Can we predict the conformational preference of amides?

Area of Science:

• Computational Chemistry
• Spectroscopy
• Organic Chemistry

Background:

• Understanding the conformational preferences of secondary amides is crucial in various chemical and biological contexts.
• Previous studies have explored amide conformations, but a definitive link between theoretical predictions and experimental validation remained to be fully established.

Purpose of the Study:

• To investigate the extent to which theoretical calculations can accurately predict the conformation of secondary amides.
• To establish a reliable experimental method for determining amide conformation using NMR spectroscopy guided by computational data.

Main Methods:

• Density Functional Theory (DFT) calculations using the B3LYP/6-31G level of theory were employed.
• Gas-phase and solvent studies were performed to evaluate conformational energies.
• (1)H Nuclear Magnetic Resonance (NMR) spectroscopy was utilized to analyze chemical shifts.

Main Results:

• Theoretical calculations consistently predicted a Z-anti conformation as the most stable for a series of acetamides.
• NMR data indicated that protons anti to the N-H group appear approximately 0.8 ppm further downfield than those gauche to it.
• A correlation was established between calculated conformational preferences and observed NMR chemical shifts.

Conclusions:

• Computational methods, specifically DFT, are effective in predicting the preferred Z-anti conformation of secondary amides.
• NMR chemical shift analysis, guided by theoretical calculations, provides a simple and convenient experimental approach to determine amide conformation.
• This integrated approach offers a valuable tool for researchers studying amide structures and dynamics.