Hello Benoit,
It sounds like you have missed at least one low energy intermediate a= long the path. You could tie everything together with some IRC's after you'= ve identified the relevant transition states.

Best,

Am= y

=A0

=A0 From: Benoit Benoit bonoit_10**yahoo.fr = <owner-chemistry(_)ccl.net>
To: "Austin, Amy J " <amy_jean_austin(_)yahoo.com>
<= b>Sent: Tuesday, February 19,= 2013 8:35 AM
style=3D"font-weight: bold;">Subject:= CCL: Negative activation energy/non Arrhenius

= =0A>
Sent to CCL by: "Benoit Benoit" [bonoit_10_+_yahoo.fr]
De= ar CCLers,

I'd like to enquire about negative activation energies th= at some reactions can have, what's the meaning of such negative value?
D= oes is mean that the reaction is barrierless?

Any help is appreciate= d

Best regards

Benoit

the strange charac= ters on the top line to the (_) sign. You can also
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=A0 =0A> =0A> =0A> =0A> -=3D This is automatically added to ea= ch message by the mailing script =3D-=0A> To recover the email address of t= he author of the message, please change> Conferences: http://server.ccl.net= /chemistry/announcements/conferences/>=0A> =0A=0AJames E. Eilers=0AProfesso= r=0AChemistry Department=0ASouthern Illinois University Edwardsville=0A(618= )650-3559=0A=0A=0A=0A-=3D This is automatically added to each message by th= e mailing script =3D-=0ATo recover the email address of the author of the m= essage, please change=0Athe strange characters on the top line to the :_: sig= n. You can also=0A=0A= =0A=0A=A0 =A0 http://www.cc= l.net/cgi-bin/ccl/send_ccl_message=0A=0AE-mail to administrators: CHEMISTRY= -REQUEST:_:ccl.net or use=0A=A0 =A0 http://www.ccl.net/cgi-bin/ccl/send_ccl_m= essage=0A=0ASubscribe/Unsubscribe:=A0 =A0 http://www.ccl.net/chemistry/sub_= unsub.shtml=0A=0A=0A= =0AJob: http://www.ccl.net/jobsConferences: http://server.ccl.net/chemistry= /announcements/conferences/=0A=0ASearch Messages: http://www.ccl.net/chemis= try/searchccl/index.shtml=0A=0AIf your mail bounces from CCL with 5.7.1 err= or, check:=0A=A0 =A0=0A=0ARTFI: http://www.= ccl.net/chemistry/aboutccl/instructions/ ---790415396-1344751577-1361486421=:89304 Content-Type: text/html; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable

Hi Benoit,

If this is a c= omputational study of the PES, there is another possibility for a negative = barrier height. DFT's underestimate barrier heights. The hybrid functionals= are better for this purpose, but B3LYP has issues due to the training set.= If you need a DFT for the computational expense, the MPWB1K might help.
Best,

Amy

= From: James Eilers jeiler= s[A]siue.edu <owner-chemistry:_:ccl.net>
To: "Austin, Amy J " <amy_jean_austin:_:yah= oo.com>
Sent: Thursday, Febr= uary 21, 2013 12:50 PM
Subject: CCL: Negative activation energy/non Arrhenius

=

Sent to CCL by: James Eilers [jeilers%%siue.edu]
One should work= with the modified Arrhenius expresion A sqrt(T) exp(-Ea/RT) before g= etting all that excited or perplexed by negative experimental activation en= ergies.

On Feb 21, 2013, at 9:55 AM, Abrash, Sam sabrash(0)richmond.= edu wrote:

