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

Abstract

Solvent effects on the C−N bond rotation process of formamide (FA) are investigated experimentally and theoretically. Temperature-dependent exchange broadened 1 H NMR line shapes of [ 15 N]FA in water, dimethyl sulfoxide- d 6, and tetrachloroethane- d 2, and as neat solution are consistent with Δ G ⧧ 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 Δ G ⧧ 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 Δ G ⧧ 298 for hydrated FA is 1.99 kcal mol -1 higher than Δ G ⧧ 298 for gas-phase FA. SC-IPCM calculations for a 1:1 FA−H 2 O complex were performed to allow for direct inclusion of intermolecular hydrogen bonding. The predicted Δ G ⧧ 298 for the hydrated 1:1 FA−H 2 O complex is ca. 2.3 kcal mol -1 higher than the predicted gas-phase Δ G ⧧ 298, in excellent agreement with experiment.