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Electrostatic calculation of the substituent effect: an efficient test on isolated molecules
1Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo n. 2, 16610 Praha 6, Czech Republic.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|October 21, 2003
Summary
Electrostatic approximations for disubstituted molecule energies are qualitatively correct but quantitatively underestimate interactions. This suggests complex electron density distributions, challenging simple models for substituent effects.
Area of Science:
- Computational Chemistry
- Quantum Chemistry
- Organic Chemistry
Background:
- Simple electrostatic models approximate disubstituted molecule energies using poles or dipoles.
- These models are often tested using acid-base properties, which can be indirect.
Purpose of the Study:
- To evaluate the accuracy of electrostatic approximations for substituent interactions in disubstituted molecules.
- To introduce a more direct and efficient model system for testing these approximations.
Main Methods:
- Calculated energies for 27 1,4-derivatives of bicyclo[2.2.2]octane using density functional theory (DFT) at the B3LYP/6-311+G(d,p) level.
- Evaluated substituent interaction energy via isodesmic homodesmotic reactions, using previously validated gas-phase acidity data as a reference.
Main Results:
- Electrostatic approximation correctly predicts the qualitative order of interaction strength (pole-pole > pole-dipole > dipole-dipole).
- Calculated energies consistently underestimate the actual interaction energies, especially for weaker interactions.
- Improvements, such as adjusting effective permittivity, failed to correct the underestimation, requiring permittivity values less than unity.
Conclusions:
- Simple electrostatic models are insufficient for accurate quantitative prediction of substituent interaction energies.
- The observed discrepancies indicate a more complex electron density distribution than accounted for by basic electrostatic models.
- The validity of conclusions regarding "through-space" substituent effects based on these models is questionable.