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Nucleophilicity-periodic trends and connection to basicity
1Department of Chemistry, University of Oslo, P. O. Box 1033, Blindern, 0315 Oslo, Norway. einer.uggerud@kjemi.uio.no
Chemistry (Weinheim an Der Bergstrasse, Germany)
|October 26, 2005
Summary
Quantum-chemical calculations reveal that barrier heights in S(N)2 reactions decrease across the periodic table. Ionization energy correlates with lower barriers, clarifying nucleophilicity and basicity relationships.
Area of Science:
- Physical Chemistry
- Quantum Chemistry
- Organic Chemistry
Background:
- Nucleophilicity and basicity are key concepts in chemical reactivity.
- Understanding the factors influencing S(N)2 reaction barriers is crucial for predicting reaction outcomes.
Purpose of the Study:
- To estimate potential energy profiles for 18 identity S(N)2 reactions.
- To investigate trends in barrier heights and geometrical requirements.
- To clarify the relationship between nucleophilicity and basicity.
Main Methods:
- Utilized G2-type quantum-chemical calculations.
- Analyzed potential energy profiles of identity S(N)2 reactions involving various substituents (X = NH2, OH, F, PH2, SH, Cl, AsH2, SeH, Br).
Main Results:
- Barrier heights and geometrical requirements were similar for both charged (X-) and neutral (XH) reactants.
- Barrier heights decreased across the periodic table (left to right).
- Higher ionization energies correlated with decreased reaction barriers.
Conclusions:
- Stronger electrostatic character in substrates leads to lower energetic barriers.
- Intrinsic nucleophilicity and basicity trends are often opposite.
- Overall nucleophilicity depends on the interplay between intrinsic nucleophilicity and basicity.