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Pressure-dependent internal rotation rate constants for [¹⁵N]formamide in gas mixtures containing 1.3 Torr of [¹⁵N]formamide and SF6 pressures ranging from 150 to 5225 Torr were determined from analysis of exchange-broadened ¹H NMR spectra obtained at 333 K. They agree well with RRKM calculations using experimental vibrational frequencies and rotational constants for the ground state and appropriately scaled 6-311++G** parameters for the transition states. The strong collision model, with a hard sphere diameter of 4.8 Å, yields satisfactory agreement. Activated formamide molecules have an average calculated energy specific rate constant, 〈k(E)〉 , of ca. 5.3 × 10⁹ s^-1 and a state density of ca. 7.2 cm^-1. Statistical internal rotation kinetics requires activated formamide molecules to have vibrational lifetimes of <200 ps and average intervibrational coupling matrix elements of ≥0.1 cm^-1. These results are compared to kinetic studies of similar low-energy unimolecular processes.

Kinetic parameters and conformational equilibrium constants characterizing the internal rotation in gaseous N-ethyl, N-methylthioformamide (EMSF), N-ethyl, N-methylthioacetamide (EMSA), and N-ethyl, N-methyltrifluorothioacetamide (EMSTFA) are determined from exchange broadened ¹H NMR spectra. The ΔG^&thermod;298 values, in kcal mol^-1, are 22.6 (0.1), 17.8 (0.1), and 17.2(0.1) for EMSF, EMSA, and EMSTFA, respectively. Conformer Gibbs free energy differences, ΔG° (ΔG° = G° (methyl group syn to thionyl sulfur) − G° (methyl group anti to thionyl sulfur)), in cal mol^-1, are −123 (24), 56 (9), and 96 (13) for EMSF, EMSA, and EMSTFA, respectively. These results are compared to solution phase values and to the corresponding oxoamides.

Solvent effects on the C−N bond rotation process of formamide (FA) are investigated experimentally and theoretically. Temperature-dependent exchange broadened ¹H NMR line shapes of [¹⁵N]FA in water, dimethyl sulfoxide-d6, and tetrachloroethane-d2, and as neat solution are consistent with ΔG^&thermod;298 values for FA internal rotation, in kilocalories per mole, of 18.2(0.1), 17.8(0.1), 17.2(0.1), and 18.0(0.1), respectively. These values are significantly higher than the gas-phase ΔG^&thermod;298 value, 16.0(0.1) kcal mol^-1, and they correlate well with empirical spectroscopic solvent polarity scales. Self-consistent isodensity polarizable continuum model (SC-IPCM) calculations at the HF/6-311++G** level of theory for FA in a series of dielectric constants ranging from 1 to 109 predict a complete reversal of the preferred internal rotation path for hydrated FA compared to gas-phase FA. Equal contributions to the internal rotation rate constant from both possible internal rotation paths is predicted to occur at a dielectric constant of 7.08. SC-IPCM calculations predict that ΔG^&thermod;298 for hydrated FA is 1.99 kcal mol^-1 higher than ΔG^&thermod;298 for gas-phase FA. SC-IPCM calculations for a 1:1 FA−H2O complex were performed to allow for direct inclusion of intermolecular hydrogen bonding. The predicted ΔG^&thermod;298 for the hydrated 1:1 FA−H2O complex is ca. 2.3 kcal mol^-1 higher than the predicted gas-phase ΔG^&thermod;298, in excellent agreement with experiment.

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^&thermod;298 = 12.2(0.1) kcal mol^-1, ΔH^&thermod;298 = 11.3(0.4) kcal mol^-1, and ΔS^&thermod;298 = −3.3(0.4) cal mol^-1 K^-1. Using a 6-31+G* basis set, HF calculations predict ΔH^&thermod;298 = 12.26 kcal mol^-1 and ΔS^&thermod;298 = −3.74 cal mol^-1 K^-1 and MP2 calculations predict ΔH^&thermod;298 = 9.99 kcal mol^-1 and ΔS^&thermod;298 = −3.89 cal mol^-1 K^-1. Stationary point MP4/6-31+G*//HF/6-31+G* calculations predict ΔH^&thermod;298 = 11.55 kcal mol^-1 and ΔS^&thermod;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.

Temperature-dependent gas-phase ¹H NMR spectra of seven thiocarbonyl-substituted N,N-dimethylthioamides (YCSN(CH3)2) obtained at 300 MHz are consistent with the following free activation energies ΔG^&thermod;298 (kcal mol^-1):  Y = H, 22.5 (0.1); CH3, 18.0 (0.1); F, 18.3 (0.1); Cl, 16.9 (0.2); CF3, 17.2 (0.1); CH2CH3, 17.6 (0.1); CH(CH3)2, 16.3 (0.1). The results are compared to condensed-phase values and to the corresponding gas-phase oxoamides.

Activation parameters and conformational stabilities characterizing the internal rotation about the peptide bond in a series of N,N-asymmetric dialkylformamides (HCONR1R2:  R1 = CH3, R2 = propyl, butyl, and isopropyl) and N,N-asymmetric dialkyltrifluoroacetamides (F3CCONR1R2:  R1 = CH3, R2 = propyl, butyl, and isopropyl) are determined from temperature-dependent gas-phase ¹H NMR spectra. Conformer free energy differences, ΔG⁰298(syn−anti), in cal mol^-1, and activation free energies, ΔG^&thermod;298, in kcal mol^-1 , for the formamides are −83(14)/19.4(0.1) for R2 = propyl, −80(14)/19.3(0.1) for R2 = butyl, and −91(13)/19.1(0.1) for R2 = isopropyl and for the trifluoroacetamides 178(24) /16.8(0.1) for R2 = propyl, 191(53)/16.6(0.1) for R2 = butyl, and 218(29)/16.3(0.1) for R2 = isopropyl. The preferred conformer in both the gas and liquid phases has the N-methyl group syn to the carbonyl oxygen in the formamide systems and the N-methyl group anti to the carbonyl oxygen in the trifluoroacetamides. The gas-phase results are compared to liquid-phase values.