From owner-chemistry@ccl.net Thu Mar 19 16:04:00 2020 From: "Abrash, Samuel sabrash,+,richmond.edu" To: CCL Subject: CCL: Negative activation energies Message-Id: <-53999-200319160234-5073-SEoj7X5InyIVFMgxC2gxYw]*[server.ccl.net> X-Original-From: "Abrash, Samuel" Content-Language: en-US Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset="utf-8" Date: Thu, 19 Mar 2020 20:02:26 +0000 MIME-Version: 1.0 Sent to CCL by: "Abrash, Samuel" [sabrash-.-richmond.edu] Hi Folks, I’ve been reading some papers on Criegee intermediates, that talk about an initio and DFT calculations resulting in negative activation energies. I know that experimentally negative activation energies are an artifact observed when the activation barrier is close to zero and the preexponential factor decreases with increasing temperature, for example in cases where the activated complex includes a van der Waals complex. But I don’t understand how a theoretical calculation of a PES can result in a negative activation energy. Any downhill PES simply has an EA of zero. The only answer I can think of is if the authors are taking their PES and calculating the EA using Eyring theory or the equivalent and that the Eyring calculation has a preexpontial factor that decreases with increasing T. (They don’t deign to explain how the negative EAs are obtained). If anyone can enlighten me it would be much appreciated. Best regards, Sam Abrash Associate Professor University of Richmond From owner-chemistry@ccl.net Thu Mar 19 17:49:00 2020 From: "Eric V. Patterson eric.patterson|,|stonybrook.edu" To: CCL Subject: CCL: Negative activation energies Message-Id: <-54000-200319174738-6342-0AQh6FE/YKWOvV8XugobHA(_)server.ccl.net> X-Original-From: "Eric V. Patterson" Content-Type: multipart/alternative; boundary="Apple-Mail=_E3A44668-7EBF-4339-9D2E-1FCA9C5B044A" Date: Thu, 19 Mar 2020 17:47:31 -0400 Mime-Version: 1.0 (Mac OS X Mail 13.0 \(3608.60.0.2.5\)) Sent to CCL by: "Eric V. Patterson" [eric.patterson()stonybrook.edu] --Apple-Mail=_E3A44668-7EBF-4339-9D2E-1FCA9C5B044A Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=utf-8 Hello Sam, There are at least a couple of ways that a negative activation barrier = might be reported from a computational method. The two I see most often = both involve errors in treating the system or interpreting the data. 1) The barrier has been measured from reactants at infinite separation = rather from a pre-reaction complex that is lower in energy than the = separated reactants. I=E2=80=99ve seen this happen most often when = people have failed to confirm their TS via IRC and incorrectly assume = the reference point. 2) The barrier is computed from free energies, and there is some = significant error in the free energy corrections. This could be from = large errors in entropy, not properly accounting for standard states in = bimolecular reactions, or so forth. I=E2=80=99m sure there are other cases. I hope you=E2=80=99re well, Eric ---------------------------------------------------- Eric V. Patterson, PhD Director of Undergraduate Laboratories Senior Lecturer > On Mar 19, 2020, at 4:02 PM, Abrash, Samuel sabrash,+,richmond.edu = wrote: >=20 >=20 > Sent to CCL by: "Abrash, Samuel" [sabrash-.-richmond.edu] > Hi Folks, >=20 > I=E2=80=99ve been reading some papers on Criegee intermediates, that = talk about an initio and DFT calculations resulting in negative = activation energies. I know that experimentally negative activation = energies are an artifact observed when the activation barrier is close = to zero and the preexponential factor decreases with increasing = temperature, for example in cases where the activated complex includes a = van der Waals complex. >=20 > But I don=E2=80=99t understand how a theoretical calculation of a PES = can result in a negative activation energy. Any downhill PES simply has = an EA of zero. The only answer I can think of is if the authors are = taking their PES and calculating the EA using Eyring theory or the = equivalent and that the Eyring calculation has a preexpontial factor = that decreases with increasing T. (They don=E2=80=99t deign to explain = how the negative EAs are obtained). >=20 > If anyone can enlighten me it would be much appreciated. >=20 > Best regards, > Sam Abrash > Associate Professor=20 > University of Richmond >=20 >=20 >=20 > -=3D This is automatically added to each message by the mailing script = =3D- > To recover the email address of the author of the message, please = change>=20>=20>=20 > Subscribe/Unsubscribe:=20>=20>=20 > Job: http://www.ccl.net/jobs=20 > Conferences: = http://server.ccl.net/chemistry/announcements/conferences/ >=20>=20>=20>=20 >=20 --Apple-Mail=_E3A44668-7EBF-4339-9D2E-1FCA9C5B044A Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=utf-8 Hello= Sam,

