Hydroxyl Radical−π Interaction in a Single Crystal

Numerous attempts for organic radical stability mostly entail steric hindrance, spin-delocalization, supramolecular interaction with the host, π-π interactions, and hydrogen bonding. To date, there is no report of single crystals containing a hydroxyl radical (^•OH). In this work, we have stabilized ^•OH in the crystal, which has been obtained from the filtrate after separating the precipitate of the chromenopyridine radical (DCP(2)^•) from the reaction mixture. DCP(2)^• abstracts a hydrogen atom from dissolved water in the ethanolic filtrate to grow the single crystal containing DCPH(2) and ^•OH in the asymmetric unit. The crystal packing and computational studies suggest that π-^•OH and ^•OH···N hydrogen-bonding interactions are responsible for stabilizing ^•OH. The presence of ^•OH has been further confirmed by mass analysis with the 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) adduct. Solid-state electron paramagnetic resonance (EPR), solution state nitroblue tetrazolium (NBT) assay, and spin trapping with 5,5-dimethyl-1-pyrroline <i>N</i>-oxide (DMPO) in the presence of super oxide dismutase suggest ^•OH formation in the single crystal.