Atomic nitrogen encapsulated in fullerenes: realization of a chemical Faraday cage

Fullerenes, C60 and C70, are ideal containers for atomic nitrogen. We will show by electron paramagnetic resonance (EPR) experiments that nitrogen in C60 keeps its atomic ground state configuration and resides in the center of the cage. This is the first time that atomic nitrogen is stabilized at ambient conditions. The inert shell of the fullerene protects the highly reactive nitrogen from undergoing chemical reactions with the surroundings. The fullerene cage is the chemical analogue of the Faraday cage in case of electrical fields, i.e. it shields off the chemical reactivity. As for the free nitrogen atom, the spins of the three p-electrons of nitrogen in C60 are parallel (S = 3/2) and the atom has spherical symmetry. Due to the center position of nitrogen in C60, extremely sharp EPR lines are observed. This reflects the absence of a strong host-guest interaction and shows that the individuality of nitrogen in the fullerenes is preserved. Further evidence for the almost interaction-free suspension of nitrogen in the fullerene cages is provided by g-factor measurements. These investigations show that magnetic shielding of the host molecules can account for the observed differences between N rate at C60 and N rate at C70. The fullerene cage can be chemically modified without destroying the endohedral complex. The chemical modifications change the symmetry of the molecule which is observed through an additional fine structure in the EPR spectrum. Influences of the modifications on the stability of N rate at C60 will be discussed. (orig.)