Hydroxyl Radical−π\nInteraction in a Single\nCrystal

Numerous attempts for organic radical stability mostly\nentail steric\nhindrance, spin-delocalization, supramolecular interaction with the\nhost, π–π interactions, and hydrogen bonding. To\ndate, there is no report of single crystals containing a hydroxyl\nradical (<sup>•</sup>OH). In this work, we have stabilized <sup>•</sup>OH in the crystal, which has been obtained from the\nfiltrate after separating the precipitate of the chromenopyridine\nradical (DCP(2)<sup>•</sup>) from the reaction mixture. DCP(2)<sup>•</sup> abstracts a hydrogen atom from dissolved water in\nthe ethanolic filtrate to grow the single crystal containing DCPH(2)\nand <sup>•</sup>OH in the asymmetric unit. The crystal packing\nand computational studies suggest that π–<sup>•</sup>OH and <sup>•</sup>OH···N hydrogen-bonding\ninteractions are responsible for stabilizing <sup>•</sup>OH.\nThe presence of <sup>•</sup>OH has been further confirmed by\nmass analysis with the 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)\nadduct. Solid-state electron paramagnetic resonance (EPR), solution\nstate 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\nsuggest <sup>•</sup>OH formation in the single crystal.