From Competition to Commensuration by Two Major Hydrogen-Bonding Motifs

Carboxylic acid–acid hydrogen-bonding dimer and acid–pyridine hydrogen-bonding motif are two competing supramolecular synthons that a molecule possessing both carboxylic acid and pyridine functional groups could form in the solid state. Their coexistence has been observed but for the molecules with the molar ratio of carboxylic acid and pyridine groups being greater than 1:1. In this crystal engineering study, 2-[phenyl­(propyl)­amino]­nicotinic acid with a 1:1 molar ratio of these two functional groups was discovered to have two polymorphs, in which one consists of unique hydrogen-bonded tetramer units bearing both acid–acid and acid–pyridine hydrogen-bonding motifs, while the other is composed of acid–pyridine hydrogen-bonded chains. Quantum mechanical calculations were employed to unravel the essence of the coexistence of the two vying counterparts as well as the origins of the tetramer and chain structures

From Competition to Commensuration by Two Major Hydrogen-Bonding Motifs

Carboxylic acid–acid hydrogen-bonding dimer and acid–pyridine hydrogen-bonding motif are two competing supramolecular synthons that a molecule possessing both carboxylic acid and pyridine functional groups could form in the solid state. Their coexistence has been observed but for the molecules with the molar ratio of carboxylic acid and pyridine groups being greater than 1:1. In this crystal engineering study, 2-[phenyl­(propyl)­amino]­nicotinic acid with a 1:1 molar ratio of these two functional groups was discovered to have two polymorphs, in which one consists of unique hydrogen-bonded tetramer units bearing both acid–acid and acid–pyridine hydrogen-bonding motifs, while the other is composed of acid–pyridine hydrogen-bonded chains. Quantum mechanical calculations were employed to unravel the essence of the coexistence of the two vying counterparts as well as the origins of the tetramer and chain structures

From Competition to Commensuration by Two Major Hydrogen-Bonding Motifs

Carboxylic acid–acid hydrogen-bonding dimer and acid–pyridine hydrogen-bonding motif are two competing supramolecular synthons that a molecule possessing both carboxylic acid and pyridine functional groups could form in the solid state. Their coexistence has been observed but for the molecules with the molar ratio of carboxylic acid and pyridine groups being greater than 1:1. In this crystal engineering study, 2-[phenyl­(propyl)­amino]­nicotinic acid with a 1:1 molar ratio of these two functional groups was discovered to have two polymorphs, in which one consists of unique hydrogen-bonded tetramer units bearing both acid–acid and acid–pyridine hydrogen-bonding motifs, while the other is composed of acid–pyridine hydrogen-bonded chains. Quantum mechanical calculations were employed to unravel the essence of the coexistence of the two vying counterparts as well as the origins of the tetramer and chain structures

From Competition to Commensuration by Two Major Hydrogen-Bonding Motifs

Carboxylic acid–acid hydrogen-bonding dimer and acid–pyridine hydrogen-bonding motif are two competing supramolecular synthons that a molecule possessing both carboxylic acid and pyridine functional groups could form in the solid state. Their coexistence has been observed but for the molecules with the molar ratio of carboxylic acid and pyridine groups being greater than 1:1. In this crystal engineering study, 2-[phenyl­(propyl)­amino]­nicotinic acid with a 1:1 molar ratio of these two functional groups was discovered to have two polymorphs, in which one consists of unique hydrogen-bonded tetramer units bearing both acid–acid and acid–pyridine hydrogen-bonding motifs, while the other is composed of acid–pyridine hydrogen-bonded chains. Quantum mechanical calculations were employed to unravel the essence of the coexistence of the two vying counterparts as well as the origins of the tetramer and chain structures

