Fragmentation of Radical Anions of Polyfluorinated Benzoates

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2000-01, Vol.104 (2), p.352-361
• Main Authors: Konovalov, Valery V, Laev, Sergey S, Beregovaya, Irina V, Shchegoleva, Lyudmila N, Shteingarts, Vitalij D, Tsvetkov, Yuri D, Bilkis, Itzhak
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp993361d

A comprehensive study of the symmetry forbidden fragmentation of short-lived radical anions (RAs) has been undertaken for the complete set of polyfluorinated benzoates (C6F n H5 - n CO2 -, n = 1−5). The decay rate constants (k c) of RAs have been determined in aqueous alkaline solution (pH 13.4) by electron photoinjection (EPI) from mercury electrodes and were found to increase dramatically from ≤3 × 103 s-1 (3-F−C6H4CO2 -) to (1.2 ± 0.8) × 109 s-1 (C6F5CO2 -). The regioselectivity of C−F bond cleavage in the RA fragmentation has been revealed by structure assignment of reduction products of the polyfluorinated benzoic acids by Na, K, and Zn in liquid NH3, as well as by Zn in aqueous NH3 and aqueous alkaline solutions. The k c values depend on the position of the cleaved fluorine to the CO2 - group generally in the order para > ortho > meta, and to sharply increase if adjacent fluorine atoms are present. The observed trends reveal that the kinetics of the RA fragmentation reaction is not controlled by the reaction thermodynamics. Semiempirical UHF/INDO calculations, the validity of which has been confirmed by ab initio ROHF/6-31+G calculations, were done to rationalize the observed trends. The reaction transition state (TS) was considered to arise from the RA's Π and Σ* states crossing avoided due to out-of-plane deviation of the cleaving C−F bond. The satisfactory linear correlation (R = 0.96) between the model reaction energy barrier E a and log k c has been achieved with modeling the local solvation of the CO2 - group by its protonation.