Gas-Phase NMR Studies of N,N-Dimethylthioamides. Influence of the Thiocarbonyl Substituent on the Internal Rotation Activation Energies

Abstract

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 ⧧ 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.

Gas-Phase Nuclear Magnetic Resonance Study of (15N)Trifluoroacetamide: Comparison of Experimental and Computed Kinetic Parameters

“…(1) Primary Amides Exhibit Medium Effects Similar to Those of Tertiary Amides . The medium dependence of the rotational barrier in amides has been examined for several tertiary amides and thioamides. − ,, ,29 Liquid barriers are dependent on solvent and concentration; high polarity and/or amide concentration favor higher amide rotational barriers. Invariably, however, rotational barriers (as measured by Δ G ⧧ 298 values) are lower in the gas phase than in the liquid phase, regardless of solvent or concentration.…”

“…Unfortunately, gas-phase dynamic NMR studies of primary amides are difficult for several reasons. First, the vapor pressure of primary amides is extremely low, usually less than that for the tertiary amides that have been more extensively studied. − Vaporization of enough amide for acquisition of gas-phase spectra is further impaired by the affinity of the amide hydrogens for the glass NMR tube. Also, whereas dimethyl amides have three equivalent protons at the exchanging sites, primary amides have only one.…”

Solvent Effects on the Barrier to C−N Bond Rotation in N,N-Dimethylaminoacrylonitrile

“…Conformational isomerization processes, such as rotation about the C−N bonds of amides, have long held great interest for organic chemists. − Some of this interest derives from the clues about electronic structure provided by the corresponding potential energy barriers. , For instance, in the case of amides, the large C−N rotational barrier provides clear experimental evidence for an interaction between the nitrogen lone pair and the carbonyl group. Another source of motivation arises from biochemical applications, such as the important role that peptide bond isomerization in proline residues can play in limiting the rate of protein folding and the observation of rotamase enzymes that catalyze this isomerization. , Solvent effects constitute a second general topic of continuing relevance .…”

Determination of the Internal Rotation Barrier of [15N]Formamide from Gas-Phase 1H NMR Spectra

“…Typically, 30 000 free induction decays were acquired and stored in 8K to produce frequency domain spectra of sufficient quality for fitting. Temperature control and measurement have been described previously . Samples equilibrated for at least 10 min prior to data acquisition.…”

“…Exchange-broadened spectra were obtained at nine temperatures between 318.3 and 333.1 K. Rate constants were calculated with the computer program DNMR5 25 which uses an iterative nonlinear least-squares regression analysis to obtain the best fit of the experimental NMR spectrum. The method used for estimating the effective T 2 for each spectrum has been described previously . It was not possible to obtain slow exchange coupling constants for gas-phase FA and previously reported coupling constants which are consistent with our measured fast exchange averages were used. , Typically 800 experimental points were used in the analysis of each spectrum.…”