Reactions of atomic transition-metal ions with long-chain alkanes

Understanding metal ion interactions with long-chain alkanes not only is of fundamental importance in the areas of organometallic chemistry, surface c hemistry, and catalysis, but also has significant implication in mass spect rometry method development for the analysis of polyethylene. Polyethylene r epresents one of the most challenging classes of polymers to be analyzed by mass spectrometry. In this work, reactions of several transition-metal ion s including Cr+, Mn+, Fe+, Co+, Ni+, Cu+, and Ag+ with long-chain alkanes, C28H58 and C36H74 are reported. A metal powder and the nonvolatile alkane a re co-deposited onto a sample target of a laser desorption/ionization (LDI) time-of-flight mass spectrometer. The metal ions generated by LDI react wi th the vaporized alkane during desorption. It is found that all these metal ions can form adduct ions with the long-chain alkanes. Fe+, Co+, and Ni+ p roduce in-source fragment ions resulting from dehydrogenation and dealkylat ion of the adduct ions. The post-source decay (PSD) spectra of the metal-al kane adduct ions are recorded. It is shown that PSD of Ag+ alkane adduct io ns produces bare metal ions only, suggesting weak binding between this meta l ion and alkane. The PSD spectra of the Fe+, Co+ and Ni+ alkane adduct ion s display extensive fragmentation. Fragment ions are also observed in the P SD spectra of Cr+, Mn+, and Cu+ alkane adduct ions. The high reactivity of Fe+, Co+, and Ni+ is consistent with that observed in small alkane systems. The unusually high reactivity of Cr+, Mn+, and Cu+ is rationalized by a re action scheme where a long-chain alkane first forms a complex with a metal ion via ion/induced dipole interactions. If sufficient internal energy is g ained during the complex formation, metal ions can be inserted into C-H and C-C bonds of the alkane, followed by fragmentation. The thermal energy of the neutral alkane is believed to be the main source of the internal energy acquired in the complex. Finally, the implication of this work on mass spe ctrometry method development for polyethylene analysis is discussed. (C) 20 01 American Society for Mass Spectrometry.