Fates of the molecular anions of SF6 and C7F14 upon recombination with positive ions

Electron attachment to C7F14, thermal detachment from C7F14(-), the electron affinity of C7F14, and neutralization of C7F14(-) by Ar+

Rate coefficients and branching fractions have been measured for electron attachment to perfluoromethylcyclohexane, C(7)F(14), along with thermal detachment rate coefficients for C(7)F(14)(-), from 300 to 630 K, using a flowing-afterglow Langmuir-probe apparatus. The attachment rate coefficient at room temperature is 4.5 ± 1.2 × 10(-8) cm(3) s(-1) and increases with temperature at a rate described by an activation energy of 50 ± 25 meV. Thermal electron detachment is negligible at room temperature, but measurable at 600 K and above, reaching 2300 ± 1300 s(-1) at 630 K. Analysis of the attachment-detachment equilibrium yields EA(C(7)F(14)) = 1.02 ± 0.06 eV, in agreement with the literature value while more than halving the uncertainty. Implications of the measurement for the electron affinity of SF(6) are discussed. The dominant product of electron attachment is the parent anion, but C(6)F(11)(-) and C(7)F(13)(-) are also observed at very low levels (<0.1%) at room temperature and increase in importance as the temperature is increased, reaching ~10% each at 630 K. In the course of this work we have also measured rate coefficients for the neutralization of C(7)F(14)(-) by Ar(+) at 300, 400, and 500 K: 4.8, 3.5, and 3.1 × 10(-8) cm(3) s(-1), respectively, with uncertainties of ±5 × 10(-9) cm(3) s(-1).

The Peculiar Trend of Cyclic Perfluoroalkane Electron Affinities with Increasing Ring Size

The adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) of cyclic perfluoroalkanes, c-C(n)F(2n) (n = 3-7), and their monotrifluoromethyl derivatives were computed using various pure and hybrid density functionals with DZP++ (polarization and diffuse function augmented double-zeta) basis sets. The theoretical AEA of c-C(4)F(8) at KMLYP/DZP++ is 0.70 eV, which exhibits satisfactory agreement with the 0.63 +/- 0.05 eV experimental value. The nonzero-point-corrected AEA of c-C(4)F(8) is predicted to be 0.41 eV at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level of theory, which shows a slight deviation of 0.11 eV from the KMLYP estimated value of 0.52 eV for the same. With the zero-point correction from the MP2/6-311G(d) [Gallup, G. A. Chem. Phys. Lett. 2004, 399, 206] level of theory combined with the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ result, the most reliable estimate of AEA of c-C(4)F(8) is 0.60 eV. c-C(3)F(6)(-), c-C(4)F(8)(-), and c-C(5)F(10)(-) are unusual in preferring planar to near planar ring structures. The ZPE-corrected AEAs of c-C(n)F(2n) increase from n = 3 (0.24 eV) to n = 5 (0.77 eV), but then dramatically fall off to 0.40 eV for both n = 6 and n = 7. All of the other functionals predict the same trend. This is due to a change in the structural preference: C(s)() c-C(6)F(12)(-) and C(1) c-C(7)F(14)(-) are predicted to favor nonplanar rings, each with an exceptionally long C-F bond. (There also is a second, higher energy D3d minimum for C(6)F(12)(-).) The SOMOs as well as the spin density plots of the c-PFA radical anions reveal that the "extra" electron is largely localized on the unique F atoms in the larger n = 6 and n = 7 rings but is delocalized in the multiatom SOMOs of the three- to five-membered ring radical anions. The computed AEAs are much larger than the corresponding VEAs; the latter are not consistent with different functionals. The AEAs are substantially larger when a c-C(n)()F(2)(n)() fluorine is replaced by a -CF(3) group. This behavior is general; PFAs with tertiary C-F bonds have large AEAs. The VDEs for all the anions are substantial, ranging from 1.89 to 3.64 eV at the KMLYP/DZP++ level.