Knowledge Bank

Peptide molecules XCO-NYY are characterized by the low potential barrier $V3$ to internal rotation of CH$3$ substituted for X and/or Y. A most conspicuous example is acetamide, for which $V3$ was previously reported to be 25.043857(19)\wn. The present study intended to clarify why $V3$ is so low in peptide molecules, by examining the effect of the out-of-plane bending or inversion of the amino/imino group on the molecular structure through deuterium substitution for amino/imino hydrogens. The potential barrier $V3$ in acetamide was found to decrease by 2.509, 2.958, and 5.422\wn, when H's at $cis$, $trans$, and both positions in the amino group were replaced by deuterium atoms, respectively. The reduction was proportional to the effective mass of the amino inversion, which was in turn ascribed to the change in electronic resonance character of the peptide linkage. The amino inversion is coupled with the CH$3$ internal rotation, producing an interaction term proportional to $\tau sin3\alpha$ ($\tau$: amino inversion, $\alpha$: methyl internal rotation). This coupling term, when the inversion is treated by second order perturbation, yields a $V6$ term, in agreement with Ilyushin $etal$., who derived an unusually large $V6$ term of -10.044874(73) \wn. Deuteration for the imino hydrogen reduces $V3$ by 11.40\wn for the $N$-methyl formamide $trans$ form, whereas for the $cis$ form it increases $V3$ by 8.09\wn. It is interesting that even a small perturbation like deuterium substitution causes a substantial change in electronic structure of the peptide systems.