The vibrational spectrum of the stable free radical 1,1,3,3-tetramethylisoindolin-2-yloxyl

Solid and solution IR and Raman spectra of a stable nitroxide radical, 1,1,3,3-tetramethylisoindolin-2-yloxyl (TMIO), are reported and compared to ab initio density functional theory calculations of the vibrational frequencies to obtain unequivocal band assignments, in particular of the NO stretching frequency, ν(NO). The band position was found to be at 1431 cm−1 for the solid, which is well outside the previously published range of 1310–1380 cm−1 for nitroxide radicals. This apparently anomalous peak position was confirmed by undertaking isotopic substitution studies through the preparation and recording of vibrational spectra of tetrakis(trideuteriomethyl)isoindolin-2-yloxyl ([2H12]-TMIO) and [2H12,15N]-TMIO analogues. Solution spectra of TMIO in methanol and CCl4 are assessed for possible solvent-dependent spin density distribution effects in the NO bond.