15min:

YOSHIYUKI KAWASHIMA, Department of Applied Chemistry, Kanagawa Institute of Technology, Atsugi, Kanagawa 243-0292, JAPAN; RICHARD D. SUENRAM, National Institute of Standards and Technology, Gaithersburg, MD 20899; AND EIZI HIROTA, The Graduate University for Advanced Studies, Hayama, Kanagawa 240-0193, JAPAN.

In order to clarify the dynamical behavior of the peptide bond, we have undertaken a systematic study of ' peptide molecules ', 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 (CH₃ of which is referred to as C-CH₃) at the carbonyl side and a methyl group (called N-CH₃) at the amide side, and aimed at unveiling how the two CH₃ groups interact with each other through the peptide bond. We have derived a rotational Hamiltonian including the two CH₃ internal rotations, and have treated the C-CH₃ internal rotation by a conventional PAM, while applying a more sophisticated approach to the N-CH₃ internal rotation. NMPA may be regarded to belong to group G₁₈ , even if its skeleton executes large-amplitude ' out-of-plane ' motions. The group consists of 6 species: A₁ , A₂ , E₁ , E₂ , E₃ , and E₄ . We have observed and analyzed A₁ (or A₂ ) and E₂ spectra, but have not detected any lines due to the first excited state of the CH₃CH₂-CO torsion of A₂ symmetry, indicating that the internal-rotation splitting is quite large. The potential barrier to the C-CH₃ internal rotation was determined to be 799 cm^-1, which may be compared with that of N-CH₃ of about 81 cm^-1. The coupling between the two CH₃'s is being analyzed by observing E₁ , E₃ , and E₄ spectra.