Subject: rotational barrier: experiment and theory

From: Jorge Seminario <[email-masked]>
Date: Thu, 29 May 1997 03:16:47 GMT

Hello dear members:

I wonder if you could help me to clear up some problems to interpret my
colleagues' experiment by using our computational tools. The molecule
consists in two benzene rings connected by two carbon atoms.
The experiment on Ph-C-C-Ph (where the central C-C bond is triple and
the adjacents are single) indicates that a sudden rotation of one of
the benzene rings with respect to the other is initiated when the
temperature reaches 30 K.  The calculations
yield a barrier of about 0.5 kcal/mol for the rotation of the benzene
rings. The argument against this theoretical barrier is that
at 30K, the kT (or RT) available for this mode is only 0.06 kcal/mol and
therefore, the rotation of the phenyl groups is not possible because the
barrier is almost ten times bigger than the available energy. So,
the calculation might be wrong.
Well, we repeated the calculation with several other methods and the
results are practically the same. However, a gross estimate, indicates
that the total thermal energy in this molecule at 30K is about .5 kcal/mol,
this mean, counting all the other vibrational modes and other
contributions to the thermal energy. Now, the main question to all of you
is if this total thermal energy can be available for the rotation? if not,
why this rotation could happen?

On this lines, [Morrison and Boyd 5thEd. page80] indicates a "lack of
understanding"  for the rotational barrier of ethane, which is 3 kcal/mol
(this would correspond to a T=1500) however at 300 K ethane seems to have
practically free rotation. I am tempted to infer that whatever explains
ethane to over-pass the barrier of 3 kcal/mol at 300K, will also explain
the experiment above. Interestingly, the total thermal energy of ethane at
300 K is about 3kcal/mol.
Sorry if this is too trivial for the net.
Your comments, suggestions and ideas of any kind are highly appreciated.
Cordially
Jorge Seminario
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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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