Gas-Phase Nuclear Magnetic Resonance Study of Berry Pseudorotation of SF₄. Comparison of Experimental and Calculated Kinetic Parameters and Falloff Kinetics

Abstract

Temperature-dependent Berry pseudorotation rate constants of SF4 gas at ca. 7.9 atm, determined from analysis of exchange-broadened 19F NMR spectra, are consistent with E ∞ = 11.9(0.2) kcal mol-1, A ∞ = 3.56(1.09) × 1012 s-1, ΔG ⧧ 298 = 12.2(0.1) kcal mol-1, ΔH ⧧ 298 = 11.3(0.4) kcal mol-1, and ΔS ⧧ 298 = −3.3(0.4) cal mol-1 K-1. Using a 6-31+G* basis set, HF calculations predict ΔH ⧧ 298 = 12.26 kcal mol-1 and ΔS ⧧ 298 = −3.74 cal mol-1 K-1 and MP2 calculations predict ΔH ⧧ 298 = 9.99 kcal mol-1 and ΔS ⧧ 298 = −3.89 cal mol-1 K-1. Stationary point MP4/6-31+G*//HF/6-31+G* calculations predict ΔH ⧧ 298 = 11.55 kcal mol-1 and ΔS ⧧ 298 = −3.91 cal mol-1 K-1. DFT calculations using the 6-31+G* basis set and the B3LYP and B3PW91 hybrid functionals are considerably less accurate. Pressure-dependent rate constants obtained at 335 K agree well with RRKM theory predictions using the strong collision assumption. The curvature and displacement of the experimental falloff curve are not significantly perturbed by effects of weak collisions and/or nonstatistical intramolecular vibrational energy redistribution.

Structures of the Octahedral Persulfuranes H4SF2, H2SF4, Me2SH4 and Me2SF4, and Activation Enthalpies for Their cis/trans Isomerization

“…[1] The data obtained in the present investigation indicate that these molecules are rather rigid, in sharp contrast to the well-known fluxionality of sulfuranes of the type SX 4 : in the case of SF 4 the barrier for pseudorotation is only ∆H # 298 ϭ 47 kJ·mol Ϫ1 . [16] In the case of the persulfurane A no cis/trans isomerization was observed within 254 h at 238°C under basic conditions. However, the compound isomerized rapidly after addition of a catalytic amount of SbF 5 , which cleaved one of the SF bonds, generating a cationic five-coordinate intermediate.…”

Pressure-Dependent NMR Spectroscopy Indicates That Internal Rotation of Gas-Phase Formamide Follows Statistical Kinetics

“…Pressure-dependent rate constants of 14 thermally initiated low-energy conformational processes studied in our laboratory, including internal rotations, ring inversions, and pseudorotations, obtained from analysis of exchangebroadened NMR spectra are consistent with RRKM predictions. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] In all these studies, rate constants were measured near 300 K where the internal energy of most of the activated molecules was less than 1 kcal mol -1 above the threshold energy for the process. Table 1 lists threshold energies (E 0 ), state densities for the activated molecules at E 0 [F(E 0 )], and average energy specific rate constants [〈k(E)〉], for these processes.…”

The Solid‐State Structure of SF4: The Final Piece of the Puzzle

“…Vibrational spectra of liquid SF 4 have also been assigned in terms of molecular C 2v point symmetry. [6] At low temperature, the exchange can be slowed down, allowing the observation of two triplets in the 19 F NMR spectrum. [6] At low temperature, the exchange can be slowed down, allowing the observation of two triplets in the 19 F NMR spectrum.…”