GAS-PHASE H-1-NMR STUDY OF N-ETHYL-N-METHYL AMIDES - TORSIONAL BARRIER AND CONFORMER EQUILIBRIA AROUND THE PEPTIDE-BOND

The conformational stabilities and barriers to internal rotation for t hree N-ethyl-N-methyl amides (XCONCH(3)-(CH2CH3); X = CF3, CH3, and H) were determined from exchange-broadened H-1 NMR spectra. Conformer fr ee energy differences, Delta G(0), in cal mol(-1) (gas/l mol % solutio n) are 204 (49)/27 (25) at 298 K for X = CF3 (in CS2), 178 (136)/35 (6 ) at 298.0/306.1 K for X = CH3 (in DMSO-d(6)), and 78 (11)/235 (8) at 301.2/303.0 K for X = H (in DMSO-d(6)). The most stable conformer has the N-ethyl group syn to the carbonyl, except for X = H where it is an ti to the carbonyl. Activation free energies Delta G(298)(double dagge r) in kcal mol(-1) (gas/solution) are 16.4 (0.1)/18.2 (0.1) for X = CF 3 (in CCl4), 15.4 (0.2)/18.5 (0.2) for X = CH3 (in DMSO-d(6)), and 19. 6 (0.2)/21.0 (0.2) for X = H (in DMSO-d(6)). Gas-phase values of Delta G(298)(double dagger) are lower the liquid-phase values by 1.4-3.1 kc al mol(-1). This is consistent with previous experimental studies.