Ab Initio Study of Some Persistent Nitroxide Radicals

Stable free radicals have a variety of applications. Particularly, the aminoxyl group is frequently used in spin-labeling experiments. Nitroxides may also exhibit intriguing chiral and magnetic properties, and can be of interest for studies of molecular parity violation. We present results for three different groups of persistent nitroxide radicals: i) acyclic: dimethylaminoxyl (Me2NO), bis(trifluoromethyl)aminoxyl (CF3)2NO, and di-tert-butyl nitroxyl [(Me3C)2NO]; ii) cyclic: aziridine-N-oxyl, azetidine-N-oxyl, pyrrolidine-N-oxyl and piperidine-N-oxyl; and iii) imino nitroxides. We used density functional and ab initio (MP2, coupled cluster) methods to obtain insight into the underlying chemistry. The molecular structures, harmonic vibrational frequencies, inversion barriers, and hyperfine coupling constants are reported. The cyclic aziridine-N-oxyl exhibits a considerable inversion barrier of ?hc 3500 cm–1 compared to only ?hc 500 cm–1 for the other examples. Stable imino nitroxides are theoretically characterized for the first time in our work. We discuss the possibilities that some of the chiral derivatives may be dominated by molecular parity violation in their dynamics.