Subject: Rotation barrier, thermal enery

From: Jorge Seminario <[email-masked]>
Date: Fri, 30 May 1997 11:09:40 GMT

Dear CCL Members:
I am very indebted to all the members who sent their replies regarding my
question of rotation. I would like to make a small summary of
what I found out based on the information I obtained from the overwhelming
responses.

Let me rephrase the original question:
The molecule in question is tolane (diphenylacetylene, or diphenylethyne),
the interest focuses in the relative rotation of one phenyl group with
respect to the other. Evidently, this rotation is around the acethylene
triple bond.

Experiments are able to observe if a very small group of molecules
rotate, and probably are also able to quantify the number of rotating
molecules by, to make it short,  observing a current on a detector.
This current, in principle is proportional to the number of
molecules that are rotating. It is measured against changes in T from
close to 0 K to 300 K.

The current increases with very small slope from 0K to 30 K, at 30 K the
current jumps 3 to 4 orders of magnitude, then after
this "jump" (I said "sudden rotation" in my previous mail), again the
current increases with small slope.

As few ccl members already recalculated, the barrier for this rotation is
about 0.5 to 0.7 kcal/mol and the experimental value is 0.57 kcal/mol.
Therefore, there is not doubt about the calculations any more.

My inquiry also touched the validity of using kT (or a factor of it)
as the only energy available for the rotation (within the statistical
sense, i.e. understanding that still a very small fraction of molecules
are going to have enough energy to overpass a barrier 10 times bigger
than kT, and tunneling, etc).

Despite of several concerns against, I agree with several responses that
kT is not the whole story. Actually, as it was pointed out, the kT
argument makes no sense, otherwise ethane should not rotate till 1500 K.
We know that it rotates rapidly at 300 K.

I have been advised to use transition state theory for this process, and
also to treat it as a unimolecular reaction using RRKM theory.

However, what is not clear yet is why the jump at 30 K. Can it be
predicted theoretically? I am tempted to suggest, as several colleagues
have indicated, that the total thermal energy (which is one order of
magnitude bigger than kT) plays a role on this jump. Wether is the energy
of the rotational barrier alone or with the inclusion of the interactions
between adjacent molecules. This barrier have to be, at least, matched by
the total thermal energy so we can have the jump in rotations at the T
corresponding to the total thermal energy.

Your comments are more than welcome

Cordially
Jorge Seminario
--------------------------------------------
Jorge M. Seminario
Department of Chemistry and Biochemistry
University of South Carolina
Columbia, South Carolina 29208
Tel: 803-777-9567
Fax: 803-777-9521
email: [email-masked]
--------------------------------------------

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From: Mike Kotelyanskii <[email-masked]>
Date: Fri, 30 May 1997 10:03:06 -0400 (EDT)

First, let me clarify what you are implying is that
the energy can be "consolidated" from the other degtrees of freedom to
overcome the barrier? Well it is possible, but is apparently improbable.
Otherwise a stone lying on the road would be able to jump up, and pretty 
high. It's thermal energy is enough to overcome it's gravity.
Doesn't happen though. This justifies the Arrhenius approach.
It's kT that gives you a possibility to overcome the barrier.
Another contribution is "entropic" and is given by the preexponent
in the Transition State theory.

Do I understand correctly that the experiment is done in condensed phase
(probably solid as I can judge form the temperatures you mention)
Then I would assume this 30K jump may come from some sort of
structural or other phase transition.
May be I am wrong but can it be the insulator - semiconductor transition?
In both cases, current increases with the temperature (opposit to 
conductors), but current in semiconductor is higher than in insulator
(provided, the same voltage is applied)

May be this helps...
Michael

-------------------------------------------------------------------------------
Michael J. Kotelyanskii				Phone (814) 863 43 81
Polymer Science Program				FAX   (814) 865 29 17
Department of Materials Science and
Engineering                                     [email-masked]
Pennsilvania State University
University Park, PA 16802, USA
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On Fri, 30 May 1997, Jorge Seminario wrote:

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From: "Peter Shenkin" <[email-masked]>
Date: Fri, 30 May 1997 15:48:20 -0400

On May 30, 10:03am, Mike Kotelyanskii wrote:
> Subject: CCL:Rotation barrier, thermal enery

> structural or other phase transition.

Yes.  In addition to everything else that's been said, it could be
an order-disorder transition, and not a unimolecular activated process.
In this situation, thermodynamic measurements, like the volume, or
heat capacity as a function of time, should be revealing.

The question is:  does the temperature dependence follow Arrhenius
behavior (log k vs. 1/T is a straight line).  If not, then other
possibilities should be considered.

	-P.

-- 
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