Reactions of cationic silicon clusters with xenon difluoride

Cationic silicon clusters, Si_1-7⁺, were observed to react bimolecularly and exothermically with xenon difluoride in the ion trap of a Fourier transform mass spectrometer. Three ionic products are observed from Si_n⁺:SiF⁺, Si_n-1⁺, and Si_nF⁺. Subsequent reactions of these products with xenon difluoride were determined as well. Si_nF⁺, n=2-6, react with xenon difluoride to form two ionic products: SiF⁺ and Si _n-1⁺. SiF_m⁺, m=1-3, react with xenon difluoride to form SiF_m+1⁺, and XeF⁺. All observed products correspond to mono- or difluorination of the clusters by xenon difluoride; in many cases the reaction was sufficiently exothermic that the fluorinated cluster fragmented immediately to produce either Si _n-1⁺ or SiF⁺. Based upon the observed trends in the product distributions, the extent of mono- vs difluorination of the clusters was obtained. The amount of monofluorination varies from 100% for SiF_m⁺, m=0-3, to 0% for Si₇⁺. By extrapolation, xenon difluoride should difluorinate bulk silicon exclusively. The reaction rates for the bare clusters differ only slightly among themselves. The monofluorinated silicon clusters, in contrast, have significant variations in reaction rate as a group. Substantially lower reaction rates are observed for Si₄F⁺ and Si₆F⁺; this is believed to derive from the greater thermodynamic stabilities of Si₄ ⁺ and Si₆⁺.