σ-Electrons Responsible for Cooperativity and Ring Equalization in Hydrogen-Bonded Supramolecular Polymers

Area of Science:

• Computational Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Hydrogen-bonded chains are fundamental supramolecular structures.
• Urea, deltamide, and squaramide are key building blocks for such chains.
• Understanding their self-assembly and structural properties is crucial.

Purpose of the Study:

• To investigate the cooperative bonding mechanisms in linear chains of urea, deltamide, and squaramide.
• To analyze the structural deformations, specifically C-C bond equalization, in deltamide and squaramide rings during polymerization.
• To elucidate the electronic origins of self-assembly and structural changes.

Main Methods:

• Quantum chemical calculations using dispersion-corrected density functional theory (DFT).
• Specific level of theory: BLYP-D3(BJ)/TZ2P.
• Utilized energy decomposition analysis and Kohn-Sham molecular orbital analysis.

Main Results:

• Confirmed cooperative effects in all studied hydrogen-bonded systems.
• Identified charge transfer from carbonyl oxygen lone pairs to N-H antibonding orbitals as the primary driving force.
• Observed elongation of C=O bonds and contraction of adjacent C-C bonds, leading to more regular rings in deltamide and squaramide.

Conclusions:

• The cooperative effect, driven by σ-electronic charge transfer, governs the self-assembly and structural properties of these chains.
• π-electron delocalization plays a minor role in the inter-monomer interactions.
• The findings provide insights into the design principles for novel supramolecular materials.