Radical Dimerization in a Plastic Organic Crystal Leads to Structural and Magnetic Bistability with Wide Thermal Hysteresis

The nitroxyl radical 1-methyl-2-azaadamantane N-oxyl (Me-AZADO) exhibits magnetic bistability arising
from a radical/dimer
interconversion. The transition from the rotationally disordered paramagnetic
plastic crystal, Me-AZADO, to the ordered diamagnetic crystalline
phase, (Me-AZADO)2, has been conclusively demonstrated
by crystal structure determination from high-resolution powder diffraction
data and by solid-state NMR spectroscopy. The phase change is characterized
by a wide thermal hysteresis with high sensitivity to even small applied
pressures. The molecular dynamics of the phase transition from the
plastic crystal to the conventional crystalline phase has been tracked
by solid-state (1H and 13C) NMR and EPR spectroscopies.