Ion chemistry in germane/fluorocompounds gaseous mixtures: a mass spectrometric and theoretical study

摘要

The ion-molecule reactions occurring in GeH4/NF3, GeH4/SF6, and GeH4/SiF4 gaseous mixtures have been investigated by ion trap mass spectrometry and ab initio calculations. While the NFx+ (x = 1-3) react with GeH4 mainly by the exothermic charge transfer, the open-shell Ge+ and GeH2+ undergo the efficient F-atom abstraction from NF3 and form GeF+ and F-GeH2+ as the only ionic products. The mechanisms of these two processes are quite similar and involve the formation of the fluorine-coordinated complexes Ge-F-NF2+ and H2Ge-F-NF2+, their subsequent crossing to the significantly more stable isomers FGe-NF2+ and F-GeH2-NF2+, and the eventual dissociation of these ions into GeF+ (or F-GeH2+) and NF2. The closedshell GeH+ and GeH3+ are instead much less reactive towards NF3, and the only observed process is the less efficient formation of GeF+ from GeH+. The theoretical investigation of this unusual H/F exchange reaction suggests the involvement of vibrationally-hot GeH+. Passing from NF3 to SF6 and SiF4, the average strength of the M-F bond increases from 70 to 79 and 142 kcal mol(-1), and in fact the only process observed by reacting GeHn+ (n = 0-3) with SF6 and SiF4 is the little efficient F-atom abstraction from SF6 by Ge+. Irrespective of the experimental conditions, we did not observe any ionic product of Ge-N, Ge-S, or Ge-Si connectivity. This is in line with the previously observed exclusive formation of GeF+ from the reaction between Ge+ and C-F compounds such as CH3F. Additionally observed processes include in particular the conceivable formation of the elusive thiohypofluorous acid FSH from the reaction between SF+ and GeH4.