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

Thickness-Dependent and Magnetic-Field-Driven Suppression of Antiferromagnetic Order in Thin V₅S₈ Single Crystals

With materials approaching the 2D limit yielding many exciting systems with intriguing physical properties and promising technological functionalities, understanding and engineering magnetic order in nanoscale, layered materials is generating keen interest. One such material is V₅S₈, a metal with an antiferromagnetic ground state below the Néel temperature <i>T</i>_N ∼ 32 K and a prominent spin-flop signature in the magnetoresistance (MR) when <i>H</i>∥<i>c</i> ∼ 4.2 T. Here we study nanoscale-thickness single crystals of V₅S₈, focusing on temperatures close to <i>T</i>_N and the evolution of material properties in response to systematic reduction in crystal thickness. Transport measurements just below <i>T</i>_N reveal magnetic hysteresis that we ascribe to a metamagnetic transition, the first-order magnetic-field-driven breakdown of the ordered state. The reduction of crystal thickness to ∼10 nm coincides with systematic changes in the magnetic response: <i>T</i>_N falls, implying that antiferromagnetism is suppressed; and while the spin-flop signature remains, the hysteresis disappears, implying that the metamagnetic transition becomes second order as the thickness approaches the 2D limit. This work demonstrates that single crystals of magnetic materials with nanometer thicknesses are promising systems for future studies of magnetism in reduced dimensionality and quantum phase transitions

Ni–C–N Nanosheets as Catalyst for Hydrogen Evolution Reaction

We report a facile nitrogenation/exfoliation process to prepare hybrid Ni–C–N nanosheets. These nanosheets are <2 nm thin, chemically stable, and metallically conductive. They serve as a robust catalyst for the hydrogen evolution reaction in 0.5 M H₂SO₄, or 1.0 M KOH or 1.0 M PBS (pH = 7). For example, they catalyze the hydrogen evolution reaction in 0.5 M H₂SO₄ at an onset potential of 34.7 mV, an overpotential of 60.9 mV (at <i>j</i> = 10 mA cm^–2) and with remarkable long-term stability (∼10% current drop after 70 h testing period). They are promising as a non-Pt catalyst for practical hydrogen evolution reaction

Potential Fluctuations at Low Temperatures in Mesoscopic-Scale SmTiO₃/SrTiO₃/SmTiO₃ Quantum Well Structures

Heterointerfaces of SrTiO₃ with other transition metal oxides make up an intriguing family of systems with a bounty of coexisting and competing physical orders. Some examples, such as LaAlO₃/SrTiO₃, support a high carrier density electron gas at the interface whose electronic properties are determined by a combination of lattice distortions, spin–orbit coupling, defects, and various regimes of magnetic and charge ordering. Here, we study electronic transport in mesoscale devices made with heterostructures of SrTiO₃ sandwiched between layers of SmTiO₃, in which the transport properties can be tuned from a regime of Fermi-liquid like resistivity (ρ ∝ <i>T</i>²) to a non-Fermi liquid (ρ ∝ <i>T</i>^5/3) by controlling the SrTiO₃ thickness. In mesoscale devices at low temperatures, we find unexpected voltage fluctuations that grow in magnitude as <i>T</i> is decreased below 20 K, are suppressed with increasing contact electrode size, and are independent of the drive current and contact spacing distance. Magnetoresistance fluctuations are also observed, which are reminiscent of universal conductance fluctuations but not entirely consistent with their conventional properties. Candidate explanations are considered, and a mechanism is suggested based on mesoscopic temporal fluctuations of the Seebeck coefficient. An improved understanding of charge transport in these model systems, especially their quantum coherent properties, may lead to insights into the nature of transport in strongly correlated materials that deviate from Fermi liquid theory