Studies of negative ion formation in fluoroethers and fluorosulphides using low-energy (≤10 eV) electron beam and electron swarm techniques

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Abstract
The attachment of low-energy (≤10 eV) electrons to four fluoroethers (CF3OCF3, CF3OCF2H, CF2HOCF2H and CF3OCH3) and two fluorosulphides (CF3SCF3 and CF3SCH3) has been studied using a time-of-flight mass spectrometer (TOFMS) and a high pressure electron swarm technique. The relative cross sections as a function of incident electron energy for all observed anions were measured by employing the former, and the total absolute electron attachment rate constants were measured by employing the latter technique. All six molecules were found to attach electrons dissociatively. The types and relative intensities of the fragment anions depend strongly on the number and relative positions of the F atoms in the molecule and on the presence of O or S atoms in the molecule. The fluorosulphides attach lower energy electrons than do the fluoroethers. The magnitude of the total electron attachment rate constants increases with increasing number of F atoms in the molecule. The observed negative ions (in decreasing order of intensity) and the positions of the peak intensities (given in parentheses in eV) are: F−(5.3) and CF3O−(4.8) from CF3OCF3; CF3O−(3.7), F−(5.7), HF−2(6.1), CFO−(5.9), and CF−3(6.7) from CF3OCF2H; CF3O−(3.0), HF−2(4.7), F−(5.0), and CFO−(4.9) from CF2HOCF2H; F−(6.7) from CF3OCH3; CF3S−(0.6) and F−(3.8) from CF3SCF3; CF3S−(∼0.0) from CF3SCH3. Energetic considerations were employed to identify possible fragmentation mechanisms of the negative ion states (NISs) leading to the production of the observed fragment negative ions and to deduce values for the electron affinities of the radicals CF3O and CF3S. CNDO/2 and MNDO molecular orbital calculations were performed on all six molecules investigated in an effort to rationalize the types, relative intensities, and positions of the maxima in the cross sections of the observed anions.

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