Subject: Re: CCL:Light induced torsional movement

From: Daniel Kost <[email-masked]>
Date: Mon, 30 May 1994 11:57:21 +0300 (IDT)

On Sun, 29 May 1994 [email-masked] wrote:

> Daniel Kost states in a reply to an inquiry by Charles Marais,
> 	" I would just like to comment that a reaction which is induced by
> UV light, and the reverse transformation by heat" is quite impossible, since it
> would violate the principle of microscopic reversibility:  At least in the 
> thermal part, BOTH the forward and reverse reactions should be "induced" to
> the same extent."
> 
> This is completely incorrect.  First consider an endothermic reaction with a 
> delta H = 50 kJ/mol, and an additional activation barrier of 10 kJ/mol.  The
> energy of activation for the forward reaction will be 60 kJ/mol, and for the
> reverse reaction 10 kJ/mol.  If the populations of both product and reactant
> are initially equal, then at a given temperature, the rate of the reverse 
> reaction will exceed the rate of the forward reaction, since a larger fraction
> of molecules will have the energy necessary to exceed the lower activation
> barrier.
> 
> This is an example of a thermal reaction where the forward and reverse 
> processes are NOT induced to the same extent.

The principle of microscopic reversibility states only that the forward 
and reverse reactions (any thermal reaction) path through the same 
transition states and intermediates. It says nothing about rates, and the 
example given above, while by itself correct, is irrelevant.  All I meant 
is that if a reaction is thermally accelerated, i.e. pathes through a 
lower transition state, the reverse reaction would be equally accelerated 
in the sense that the same lower transition state would have to be reached.
In the above example, the different rates are the consequence of the mass 
law, i.e. different concentrations, and also the obvious difference in 
rate constants. Microscopic reversibility would only require that IF the 
activation barrier for the forward reaction is lowered by, say, 5kJ, the 
reverse would also be lowered by the same amount.

> 
> But what of microscopic reversibilty?  The answer lies in the precise state-
> ment of this principle.  Laidler in his textbook on chemical kinetics, states
> it as follows - "In a system at equilibrium, any molecular process and the
> reverse of that process occur on the average at the same rate".  The key words
> here are AT EQUILIBRIUM.  For the type of system above, at equilibrium, the

The above, unfortunately, has nothing at all to do with microscopic 
reversibility.  It simply defines the kinetic conditions for equilibrium: 
equal forward and reverse rates.
Microscopic reversibility is not at all restricted to equilibrium conditions.
It applies equally to non-equilibrium situations.

> probability of a given reactant molecule passing over the activation barrier
> to form a product is still much smaller than the probability of a given product
> molecule passing over its smaller activation barrier to form products.  The
> reason that the overall rates of the forward and reverse reactions will
> be equal at equilibrium is because in such a system the number of molecules 
> in the form of reactants will far exceed the number of molecules in the form 
> of products, and thus the population weighting balances out the energetic 
> weighting.
> 
> This tells us much about the nature of the system described by our original 
> correspondent.  First, the initial reaction does not lead to an equilibrium 
> product, but to a system trapped in a state of disequilibrium.  Thus the
> nascent distribution of products will not be described by microscopic revers-
> ibility.  Second, this disequilibrium state is populated not thermally, since
> [ignore the word since]  but by a reaction which occurs on an excited state.
> There are several excited state potentials which can result in this type of
> disequilibrium.  One of the most common has the endothermicity of the 
> ground state reversed, and has a potential minimum at a nuclear configuration
> on the product side of the ground state potential.  REaction on such a surface
> followed by relaxation to the ground state would result in the type of 
> photon forward, thermal backward reaction described by our initial 
> correspondent.
> 
> 				Sam Abrash
> 				[email-masked]
> 

Otherwise, I agree with the writer.

Daniel Kost
[email-masked]