NORMAL VIBRATION CALCULATION OF MONOSUBSTITUTED AMIDES

Abstract

“Monosubstituted amide molecules, in which carbonyl oxygen and amide hydrogen are in the trans position, give six characteristic absorption bands in the region $1800-400 cm^{-1}$. These are 1650, 1550, 1270, 730, 630, and $600 cm^{-1}$ in the case of $R-CONH-R^{\prime}$, and 1650, 1450, 950, 630, 620, and $500 cm^{-1}$ in the case of $R-COND-R^{\prime}$. Of these the first four are due to the in-plane vibrations and are related to the C $=$ O and C - N stretching, and O $=$ C N and $N-$H(N$-$D) bending modes. The other two are due to the out-of-plane vibrations and are related to $N-$H(N$-$D) and C $=$ O bending modes. A normal coordinate treatment has been made in order to elucidate the nature of the characteristic in-plane vibrations. An intramolecular potential function of the Urey-Bradley type has been used, and the force constants have been transferred from other molecules which are closely related to peptide molecules. Diformylhydrazine ($H-CONH-)_{2}$ and its N-deuterated compound have been studied first. This molecule possesses a simple absorption spectrum, giving only six bands in the region $1800-400 cm^{-1}$, four of which are due to the in-plane vibrational modes of the peptide group. Frequencies and normal modes as well as potential energy and kinetic energy distributions have been calculated for the infra-red active in-plane vibrations ($B_{u}$) of ($H-CONH-)_{2}$ and ($H-COND-)_{2}$. Observed and calculated frequencies agree within 2% except for one case. Similar calculations have been carried out for the trans and cis isomers of N-methylacetamide, $CH_{3}-CONH-CH_{3}$ and $CH_{3}-COND-CH_{3}$ with the methyl groups being treated as point masses. In this case, frequency agreement is within 4% except for two cases. Calculated results of normal modes and energy distributions give a clear picture of the nature of the characteristic vibrations.”