Chemical Dynamics, Kinetics, and Transition State Theory
From potential energy surfaces to rates
Classical transition state theory
Transition state theory estimates the rate constant by assuming quasi-equilibrium between reactants and the activated complex. In its simplest form, where captures transmission and dynamical corrections.
What really controls rates
Barrier height matters, but so do tunneling, recrossing, solvent friction, entropy, and dynamical bottlenecks.
Beyond simple barriers
Many reactions are not well described by a single saddle point. Enzymatic reactions, barrierless association, roaming mechanisms, and frustrated intermediates require variational rate theory, master equations, or direct dynamics.
What does the factor in transition state theory represent?
It accounts for deviations from idealized transition state crossing, including recrossing and tunneling.
Correct answer: The transmission coefficient correcting for dynamical effects
Why can entropy significantly affect reaction rates?
Large entropic penalties can slow reactions even when enthalpic barriers are modest.
Correct answer: Because the rate depends on the free-energy barrier , not just the enthalpy barrier.
Rate-theory toolkit
| Approach | Best use | Key assumption |
|---|---|---|
| Transition state theory | Simple barriers | Quasi-equilibrium at dividing surface |
| Variational TST | Flexible barriers | Choose dividing surface to minimize rate |
| Marcus theory | Electron transfer | Parabolic free-energy surfaces |
| RRKM theory | Unimolecular reactions | Energy randomization in a microcanonical ensemble |
Kinetic descriptions
Microscopic dynamics
- Tracks trajectories or quantum wavepackets
- Reveals recrossing and nonadiabatic effects
- Computationally intensive
Statistical rate theory
- Uses ensembles and free-energy barriers
- Efficient for many systems
- May miss rare dynamical phenomena
Which factor is most likely to make TST overestimate a rate?
If trajectories frequently recross, the ideal TST flux assumption is violated and rates are overestimated.
Correct answer: Strong recrossing of the dividing surface
Research frontier
Current work focuses on combining ab initio dynamics, machine-learned potentials, path-integral methods, and enhanced sampling to compute reaction rates in complex environments with chemical accuracy.