Enhanced Proton Loss from Neutral Free Radicals: Toward Carbon-Centered Superacids

Area of Science:

• Physical Chemistry
• Organic Chemistry
• Computational Chemistry

Background:

• Proximity of radical centers to protons is known to enhance proton dissociation.
• The radical enhanced deprotonation (RED-shift) phenomenon's generality and structural scope require investigation.

Purpose of the Study:

• To investigate the generality of the RED-shift phenomenon.
• To determine the structural requirements for RED-shift.
• To compute pKa values for various species with adjacent radical centers.

Main Methods:

• Density Functional Theory (DFT) method used to compute pKa values from free energies of deprotonation.
• Investigated sulfinic, sulfonic, pentan-2,4-dione, and Meldrum's acid species with C-, N-, and O-centered radicals.

Main Results:

• All investigated series exhibited significant RED-shifts.
• RED-shifts increased with the electronegativity of the radical center.
• A Meldrum's acid with an alkoxyl radical substituent displayed a superacidic pKa.
• Ethyne units effectively enhanced acidity and conducted RED-shifts over long molecular chains (>20 atoms).

Conclusions:

• Radical enhanced deprotonation is a general phenomenon applicable to various molecular structures.
• Electronegativity of the radical center modulates the extent of RED-shift.
• Long conjugated systems, particularly those with ethyne units, can transmit RED-shifts to remote acidic sites.
• RED-shifted species feature conjugate radical anions with site-exchanged spin and electronic charge.