Foundations of Chemical Kinetics
Lecture 14: Transition-state theory:
Evaluation
Marc R. Roussel
Department of Chemistry and Biochemistry
What works
I
The thermodynamic formalism of transition-state theory
connects well to other parts of physical chemistry, making the
theory conceptually useful.
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For very simple reactions, transition-state theory often gives
fairly good values.
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Even when the theory doesn’t do a great job of predicting the
value of a rate constant, it often predicts the relative effect of
various factors on the rate constant correctly, e.g. isotope
effects.
I
It’s an important starting point for understanding reactions
under non-ideal conditions, e.g. the effect of ionic strength.
What doesn’t work
I
Transition-state theory often badly overestimates rate
constants.
This is thought to result from trajectories that cross the
saddle point and then turn around and come back, which are
not properly accounted for in the theory.
This is sometimes fixed by multiplying transition-state rate
constants by a transmission coefficient (κ), which is
unfortunately not much more than a fudge factor.
kB T Q ‡
∆E0
k =κ ◦
N exp −
c h QX QY
RT
What doesn’t work (continued)
I
One part of transition-state theory, the idea of passage
through the transition state from reactants to products, is an
irreducibly classical mechanical idea.
Of course, molecules are quantum mechanical objects, so the
idea of a trajectory through the transition state can’t be right.
This leads to a number of difficulties which have not, to this
day, been resolved.
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What exactly do we mean by passage through the transition
state?
Tunneling is important in reactions involving motions of light
particles (electron-transfer reactions, proton-transfer reactions,
etc.).
Separability of the reaction coordinate is not guaranteed.
What doesn’t work (continued)
I
The equilibrium approximation at the heart of transition-state
theory is only valid for high barriers.
I
In most reactions, the reactants have to be in a fairly specific
quantum state to react (e.g. one specific vibrational mode has
to have enough energy, and the energy in the other modes
doesn’t matter).
This will tend to have the effect of creating a non-equilibrium
(i.e. non-Boltzmann) distribution of energy. If this effect is
sufficiently strong, perhaps because molecules energized in the
correct vibrational mode react very quickly, then again the
equilibrium approximation might not be valid, regardless of
the height of the barrier.
In either of the above cases, transition-state theory might give
results that are not even qualitatively correct.