FOURIER TRANSFORM MICROWAVE SPECTROSCOPY OF N-METHYLPROPIONAMIDE

In order to clarify the dynamical behavior of the peptide bond, we have undertaken a systematic study of$^{\\prime}$ peptide molecules $^{\\prime}$, which consist of (a) peptide bond(s) with internal-rotation groups at the both ends of the bond(s). In the present investigation we focused attention to the molecule (NMPA) shown in the title, which has an ethyl group ($CH_{3}$ of which is referred to as $C-CH_{3}$) at the carbonyl side and a methyl group (called $N-CH_{3}$) at the amide side, and aimed at unveiling how the two $CH_{3}$ groups interact with each other through the peptide bond. We have derived a rotational Hamiltonian including the two $CH_{3}$ internal rotations, and have treated the $C-CH_{3}$ internal rotation by a conventional PAM, while applying a more sophisticated approach to the $N-CH_{3}$ internal rotation. NMPA may be regarded to belong to group $G_{18}$, even if its skeleton executes large-amplitude $^{\\prime}$ out-of-plane $^{\\prime}$ motions. The group consists of 6 species: $A_{1}, A_{2}, E_{1}, E_{2}, E_{3}$, and $E_{4}$. We have observed and analyzed $A_{1}$ (or $A_{2}$) and $E_{2}$ spectra, but have not detected any lines due to the first excited state of the $CH_{3}CH_{2}-CO$ torsion of $A_{2}$ symmetry, indicating that the internal-rotation splitting is quite large. The potential barrier to the $C-CH_{3}$ internal rotation was determined to be $799 cm^{-1}$, which may be compared with that of $N-CH_{3}$ of about $81 cm^{-1}$. The coupling between the two $CH_{3}^{\\prime}$s is being analyzed by observing $E_{1}, E_{3}$, and $E_{4}$ spectra.