Gas-phase reactions of XH3+ (X = C, Si, Ge) with NF3: a comparative investigation on the detailed mechanistic aspects

The gas-phase reaction of CH3+ with NF3 was investigated by ion trap mass spectrometry (ITMS). The observed products include NF2+ and CH2F+. Under the same experimental conditions, SiH3+ reacts with NF3 and forms up to six ionic products, namely (in order of decreasing efficiency) NF2+, SiH2F+, SiHF2+, SiF+, SiHF+, andNHF+. TheGeH3+ cation is instead totally unreactive toward NF3. The different reactivity of XH3+ (X=C, Si, Ge) toward NF3 has been rationalized by ab initio calculations performed at the MP2 and coupled cluster level of theory. In the reaction of both CH3+ and SiH3+, the kinetically relevant intermediate is the fluorine-coordinated isomer H3X-F-NF2+ (X = C, Si). This species forms from the exoergic attack of XH3+ to one of the F atoms of NF3 and undergoes dissociation and isomerization processes which eventually result in the experimentally observed products. The nitrogen-coordinated isomers H3X-NF3+ (X = C, Si) were located as minimum-energy structures but do not play an active role in the reaction mechanism. The inertness of GeH3+ toward NF3 is also explained by the endoergic character of the dissociation processes involving the H3Ge-F-NF2+ isomer.