Structures, thermochemistry, vibrational frequencies and integrated infrared intensities of SF5CF3 and SF5CF3-, with implications for global temperature patterns

The molecular structures and energetics of the potent greenhouse gas SF5CF3/SF5CF3- species have been examined using nine hybrid and pure density functional theory (DFT) methods, with the basis sets of double-zeta plus polarization quality plus additional diffuse s- and p-type functions, denoted as DZP++. The geometries are fully optimized with each DFT method independently. The three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity (EA(ad)), the vertical electron affinity (EA(vert)) and the vertical detachment energy (VDE). The dissociation energies of the SF5CF3 and SF5CF3- species as well as the harmonic vibrational frequencies are reported. The neutral SF5CF3 global minimum has C-S symmetry in its (1)A' electronic ground state. The S-C bond distance for SF5CF3 is predicted to be 1.898Angstrom (BHLYP) and the torsional barrier around the S-C bond is 17 cm(-1) (B3LYP). For the SF5CF3- anion, there are three minima. Structure a is geometrically similar to the neutral, while structure b has an energy close to a, but is best described as SF5-...CF3. Charge distribution analysis indicates that structure a for SF5CF3- also has some ion-dipole character. Structure c has a lower energy, but it is a loose complex of the type SF4-...CF4. The predicted EA(ad) values (from the neutral to the anionic structure a) for SF5CF3 range from 1.59-3.00 eV, and the value 1.59 eV (KMLYP) is thought to be most reliable. The S-C bond dissociation energy D(SF5-CF3) is predicted to be 2.21 eV (B3LYP). For the SF5CF3- anion (structure a), the theoretical energy for dissociation to SF5-+CF3 is 0.06eV (B3LYP), significantly smaller than other dissociation pathways. The IR absorptions of SF5CF3 agree well with available experiments with average errors 22 (B3P86), 24 (B3PW91), 29 (B3LYP), 52 (BP86); 52 (BPW91), 61 (KMLYP) and 68 (BLYP)cm(-1). The theoretical results for the IR intensities show that SF5CF3 may be an effective greenhouse gas and hence have a significant impact on global warming. These results are consistent with Ball's suggestion that SF5CF3 may not have as long a lifetime as SF6 in the atmosphere, and that its GWP might be overestimated.