CCL: Barrierless channel

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!A!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
 >
 >
 >
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