CCL: Barrierless channel



 Sent to CCL by: "Eric V. Patterson" [eric.patterson() stonybrook.edu]
 Dear Benoit,
 So it seems to me that you definitely need to locate stationary points for the
 complexes.  That may change the reaction energy diagram such that there is a
 barrier from pre-TS complex to TS, or maybe not.  Perhaps you have indeed found
 a barrierless channel.  A difference of less than 2 kcal/mol is just enough to
 be worth mentioning, but not enough to lead to firm conclusions unless you are
 using very accurate methods.  The data suggest a rather flat PES with little
 distinction among various stationary points.
 Cheers,
 Eric
 > On Jul 12, 2019, at 4:14 PM, bonoit bonoit ablux13%a%gmail.com
 <owner-chemistry- -ccl.net> wrote:
 >
 >
 > 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]*[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>
 >