?Vill=E0?= <jorgevil' at \`usc.edu>
 Dear Atte,
 take a look at GAUSSRATE (http://comp.chem.umn.edu/sds/), which you can use for
 calculating the free energy profile for your reaction using Variational
 Transition
 State Theory (VTST), even if you don't have any maximum on the PES. In principal
 you have two general approaches to use for solving your problem using VTST. The
 first is the use of the geometries generated by an intrinsic reaction path (IRC)
 calculation and the second is using a simple distinguished reaction coordinate
 (DCP). Both of these possibilities can be readily explored using GAUSSRATE (for
 automatically calculate the needed structures if you have Gaussian installed in
 your computer) or its father POLYRATE (if you have previously calculated those
 paths and you kept first and second derivatives along them)
 Jordi
 --
 Jordi Villa i Freixa
 Department of Chemistry, University of Southern California
 3620 S McClintock Av. #418; Los Angeles, CA, USA, 90089-1062
 Tlf: 1-(213)-740 7671 Fax: 1-(213)-740 2701
 jorgevil' at \`usc.edu      http://laetro.usc.edu/jorgevil
 ----------8<-----------------------
 From: michael braunschweig <charly' at \`Citrin.Chemie.Uni-Dortmund.de>
 hi atte,
 to obtain an trasition state try to calculate a first order saddle point
 on the pes without making any constrains. to find the start structure
 make a quadratic synconous transit. to be sure this is a valid
 transition state make a frequency calculation (one imaginary frequency)
 and compute an irc-run. in g98 the keywords are: opt=qst2 (or qst3),
 opt=ts and irc.
 hope this will help
 michael braunschweig
 university of dortmund
 department of chemistry
 germany
 	atte.sillanpaa' at \`oulu.fi		+358 (0)8 553 1681 (work), KE 368
 	Dept. of Physical Chemistry	+358 (0)40 592 7369 (gsm)
 	Po-BOX 3000
 	FIN-90014 Oulun Yliopisto
 	FINLAND
                   |   Entropy requires no maintenance.   |