From Competition to Commensuration by Two Major Hydrogen-Bonding Motifs

Carboxylic acid–acid hydrogen-bonding dimer and acid–pyridine hydrogen-bonding motif are two competing supramolecular synthons that a molecule possessing both carboxylic acid and pyridine functional groups could form in the solid state. Their coexistence has been observed but for the molecules with the molar ratio of carboxylic acid and pyridine groups being greater than 1:1. In this crystal engineering study, 2-[phenyl­(propyl)­amino]­nicotinic acid with a 1:1 molar ratio of these two functional groups was discovered to have two polymorphs, in which one consists of unique hydrogen-bonded tetramer units bearing both acid–acid and acid–pyridine hydrogen-bonding motifs, while the other is composed of acid–pyridine hydrogen-bonded chains. Quantum mechanical calculations were employed to unravel the essence of the coexistence of the two vying counterparts as well as the origins of the tetramer and chain structures

From Competition to Commensuration by Two Major Hydrogen-Bonding Motifs

Carboxylic acid–acid hydrogen-bonding dimer and acid–pyridine hydrogen-bonding motif are two competing supramolecular synthons that a molecule possessing both carboxylic acid and pyridine functional groups could form in the solid state. Their coexistence has been observed but for the molecules with the molar ratio of carboxylic acid and pyridine groups being greater than 1:1. In this crystal engineering study, 2-[phenyl­(propyl)­amino]­nicotinic acid with a 1:1 molar ratio of these two functional groups was discovered to have two polymorphs, in which one consists of unique hydrogen-bonded tetramer units bearing both acid–acid and acid–pyridine hydrogen-bonding motifs, while the other is composed of acid–pyridine hydrogen-bonded chains. Quantum mechanical calculations were employed to unravel the essence of the coexistence of the two vying counterparts as well as the origins of the tetramer and chain structures

Tautomeric Polymorphism of 4‑Hydroxynicotinic Acid

4-Hydroxynicotinic acid (4-HNA) was discovered to exist in the solid state as either 4-HNA or its tautomer 4-oxo-1,4-dihydropyridine-3-carboxylic acid (4-ODHPCA) in three polymorphs and two hydrates. Packing motifs differ as each of the three oxygen atoms acts as the hydrogen-bond acceptor, respectively, in the anhydrate forms, while in the hydrate forms, water molecules participate in hydrogen bonding with 4-HNA. Phase behaviors of the forms were characterized by differential scanning calorimetry (DSC), hot-stage microscopy (HSM), and thermogravimetric analysis (TGA). It was found that anhydrates I and II converted into III during heating; the two hydrate forms dehydrated at different temperatures and eventually transformed into anhydrate III, and sublimation of all five forms led to form III when the crystals were heated. Quantum mechanical calculations were performed providing further insight into the polymorphism

Facile Design of Green Engineered Cellulose/Metal Hybrid Macrogels for Efficient Trace Phosphate Removal

Cellulose/metal hybrid macrogels were prepared by immobilizing uniformly dispersed thiolate-modified Fe₂O₃ nanoparticles into a cellulose matrix. The structure and properties of the hybrid macrogels were characterized using SEM, EDS, water contact angle, XRD, FTIR, XPS, and adsorption tests. The hybrid macrogels exhibited good stability, convenient operation, and high selectivity for trace phosphate removal, with a remaining phosphorus concentration of only 0.15 mg L^–1 in 200 min (5 mg L^–1 initial concentration). The influences of pH, ionic strength, and competitive anions were also investigated. The hybrid macrogels could be recovered by simple and rapid magnetic separation and regenerated in NaOH solution. During 5 cycles of the adsorption–elution–regeneration stability test, the hybrid macrogels still retained 80% adsorption capacity of trace phosphate. In this work, utilization of natural polymer was combined with green and sustainable technology to develop economically sustainable, eco-friendly, and cost-effective cellulose/metal hybrid macrogels for the application of trace phosphate removal from water

Structural Isomerization of 2‑Anilinonicotinic Acid Leads to a New Synthon in 6‑Anilinonicotinic Acids

Through structural modification of 2-anilinonicotinic acid by isomerization, a new synthon, acid-aminopyridine, is created, and the two original synthons, i.e., the acid–acid homosynthon and acid–pyridine heterosynthon are no longer observed in the newly designed 6-anilinonicotinic acids. The new synthon has a hydrogen-bond strength rivaling that of the acid–acid homosynthon and the acid–pyridine heterosynthon, as suggested by theoretical calculations, which explains its formation