Reaction Mechanisms and Molecular Kinetics
From Potential Surfaces to Rates
Transition State Theory
Reaction rates can often be approximated by transition state theory,<br><br>where is the activation free energy. This connects molecular structure to kinetic observables.
Mechanistic Concepts
Mechanistic analysis identifies intermediates, transition states, rate-determining steps, and competing pathways. For solution-phase and biomolecular chemistry, kinetics are influenced by diffusion, solvation, conformational gating, and dynamic disorder.
Kinetic Terms
In transition state theory, which quantity most directly controls the exponential factor in the rate constant?
The barrier height in free-energy terms sets the dominant exponential dependence of .
Correct answer: The activation free energy
What is the purpose of identifying a transition state?
A transition state approximates the bottleneck governing reaction kinetics.
Correct answer: to locate the highest point along the minimum-energy reaction path
Energy Terms in Mechanisms
Potential energy barrier
- Electronic-structure based
- Useful for gas-phase paths
Free-energy barrier
- Includes entropy and solvent effects
- More relevant to observed rates
Kinetic Isotope Effects
Replacing hydrogen with deuterium changes vibrational zero-point energies and tunneling probabilities, often altering reaction rates. Such effects are powerful probes of bond breaking, proton transfer, and tunneling-driven mechanisms.
Mechanistic Investigation Workflow
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1
Map the potential or free-energy landscape.
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2
Locate minima, transition states, and competing channels.
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3
Compute or measure rate constants across conditions.
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4
Test isotopic, substituent, or solvent effects.
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5
Refine the mechanism until it explains all observables.