In isopentane at 25° a 0.1 M solution of nortropane N-oxyl (1) contains 0.0618 M of the free radical and a 0.1 M solution of 9-azabicyclo[3.3.1]nonane N-oxyl (4) contains 0.0854 M of radical. Measurements at lower temperatures show that the thermodynamic parameters for the dimerization of these two nitroxides are very similar. However, the solid-state properties of 1 and 4 show striking differences. Thus, at room temperature 1 is yellow, diamagnetic, and has a strong band in its infrared spectrum attributable to the NO group at 1332 cm-1 in KBr. In contrast, 4 is red, paramagnetic, and has only a few weak bands in this region.
Journal of the American Chemical Society95, no. 19 (1973): 6390–6394.