Hydrolysis of sulphur fluorides SFn (n = 3 - 5) in the gas phase

One of the important reasons for the SF6 decomposition during exploitation is the “oxygenation” of the SFn family [1]. This is particularly important for arc quenching when oxygen containing compounds appear by radical reactions with O and OH particles [1]. The high reactivity of the smaller neutral or charged species can be estimated from thermodynamic data; hence, a direct reaction between neutral SFn, n < 6, or its anions and H2O becomes possible. The easy hydrolysis was confirmed for SF4, but not for the SF4- [2] and SF6- anions as the latter form SF6-(H2O)n clusters [3]. The absense of H0298 data for the SOF-n anions does not allow to confirm their hydrolysis for other n values than n = 2, 3, 5. To our knowledge, the hydrolysis mechanisms for all SFn species have not yet been studied although it is important for the isolation application of the SF6 gas under normal conditions. The possibility of hydrolysis can be confirmed for a wide SFn series by a simple thermodynamic analysis of the heats reactions: SFn + H2O = SOFn-2 + 2HF. The H0298 are negative for all neutral forms and anions for n = 3 - 6 and, surprisingly, even for SF6, the heat of reaction ranges from -36.5 to -49.5 kJ/mol on the basis of different source estimates [4-6]. The well known SF6 stability is in contradiction with the H0298 estimates. An explanation of this fact could be related with a high activation barrier. But should the barrier be so high for the smaller SFn species and respective anions ? Hydrolysis of the SFn molecules is studied at different levels of molecular orbital theory. First, the intrinsic reaction coordinate approach was applied with the full chain technique implemented in AMPAC [7]. Then, isolated points were re-calculated at higher DFT and MP2 levels. Despite a distinction between the relative energies for the reagents, intermediates (INT), and transitions states (TS1 and TS2), as compared to the higher level methods, the semiempirical scheme allows to define HOSFn intermediates for hydrolysis of neutral SFn molecules for n = 3 - 5 with moderate activation energies, which are confirmed by the calculations performed at higher levels (case n = 4 is shown in Table). This leads to reaction pathways under normal conditions wherein the radical reaction with OH is negligible as compared to the SFn hydrolysis. Acknowledgements. The authors wish to thank the FUNDP for the use of the Namur Scientific Computing Facility (SCF) Centre. They are grateful for the financial support of the European Community (project No ENK6-CT-2000-00087). References. 1. J.T. Herron, R.J. Van Brunt, Proc 9th Int. Symp. on Plasma Chem., University of Bari, Italy, 1989. 2. A.E.S. Miller, T.M. Miller, A.A. Viggiano, R.A. Morris, J.M. Van Doren, S.T. Arnold, J.F. Paulson, J. Chem. Phys. 102 (1995) 8865. 3. L.W. Sieck, J. Phys. Chem. 90 (1986) 6684. 4. Y.-S. Cheung, Y.-J. Chen, C.Y. Ng, S.-W. Chiu, W.-K. Li, J. Amer. Chem. Soc., 117 (1995) 9725. 5. Handbook of Chemistry and Physics, 51st Edition, CRC, 1970-1971. 6. J.T. Herron, J. Phys. Chem. Ref. Data, 16 (1987) 1. 7. AMPAC 6.0, Manual, Semichem. Inc. 1997.