9 research outputs found
Numerical Study of Vapor Condensation on Patterned Hydrophobic Surfaces Using the String Method
Vapor condensation on solid surfaces
plays a crucial role across
a wide range of industrial applications. Recent advances of nanotechnology
have made possible the manipulation of the condensation process through
the control of surface structures. In this work, we study vapor condensation
on hydrophobic surfaces patterned with microscale pillars. The critical
nuclei, the activation barriers, and the minimum energy paths are
computed using the climbing string method. The effects of pillar height,
interpillar spacing, the level of supersaturation, and the intrinsic
wettability of the solid surface on the nucleation process are investigated.
Two nucleation scenarios are obtained from the computation. In the
case of high pillar, narrow interpillar spacing, low supersaturation,
and/or low surface wettability, the critical nucleus prefers the suspended
Cassie state; otherwise, it prefers the impaled Wenzel state. A comparison
of the nucleation barrier with that on a flat surface of the same
material reveals that vapor condensation is inhibited by the microstructures
in the former case, while enhanced in the latter case. The critical
values of the pillar height, the interpillar spacing, and the supersaturation
at which the critical nucleus changes from the Cassie state to the
Wenzel state are identified from the phase diagram of the critical
nucleus. It is found that the dependence of the critical interpillar
spacing on the supersaturation follows closely the curve of the critical
radii in a homogeneous nucleation. The relaxation dynamics of the
condensate after the critical nucleus is formed is computed by solving
the steepest descent equation. It is observed that when the pillar
is low and/or the interpillar spacing is wide, a condensate initially
in the Cassie state may evolve into the Wenzel state during the relaxation
New bisesquiterpenoid lactone from the wild rhizome of <i>Atractylodes macrocephala</i> Koidz grown in Qimen
<p>The rhizomes of <i>Atractylodes macrocephala</i> are used as both a food source and traditional Chinese medicine in China. A phytochemical investigation was carried out on wild <i>A. macrocephala</i> grown in Qimen County in eastern China, and yielded a novel bisesquiterpenoid lactone, namely, biatractylenolide II (<b>1</b>), along with two known compounds, atractylenolide II (<b>2</b>) and taraxeryl acetate (<b>3</b>). The structure and relative configuration of the new compound were elucidated mainly by 1D and 2D NMR spectroscopic methods in combination with HRESIMS experiments. This paper describes the isolation and structural elucidation of the new bisesquiterpenoid lactone (<b>1</b>).</p
Generalized Energy-Based Fragmentation Approach for Localized Excited States of Large Systems
We have extended
the generalized energy-based fragmentation (GEBF)
approach to localized excited states of large systems. In this approach,
the excited-state energy of a large system could be expressed as the
combination of the excited-state energies of âactive subsystemsâ,
which contains the chromophore center, and the ground-state energies
of âinactive subsystemsâ. The GEBF approach has been
implemented at the levels of time-dependent density functional theory
(TDDFT) and approximate coupled cluster singles and doubles (CC2)
method. Our results show that GEBF-TDDFT can reproduce the TDDFT excitation
energies and solvatochromic shifts for large systems and that GEBF-CC2
could be used to validate GEBF-TDDFT result (with different functionals).
The GEBF-TDDFT method is found to be able to provide satisfactory
or reasonable descriptions on the experimental solvatochromic shifts
for the <i>n</i> â Ď* transitions of acetone
in various solutions, and the lowest Ď â Ď* transitions
of pyridine and uracil in aqueous solutions
Benchmark Relative Energies for Large Water Clusters with the Generalized Energy-Based Fragmentation Method
The
generalized energy-based fragmentation (GEBF) method has been
applied to investigate relative energies of large water clusters (H<sub>2</sub>O)<sub><i>n</i></sub> (<i>n</i> = 32,
64) with the coupled-cluster singles and doubles with noniterative
triple excitations (CCSDÂ(T)) and second-order MøllerâPlesset
perturbation theory (MP2) at the complete basis set (CBS) limit. Here
large water clusters are chosen to be representative structures sampled
from molecular dynamics (MD) simulations of liquid water. Our calculations
show that the GEBF method is capable of providing highly accurate
relative energies for these water clusters in a cost-effective way.
