Acid and base strength variations: rationalization for cyclic amine bases and acidic aqua cations

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Thermodynamics

Background:

• Acid and base strength variation is traditionally analyzed using various chemical concepts.
• Thermodynamic and thermochemical cycles offer a framework for interpreting these variations.
• Understanding inherent acidity and basicity is crucial for predicting chemical behavior.

Purpose of the Study:

• To highlight and evaluate recent developments in the thermodynamic approach for analyzing acid and base strength trends.
• To interpret acid and base strength rankings using thermodynamic cycles and identify dominant steps.
• To rationalize energy changes in terms of structural chemical concepts and explore solvent effects.

Main Methods:

• Utilizing thermodynamic/thermochemical cycles, each with three defined steps, to analyze series of bases and acids.
• Employing computational chemistry and mass spectrometric techniques to compare inherent basicity/acidity.
• Incorporating gas-phase energy cycles and extended free energy cycles that include dehydration/hydration changes.

Main Results:

• Identified unique dual functions for electronegativity and charge density indicators in evaluating thermodynamic cycles.
• Demonstrated the application of these methods to N-heterocyclic amines and period 4/group 2 aqua dications.
• Found that the transfer of M(H2O)n^2+ complex hydrates from gas-phase to bulk water is a dominant step in extended cycles.

Conclusions:

• The thermodynamic approach, utilizing energy cycles, provides a robust method for analyzing acid and base strength variations.
• Electronegativity and charge density are key indicators for rationalizing thermodynamic cycles, particularly for aqua cations.
• Computational and experimental advancements facilitate a deeper understanding of inherent chemical properties and solvent effects.