Dynamic Structure of Organic Compounds in Solution According to NMR Data and Quantum Mechanical Calculations: IV. Benzamide

Abstract

To study the structure and dynamics of nitrogen-containing compounds, NMR parameters with directly involved nitrogen can provide valuable structure information. However, this information can only be obtained using15N-enriched compounds due to low natural abundance of 15N and extremely short relaxation of 14N nuclei. For synthesis of these compounds from 15N-ammonium salts, 15N-enriched benzamides often used as intermediates. In the present work, we study the dynamic structure of benzamide caused by two independent factors: hindered internal rotation of the NH2 group around the C(O)-N bond and the amide group as a whole relative to the benzene ring. This deeper knowledge of mechanism and parameters of these processes in amides is important for meaningful interpretation and prediction of the biological activity of aromatic amides in living systems, the strength and conformation of its supramolecular complexes with lanthanide and actinide ions. Double enriched [2H5, 15N]benzamide was synthesized to avoid unwanted superposition of intense aromatic multiplet on the amide signals in the 1H NMR spectra. In the 1H spectrum of this compound observed only intense signals of amide protons, which allowed accurate quantitative characterization of the parameters of the dynamic processes under study. The experimental data obtained is in good agreement with our results of simulation by quantum molecular dynamics techniques.