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

## TL;DR

This study explores how halogenation affects the solvation of a stable organic radical called TEMPO by benzyl alcohol.

## Contribution

The novel finding is that the position of halogen atoms in benzyl alcohol influences which side of the TEMPO radical is preferentially solvated.

## Key findings

- Para-halogenated benzyl alcohols show increased preference for the tightly arranged side of TEMPO.
- Ortho-halogenated benzyl alcohols invert this preference.
- Some benzyl alcohol reduces TEMPO to TEMPO-H, forming a detectable complex.

## Abstract

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.