Studies of rare gas endohedral fullerene complexes: The dynamics of gas phase acid-base reactions

Incorporation of noble gas atoms into the cavity of fullerene molecules is achieved by a high temperature and pressure thermal process, yielding a filled fraction of {dollar}sim{dollar}0.1%. This fraction is significantly lower than the theoretically predicted equilibrium composition of about 10% for He and Ne. It is observed that while prolonged exposure to the pressurized rare gas does not increase the yield, using the dried solvent-extracted fraction from one run as starting material for a consecutive run, increases the yield linearly with the number of labeling runs. It is found that the thermal release of Ne from within C{dollar}sb{lcub}60{rcub},{dollar} that preserves the fullerene cage, is a very slow process when pristine samples of Ne@C{dollar}sb{lcub}60{rcub}{dollar} are used. This is consistent with theoretical predictions for a high energy barrier for a pure bimolecular incorporation process. These observations suggest an involvement of a catalyst in both the incorporation and release processes. We propose a modified window mechanism in which a radical adds to the cage and weakens its bonds, allowing easier opening of a window through which the rare gas atom may pass. An increase in the filled fraction (of predominantly the heavy rare gases) can be achieved by chromatographic fractionation of the mixture. For He and Ne, a new method of incorporation by beam implantation was developed. A 0.45% incorporation is achieved for a 60 eV beam of He{dollar}sp+.{dollar} Surprisingly, exposing C{dollar}sb{lcub}60{rcub}{dollar} to the neutral output of the discharge source also results in incorporation, albeit lower. We suggest that electronically excited metastable atoms at thermal kinetic energies are the reacting species in this case.; Using crossed, seeded nozzle beams, we have studied the detailed dynamics for the reaction of HI with three amines. Reactions with tri-n-butylamine and with quinuclidine give product distributions which are forward peaked in the center-of-mass system, indicating a modified stripping mechanism. Reaction with tetrakis(dimethylamino)ethylene gives a symmetric distribution, indicating a long-lived complex. However, at high relative energies, a fragment ion is produced which is forward scattered. This suggests that protonation can occur on two different sites: at low energies the ethylenic double bond is protonated via a long-lived complex mechanism, and at high energies the aminic nitrogen is protonated via a modified stripping reaction that subsequently produces the forward scattered fragment.