One electron less or one proton more: how do they differ?

Area of Science:

• Physical Chemistry
• Computational Chemistry

Background:

• Ionization Energies (IE) and Proton Affinities (PA) are critical molecular properties.
• Understanding the relationship between IE and PA provides insights into molecular stability and reactivity.

Purpose of the Study:

• To compile and analyze a large dataset of IE and PA values.
• To investigate the relationship between IE and PA across diverse molecular families.
• To assess the thermodynamic stability of protonated molecules.

Main Methods:

• Data compilation from NIST and scientific literature.
• Analysis of trends in PA versus IE plots.
• Calculation of hydrogen atom affinities (HA) from IE and PA data.

Main Results:

• A dataset of 3,052 IE and 1,670 PA values was compiled.
• Plots of PA versus IE show slopes generally less negative than -1 for many molecular families.
• Metals, oxides, and hydroxides exhibit slopes near -1, while hydrogen halides and noble gases show slopes around -0.3.
• Protonated nitriles demonstrate exceptional stability due to high hydrogen atom affinities of their radical cations.

Conclusions:

• The relationship between IE and PA influences the thermodynamic stability of protonated species.
• Protonation thermodynamics are linked to both neutral molecule enthalpies and radical cation ion enthalpies.
• Collected data facilitate comparisons of homolytic and heterolytic bond cleavage stability in protonated molecules.