We demonstrate that the relative energies from GEBF-MP2/CBS are in
excellent agreement with those from GEBF-CCSDÂ(T)/CBS for these water
clusters. With the GEBF-CCSDÂ(T)/CBS relative energies as the benchmark
results, we have assessed the performance of several theoretical methods
widely used for <i>ab initio</i> MD simulations of liquids
and aqueous solutions. These methods include density functional theory
(DFT) with a number of different functionals, MP2, and density functional
tight-binding (the third generation, DFTB3 in short). We find that
MP2/aug-cc-pVDZ and several DFT methods (such as LC-ĎPBE-D3
and ĎB97XD) with the aug-cc-pVTZ basis set can provide satisfactory
descriptions for these water clusters. Some widely used functionals
(such as B3LYP, PBE0) and DFTB3 are not accurate enough for describing
the relative energies of large water clusters. Although the basis
set dependence of DFT is less than that of <i>ab initio</i> electron correlation methods, we recommend the combination of a
few best functionals and large basis sets (at least aug-cc-pVTZ) in
theoretical studies on water clusters or aqueous solutions
New neolignan glycoside from the root of <i>Aralia echinocaulis</i> Hand. -Mazz
<p>A phytochemical investigation was carried out to the root of <i>Aralia echinocauis</i>, and a new neolignan glycoside, 3-{3,5-dimethoxy-4β-d-Glucopyranoside-2-[3-(3-β-d-Glucopyranoside-4-methoxy-phenyl)-allyl]-phenyl}-prop-2-en-1-ol (<b>1</b>), together with a known saponin, araliasaponin II (<b>2</b>) for the first time was obtained<i>.</i> The chemical structure of compound <b>1</b> was identified mainly by the analysis of NMR including 1D and 2D NMR in combination with High Resolution Electrospray Ionisation Mass (HR-ESI-MS). This paper herein describes the isolation and structural elucidation of compound <b>1</b>.</p
Regulating Circularly Polarized Luminescence Signals of Chiral Binaphthyl-Based Conjugated Polymers by Tuning Dihedral Angles of Binaphthyl Moieties
A series
of chiral binaphthyl-based conjugated polymers enantiomers incorporating
boron dipyrromethene (BODIPY) chromophore in the main chain backbone
were designed and synthesized by Pd-catalyzed Sonogashira cross-coupling
reaction. All of them can exhibit strong Cotton effects and circularly
polarized luminescence (CPL) emission signals in THF solution. The
CD absorption dissymmetry factors (<i>g</i><sub>abs</sub>) and the luminescence dissymmetry factors (<i>g</i><sub>lum</sub>) can be regulated by tuning the dihedral angles of binaphthyl
arising from different substitutions of BINOL hydroxyls. Interestingly,
the chiral polymers can exhibit the gradual increase of both <i>g</i><sub>abs</sub> and <i>g</i><sub>lum</sub> as
the decrease of dihedral angles of the chiral binaphthyl moiety. This
work can provide a new strategy for the development of CPL emission
materials
Molecular Dynamics Simulations of Hydrogen Bond Dynamics and Far-Infrared Spectra of Hydration Water Molecules around the Mixed Monolayer-Protected Au Nanoparticle
Molecular dynamics simulations have
been performed to systematically investigate the structure and dynamics
properties, hydrogen bond (HB) dynamics, and far-infrared (far-IR)
spectra of hydration water molecules around the mixed monolayer-protected
Au nanoparticles (MPANs) with different ligand compositions and length.
Our simulation results demonstrate that the translational and rotational
motions of hydration water molecules in the proximity of charged terminal
NH<sub>3</sub><sup>+</sup> and COO<sup>â</sup> groups are suppressed
significantly with respect to the bulk water. Compared to the bulk
water, meanwhile, longer structural relaxation times of hydration
H<sub>2</sub>OâH<sub>2</sub>O HBs indicate enhanced strength
of H<sub>2</sub>OâH<sub>2</sub>O HBs at the interface of mixed
MPANs. Accordingly, these hydration water molecules around the charged
terminal groups can exhibit a considerable blue-shift in far-IR spectra
for all ligand compositions and length studied here. A series of detailed
HB analyses manifest that above restricted behavior of hydration water
molecules can be attributed to the stronger H<sub>2</sub>OâNH<sub>3</sub><sup>+</sup> and H<sub>2</sub>OâCOO<sup>â</sup> HBs and the corresponding structural relaxation times are much greater
than those of hydration H<sub>2</sub>OâH<sub>2</sub>O HBs.
