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, pi-pi interactions, and hydrogen bonding. To date, there is no report of single crystals containing a hydroxyl radical (center dot OH). In this work, we have stabilized center dot OH in the crystal, which has been obtained from the filtrate after separating the precipitate of the chromenopyridine radical (DCP(2)center dot) from the reaction mixture. DCP(2)center dot abstracts a hydrogen atom from dissolved water in the ethanolic filtrate to grow the single crystal containing DCPH(2) and center dot OH in the asymmetric unit. The crystal packing and computational studies suggest that pi-center dot OH and center dot OH<middle dot><middle dot><middle dot>N hydrogen-bonding interactions are responsible for stabilizing center dot OH. The presence of center dot 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 center dot OH formation in the single crystal.