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Can a secondary isotope effect be larger than a primary?
Charles L Perrin1, Kathryn D Burke1
1Department of Chemistry, University of California-San Diego, La Jolla, California 92093-0358, United States.
The Journal of Physical Chemistry. A
|April 17, 2015
Summary
This study reveals that secondary oxygen-18 isotope effects on acidities are often larger than primary effects, a counterintuitive finding explained by molecular rotation and vibrational frequencies.
Area of Science:
- Computational chemistry
- Physical chemistry
- Isotope effects
Background:
- Equilibrium isotope effects provide insights into chemical bonding and molecular structure.
- Understanding primary and secondary isotope effects is crucial for interpreting reaction mechanisms.
Purpose of the Study:
- To compute primary and secondary (18)O equilibrium isotope effects on Brønsted and Lewis acidities.
- To investigate the factors governing the relative magnitudes of these isotope effects.
Main Methods:
- Density-functional theory (DFT) calculations were employed.
- Calculations focused on acids involving carbon, boron, nitrogen, and phosphorus centers.
Main Results:
- Secondary (18)O isotope effects were frequently larger than primary effects.
- This counterintuitive observation was linked to vibrational frequencies and zero-point energies.
- The moment-of-inertia factor was identified as the dominant contributor to larger secondary effects.
Conclusions:
- The study explains the prevalence of larger secondary (18)O isotope effects.
- Molecular rotation and the position of the oxygen-18 atom are key factors.
- Findings advance the understanding of isotope effects in acid-base chemistry.
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