Furthermore, we find that increasing ligand length can affect much
the morphology of self-assemble monolayers in water owing to enhanced
hydrophobic interactions between alkane chains and water molecules
and favor the translational mobility of hydration water molecules
owing to weaken electrostatic interactions. Unlike the translational
motions, our comparison results among different ligand lengths clearly
confirm that the rotational relaxation of hydration water molecules
should be dominated by the local and directional HBs at the interfaces,
rather than the previous explanation of the ratio between hydrophobic/hydrophilic
exposed regions. More importantly, our simulations reveal at a molecular
level that the ligand composition has a little influence on the structure,
dynamics, HBs, and far-IR spectra of hydration water molecules around
the mixed MPANs mainly due to the comparable strength between H<sub>2</sub>OâNH<sub>3</sub><sup>+</sup> and H<sub>2</sub>OâCOO<sup>â</sup> HBs
Structural Properties and Vibrational Spectra of Ethylammonium Nitrate Ionic Liquid Confined in Single-Walled Carbon Nanotubes
The
structures and relevant vibrational spectra of an ethylammonium
nitrate (EAN) ionic liquid (IL) confined in single-walled carbon nanotubes
(SWCNTs) with various diameters have been investigated in detail by
using classical molecular dynamics simulation. Our simulation results
demonstrate that the EAN IL confined in larger SWCNTs can form well-defined
multishell structures with an additional cation chain located at the
center. However, a different single-shell hollow structure has been
found for both the cations and the anions in the 1 nm SWCNT. For the
cations confined in SWCNTs, the CH<sub>3</sub> groups stay closer
to the nanotube walls because of their solvophobic nature, while the
NH<sub>3</sub><sup>+</sup> groups prefer to point toward the central
axis. Accordingly, the NO<sub>3</sub><sup>â</sup> anions tend
to lean on the SWCNT surface with three O atoms facing the central
axis to form hydrogen bonds (HBs) with the NH<sub>3</sub><sup>+</sup> groups. In addition, in the 1 nm SWCNT, the CH<sub>3</sub> groups
of cations exhibit an obvious blue shift of around 16 cm<sup>â1</sup> for the CâH stretching mode with respect to the bulk value,
and the NâH stretching mode of NH<sub>3</sub><sup>+</sup> groups
is split into two characteristic peaks with one peak appearing at
a higher frequency. Such a blue shift is attributed to the existence
of more free space for the CâH bonds of confined CH<sub>3</sub> groups, while the splitting phenomenon is due to the fact that more
than 60% of the confined NH<sub>3</sub><sup>+</sup> groups have one
dangling NâH bond. For the anions confined in the 1 nm SWCNT,
the NâO stretching mode of NO<sub>3</sub><sup>â</sup> has a maximum red shift of around 24 cm<sup>â1</sup> with
respect to the bulk value, which is attributed to enhanced HBs between
anions and cations. Our simulation results reveal a molecular-level
correlation between confined structural configurations and the corresponding
vibrational spectra changes for the ILs confined in nanometer scale
environments
Molecular-Level Insights into Size-Dependent Stabilization Mechanism of Gold Nanoparticles in 1âButyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid
Here
we report a series of classical molecular dynamics simulations
for the icosahedral Au nanoparticles with four different diameters
of 1.0, 1.4, 1.8, and 2.3 nm in 1-butyl-3-methylimidazolium tetrafluoroborate
([bmim]Â[BF<sub>4</sub>]) room-temperature ionic liquid (RTIL). Our
simulation results reveal for the first time a size-dependent stabilization
mechanism of the Au nanoparticles in the [bmim]Â[BF<sub>4</sub>] RTIL,
which may help to clarify the relevant debate on the stabilization
mechanism from various experimental observations. By comparison, the
alkyl chains in the [bmim]<sup>+</sup> cations are found to dominate
the stabilization of the smallest Au<sub>13</sub> nanoparticle in
the RTIL while the imidazolium rings should be mainly responsible
for the stabilization of other larger nanoparticles in the RTIL. Compared
to the [bmim]<sup>+</sup> cations, the [BF<sub>4</sub>]<sup>â</sup> anions are found to have an indirect influence on stabilizing the
Au nanoparticles in the RTIL because of the weak interaction between
the Au nanoparticles and the anions. However, such differences in
the stabilization mechanism between the small and the large Au nanoparticles
can be attributed to the unique hydrogen bond (HB) network between
the cations and the anions in the first solvation shell. Meanwhile,
increasing the particle size can lead to the enhanced HBs on the surface
of Au nanoparticles, so slower rotational motions and more pronounced
orientation distribution of cations can be observed around the larger
nanoparticles. Our simulation results in this work provide a molecular-level
understanding of the unique size-dependent stabilization mechanism
of the Au nanoparticles in the imidazolium-based RTILs