Aromaticity-Modulated Noncovalent Interactions in Small Organic Molecules



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This dissertation focuses on computational explorations of the relationships between aromaticity and noncovalent interactions (e.g., hydrogen bonding, π-stacking, and σ-hole bonding) in self-assembling organic systems. Even though the concepts of (anti)aromaticity and noncovalent interactions are typically considered in organic chemistry textbooks to be largely separate, theoretical investigations based on hydrogen bonded purine and pteridine quartets, σ-hole bonded ketocyclopolyenes, as well as hydrogen bonded and π-stacked assemblies of deltamide, squaramide, and croconamide derivatives show that these effects are mutually influential. Noncovalent interactions that polarize the ring π-electrons of monomers and increase aromatic character are strengthened, while those that decrease aromatic character in monomers are weakened. Geometric, magnetic, and energetic criteria of aromaticity were employed to quantify the effects of different types of noncovalent interactions on the aromatic and antiaromatic character of self-assembling monomers. We found that purine and pteridine quartets that associate through four pairs of hydrogen bonding interactions can display markedly different association strengths, even with the same secondary electrostatic interactions present, and show instead that these variations are the result of aromaticity gain in monomers. We found that tetrel, pnictogen, chalcogen, and halogen bonding interactions are stronger if they increase the aromatic character of interacting substrates but are weaker if they increase the antiaromatic character of interacting substrates. Hydrogen bonded and π-stacked monomers of deltamide, squaramide, and croconamide derivatives show increased aromatic character. Together these findings suggest the importance of considering “aromaticity gain” as an important structural feature in the rational design of self-assembling monomers and their chemical properties.



Aromaticity, σ-hole bonding


Portions of this document appear in: Paudel, Hari Ram, Ranjita Das, Chia-Hua Wu, and Judy I. Wu. "Self-assembling purine and pteridine quartets: how do π-conjugation patterns affect resonance-assisted hydrogen bonding?." Organic & biomolecular chemistry 18, no. 6 (2020): 1078-1081; and in: Paudel, Hari Ram, Lucas José Karas, I. Judy, and Chia Wu. "On the reciprocal relationship between σ-hole bonding and (anti) aromaticity gain in ketocyclopolyenes." Organic & biomolecular chemistry 18, no. 27 (2020): 5125-5129.