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 N O stretching frequency, ν(N O ). 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 ([ 2H 12]-TMIO) and [ 2H 12, 15N]-TMIO analogues. Solution spectra of TMIO in methanol and CCl 4 are assessed for possible solvent-dependent spin density distribution effects in the N O bond.