Fluorine Hyperfine Splittings in the Electron Spin Resonance (ESR) Spectra of Aromatic Radicals. An Experimental and Theoretical Investigation

摘要

This article reports our experimental and theoretical investigations of fluorine hyperfine coupling constants (hfcc's) in the anion and cation radicals of a number of fluorinated benzenes, naphthalenes, and anthracenes. We have obtained electron spin resonance (ESR) spectra and hfcc's for the electrolytically generated anion radicals of 1,2,3,4-tetrafluoronaphthalene, 1,2,3,4-tetrafluoroanthracene, and 9,10-perfluoroanthraquinone. The experimental values of the hfcc's of these radicals, along with the hfcc's of several cation radicals of fluorinated benzenes and naphthalenes currently available in the literature, have been compared to our theoretical predictions using the UB3LYP density functional method in conjunction with a variety of basis sets. The EPR-III basis set usually gave the best agreement between theory and experiment for the fluorine splittings with an average relative error of 15%. We also find that it is possible to correlate the experimental fluorine hfcc's with the calculated π- and total electron spin populations ρ on the fluorine atom, the adjacent carbon atom, and the carbon-fluorine bond, thus providing some chemical insight into the origin of the interactions. The best correlation is obtained with a two-parameter equation of the form A_F = Q_(CF)ρ_C~π + Q_(FC)ρ_F~π. The fit to 21 fluorine splittings using the EPR-III basis set and Mulliken π-electron spin populations gives an average error of only 9%. The average error obtained with EPR-II and NBO π-electron spin populations is 8%. Roughly 80% of the fluorine hfcc can be attributed to π-electron spin population on the fluorine atom. Our results indicate that conjugation of the fluorine atom with the ring is the primary source of the unpaired electron density on fluorine and that the often-assumed separability of σ- and π-electrons in aromatic systems is justified in these radicals as well.