There are at = least a couple of ways that a negative activation barrier might be = reported from a computational method.  The two I see most often = both involve errors in treating the system or interpreting the = data.

1) The = barrier has been measured from reactants at infinite separation rather = > from a pre-reaction complex that is lower in energy than the separated = reactants.  I=E2=80=99ve seen this happen most often when people = have failed to confirm their TS via IRC and incorrectly assume the = reference point.

2) The barrier is computed from free energies, and there is = some significant error in the free energy corrections.  This could = be from large errors in entropy, not properly accounting for standard = states in bimolecular reactions, or so forth.

I=E2=80=99m sure there are other = cases.

I hope = you=E2=80=99re well,
Eric

----------------------------------------------------
Eric V. Patterson, PhD
Director of Undergraduate Laboratories
Senior = Lecturer





On Mar 19, 2020, at 4:02 PM, Abrash, Samuel sabrash,+,richmond.edu <owner-chemistry[*]ccl.net> wrote:


Sent to CCL by: "Abrash, Samuel" [sabrash-.-richmond.edu]
Hi= Folks,

I=E2=80=99ve been reading some = papers on Criegee intermediates, that talk about an initio and DFT = calculations resulting in negative activation energies.  I know = that experimentally negative activation energies are an artifact = observed when the activation barrier is close to zero and the = preexponential factor decreases with increasing temperature, for example = in cases where the activated complex includes a van der Waals = complex.

But I don=E2=80=99t understand how = a theoretical calculation of a PES can result in a negative activation = energy.  Any downhill PES simply has an EA of zero.  The only = answer I can think of is if the authors are taking their PES and = calculating the EA using Eyring theory or the equivalent and that the = Eyring calculation has a preexpontial factor that decreases with = increasing T. (They don=E2=80=99t deign to explain how the negative EAs = are obtained).

If anyone can enlighten me = it would be much appreciated.