>
> Sent to CCL by: "Abrash, Sam" [sabrash/./ri= chmond.edu]
> Hi Benoit,
>
> Amy is correct if you deter= mined the negative activation energy by theoretical means. If, on the= other hand, it is an experimental result, what it means is that you have a= n activation barrier either of zero or sufficiently close to zero that the = temperature dependence of the kinetic pre-exponential factor, A, dominates over that of the activation energy. The temperature depend= ence of the preexpontial factor will look like a negative activation energy= .
>
> Best regards,
> Sam Abrash
>
> Samuel= A. Abrash
> Department of Chemistry
> Coordinator, Environment= al Studies Progrram
> University of Richmond
> Richmond, VA 231= 73
> Phone (Gottwald): 804-289-8248
> Phone (Internationa= l Center): 804-484-1473
> Fax: 804-287-1897
> E-mail:&nbs= p; sabrash~!~richmond.edu
> Web-page: http://www.richmond.edu/~sabrash<= br>> "That's one of the tragedies of this life, that the men who are mos= t in need of a beating are always enormous." Rudy Valee, as J. D. Hackensac= ker, III, in the Preston Sturges Film "The Palm Beach Story"
>
&g= t; -----Original Message-----
>> From: owner-chemistry+sabrash=3D=3Drichmond.edu~!~ccl.net [mailto:owner-chemistr= y+sabrash=3D=3Drichmond.edu~!~ccl.net] On Behalf Of Amy Austin amy_jean_aus= tin*yahoo.com
> Sent: Tuesday, February 19, 2013 8:54 PM
> To: = Abrash, Sam
> Subject: CCL: Negative activation energy/non Arrhenius<= br>>
> Hello Benoit,
>
> It sounds like you have mis= sed at least one low energy intermediate along the path. You could tie ever= ything together with some IRC's after you've identified the relevant transi= tion states.
>
> Best,
>
> Amy
>
> <= br>>
>
> ________________________________
> From: Be= noit Benoit bonoit_10**yahoo.fr <owner-chemistry(_)ccl.net>
> T= o: "Austin, Amy J " <amy_jean_austin(_)yahoo.com>
> Sent: Tuesd= ay, February 19, 2013 8:35 AM
> Subject: CCL: Negative activation ene= rgy/non Arrhenius
>
>
> Sent to CCL by: "Benoit Benoit" [bonoit_10_+_yahoo.fr]
> Dear CCLers,
>
> I'd l= ike to enquire about negative activation energies that some reactions can h= ave, what's the meaning of such negative value?
> Does is mean that t= he reaction is barrierless?
>
> Any help is appreciated
>= ;
> Best regards
>
> Benoitthe strange characters on th= e top line to the (_) sign. You can also
>
>
>
>= E-mail to administrators: CHEMISTRY-REQUEST(_)ccl.net = or use
>
>
> --752418115-821153590-1361325269=3D1634> Content-Type: text/html; charset=3Do-8859-1
> Content-Transfer-= Encoding: quoted-printable
>
> <html><body><div= style=3D"color:#000; background-color:#fff; font-family:arial, helvetica, = sans-serif;font-size:12pt">Hello Benoit, It sounds li= ke you have missed at least one low energy intermediate along the path. You could tie everything together with some IRC's after you've identified = the relevant transition states. Best,  <br&g= t; Amy <div> <= /div><div> </div> <div style=3D"font-famil= y: arial, helvetica, sans-serif; font-size: 12pt;"> <div style=3D"fon= t-family: times new roman, new york, times, serif; font-size: 12pt;"> &l= t;div dir=3D"ltr"> <hr size=3D= "1"> From:</sp= an> Benoit Benoit bonoit_10**yahoo.fr <owner-chemistry(= _)ccl.net> "Austin, Amy J " <amy_jean_austin(_)ya= hoo.com> Tuesday, February 19, 2013 8:35 AM style=3D"font-weight: bold;">Subject:</spa= n> CCL: Negative activation energy/non Arrhenius <= ;/font> </div> 
> Sent to CCL by: "Benoi= t  Benoit" [bonoit_10_+_yahoo.fr] Dear CCLers, &= lt;br>I'd like to enquire about negative activation energies that some r= eactions can have, what's the meaning of such negative value? Does= is mean that the reaction is barrierless? Any help is a= ppreciated Best regards Benoit<br&= gt; <br<br the strange characters on the = top line to the (_) sign. You can also<br E-mail to s= ubscribers: <a ymailto=3D"mailto:CHEMISTRY(_)ccl.net" href=3D"mailto:CHE= MISTRY(_)ccl.net">CHEMISTRY(_)ccl.net</a> or use: &nb= sp;     E-mail to administrators: <a ymailto=3D"mailto:CHEMISTRY-REQUEST(_)ccl.net"
> href=3D"mailt= o:CHEMISTRY-REQUEST(_)ccl.net">CHEMISTRY-REQUEST(_)ccl.net</a> or = use       <a href=3D"http://www.c= cl.net/cgi-bin/ccl/send_ccl_message" target=3D"_blank">http://www= .ccl.net/cgi-bin/ccl/send_ccl_message</a> <br<br&g= t;      <br <br<br<= ;br><br <br      <= br><br </div>= </div> </div></body></html>
>
>=
>
> -=3D This is automatically added to each message by the = mailing script =3D-
> To recover the email address of the author of t= he message, please change> Conferences: http://server.ccl.net/chemist= ry/announcements/conferences/>
>

James E. Eilers
Pr= ofessor
Chemistry Department
Southern Illinois University Edwardsvill= e
(618)650-3559

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