COMBINING THEORY WITH EXPERIMENT - ASSESSMENT OF THE THERMOCHEMISTRY OF SFN, SFN-, N=1-6(, AND SFN)

The Gaussian-2 [(G2) or G2(MP2)] ab initio quantum chemical calculatio ns allow the examination of the consistency of experimental thermochem ical data for SFn, SFn+, and SFn- (n = 1-6) in the literature. Based o n comparisons between G2 and G2(MP2) theoretical predictions and exper imental measurements, we recommend the following self-consistent set o f experimental heats of formation at 0 K (Delta(f)H degrees(0)) for SF n, SFn+, and SFn- (n = 1-6): Delta(f)H degrees(0)(SF) = 2.9 +/- 1.4 kc al/mol, Delta(f)H degrees(0)(SF+) = 240.9 +/- 1.2 kcal/mol, Delta(f)H degrees(0)(SF-) = -43 +/- 13 kcal/mol, Delta(f)H degrees(0)(SF2) = -69 .4 +/- 2.6 kcal/mol, Delta(f)H degrees(0)(SF2+) = 163.2 +/- 2.6 kcal/m ol, Delta(f)H degrees(0)(SF3) = -103 +/- 5 kcal/mol, Delta(f)H degrees (0)(SF3+) = 85.6 +/- 5.0 kcal/mol, Delta(f)H degrees(0)(SF3-) = -174.5 +/- 6.8 kcal/mol, Delta(f)H degrees(0)(SF4) = -181 +/- 5 kcal/mol, De lta(f)H degrees(0)(SF4+) = 93 +/- 5 kcal/mol, Delta(f)H degrees(0)(SF4 -) = -215.6 +/- 6.8 kcal/mol, Delta(f)H degrees(0)(SF5) = -205.9 +/- 3 .4 kcal/mol, Delta(f)H degrees(0)(SF5+) = 15.5 +/- 3.6 kcal/mol, Delta (f)H degrees(0)(SF5-) = -291 +/- 5.7 kcal/mol, Delta(f)H degrees(0)(SF 6) = -288.4 +/- 0.2 kcal/mol, and Delta(f)H degrees(0)(SF6-) = -314.5 +/- 2.4 kcal/mol. For Delta(f)H degrees(0)(SF2-), which is not known e xperimentally, we recommend a G2 value of -102.4 kcal/mol. At the MP2/ 6-31+G(d) level, SF6+ is found to be unstable with respect to dissocia tion forming lower sulfur fluoride cations. We have rationalized the t heoretical structures for SFn, SFn+, and SFn- (n = 1-6) using the vale nce-shell-electron-pair-repulsion theory. The alternating patterns of high and low values observed for the SFn-1-F (n = 2-6), SFn-1+-F (n = 3-5), and SFn-1-F (n = 2-6) bond dissociation energies at 0 K and for the ionization energies and electron affinities of SFn (n = 1-6) are a ttributed to special stabilities for closed-shell molecular species, ( SF3+, SF2, and SF-), (SF5+, SF4, and SF3-), and (SF6 and SF5-), with f ully-filled 8, 10, and 12 valence electron shells around the central S atoms, respectively.