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 N-oxide (DMPO) in the presence of super oxide dismutase suggest •OH formation in the single crystal.