Gas-phase reactions of SiHn+ (n = 1,2) with NF3: a computational investigation on the detailed mechanistic aspects

The mechanism of the gas-phase reactions of SiHn+ (n=1,2) with NF3 have been investigated using ab initio calculations at the MP2 and CAS-MCSCF level of theory. In the reaction of SiH+ the kinetically relevant intermediates are the two fluorine-coordinated isomers HSi-F-NF2. These species forms from the exoergic attack of SiH+ to one of the F atoms of NF3 and undergo isomerisation and subsequent dissociation into SiF+ and NHF2. The fluorine-coordinated isomers, by means of the competitive crossing between the singlet and triplet surfaces, forms also the spin forbidden HSiF+ and NF2 products. The reaction of SiH2+ with NF3 involve the concomitant formation of the nitrogen-coordinated and fluorine-coordinated complex. The latter isomer undergo the direct dissociation into NF2+ and H2SiF products. The nitrogen-coordinated isomer undergo two competitive processes, namely the isomerisation into the fluorine-coordinate complex which directly dissociates into its constituting fragments, and the passage through the conical intersection to form the isomer H2SiF-NF2 which directly dissociate into H2SiF+ and NF2 products.