The most stable conformer has the //-ethyl group syn to the carbonyl, except for X = H where it is anti to the carbonyl. Activation free energies AG*29s in kcal mol-1 (gas/solution) are 16.4 (0.1)/18.2 (0.1) for X = CF3 (in CCL,), 15.4 (0.2)/18.5 (0.2) for X = CH3 (in DMSO-t/6), and 19.6 (0.2)/21.0 (0.2) for X = H (in DMSO-i/e). Gas-phase values of AG*298 are lower the liquid-phase values by 1.4-3.1 kcal mol-1. This is consistent with previous experimental studies.
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.
Rate constants obtained from total line-shape analysis of 11 temperature-dependent gas-phase NMR
spectra of (15N)trifluoroacetamide yielded the following kinetic parameters: ΔG
⧧
298 = 15.1(0.36) kcal mol-1,
ΔH
⧧ = 13.3(1.3) kcal mol-1, and ΔS
⧧ = −5.9(4.5) cal mol-1 K-1. Ab initio calculations performed at the
MP2 level using the 6-311++G(d,p) basis set calculated ΔG
⧧
298 = 15.19 kcal mol-1, ΔH
⧧ = 14.35 kcal
mol-1, and ΔS
⧧ = −2.82 cal mol-1 K-1 in reasonable agreement with experiment. Hartree−Fock calculations
using that basis set were also in reasonable agreement with experiment, yielding ΔG
⧧
298 = 16.03 kcal mol-1,
ΔH
⧧ = 14.35 kcal mol-1, and ΔS
⧧ = −5.62 cal mol-1 K-1. DFT calculations (B3-PW91) using the same basis
set gave results that were considerably less accurate.
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