Atomic nitrogen encapsulated in fullerenes: Effects of cage variations

The synthesis and EPR spectroscopic investigations of a family of six endohedral fullerenes, namely, N@C-60 (1), N@C-61(COOC2H5)(2) (2), N@C-66(COOC2H5)(12) (3), N@C-66(COOC2D5)(12) (4), N@C-61(COOC2D5)(2) (5), and N@C-70 (6), containing atomic nitrogen in the S-4(3/2) ground state is described. The parent systems N@C-60 (1) and N@C-70 (6) were synthesized by nitrogen ion implantation. The syntheses of the C-2 upsilon-symmetric monoadducts 2 and 5 and the Ti, symmetric hexaadducts 3 and 4 exhibiting well-defined cage distortions were accomplished via cyclopropanation with the corresponding malonates, With respect to these additions the reactivity of 1 is indistinguishable from that of empty C-60 The quartet electronic spin of the encapsulated N-atoms is a very sensitive probe for cage modifications. In the monoadducts 2 and 5 a permanent zero field splitting (ZFS) tensor with rigidly aligned axes was revealed reflecting the intrinsic droplet like cage distortion. In contrast, no fine structure due to intrinsic distortions was monitored in the ESR spectra of the highly symmetric hexaadducts 3 and 4. In these cases only matrix-induced distortions of the cage lead to ZFS interactions. In solution fluctuations of the ZFS tensor are the major source of spin relaxation. The root-mean-square value of this collision-induced fluctuating ZFS interaction as estimated from relaxation data fur N@C-60 (1) and the hexaadduct 3 is in the range of the ZFS interaction measured for the monoadduct 2.