TY - JOUR
T1 - Gas-phase 1H NMR study of N-ethyl-N-methyl amides
T2 - Torsional barrier and conformer equilibria around the peptide bond
AU - Suarez, Cristina
AU - Tafazzoli, Mohsen
AU - True, Nancy S.
AU - Gerrard, Sonja R.
AU - LeMaster, Clifford B.
AU - LeMaster, Carole L.
PY - 1995/5/1
Y1 - 1995/5/1
N2 - The conformational stabilities and barriers to internal rotation for three N-ethyl-N-methyl amides (XCONCH;-(CH2CH3); X = CF3, CH3, and H) were determined from exchange-broadened 1H NMR spectra. Conformer free energy differences, ΔG0, in cal mol-1 (gas/1 mol % solution) 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-d6), and 78 (11)/235 (8) at 301.2/303.0 K for X = H (in DMSO-d6). The most stable conformer has the N-ethyl group syn to the carbonyl, except for X = H where it is anti to the carbonyl. Activation free energies ΔG‡298 in kcal mol-1 (gas/solution) are 16.4 (0.1)/18.2 (0.1) for X = CF3 (in CCl4), 15.4 (0.2)/18.5 (0.2) for X = CH3 (in DMSO-d6), and 19.6 (0.2)/21.0 (0.2) for X = H (in DMSO-d6). Gas-phase values of ΔG‡298 are lower the liquid-phase values by 1.4-3.1 kcal mol-1. This is consistent with previous experimental studies. © 1995 American Chemical Society.
AB - The conformational stabilities and barriers to internal rotation for three N-ethyl-N-methyl amides (XCONCH;-(CH2CH3); X = CF3, CH3, and H) were determined from exchange-broadened 1H NMR spectra. Conformer free energy differences, ΔG0, in cal mol-1 (gas/1 mol % solution) 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-d6), and 78 (11)/235 (8) at 301.2/303.0 K for X = H (in DMSO-d6). The most stable conformer has the N-ethyl group syn to the carbonyl, except for X = H where it is anti to the carbonyl. Activation free energies ΔG‡298 in kcal mol-1 (gas/solution) are 16.4 (0.1)/18.2 (0.1) for X = CF3 (in CCl4), 15.4 (0.2)/18.5 (0.2) for X = CH3 (in DMSO-d6), and 19.6 (0.2)/21.0 (0.2) for X = H (in DMSO-d6). Gas-phase values of ΔG‡298 are lower the liquid-phase values by 1.4-3.1 kcal mol-1. This is consistent with previous experimental studies. © 1995 American Chemical Society.
UR - https://www.scopus.com/pages/publications/0000168754
U2 - 10.1021/j100020a045
DO - 10.1021/j100020a045
M3 - Article
SN - 0022-3654
VL - 99
SP - 8170
EP - 8176
JO - Journal of Physical Chemistry
JF - Journal of Physical Chemistry
IS - 20
ER -