DFT study of the EPR spectral pattern of propagating methacrylic radicals

The EPR spectral pattern observed in the bulk polymerization of methacrylic monomers was theoretically investigated by DFT methodology. The conformational analysis of the propagating radical by the rotation around the Cα-Cβ bond, was performed using the B3LYP/6-31G∗ computational scheme. To obtain accurate values of the isotropic hyperfine coupling constants (hfccs) a higher level protocol, B3LYP/TZVP//B3LYP/6-31G∗, was applied. The experimental 13-line spectrum registered at the first stage of the polymerization was assumed to correspond to a free rotating radical in a fluid medium and it was simulated just considering the most stable conformation. The 9-line spectrum registered at high conversions was interpreted in terms of highly hindered rotational conformers frozen in the very viscous medium. This spectrum was well reproduced by a model which considers the sum of the individual spectra of the conformations spread around the most probable. Each of these contributing spectra was obtained on the basis of the computed hfccs for the considered conformations and weighted by his relative Boltzmann population according to the DFT analysis. Besides, the calculated hfccs showed an excellent agreement with those predicted by the Heller-McConnell relationship, which confirms the coherence of the DFT methodology for this kind of calculations.