*From*: "Eric V. Patterson" <eric.patterson|stonybrook.edu>*Subject*: CCL: Barrierless channel*Date*: Fri, 12 Jul 2019 21:09:03 -0400

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> >