Abstract
To augment previous theoretical studies of thermochemical properties such as the electron affinity (EA) and bond dissociation enthalpy (D298o) of neutral and anionic SFn (with n = 1–6), further extensive theoretical computations using Gaussian-4 and Weizmann-1 and Weizmann-2 (G4/W1/W2) methods were carried out with extensive consideration of the role of the metastable conformational isomer of the SF4− anion. The energy of the metastable conformer is 39 kJ/mol higher than that of the global minimum structure, and the barrier height between the metastable conformer and its global minimum was calculated to be 27 kJ/mol by the CCSD(T)/Aug-cc-pvQZ+d//MP2/cc-pvQZ+d method. Many of the discrepancies that have persisted between previous theoretical and experimental data can be explained more adequately by considering the metastable conformer. The difference in the EA calculated using the Gaussian-3 (G3) vs the W2 method for SFn with n = 2–6 ranges from 0.12 eV to 0.21 eV, which is much larger than ±0.041 eV of the expected error for the G3 method. The difference in D298o calculated using G3 vs W2 was also significant in several cases, especially for hypervalent fluorides with n = 3–6. The final results obtained with the W2 procedure are sufficiently converged to a chemical accuracy of ±4 kJ/mol ≈ ±0.04 eV for not only the EA but also D298o, for all neutral and anionic SFn species with n = 1–6.
Highlights
IntroductionIntensive and elaborate experimental studies of the thermochemical properties of SFn systems were actively conducted by numerous research groups during the 1980s and 1990s, and all of the results up to early 2000 were well summarized and assessed by Miller and co-workers; they compared the experimental results with theoretical data generated through composite procedures employing quantum chemical computations on the basis of the Gaussian-2 and Gaussian-3 (G2/G3) theories
It is shown that the existence of the metastable SF−4 anion and its possible roles are very important in understanding the experimental results for neutral and anionic SF4 and SF5 systems, and many of the previous discrepancies that have persisted between theoretical and experimental studies for almost two decades can be explained better than ever before on the basis of the present theoretical results that consider the metastable isomer of the SF−4 anion
We compared the results obtained by the G2, G3, G4, CCSD(T)-complete basis-set (CBS), W1, CCS(DTQ), and W2 methods for the electron affinity (EA) and D○298 K of neutral and anionic SFn
Summary
Intensive and elaborate experimental studies of the thermochemical properties of SFn systems were actively conducted by numerous research groups during the 1980s and 1990s, and all of the results up to early 2000 were well summarized and assessed by Miller and co-workers; they compared the experimental results with theoretical data generated through composite procedures employing quantum chemical computations on the basis of the Gaussian-2 and Gaussian-3 (G2/G3) theories.. Miller and co-workers estimated the accuracy of their results by the G3 method within 0.1 eV for sulfur fluorides excluding SF6.9 A few further theoretical studies with other variants of more elaborate theoretical methods were conducted thereafter but were limited to either the simplest system, i.e., sulfur monofluoride SF, or the neutral systems of SFn only.
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