Knowledge Bank

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 $Caminati,a$ who assigned and analyzed only the A state of the $CH3$ internal rotation and thus reported an estimated potential barrier $V3$. Kitano and $Kuchitsub$ 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 $50\circ$C. 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 $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C,15N$, and $\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}O$ were observed to derive an $rs$ structure, which is in good agreement with the electron diffraction and ab initio results. The $V3$ value of trans is approximately $60cm-1$, which is 4.5 times smaller than the value of cis form which was determined to be $279cm-1$.