Dynamical Structure of Peptide Molecules: Fourier Transform Microwave Spectroscopy of N-Methylpropionamide
Y Kawashima, R D. Suenram, E Hirota
To clarify the dynamical aspects of the peptide structure, N-methylpropionamide (NMPA) was investigated as an example of peptide molecules: XCONHY (X=CH3CH2 and Y = CH3 for NMPA), paying special attention to the internal rotation of the two methyl groups. NMPA was found to have an almost planar skeleton with an extended syn/trans conformation, as indicated by the observed value of Iaa + Ibb -Icc, and its rotational spectra were interpreted in terms of group G18 consisting of six symmetry species: A1, A2, E1, E2, E3, and E4. The A1 and E2 spectra were observed to be split for most of the b-type transitions, yielding the internal-rotation potential barrier V3 of 796 (21) cm-1 for CH3 in the ethyl group referred to as C-CH3. The spectra of the three E species: E1, E3, and E4 are separated several tens to thousands of MHz from the corresponding A1 spectrum, suggesting a low internal-rotation potential barrier of the CH3 bonded to the nitrogen, called N-CH3. In sharp contrast with the A1 spectra, which were well fitted to the ordinary asymmetric-rotor spectral pattern, a few higher-order terms were required to reproduce the E1 spectrum, presumably because of the low N-CH3 barrier. The spectral analysis thus performed led to a V3 of 80.06487 (14) cm-1, an order of magnitude lower than that of C-CH3. The E3 and E4 spectra are found to form triplets with the corresponding E1 lines at the center, and the E-3-E1 and E4-E1 splittings are explained essentially by the contributions of the C-CH3 internal rotation combined with the kinetic-energy coupling between the two ethyl groups. The torsion around the C - C bond between the ethyl and carbonyl groups is suggested by an ab initio calculation to be of double minimum nature, but the observed A1 spectrum did not show any indication of such a double-minimum potential for the C-C torsion, although the possibility of a small hump being present at a planar conformation can not be entirely eliminated.
, Suenram, R.
and Hirota, E.
Dynamical Structure of Peptide Molecules: Fourier Transform Microwave Spectroscopy of N-Methylpropionamide, Journal of Molecular Spectroscopy
(Accessed December 2, 2023)