Gas-phase ion chemistry of NF 3/SO 2 mixtures: A mass spectrometric and theoretical investigation

Abstract

The gas-phase ion chemistry of NF 3/SO 2 mixtures has been investigated by ion trap mass spectrometry and theoretical calculations. SO + and SO 2 + react efficiently with NF 3 giving F (SO) + and F (SO 2) +. CAD experiments and thermochemical considerations support the exclusive formation of the sulfur–fluorine cations F SO + and F SO 2 +. NF 2 + is unreactive toward SO 2, and NF 3 + undergoes exclusively the efficient charge transfer. On the other hand, NF + activates the OS O bond, with formation of SO + and NO +. DFT and coupled cluster calculations indicate that these ionic products arise from a SO +–(FNO) ion–dipole complex, which dissociates into SO + and FNO or NO + and FSO. This intermediate is more stable than NF + and SO 2 by nearly 60 kcal mol −1. We have also located a less stable sulfur–nitrogen complex FN SO 2 +, whose formation explains the less efficient observed charge transfer between NF + and SO 2. The only observed negative ion–molecule reaction is the formation of F SO 2 − from the reaction between SO 2 − and NF 3. Our investigated processes may be of interest for the plasma and the atmospheric chemistry of NF 3, one of the gaseous compounds most extensively used in the electronic industry to perform etching and cleaning processes.