Electronic Effect on the Molecular Motion of Aromatic Amides: Combined Studies Using VT-NMR and Quantum Calculations.

Sungsoo Kim,Kyungwon Kwak,Wansik Cha,Daeun Won,Jungyu Kim,Sangdoo Ahn,Suk-Kyu Chang,Jieun Kim,Keunhong Jeong

doi:10.3390/molecules23092294

Abstract

Rotational barrier energy studies to date have focused on the amide bond of aromatic compounds from a kinetic perspective using quantum calculations and nuclear magnetic resonance (NMR). These studies provide valuable information, not only regarding the basic conformational properties of amide bonds but also the molecular gear system, which has recently gained interest. Thus, we investigate the precise motion of the amide bonds of two aromatic compounds using an experimental rotational barrier energy estimation by NMR experiments and a theoretical evaluation of the density functional theory calculation. The theoretical potential energy surface scan method combined with the quadratic synchronous transit 3 method and consideration of additional functional group rotation with optimization and frequency calculations support the results of the variable temperature 1H NMR, with deviations of less than 1 kcal/mol. This detailed experimental and theoretical research strongly supports molecular gear motion in the aromatic amide system, and the difference in kinetic energy indicates that the electronic effect from the aromatic structure has a key role in conformational movements at different temperatures. Our study provides an enhanced basis for future amide structural dynamics research.

Highlights

The amide bond, a basic unit of proteins, has unique steric and energetic characteristics [1,2].studying the dynamics of these bonds is important to understand protein dynamics.Conformational studies of aromatic amide bonds, in particular, have been extensively carried out owing to their potential applications in fields such as asymmetric synthesis, molecular gear systems, single-molecule motors, and single-molecule devices [3].An aromatic amide has two rotational motions [4] around the central carbonyl group: the rotation around aryl-CO and the rotation around the C-N bond in the amide unit
We investigated the energy based on the geometric optimization of scan coordinates, changing the dihedral angles from −170◦ to 200◦ for the C-N bond and aryl-CO bond, respectively, in N,N-diethylamide derivatives (1), (2)
As indicated in the 2D potential energy surface (PES) scan with frequency calculation in Figure 5, we investigated independent aryl-CO and C-N dihedral angles for concerted aryl-CO/C-N bond rotation

Summary

Introduction

The amide bond, a basic unit of proteins, has unique steric and energetic characteristics [1,2].studying the dynamics of these bonds is important to understand protein dynamics.Conformational studies of aromatic amide bonds, in particular, have been extensively carried out owing to their potential applications in fields such as asymmetric synthesis, molecular gear systems, single-molecule motors, and single-molecule devices [3].An aromatic amide has two rotational motions [4] around the central carbonyl group: the rotation around aryl-CO and the rotation around the C-N bond in the amide unit. The amide bond, a basic unit of proteins, has unique steric and energetic characteristics [1,2]. Studying the dynamics of these bonds is important to understand protein dynamics. Conformational studies of aromatic amide bonds, in particular, have been extensively carried out owing to their potential applications in fields such as asymmetric synthesis, molecular gear systems, single-molecule motors, and single-molecule devices [3]. An aromatic amide has two rotational motions [4] around the central carbonyl group: the rotation around aryl-CO and the rotation around the C-N bond in the amide unit. The basic structural change of an aromatic amide involves three bond rotational processes [4,5] that compose the interconversions of the conformers. The two independent rotations of the C-N bond or aryl-CO

Methods

Results

Conclusion