Circular dichroism studies of crystalline chiral and achiral alpha-nitronyl nitroxide radicals in a KBr matrix

The X-ray crystal structures of a series of four chiral phenyl alpha -nitronyl nitroxides substituted in the aromatic ring with an (R)-lactate moiety are compared, and reveal the presence of all four gross diastereomeric conformations resulting from the twisted conformations of the phenyl and alpha -nitronyl nitroxide moieties. The use of KBr discs as solid solutions for the study of the circular dichroism (CD) spectra of these radical crystals provides useful information concerning the conformational stereoisomerism displayed by the compounds, both intrinsically chiral and other achiral radicals which crystallise in chiral space groups. The optimum conditions for obtaining the solid state CD spectra have been investigated, and require a short ball-milling of the mixture followed by careful recording of the spectrum. The Cotton effects observed in the CD spectra have been correlated with the observed enantiomeric forms of the pseudo-anti and pseudo-eclipsed conformers in the crystals of chiral lactic acid-derived phenyl alpha -nitronyl nitroxides and an achiral phenyl alpha -nitronyl nitroxide that crystallises in a chiral space group.