Unimolecular Dissociations of C70+ and Its Noble Gas Endohedral Cations Ne@C70+ and Ar@C70+: Cage-Binding Energies for C2 Loss

The energetics and dynamics of unimolecular decompositions of C 7 0 + and its noble gas endohedral cations, Ne@C 7 0 + and Ar@C 7 0 + , have been studied using tandem mass spectrometry techniques. The high-resolution mass-analyzed ion kinetic energy (HR-MIKE) spectra for the unimolecular reactions of C 7 0 + , Ne@C 7 0 + , and Ar@C 7 0 + were recorded by scanning the electrostatic analyzer and using single-ion counting that was achieved by combination of an electron multiplier, amplifier/discriminator, and multichannel analyzer. These cations dissociate unimolecularly via loss of a C 2 unit, and no endohedral atom is observed as fragment. The activation energies for C 2 evaporation from Ne@C 7 0 + and Ar@C 7 0 + are lower than those for elimination of the endohedral noble gas atoms. The kinetic energy release distributions (KERDs) for the C 2 evaporation have been measured and, by use of the finite heat bath theory (FHBT), the binding energies for the C 2 emission have been deduced from the KERDs. The C 2 evaporation energies increase in the order ΔE v a p (C 7 0 + ) < ΔE v a p (Ne@C 7 0 + ) < ΔE v a p -(Ar@C 7 0 + ), but no big difference in the cage binding was observed for C 7 0 + , Ne@C 7 0 + , and Ar@C 7 0 + , indicating incorporations of the Ne and Ar atoms into C 7 0 contribute a little to the stability of C 7 0 toward C 2 loss, which is in good agreement with theoretical calculations but contrasts with the findings in their C 6 0 analogues and in metallofullerenes that the decay energies of the filled fullerenes are much higher than those of the corresponding empty cages.