Gas-phase nuclear magnetic resonance study of berry pseudorotation of SF₄. Comparison of experimental and calculated kinetic parameters and falloff kinetics

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 functional 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.