Abstract:The application of cluster model calculations to understand the interaction of water molecules with the VPI-5 lattice is presented. The energetically favorable location of water molecules and their influence in altering the local symmetry of the VPI-5 framework are reported. The results indicate the presence of a water molecule in the vicinity of All and P2 sites which are correlating with M A S N M R and X-ray powder diffraction studies.W e also carried out thermal analysis and quantified seven different type… Show more
“…The important point is to show that the hydrophilic site exists with a sufficiently attractive interaction energy; the hydrophilicity can be explained if the model is adequate. Thus Prasad and Vetrivel used similar small models to ours and found the energetically favorable location of water molecules. Although they found only positive interaction energies, this might be due to that their calculations were limited to only 1.5 Å. 6 Directions of approach of a water molecule to the aluminum site in the larger model.…”
The hydrophilic property of VPI-5 was investigated using cluster
models by an ab initio MO method. The
four cluster models of Al(OH)4 studied differ from
each other in the original locations of the aluminum atom
and in whether they are for the hydrate or the dehydrate. Only one
model, which has an aluminum site at the
center of the fused 4-membered rings in the hydrated VPI-5, can adsorb
a water molecule with an attractive
interaction energy of over 25 kcal/mol. The other models show only
repulsive interaction energies. The
origin of the hydrophilicity was investigated by an energy
decomposition analysis. The analysis concluded
that (1) for three of the four kinds of Al sites, oxygen atoms around
the Al atom prevent any water molecule
from coming to the Al site by a large exchange interaction, and (2) for
one kind of the Al site, electrostatic
and charge transfer terms between the oxygen atom of the water molecule
and the Al atom are stronger than
the exchange interaction mentioned above. Consequently, the Al
site works well as a Lewis acid site.
“…The important point is to show that the hydrophilic site exists with a sufficiently attractive interaction energy; the hydrophilicity can be explained if the model is adequate. Thus Prasad and Vetrivel used similar small models to ours and found the energetically favorable location of water molecules. Although they found only positive interaction energies, this might be due to that their calculations were limited to only 1.5 Å. 6 Directions of approach of a water molecule to the aluminum site in the larger model.…”
The hydrophilic property of VPI-5 was investigated using cluster
models by an ab initio MO method. The
four cluster models of Al(OH)4 studied differ from
each other in the original locations of the aluminum atom
and in whether they are for the hydrate or the dehydrate. Only one
model, which has an aluminum site at the
center of the fused 4-membered rings in the hydrated VPI-5, can adsorb
a water molecule with an attractive
interaction energy of over 25 kcal/mol. The other models show only
repulsive interaction energies. The
origin of the hydrophilicity was investigated by an energy
decomposition analysis. The analysis concluded
that (1) for three of the four kinds of Al sites, oxygen atoms around
the Al atom prevent any water molecule
from coming to the Al site by a large exchange interaction, and (2) for
one kind of the Al site, electrostatic
and charge transfer terms between the oxygen atom of the water molecule
and the Al atom are stronger than
the exchange interaction mentioned above. Consequently, the Al
site works well as a Lewis acid site.
“…Similar results were reported by Tripathy's group − Since the discovery of the photorefractive effect in organic polymers, carbazole-based polymers − have attracted much attention because of their photoconductivity. Most recently, a poly( N -vinylcarbazole)-based polymer composite with high azo chromophore concentration (50 wt %) was prepared.…”
Poly{(4-nitrophenyl)-3-[N-[2-(methacryloyloxy)ethyl]carbazolyl]]diazene}
was prepared and
its photoinduced birefringence, diffraction grating, and
photorefractive asymmetric two-beam gain coupling
were studied. The monomer was obtained by performing azo coupling
in a two-phase water−dichloromethane system in the presence of a phase transfer catalyst.
Photoinduced birefringence of up
to 0.09 was observed and diffraction efficiencies of up to 25% were
obtained, with atomic force microscopy
studies revealing that the grating profile exhibited a sinusoidal
shape. Asymmetric two-beam coupling
is observed, indicating photorefractive properties, but the energy
transfer phenomena are more complex,
involving also diffraction by the photoinduced gratings. The
combination of these three optical
propertiesphotoinduced birefringence, surface gratings, and two-beam
couplingis believed to be unique
and may eventually produce an all-optical device built of a single
polymer film.
“…In the last years there has been considerable interest in the behavior of water molecules adsorbed in the aluminophosphate molecular sieves. − VPI-5 is a crystalline large-pore aluminophosphate containing non-interconnected unidimensional channels made by 18-membered rings with diameter of ≈ 12 Å. Several interesting features of the water−VPI-5 system have been reported and are worth noting.…”
The complex structural pattern described by water molecules in the channels of the large-pore VPI-5
aluminophosphate has been investigated through Car-Parrinello ab initio molecular dynamics simulations.
Water molecules occupy crystallographic positions but considerably rotate and oscillate around their equilibrium
positions. The extended network of strong hydrogen bonds among water in the channels leads to nonnegligible
collective motions. The conformation of the triple helix formed by water molecules has been thoroughly
examined; the helices run close to the channel walls and avoid the channel center. The high thermal motion
of the water molecules makes such chains rather flexible and the helices are not easily detectable during the
dynamics, but their shape emerges only when the average atomic positions are considered.
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