Gas-phase ion chemistry of NF3/SO2 mixtures: A mass spectrometric and theoretical investigation

The gas-phase ion chemistry of NF3/SO2 mixtures has been investigated by ion trap mass spectrometry and theoretical calculations. SO+ and SO2+ react efficiently with NF3 giving F(SO)+ and F(SO2)+. CAD experiments and thermochemical considerations support the exclusive formation of the sulfur–fluorine cations FSO+ and FSO2+. NF2+ is unreactive toward SO2, and NF3+ undergoes exclusively the efficient charge transfer. On the other hand, NF+ activates the OSO 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 SO2 by nearly 60 kcal mol−1. We have also located a less stable sulfur–nitrogen complex FNSO2+, whose formation explains the less efficient observed charge transfer between NF+ and SO2. The only observed negative ion–molecule reaction is the formation of FSO2− from the reaction between SO2− and NF3. Our investigated processes may be of interest for the plasma and the atmospheric chemistry of NF3, one of the gaseous compounds most extensively used in the electronic industry to perform etching and cleaning processes.