Re: Are the rings in ferrocene constantly rotating?

Date: Wed Feb 2 16:34:01 2000
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
ID: 948442388.Ch

Message:

My textbook says that the rings in ferrorcene are constantly rotating, but all of the sources I have looked at indicated that the rings are staggered or eclipsed. How can they be constantly rotating if the ring orientation can be known? Also, according to the references I have found, ferrocene has an internal rotation barrier of 4-10kJ/mol. Is this barrier just for the change of staggered to eclipsed or is it the barrier for the rings to be constantly rotating?

Please include any references used.

The staggered structure of ferrocene is an energy minimum; the all-eclipsed version is a transition state which is not very much higher in energy than the minimum. The barrier for rotation is, as you have said, only a few kJ/mol; what this means is that at room temperature, in the gas phase and in solution, ferrocene's cyclopentadienyl rings are spinning like tops. In the crystal, ferrocene exists as a staggered molecule; interactions with neighboring molecules seem to damp down the spin enough for us to get good X-ray fixes on the ring carbon atoms.

It's easy to estimate the rate for any reaction, for which the activation energy is known or estimated. One just uses the pseudo-Arrhenius relationship shown here (taken from Carey & Sundberg, Advanced Organic Chemistry, Part A, p.191-192), and plug in the temperature you're interested in. At 300 K and assuming 5 kJ/mol, I get a rate constant of 8´10¹¹ per second, or pretty fast.

When I was in grad school I learned a rule of thumb that a reaction had to have an activation energy of 20 kcal/mol or less (about 85 kJ/mol) to have a reasonable rate at room temperature. For 85 kJ/mol I get a rate constant of 10^-2 per second.

Dan Berger

Bluffton College

http://cs.bluffton.edu/~berger

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