Magic angle spinning H-1-NMR study of the spin density distribution of pyridyl nitronyl nitroxides in the crystalline phase

The electron spin density distribution was investigated for p- and m-pyridy l nitronyl nitroxides (p-PYNN and m-PYNN) in the crystalline phase by the t emperature dependence of the solid state high resolution H-1-MAS NMR spectr um. The results were compared with that of phenyl nitronyl nitroxide (PNN) for elucidating the effect of incorporation of a nitrogen atom into the aro matic group. For p-PYNN, the magnitude of the negative spin density at 3 an d 5 positions of the pyridyl group was suppressed by 30% in comparison with that of PNN and the positive spin density at 2 and 6 positions was slightl y enhanced by 10%. On the other hand, the positive spin density at 2, 4 and 6 positions of pyridyl group of m-PYNN was suppressed by 30% in average an d the negative one at 5 was also suppressed by 20%. The DFT calculation at UBLYP/6-31G(d, p) level suggested that the molecular geometry largely contr ibuted to the change of the spin density in addition to the effect of incor poration of the nitrogen atom. In fact, the spin density distribution of th e aromatic ring of p-PYNN was remarkably reduced in solution compared with that in the crystalline phase.