The effect of deuterium substitution in the amino group on the internal-rotation barrier of acetamide

Abstract

Peptide molecules XCO–NYY′ are characterized by low potential barrier V 3 to internal rotation of a methyl group substituted for X and/or Y. A most conspicuous example is acetamide, for which V 3 was previously reported to be 25.043857(19) cm −1 [8]. The present study intended to clarify why V 3 is so low in acetamide, by examining the effect of the out-of-plane bending or inversion of the amino group on the molecular structure through deuterium substitution for amino hydrogens. The potential barrier V 3 in acetamide was found to decrease by 2.630, 2.986, and 5.532 cm −1, 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 out-of-plane bending mode of the amino group (hereafter referred to as 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 τ sin 3 α, where τ and α denote the amino inversion and methyl internal rotation angles, respectively. This coupling term, when the inversion is treated by second-order perturbation, yields a V 6 term in the internal-rotation potential function of the methyl group, in agreement with the finding of Ilyushin et al. [8], who derived an unusually large V 6 term of −10.044874(73) cm −1. It is quite interesting that even a small perturbation such as deuterium substitution causes a substantial change in electronic structure of the peptide linkage.