On the electron affinities of perfluoroalkanes
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The electron affinities of several perfluorolalkane (PFA) molecules have been investigated employing hybrid and pure density functional methods. The optimum structures of the neutral PFAs and their corresponding radical anions have been predicted employing pure and hybrid density functionals in conjunction with a double zeta basis set augmented with polarization and diffuse functions (DZP++). Electron affinities, structural features of neutrals and anions of (a) linear chain PFAs (n-PFAs) and branched chain PFAs with tertiary C-F bonds, (b) mono-cyclic PFAs (c-PFAs) and CF3- substituted c-PFAs, and (c) perfluoro-bicyclo[n, n, 0]alkanes (n,n,-BCPFAS), have been explored. Adiabatic electron affinity (AEA) trends for n-PFAs (general formula, n-CnF2n+2, with “n” corresponding to the carbon chain length) reveal that AEAs show a drastic enhancement moving from n=2 to n=3, beyond that they exhibit a slow increase with increments falling of steadily with extending chain length, terminating at n=7. The radical anions of n-PFAs show a characteristic structural feature, an exceptionally long C-F bond in the middle carbon of the chain. Branched PFAs with a tertiary C-F bond are found to possess higher AEA than the linear chain PFAs. Mono-cyclic PFAs (general formula of c-PFA, c-CnF2n, and “n” corresponds to the carbon ring size) exhibit a peculiar trend of AEAs with increasing ring size. The AEAs of c-CnF2n increase from n=3 to n=5 but then dramatically fall off for both n=6 and n=7. It was noted that there is a change in the mode of binding the “extra electron” beyond the 5-memebred ring. CF3- substituted c-PFAs display enhanced adiabatic electron affinity due to the presence of tertiary C-F bonds. Adiabatic and vertical electron affinities were computed for perfluoro-bicyclo[n, n, 0]alkanes, with “n” ranging from 1 to 4. All the n.n-BCPFAs have tertiary C-F bonds. However, the mode of binding the “unpaired electron” changes significantly over the different ring sizes for these bicyclic radical anions. The highly strained 1, 1-BCPFAis predicted to have the highest AEA among the family of BCPFA molecules that were examined.