Acidity and basicity interplay in amide and imide self-association

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Physical Chemistry

Background:

• The Jorgensen Secondary Interactions Hypothesis (JSIH) explains amide/imide dimerization via spectator and H-bonded carbonyl groups.
• Previous studies suggest JSIH may not fully capture all intermolecular interactions.
• The role of spectator carbonyl groups in disrupting resonance-assisted hydrogen bonds in imides was investigated.

Purpose of the Study:

• To investigate the self-association of amides and imides.
• To clarify the factors influencing dimerization strength.
• To propose an updated model for hydrogen bonding in amides and imides.

Main Methods:

• Proton Nuclear Magnetic Resonance (¹H-NMR) and Proton Nuclear Magnetic Resonance with Diffusion Ordered Spectroscopy (¹H-DOSY) experiments.
• Density Functional Theory (DFT) calculations.
• Quantum Theory of Atoms in Molecules (QTAIM) topological analyses and Independent Gradient Model (IGM) analysis.

Main Results:

• Repulsions between spectator (O_S) and hydrogen-bonded (O_HB) carbonyl groups were observed, consistent with JSIH.
• The spectator carbonyl group (C=O_S) exhibits an overall attractive interaction with the interacting molecule.
• Self-association strength correlates with N-H acidity and C=O basicity, with imides showing weaker dimerization due to lower carbonyl basicity.

Conclusions:

• Imides dimerize less strongly than amides due to the lower basicity of their carbonyl groups.
• The basicity of carbonyl fragments is a crucial, often overlooked, factor in hydrogen bonding studies.
• The proposed model offers insights into supramolecular chemistry, particularly for systems with amide or imide functionalities.