Best = regards,
Sam Abrash
Associate Professor
University of Richmond



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= --Apple-Mail=_E3A44668-7EBF-4339-9D2E-1FCA9C5B044A-- From owner-chemistry@ccl.net Thu Mar 19 19:26:00 2020 From: "David Shobe shobedavid _ gmail.com" To: CCL Subject: CCL: Negative activation energies Message-Id: <-54001-200319192407-10052-YsFspdSk0tyOl+WKMfqfyA#server.ccl.net> X-Original-From: David Shobe Content-Type: multipart/alternative; boundary="000000000000bef49c05a13d76aa" Date: Thu, 19 Mar 2020 18:23:50 -0500 MIME-Version: 1.0 Sent to CCL by: David Shobe [shobedavid+/-gmail.com] --000000000000bef49c05a13d76aa Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable 3) If the TS and reactants are geometry optimized at one level, and then single point energies calculated at a higher level, one can get a negative activation energy. Of course, this is a computational artifact. --David Shobe On Thu, Mar 19, 2020, 6:13 PM Eric V. Patterson eric.patterson|,| stonybrook.edu wrote: > Hello Sam, > > There are at least a couple of ways that a negative activation barrier > might be reported from a computational method. The two I see most often > both involve errors in treating the system or interpreting the data. > > 1) The barrier has been measured from reactants at infinite separation > rather > from a pre-reaction complex that is lower in energy than the > separated reactants. I=E2=80=99ve seen this happen most often when peopl= e have > failed to confirm their TS via IRC and incorrectly assume the reference > point. > > 2) The barrier is computed from free energies, and there is some > significant error in the free energy corrections. This could be from lar= ge > errors in entropy, not properly accounting for standard states in > bimolecular reactions, or so forth. > > I=E2=80=99m sure there are other cases. > > I hope you=E2=80=99re well, > Eric > > ---------------------------------------------------- > Eric V. Patterson, PhD > Director of Undergraduate Laboratories > Senior Lecturer > > > > > > On Mar 19, 2020, at 4:02 PM, Abrash, Samuel sabrash,+,richmond.edu < > owner-chemistry%a%ccl.net > wrote: > > > Sent to CCL by: "Abrash, Samuel" [sabrash-.-richmond.edu] > Hi Folks, > > I=E2=80=99ve been reading some papers on Criegee intermediates, that talk= about an > initio and DFT calculations resulting in negative activation energies. I > know that experimentally negative activation energies are an artifact > observed when the activation barrier is close to zero and the > preexponential factor decreases with increasing temperature, for example = in > cases where the activated complex includes a van der Waals complex. > > But I don=E2=80=99t understand how a theoretical calculation of a PES can= result > in a negative activation energy. Any downhill PES simply has an EA of > zero. The only answer I can think of is if the authors are taking their > PES and calculating the EA using Eyring theory or the equivalent and that > the Eyring calculation has a preexpontial factor that decreases with > increasing T. (They don=E2=80=99t deign to explain how the negative EAs a= re > obtained). > > If anyone can enlighten me it would be much appreciated. > > Best regards, > Sam Abrash > Associate Professor > University of Richmond > > > > -=3D This is automatically added to each message by the mailing script = =3D-> the strange characters on the top line to the %a% sign. You can also > > E-mail to subscribers: CHEMISTRY%a%ccl.net or use= :> > E-mail to administrators: CHEMISTRY-REQUEST%a%ccl.net > or use> > > > --000000000000bef49c05a13d76aa Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
3) If the TS and reactants are geometry optimized at one = level, and then single point energies calculated at a higher level, one can= get a negative activation energy. Of course, this is a computational artif= act.=C2=A0

--David Shobe
=

= On Thu, Mar 19, 2020, 6:13 PM Eric V. Patterson eric.patterson|,|stonybrook.edu <owner-chemistry^ccl.net> wrote:
Hello Sam,

There are at least a couple of ways = that a negative activation barrier might be reported from a computational m= ethod.=C2=A0 The two I see most often both involve errors in treating the s= ystem or interpreting the data.

1) The barrier has= been measured from reactants at infinite separation rather > from a pre= -reaction complex that is lower in energy than the separated reactants.=C2= =A0 I=E2=80=99ve seen this happen most often when people have failed to con= firm their TS via IRC and incorrectly assume the reference point.

2) The barrier is computed from free energies, and there is= some significant error in the free energy corrections.=C2=A0 This could be= from large errors in entropy, not properly accounting for standard states = in bimolecular reactions, or so forth.

I=E2=80=99m= sure there are other cases.

I hope you=E2=80=99re= well,
Eric

--------------= --------------------------------------
Eric V. Patterson, PhD
Director of Undergraduate Laboratories
Senior Lecturer





On Mar 19, 2020, at 4:02 PM, Abrash= , Samuel sabrash,+,richmond.edu <owner-chemistry%a%ccl.net>= ; wrote:


Sent to CCL by: "Abrash, Samuel" = [sabrash-.-richmond.edu]
Hi Folks,

I=E2=80=99ve been reading some p= apers on Criegee intermediates, that talk about an initio and DFT calculati= ons resulting in negative activation energies.=C2=A0 I know that experiment= ally negative activation energies are an artifact observed when the activat= ion barrier is close to zero and the preexponential factor decreases with i= ncreasing temperature, for example in cases where the activated complex inc= ludes a van der Waals complex.

But I don=E2=80=99t understand how a = theoretical calculation of a PES can result in a negative activation energy= .=C2=A0 Any downhill PES simply has an EA of zero.=C2=A0 The only answer I = can think of is if the authors are taking their PES and calculating the EA = using Eyring theory or the equivalent and that the Eyring calculation has a= preexpontial factor that decreases with increasing T. (They don=E2=80=99t = deign to explain how the negative EAs are obtained).

If anyone can e= nlighten me it would be much appreciated.

Best regards,
Sam Abras= h
Associate Professor
University of Richmond



-=3D Thi= s is automatically added to each message by the mailing script =3D-
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