Hammett equation and generalized Pauling's electronegativity equation.

Area of Science:

• Computational Chemistry
• Physical Organic Chemistry
• Quantum Chemistry

Background:

• Substituent effects significantly influence molecular properties.
• Predicting these effects accurately is crucial for molecular design.
• Existing models like Hammett's equation have limitations in scope.

Purpose of the Study:

• To define and investigate Substituent Interaction Energy (SIE).
• To develop a generalized equation for predicting SIE across various molecular systems.
• To explore the relationship between SIE, Hammett's equation, and Pauling's electronegativity.

Main Methods:

• Definition of SIE via isodesmic reactions.
• Development and application of the SIE equation: SIE(X,Y) = -ksigma(X)sigma(Y).
• Analysis of applicability to diverse systems (bicyclo[2.2.2]octanes, benzenes, etc.).
• Separation of electronic effects into field/inductive and resonance components.

Main Results:

• The SIE equation accurately predicts interactions in various disubstituted systems.
• Hammett's equation and Pauling's electronegativity equation are shown to be special cases of the generalized SIE equation.
• Electronic substituent effects were successfully separated into field/inductive and resonance terms, revealing their interaction.
• Electronic substituent effects in multi-substituted systems are pairwise and additive.

Conclusions:

• A generalized equation for Substituent Interaction Energy provides a unified approach to understanding substituent effects.
• The framework successfully integrates and extends existing models like Hammett's and Pauling's.
• Understanding the interplay between field/inductive and resonance effects is key to predicting molecular behavior.