Toward an Understanding of the Ambiguous Electron Paramagnetic Resonance Spectra of the Iminoxy Radical from <i>o</i>‑Fluorobenzaldehyde Oxime: Density Functional Theory and <i>ab Initio</i> Studies

Abstract

Iminoxy radicals (R₁R₂CNO^•) possess an inherent ability to exist as <i>E</i> and <i>Z</i> isomers. Although isotropic hyperfine couplings for the species with R₁ = H allow one to distinguish between <i>E</i> and <i>Z</i>, unequivocal assignment of the parameters observed in the EPR spectra of the radicals without the hydrogen atom at the azomethine carbon to the right isomer is not a simple task. The iminoxyl derived from <i>o</i>-fluoroacetophenone oxime (R₁ = CH₃ and R₂ = <i>o</i>-FC₆H₅) appears to be a case in point. Moreover, for its two isomers the rotation of the <i>o</i>-FC₆H₅ group brings into existence the <i>syn</i> and <i>anti</i> conformers, depending on the mutual orientation of the F atom and CNO^• group, making a description of hyperfine couplings to structure even more challenging. To accomplish this, a vast array of theoretical methods (DFT, OO-SCS-MP2, QCISD) was used to calculate the isotropic hyperfine couplings. The comparison between experimental and theoretical values revealed that the <i>E</i> isomer is the dominant radical form, for which a fast interconversion between <i>anti</i> and <i>syn</i> conformers is expected. In addition, the origin of the significant <i>A</i>_F increase with solvent polarity was analyzed