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

Abstract

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 F 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 formThe 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.

Modulating the Electronic and Solid‐State Structure of Organic Semiconductors by Site‐Specific Substitution: The Case of Tetrafluoropentacenes

“…[a] Significant deviations for sim. and calcd hyperfine coupling constants for fluorine have been reported for other fluorinated acenes . [b] Interchanging these values does not change the correlation ( R 2 ) between simulation and experimental spectrum in a significant way.…”

“…and calcd hyperfine couplingc onstants for fluorine have been reported for other fluorinated acenes. [53] [b] Interchanging these values does not change the correlation (R 2 )b etween simulation and experimental spectrum in asignificant way. cene shows intense Bragg reflections as expected, because it grows in polycrystalline fashion on SiO 2 .…”

The effect of electron-withdrawing groups in the fragmentation of the radical anions of benzyl phenyl ethers

“…Application of density functional methods to compute Electron Paramagnetic Resonance parameters, as hfcc's, has been fruitful in the past. Out of a multitude of functionals investigated to date, the B3LYP scheme of Becke in many cases provided the highest accuracy of the calculated hfcc's [8].…”

“…The application of EPR spectroscopy to the study of matrix isolated radical anions and radicals is well established as a powerful technique, providing in many instances not only clear identification but also structural and mechanistic information that would otherwise be difficult to obtain [7]. The hyperfine coupling constants (hfcc's) obtained from EPR spectroscopy provide important information about the electronic structure and properties of organic radicals [8]. This technique has been widely used to the study of many types of organic radicals and radical anions but to our knowledge it has not been applied extensively to the analysis of radical anions of substituted-phenyl benzyl ethers and substituted-benzyl phenyl ethers.…”

5-Fluorolysine as alternative substrate of lysine 5,6-aminomutase: A computational study

“…Fluorine ( 19 F: I = ½, natural abundance 100%, c = 26.54 [33]) has favorable parameters to act as a probe for EPR and ENDOR [34][35][36][37][38][39][40] identification of radical intermediates by introducing hyperfine splittings. Electron withdrawing effect of fluorine is helpful to stabilize radicals.…”