15min:

YOSHIYUKI KAWASHIMA, TSUYOSHI USAMI, 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 understand the dynamical behavior of the peptide bond in ``peptide molecules: X-CONH-Y'', we have chosen N-methylformamide (N-MFA) as a simplest system with X = H and Y = CH₃. The microwave spectrum of the most stable trans form of N-MFA was studied by Fantoni and Caminati, who assigned and analyzed only the A state of the CH₃ internal rotation and thus reported an estimated potential barrier V₃ . Kitano and Kuchitsu determined the molecular structure of trans N-MFA by using electron diffraction, but they could not detect the higher energy cis conformer. We report here the rotational spectrum of both the trans and cis forms of N-MFA using FTMW spectrometers at NIST and KAIT, in the frequency region from 5 to 36 GHz, with either Ne or Ar as a buffer gas in a reservoir nozzle maintained at about 50^oC. We confirmed the assignment of Ref.a for the trans A state and extended the observation to low- J transitions. The spectra of four trans isotopomers ¹³C, ¹⁵N, and ¹⁸O were observed to derive an r_s structure, which is in good agreement with the electron diffraction and ab initio results. The V₃ value of trans is approximately 60 cm^-1, which is 4.5 times smaller than the value of cis form which was determined to be 279 cm^-1.