Experimental 1H NMR and Computational Studies of Internal Rotation of Solvated Formamide

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 104; no. 13; pp. 2985 - 2993
• Main Authors: Taha, Angela N, True, Nancy S
• Format: Journal Article
• Language: English
• Published: American Chemical Society 06.04.2000
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp993915c

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-d 6, and tetrachloroethane-d 2, and as neat solution are consistent with ΔG ⧧ 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 ⧧ 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 ⧧ 298 for hydrated FA is 1.99 kcal mol-1 higher than ΔG ⧧ 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 ⧧ 298 for the hydrated 1:1 FA−H2O complex is ca. 2.3 kcal mol-1 higher than the predicted gas-phase ΔG ⧧ 298, in excellent agreement with experiment.