CCL: Barrierless channel



 Sent to CCL by: bonoit bonoit [ablux13]*[gmail.com]
 Dear Eric,
 Thank you very much for your fruitful remarks. Below, I give answers
 for questions you raised pointby point.
 1) Have you confirmed the TS via IRC calculations?  If not, you
 should.  This will tell you if it is the TS you hoped to locate and
 also give you reasonable starting geometries for optimizing the pre-
 and post-TS complexes.
 Yes, I did and the TS is really the one I hoped.
 2) Pre- and post-TS complexes (minima) are very commonly found as
 stationary points on the Born-Oppenheimer surface.  My personal
 protocol is to look for them every time, using the end points from
 IRCs as my initial geometries.
 It's the same protocol I follow.
 3) You’ve not said what type of energy (electronic, enthalpic,
 Gibbs…).  This makes a difference, especially if entropy is a factor.
 For example, consider a pre-TS complex that is a minimum on the BO
 surface, with an electronic energy lower than separated reactants.
 Once corrected to Gibbs free energy, you may find that the complex is
 higher in energy than the separated reactants.  Similar may be true
 for post-TS complexes.
 I used the Gibbs energy. Hence, the intermediate and the TS are still
 lower than separated reactants.
 4) If your intermediate is higher in energy than the TS and higher
 than separated reactants, you have not found a barrierless process.
 You have a found a process where the barrier is equivalent to the
 difference between intermediate and reactants.  A barrierless process
 would have reactants as the highest point with everything else lying
 below.
 The intermediate and the TS are lower than separated reactants. So,
 it's a barrierless channel.
 5) How big is the energy difference?  Is it within the accepted error
 bars of your method?
 Well, the energy difference is less than 2 kcal/mol.
 6) Finally, this is a common shortcoming of BO calculations.  Unless
 you are comparing to experiments conducted near absolute zero, the BO
 surface, even corrected to Gibbs free energy, is a crude approximation
 of reality.  Nuclear dynamics can prove to be the determining factor
 when considering competing channels.
 Cheers,
 Benoit
 2019-07-11 17:30 UTC+02:00, Eric V. Patterson
 eric.patterson**stonybrook.edu <owner-chemistry(0)ccl.net>:
 > Benoit,
 >
 > I offer a very observations in no particular order.
 >
 > 1) Have you confirmed the TS via IRC calculations?  If not, you should.
 > This will tell you if it is the TS you hoped to locate and also give you
 > reasonable starting geometries for optimizing the pre- and post-TS
 > complexes.
 >
 > 2) Pre- and post-TS complexes (minima) are very commonly found as
 stationary
 > points on the Born-Oppenheimer surface.  My personal protocol is to look
 for
 > them every time, using the end points from IRCs as my initial geometries.
 >
 > 3) You’ve not said what type of energy (electronic, enthalpic,
 Gibbs…).
 > This makes a difference, especially if entropy is a factor.  For example,
 > consider a pre-TS complex that is a minimum on the BO surface, with an
 > electronic energy lower than separated reactants.  Once corrected to Gibbs
 > free energy, you may find that the complex is higher in energy than the
 > separated reactants.  Similar may be true for post-TS complexes.
 >
 > 4) If your intermediate is higher in energy than the TS and higher than
 > separated reactants, you have not found a barrierless process.  You have a
 > found a process where the barrier is equivalent to the difference between
 > intermediate and reactants.  A barrierless process would have reactants as
 > the highest point with everything else lying below.
 >
 > 5) How big is the energy difference?  Is it within the accepted error bars
 > of your method?
 >
 > 6) Finally, this is a common shortcoming of BO calculations.  Unless you
 are
 > comparing to experiments conducted near absolute zero, the BO surface, even
 > corrected to Gibbs free energy, is a crude approximation of reality.
 > Nuclear dynamics can prove to be the determining factor when considering
 > competing channels.
 >
 > Cheers,
 > Eric
 >
 >
 >> On Jul 11, 2019, at 7:21 AM, Benoit Gearald ablux13-x-gmail.com
 >> <owner-chemistry- -ccl.net> wrote:
 >>
 >>
 >> Sent to CCL by: "Benoit  Gearald" [ablux13(a)gmail.com]
 >> Dear CCLers,
 >>
 >> I'm studying a multichannel reaction. I've found a barrierless channel
 >> i.e.
 >> the transition state is below the intermediate and I have confirmed
 that
 >> by
 >> different methods of calculation.
 >> My question is whether I have to locate a molecular complex that links
 the
 >>
 >> intermediate to the TS or it's ok as it is?
 >>
 >> I'm really confused on that point and need some explanations and
 >> clarifications.
 >>
 >> Best regards,
 >>
 >> Benoit>> > > >
 >>
 >
 >