Atomic Nitrogen and Phosphorus can be encapsulated in fullerenes acting as highly stable chemical traps. In solution, the neutral atoms in their quartet spin ground state are weakly coupled to the surrounding by zero-field-splitting interaction, resulting either from collision-induced deformations of the fullerene cage or by intrinsic deviations from spherical symmetry. From an analysis of the spin relaxation times, values for the correlation time and the mean square value of the interaction can be deduced.