OH-Detected Aromatic Microsolvation of an Organic NO Radical: Halogenation Controls the Solvation Side

The persistent organic radical 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) protects its NO radical center by four methyl groups. Two of them are arranged tightly (t) on one side of the six-membered puckered heterocycle, and the other two more openly (o) on the other side. It is shown by OH stretching infrared spectroscopy in heated supersonic jet expansions that the hydrogen bond and aromatic ring of a first solvating benzyl alcohol have almost no preference for either side. An increased preference for the t side develops in para-halogenated benzyl alcohols, and it is inverted for ortho-halogenated benzyl alcohols. The experimental dependence on the actual halogen (Cl, Br, and I) is weak, whereas different quantum chemical approaches predict more or less pronounced trends along the halogen series. Some of the benzyl alcohol in the pre-expansion reservoir reduces the TEMPO radical to the closed-shell heterocyclic hydroxylamine TEMPO-H (1-hydroxy-2,2,6,6-tetramethylpiperidine), to the extent that the TEMPO-H···TEMPO complex is observed as an impurity.