Solvent Effects on the Thioamide Rotational Barrier:  An Experimental and Theoretical Study

Abstract

The solvent effect on the C-N rotational barriers of N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA) has been investigated using ab initio theory and NMR spectroscopy. Selective inversion recovery NMR experiments were used to measure rotational barriers in a series of solvents. These data are compared to ab initio results at the G2(MP2) theoretical level. The latter are corrected for large amplitude vibrational motions to give differences in free energy. The calculated gas phase barriers are in very good agreement with the experimental values. Solvation effects were calculated using reaction field theory. This approach has been found to give barriers that are in good agreement with experiment for many aprotic, nonaromatic solvents that do not engage in specific interactions with the solute molecules. The calculated solution-phase barriers for the thioamides using the above solvents are also in good agreement with the observed barriers. The solvent effect on the thioamide rotational barrier is larger than that for the amides because the thioamides have a larger ground-state dipole moment, and there is a larger change in dipole moment with increasing solvent polarity. The transition-state dipole moments for the amides and thioamides are relatively similar. The origin of the C-N rotational barrier and its relation to the concept of amide "resonance" is examined.

The effects of conformation and solvation on optical rotation: Substituted epoxides

“…43,44 As such, they have been found to provide reasonable descriptions for aprotic and nonaromatic solvents that do not engage in specific solute-solvent interactions. [57][58][59][60] The particular PCM implementation selected for the present work incorporates electron correlation effects at the B3LYP/6-3111G* level of density functional theory and constructs the solute cavity in a self-consistent manner based upon the corresponding electron isodensity surface 55 (isodensity value of 4 3 10 24 ). The relative Gibbs free energies for individual conformers, DG h , thus can be calculated taking into account the additional stabilization afforded by the surrounding dielectric environment.…”

Resonance Structures of the Amide Bond: The Advantages of Planarity

“…[29] Thus, a qualitatively different electronic charge transfer is predicted: a transfer of electronic charge from the nitrogen to the carbon atom, but with small changes within the oxygen atom. It has been argued [30] that the differences between the two analyses can be reconciled if one takes into account the high polarity of the C=O bond, thus observing the importance of resonant structure III. [25] Nevertheless, the controversy regarding the use of Bader atomic charges and their interpretation is not new, [31][32][33] and some authors [18] have even claimed that Bader atomic charges are not suitable for this kind of analysis.…”

Geometry, transition states, and vibrational spectra of boron isostere of N‐methylacetamide by ab initio calculations

“…This imparts a partial double bond character to the CON bond. But recent experimental and theoretical studies [41][42][43][44] tell a different story. The electron delocalization in the amide system has been attributed to second-order orbital interactions namely, n O 3 * CON (delocalization from lone pairs on oxygen into the sigma antibonding orbital of the CON bond, i.e., negative hyperconjugation) and n N 3 * COO (delocalization from the lone pair on nitrogen to the pi-antibonding orbital of the carbonyl group).…”