Structures, thermochemistry, vibrational frequencies and integrated infrared intensities of SF₅CF₃ and SF₅ , with implications for global temperature patterns

Abstract

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-ζ 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

species as well as the harmonic vibrational frequencies are reported.

The neutral SF₅CF₃ global minimum has C_s symmetry in its

electronic ground state. The S–C bond distance for SF₅CF₃ is predicted to be 1.898 Å (BHLYP) and the torsional barrier around the S–C bond is 17 cm⁻¹ (B3LYP). For the

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

. Charge distribution analysis indicates that structure a for

also has some ion-dipole character. Structure c has a lower energy, but it is a loose complex of the type

.

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

anion (structure a), the theoretical energy for dissociation to

+CF₃ is 0.06 eV (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.

Notes

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Additional information

Notes on contributors

Henry F. Schaefer

e-mail: [email protected]