A study on mass spectrometry of methylated [60] fullerenes using the “in-beam” electron impact technique

Abstract

Mass spectra of the methylated [60]fullerenes were obtained by EI mass spectrometry using "desorption" or "in-beam" technique. The mass spectra of the methylated fullerenes, C 60 Me n , have the molecular ion peak M ϩ indicating that the product is stable under the MS (EI) conditions. The appearance of an intense peak at m/z 360 was assigned to the formation of fullerene dication C 60 ϩϩ . The remaining peaks were assigned to successive loss of methyl groups from molecular monocation and dication. A lkylation involves reaction of fullerene anions with positive alkyl groups. This leads to different addition patterns, which vary also according to the alkylation technique. These compounds are exceptionally soluble fullerenes, and promise to be useful cross-linking additives, for which the parent fullerenes are too insoluble.Their relative simplicity aids structural characterization, and many of them form suitable crystals for X-ray structure determination [1,2]. Although the small size of the methyl group and the reasonable stability of the methylfullerenes toward electron impact (EI) mass spectrometry made the reaction suitable for elucidating mechanistic features of fullerene additions, the polymethylation was a disincentive to further work [3]. The availability of high-pressure liquid chromatography (HPLC) columns dedicated to fullerene separation has enabled progress to be made.The methylated fullerenes were prepared in our previous works by different methods [1-4]: (1) reaction of fullerene with lithium followed by MeI, (2) reduction of fullerene with sodium n-propylthiolate followed by quenching with MeI, (3) reduction of fullerene by Al-Ni alloy and aqueous NaOH-dimethyl sulfoxide followed by quenching with MeI, and (4) methylation by nucleophilic substitution of halogenated fullerene C 60 Cl 6 . The resulting methylated fullerenes differ according to the method used, indicating different addition pathways: Separation of methylated fullerenes was performed with HPLC using a Cosmosil Buckyprep column with either heptane, toluene, toluene/heptane, or cyclohexane/toluene mixtures, with UV detection at 285 nm and a flow rate of 4 mL/min.Mass spectra were obtained using a VG Autospec (Sussex University, UK) mass spectrometer using 70-eV EI ionization. The samples were introduced into the mass spectrometer using the direct insertion probe with desorption CI (chemical ionization) tip operated at a temperature between 50 and 500°C. The samples were coated on a surface and inserted directly inside an ion source, as close to the ionization area as possible, by means of an extended direct insertion probe. The Љin-beamЉ technique gives a better chance of observing molecular ions in the mass spectra at the lower temperature and small sample size [5].