16 research outputs found
From Competition to Commensuration by Two Major Hydrogen-Bonding Motifs
Carboxylic
acidâacid hydrogen-bonding dimer and acidâpyridine
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-[phenylÂ(propyl)Âamino]Ânicotinic 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 acidâacid and acidâpyridine hydrogen-bonding motifs,
while the other is composed of acidâpyridine 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
acidâacid hydrogen-bonding dimer and acidâpyridine
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-[phenylÂ(propyl)Âamino]Ânicotinic 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 acidâacid and acidâpyridine hydrogen-bonding motifs,
while the other is composed of acidâpyridine 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
acidâacid hydrogen-bonding dimer and acidâpyridine
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-[phenylÂ(propyl)Âamino]Ânicotinic 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 acidâacid and acidâpyridine hydrogen-bonding motifs,
while the other is composed of acidâpyridine 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
acidâacid hydrogen-bonding dimer and acidâpyridine
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-[phenylÂ(propyl)Âamino]Ânicotinic 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 acidâacid and acidâpyridine hydrogen-bonding motifs,
while the other is composed of acidâpyridine 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
acidâacid hydrogen-bonding dimer and acidâpyridine
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-[phenylÂ(propyl)Âamino]Ânicotinic 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 acidâacid and acidâpyridine hydrogen-bonding motifs,
while the other is composed of acidâpyridine 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
acidâacid hydrogen-bonding dimer and acidâpyridine
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-[phenylÂ(propyl)Âamino]Ânicotinic 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 acidâacid and acidâpyridine hydrogen-bonding motifs,
while the other is composed of acidâpyridine 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 4âHydroxynicotinic 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 V<sub>5</sub>S<sub>8</sub> 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
V<sub>5</sub>S<sub>8</sub>, a metal with an antiferromagnetic ground
state below the NeÌel temperature <i>T</i><sub>N</sub> ⌠32 K and a prominent spin-flop signature in the magnetoresistance
(MR) when <i>H</i>â„<i>c</i> ⌠4.2
T. Here we study nanoscale-thickness single crystals of V<sub>5</sub>S<sub>8</sub>, focusing on temperatures close to <i>T</i><sub>N</sub> and the evolution of material properties in response
to systematic reduction in crystal thickness. Transport measurements
just below <i>T</i><sub>N</sub> 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 âŒ10 nm coincides with systematic changes in the magnetic
response: <i>T</i><sub>N</sub> 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
NiâCâN Nanosheets as Catalyst for Hydrogen Evolution Reaction
We report a facile
nitrogenation/exfoliation process to prepare
hybrid NiâCâN 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
H<sub>2</sub>SO<sub>4</sub>, or 1.0 M KOH or 1.0 M PBS (pH = 7). For
example, they catalyze the hydrogen evolution reaction in 0.5 M H<sub>2</sub>SO<sub>4</sub> at an onset potential of 34.7 mV, an overpotential
of 60.9 mV (at <i>j</i> = 10 mA cm<sup>â2</sup>)
and with remarkable long-term stability (âŒ10% 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 SmTiO<sub>3</sub>/SrTiO<sub>3</sub>/SmTiO<sub>3</sub> Quantum Well Structures
Heterointerfaces
of SrTiO<sub>3</sub> 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 LaAlO<sub>3</sub>/SrTiO<sub>3</sub>, support a high carrier density electron gas at
the interface whose electronic properties are determined by a combination
of lattice distortions, spinâorbit coupling, defects, and various
regimes of magnetic and charge ordering. Here, we study electronic
transport in mesoscale devices made with heterostructures of SrTiO<sub>3</sub> sandwiched between layers of SmTiO<sub>3</sub>, in which
the transport properties can be tuned from a regime of Fermi-liquid
like resistivity (Ï â <i>T</i><sup>2</sup>)
to a non-Fermi liquid (Ï â <i>T</i><sup>5/3</sup>) by controlling the SrTiO<sub>3</sub> 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