Proton Affinities and Gas-Phase Basicity in Organic Molecules
Summary
Proton affinity and gas-phase basicity define the intrinsic tendency of a neutral organic molecule to accept a proton in vacuo, providing fundamental thermodynamic metrics of basic strength beyond solution chemistry. Derived from Brønsted-Lowry theory, proton affinity quantifies the enthalpy change when a molecule binds H⁺, while gas-phase basicity expresses the corresponding Gibbs free energy change. These parameters are determined experimentally by mass spectrometric techniques and complemented by high-level quantum-chemical calculations. Variations in hybridisation, resonance stabilisation, inductive and mesomeric effects, aromaticity and ring strain all modulate basicity, enabling the design of neutral superbases with proton affinities exceeding 300 kcal mol⁻¹. Insight into gas-phase acid–base equilibria informs fields as diverse as catalysis, atmospheric ion chemistry, interstellar processes and advanced synthetic methodologies.
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Proton Affinities and Gas-Phase Basicity in Organic Molecules publication trend
The graph below shows the total number of articles in proton affinities and gas-phase basicity in organic molecules across all publications each year (not limited to Nature Index journals).
Technical terms
Proton affinity: The enthalpy change associated with the addition of a proton to a neutral molecule in the gas phase, typically expressed in kJ mol⁻¹ or kcal mol⁻¹.
Gas-phase basicity: The Gibbs free energy change for protonation of a base in vacuo, reflecting its thermodynamic propensity to accept a proton under standard conditions.
Push–pull effect: A resonance phenomenon in which electron-donating and electron-withdrawing substituents enhance basicity by stabilising the protonated form through charge delocalisation.
Superbase: An organic base exhibiting exceptionally high proton affinity, often surpassing conventional amines and enabling unique catalytic or synthetic applications.
References
- Strong Bases and beyond: The Prominent Contribution of Neutral Push–Pull Organic Molecules towards Superbases in the Gas Phase. International Journal of Molecular Sciences (2024).
- Can an Amine Be a Weaker and a Stronger Base at the Same Time? Curious Cases of Chameleonic Ionization. ACS Physical Chemistry Au (2023).
- Push–Pull Effect on the Gas-Phase Basicity of Nitriles: Transmission of the Resonance Effects by Methylenecyclopropene and Cyclopropenimine π-Systems Substituted by Two Identical Strong Electron Donors. Symmetry (2021).
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