Re: Can you explain transition state theory?

Heather, this is rather a hard call, but I will have a go.

This is very technical stuff, that is usually introduced to specialist 
chemists either in the final undergraduate year or as graduate students. 
There is another difficulty:
"Transition state theory" is not a single theory that can easily be 
described and written down. It is really a label for a whole family of 
theories that look quite different in the detail of the way that you use 
them, but are based on a common simple physical picture.
This has been complicated even more because recently work in areas like 
enzyme-catalyzed reactions or mass spectrometry has taken over the term, and 
slightly stretched the way it is used even further.

Originally the term "transition state theory" applied to an approach to 
explaining the rates of gas phase unimolecular reactions of medium sized 
molecules (5 to 30 atoms). "Unimolecular" means a reaction where only one 
molecule seems to be involved, like when a molecule breaks up into smaller 
molecules:

C2H5Cl --> C2H4 + HCl

or when a molecule rearranges to an isomeric alternative:

CH3NC --> CH3CN

The idea was readily and rapidly extended to deal with reactions in 
solution, reactions where more than one molecule seemed to be involved, and 
features of the reaction system other than just the rates.

The essence of transition state theory is the following: when a chemical 
reaction occurs, atoms are not created nor destroyed, they simply rearrange 
into different molecules. For any elementary (= single step) reaction, we 
can decide whether we have reactants or products present according to the 
positions of the atoms. We look at which atoms are close to which other 
atoms, and that will tell us where the bonds go, and that in turn will tell 
us whether to call the arrangement "reactants" or "products". But if we are 
going to divide arrangements in this way, there must be some sort of border-
line which sits exactly between reactants and products. If we move the atoms 
just a tiny bit in one way, we would say that we had products, but just a 
tiny bit in the other direction and we would say reactants. This borderline 
we call the "transition state". Somewhere in going from reactants to 
products, the reaction system must cross this borderline. Usually it is at a 
local maximum of potential energy (actually a saddlepoint).

To work out the rate of reaction, we argue in the following way: if we have 
put the borderline in the right place, then when the group of atoms gets 
into the borderline arrangement it is equally likely to go either way -- 
onward to products, or back to reactants. So what really matters is how long 
it takes on average for the system to find its way from reactants to 
transition state. The average reaction time will be just double that 
(because half the time the system will go back to reactants even if it finds 
its way to the transition state). So, to be able to work out the rates of 
reaction, we need to find information about the reactants and about the 
transition state. The relative potential energy is the most important thing, 
but for a sophisticated calculation we need to know a lot more about the 
shapes and energies associated with the transition state as well. The really 
interesting thing is that it all depends on reactants and transition state, 
but not on products!

Now that is about as far as I can go with an overview, before we start 
getting into too much technical detail. You can find some of this detail for 
the general approach in advanced Physical Chemistry textbooks like Atkins, 
or Laidler & Meiser. To go any further with it, you would need to look at 
specialist textbooks like those of Harold S Johnston, Th Forst, or P 
Robinson & K Holbrook.