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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 122; no. 5; pp. 1422 - 1431
• Main Author: Walton, John C.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 08.02.2018
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.7b11796

Radical centers close to protons are known to enhance their dissociation. Investigation of the generality of this radical enhanced deprotonation (RED-shift) phenomenon, and the kinds of structures in which it operates, are reported. The pK as for sulfinic, sulfonic, pentan-2,4-dione, and Meldrum’s acid species, with adjacent radicals centered on C-, N-, and O atoms, were computed by a DFT method from free energies of deprotonation. All series showed significant RED-shifts that increased with the electronegativity of the radical center. The hugely negative pK a obtained for a Meldrum’s acid with an alkoxyl radical substituent showed it to belong to the superacid class. The ethyne unit was found to be uniquely effective at enhancing acidity and conducting RED-shifts through chains up to and beyond 20 atoms. These connector units enable a radical center to alter the pK a of a spatially remote acidic group. RED-shifted species were characterized by conjugate radical anions displaying site exchange of spin with electronic charge.