wrote: >=20 > Dear All, >=20 > I have the following question. >=20 > What is the range of activation energy that can happen at room = temperature. > For example, in one paper i see 0.4 eV and in some cases 1+ eV. > Sometimes more than 1.5 eV when plane wave basis sets are used. > Are there any papers that correlates activation energy to temperature.=20= > If anyone has a script/methodology, can you please share it with me. >=20 > Thanks in advance. > Vimal >=20 >=20 --Apple-Mail=_0F9F7581-1094-4572-9BFC-3627A8D2B98B Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=utf-8 Dear = Vimal,

The rate = constant and the activation energy or free energy of activation are = related by the Arrhenius equation and the Eyring equation, respectively. = These are easily found in any physical chemistry textbook / = Wikipedia.

=46ro= m there, it is much less trivial. To know how fast a reaction = really is, one must know the rate law and concentrations of the species = in the rate law. Computational chemists rarely have the ability to = apply true experimental conditions to our simulations. = Furthermore, a computed activation barrier may easily have an = error of =C2=B10.05 eV or more. This will lead to an error in the = absolute rate constant approaching a full order of magnitude. For = this reason, the ratio of rate constants (the relative rates) are more = typically discussed, since it is hoped the errors cancel in the = division.

However, it is certainly possible to put numbers into = context. Consider the simplest case of a unimolecular rate law and = k determined from the Eyring equation at 298 K. A barrier of ~0.4 = eV gives a rate constant on the order of 1E5 1/sec. That=E2=80=99s = fast. Very fast. A barrier of ~1 eV gives a rate constant on = the order of 1E-5 1/sec. That=E2=80=99s not nearly as slow as it = might seem. A barrier of ~1.5 eV gives a rate constant on the = order of 1E-14 1/sec. That=E2=80=99s very slow.

Now consider the = half-lives of those three examples, which provides the same information = > from a different perspective: ~1E-6 sec, ~1E4 sec, ~1E13 sec. = The first is still obviously very, very fast. This is = similar to the barrier to the chair flip in cyclohexane, which requires = cryogenic temperatures to inhibit. It is now a bit more clear that = the 1 eV barrier has a half life that is still perfectly reasonable if = one is willing to wait a few days for the reaction to complete. = The 1.5 eV barrier, though=E2=80=A6 A half life of about 1 = million years is no one=E2=80=99s idea of =E2=80=9Cfast=E2=80=9D except = on a geologic or cosmic timescale (all things are relative!).

In summary, it is = trivial to compute a unimolecular rate constant from an activation = barrier. After that, use the specifics of your system to decide = what is =E2=80=9Cfast=E2=80=9D and what isn=E2=80=99t.

Cheers,

Eric

----------------------------------------------------
Eric V. Patterson, PhD

Director of Undergraduate Laboratories
Senior = Lecturer

Stony Brook University
Department of = Chemistry
3400 SUNY
Stony Brook, NY = 11794-3400

465 Chemistry
eric.patterson*o*stonybrook.edu

https://www.stonybrook.edu/commcms/chemistry/faculty/patterson.= eric.html

https://sites.google.com/a/stonybrook.edu/evpatterson

voice: (631) = 632-7449
FAX: (631) 632-7960

On Mar 8, 2019, at 2:35 PM, Jeya Vimalan jeyavimalan2k#gmail.com <owner-chemistry*o*ccl.net> wrote:

Dear All,

I= have the following question.

What is the range of activation energy = that can happen at room temperature.
For example, = in one paper i see 0.4 eV and in some cases 1+ eV.
Sometimes more than 1.5 eV when plane wave basis sets = are used.
Are there any papers that correlates = activation energy to temperature.
If anyone = has a script/methodology, can you please share it with me.

Thanks in = advance.
Vimal

= --Apple-Mail=_0F9F7581-1094-4572-9BFC-3627A8D2B98B--