CHEMICAL IONIZATION OF FULLERENES

Chemical ionization of buckminsterfullerence and higher mass fullerenes has been studied by using triple quadrupole mass spectrometry (TQMS) and Fourier transform mass spectrometry (FTMS). High-pressure chemical ionization generates both protonated fullerenes (e.g. C60H+) and adducts of the fullerenes with reagent ions. The proton affinities (PA) of C50 and C70 are determined to be between 204 kcal/mol and 207 kcal/mol by proton-transfer bracketing studies in the FTMS. Reactivity data also suggest that PA(C60) < PA(C70). The stabilities of the protonated fullerenes are probed by collision-induced dissociation, which shows that these species are stable up to relatively high collision energies (approximately 150 eV, lab), above which loss of a hydrogen atom occurs. Higher-mass fullernes (C(n)) are observed in both positive (n = 84) and negative (n = 124) chemical ionization mass spectra. An anomalously high abundance of C70- is typically observed in the negative ion mass spectra and is attributed to a greater electron capture cross section of C70 with respect to that of C60.