text: Endohedral complexes He@C and Ne@C are prepared by pressurizing C to ca. 3 bar in a noble-gas atmosphere. Under these conditions about one out of every 650,000 C cages was doped with a helium atom. 
  The formation of endohedral complexes with helium, neon, argon, krypton and xenon as well as numerous adducts of the He@C compound was also demonstrated with pressures of 3 kbars and incorporation of up to 0.1% of the noble gases.
  While noble gases are chemically very inert and commonly exist as individual atoms, this is not the case for nitrogen and phosphorus and so the formation of the endohedral complexes N@C, N@C and P@C is more surprising.  
  The nitrogen atom is in its electronic initial state (S) and is highly reactive. Nevertheless, N@C is sufficiently stable that exohedral derivatization from the mono- to the hexa adduct of the malonic acid ethyl ester is possible. 
  In these compounds no charge transfer of the nitrogen atom in the center to the carbon atoms of the cage takes place. Therefore, C-couplings, which are observed very easily with the endohedral metallofullerenes, could only be observed in the case of the N@C in a high resolution spectrum as shoulders of the central line.
  The central atom in these endohedral complexes is located in the center of the cage. While other atomic traps require complex equipment, e.g. laser cooling or magnetic traps, endohedral fullerenes represent an atomic trap that is stable at room temperature and for an arbitrarily long time. Atomic or ion traps are of great interest since particles are present free from (significant) interaction with their environment, allowing unique quantum mechanical phenomena to be explored.  For example, the compression of the atomic wave function as a consequence of the packing in the cage could be observed with ENDOR spectroscopy. The nitrogen atom can be used as a probe, in order to detect the smallest changes of the electronic structure of its environment.
  Contrary to the metallo endohedral compounds, these complexes cannot be produced in an arc. Atoms  are implanted in the fullerene starting material using gas discharge (nitrogen and phosphorus complexes) or by direct ion implantation. Alternatively, endohedral hydrogen fullerenes can be produced by opening and closing a fullerene by organic chemistry methods.
  A recent example of endohedral fullerenes includes single molecules of water encapsulated in C.
  Noble gas endofullerenes are predicted to exhibit unusual polarizability. Thus, calculated values of mean polarizability of Ng@C do not equal to the sum of polarizabilities of a fullerene cage and the trapped atom, i.e. exaltation of polarizability occurs.,. The sign of the Δα polarizability exaltation depends on the number of atoms in a fullerene molecule: for small fullerenes (), it is positive; for the larger ones (), it is negative (depression of polarizability). The following formula, describing the dependence of Δα on n, has been proposed: Δα = α(2e−1). It describes the DFT-calculated mean polarizabilities of Ng@C endofullerenes with sufficient accuracy. The calculated data allows using C fullerene as a Faraday cage, which isolates the encapsulated atom from the external electric field. The mentioned relations should be typical for the more complicated endohedral structures (e.g., C@C and giant fullerene-containing "onions" ).