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Entropy considerations in kinetic method experiments.

Chrys Wesdemiotis1

  • 1Department of Chemistry, University of Akron, Akron, Ohio 44325-3601, USA. wesdemiotis@uakron.edu

Journal of Mass Spectrometry : JMS
|September 24, 2004
PubMed
Summary

This study introduces an extended kinetic method to determine binding affinities and entropies using ion-bound dimer dissociation. The method accurately measures binding enthalpies but reveals that apparent relative entropies do not match thermodynamic values due to transition state switching.

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Area of Science:

  • Physical Chemistry
  • Chemical Kinetics
  • Mass Spectrometry

Background:

  • The kinetic method is a powerful technique for determining relative binding enthalpies (affinities) and entropies of ions.
  • Understanding ion-molecule interactions is crucial in various chemical and biological processes.

Purpose of the Study:

  • To refine the extended kinetic method for accurate determination of binding enthalpies and apparent relative entropies.
  • To investigate the discrepancy between experimentally derived apparent relative entropies and thermodynamic entropy differences.

Main Methods:

  • Formation of ion-bound dimers (A-X-B(i)) with a central ion (X) bridging a sample (A) and reference molecules (B(i)).
  • Determination of branching ratios via collisionally activated dissociation at varying collision energies.

Related Experiment Videos

  • Analysis of the dependence of branching ratios on reference molecule binding enthalpies to deduce sample binding enthalpies and apparent relative entropies.

    • Main Results:

    • The extended kinetic method accurately yields binding enthalpies for proton- and Na(+)-bound dimers.
    • Derived apparent relative entropies (Delta(DeltaS(app))) do not correlate with thermodynamic entropy differences.
    • Observed trends are explained by the transition state switching model, highlighting the role of 'entropy bottlenecks' in barrierless dissociations.

    Conclusions:

    • The extended kinetic method is reliable for accurate binding enthalpy measurements.
    • The transition state switching model successfully reconciles kinetic observations with thermodynamic discrepancies, particularly concerning entropy.
    • The study emphasizes the importance of considering transition state dynamics in interpreting dissociation kinetics.