8 research outputs found
Ab Initio Design of Low Band Gap 2D Tin Organohalide Perovskites
Four
layered hybrid perovskites, based on tin iodide sheets intercalated
by divalent organic cations (ethylenediammonium, 2,2′-biaziridinium,
2,2′-biimidazolium, and 4,4′-bipyridinium), have been
modeled with ab initio techniques. The crystal structures have been
optimized at the DFT level, not including thermal effects, and finding
and characterizing three minima for each cation; with respect to the
analogues with monovalent cations, the structures are more distorted
and mostly in a staggered arrangement. The interlayer distances are
quite small for all of the systems, due to the single layer of strongly
charged cations between the inorganic sheets. The band profiles and
the band gaps, computed with an additive approach including the effects
of spin orbit coupling and post-DFT correlation corrections, show
an unexpected and interesting feature: with two of the cations some
nearly degenerate low energy levels appear at the bottom of the conduction
band. As a consequence, these systems present unusually low band gaps
(the minimum value being 1.34 eV), suggesting the possibility of light
adsorption in the visible or near-IR regions. The existence of these
low-lying levels has been correlated to the charge and the aromatic
nature of the organic ions, and a simple molecular descriptor, based
on the LUMO energy of the isolated cations, is proposed to design
other tin iodide perovskites with this characteristic
Accurate Evaluation of the Dispersion Energy in the Simulation of Gas Adsorption into Porous Zeolites
The force fields
used to simulate the gas adsorption in porous
materials are strongly dominated by the van der Waals (vdW) terms.
Here we discuss the delicate problem to estimate these terms accurately,
analyzing the effect of different models. To this end, we simulated
the physisorption of CH<sub>4</sub>, CO<sub>2</sub>, and Ar into various
Al-free microporous zeolites (ITQ-29, SSZ-13, and silicalite-1), comparing
the theoretical results with accurate experimental isotherms. The
vdW terms in the force fields were parametrized against the free gas
densities and high-level quantum mechanical (QM) calculations, comparing
different methods to evaluate the dispersion energies. In particular,
MP2 and DFT with semiempirical corrections, with suitable basis sets,
were chosen to approximate the best QM calculations; either Lennard-Jones
or Morse expressions were used to include the vdW terms in the force
fields. The comparison of the simulated and experimental isotherms
revealed that a strong interplay exists between the definition of
the dispersion energies and the functional form used in the force
field; these results are fairly general and reproducible, at least
for the systems considered here. On this basis, the reliability of
different models can be discussed, and a recipe can be provided to
obtain accurate simulated adsorption isotherms
Monte Carlo Modeling of Carbon Dioxide Adsorption in Porous Aromatic Frameworks
The adsorption isotherms of CO<sub>2</sub> in several porous aromatic
frameworks (PAFs) have been simulated with Grand Canonical Monte Carlo
technique, to support the synthesis of new materials for efficient
carbon dioxide capture and storage. The simulations covered the 0–60
bar pressure range and were repeated at 273, 298, and 323 K. The force
field employed in the simulations was optimized to fit the correct
behavior of the free gas and to reproduce the CO<sub>2</sub>–phenyl
interactions computed at high quantum mechanical level. PAFs are based
on the diamond structure, with polyaromatic chains inserted in C–C
bonds. We examined four PAF-30<i>n</i> (<i>n</i> being the number of phenyl rings in the aromatic linkers), finding
that PAF-302 is overall the best performing, although PAF-301 provides
higher adsorbed densities at very low pressure. The CO<sub>2</sub> adsorption then was simulated in a number of modified PAF-302, with
different functional groups (aminomethane, toluene, pyridine, and
imidazole) attached to the phenyl chains; different degrees of substitution
(25%, 50%, and 100% derivatized rings) were considered. The effects
of functionalization and the dependence on the substitution degree
are carefully discussed, to determine the most promising materials
at low, intermediate, and high pressures
First Principle Study of Capping Energies and Electronic States in Stoichiometric and Nonstoichiometric PbSe Nanoclusters
A large
variety of PbSe nanoclusters have been modeled at the DFT
level, to study their structure, their affinity for different ligands
and their electronic properties, also depending on surface passivation.
The clusters are extracted from the bulk rock salt structure with
cubic, prism, truncated cubic, cuboctahedral and octahedral shape
and they are fully relaxed, before computing the addition energies
of methylamine and formate anions in different positions, to model
the process of surface passivation. Then the density of states of
all the clusters is computed, to study in particular the band gap
and the behavior of the so-called intragap states, which affect the
photophysical properties of the nanoparticles, also acting as trap
states for charge carriers. We confirm the strong relationship between
nanocluster off-stoichiometry and intragap states: such states can
be localized on the surface, in the bulk or delocalized over the nanoparticle,
according to the source of off-stoichiometry. The ability of different
ligands to eliminate the intragap states are tested and discussed,
also proposing nonstandard capping molecules
Table_1_Optimizing the Relaxivity of MRI Probes at High Magnetic Field Strengths With Binuclear GdIII Complexes.PDF
<p>The key criteria to optimize the relaxivity of a Gd(III) contrast agent at high fields (defined as the region ≥ 1.5 T) can be summarized as follows: (i) the occurrence of a rotational correlation time τ<sub>R</sub> in the range of ca. 0.2–0.5 ns; (ii) the rate of water exchange is not critical, but a τ<sub>M</sub> < 100 ns is preferred; (iii) a relevant contribution from water molecules in the second sphere of hydration. In addition, the use of macrocycle-based systems ensures the formation of thermodynamically and kinetically stable Gd(III) complexes. Binuclear Gd(III) complexes could potentially meet these requirements. Their efficiency depends primarily on the degree of flexibility of the linker connecting the two monomeric units, the absence of local motions and the presence of contribution from the second sphere water molecules. With the aim to maximize relaxivity (per Gd) over a wide range of magnetic field strengths, two binuclear Gd(III) chelates derived from the well-known macrocyclic systems DOTA-monopropionamide and HPDO3A (Gd<sub>2</sub>L1 and Gd<sub>2</sub>L2, respectively) were synthesized through a multistep synthesis. Chemical Exchange Saturation Transfer (CEST) experiments carried out on Eu<sub>2</sub>L2 at different pH showed the occurrence of a CEST effect at acidic pH that disappears at neutral pH, associated with the deprotonation of the hydroxyl groups. Then, a complete <sup>1</sup>H and <sup>17</sup>O NMR relaxometric study was carried out in order to evaluate the parameters that govern the relaxivity associated with these complexes. The relaxivities of Gd<sub>2</sub>L1 and Gd<sub>2</sub>L2 (20 MHz, 298 K) are 8.7 and 9.5 mM<sup>−1</sup> s<sup>−1</sup>, respectively, +77% and +106% higher than the relaxivity values of the corresponding mononuclear GdDOTAMAP-En and GdHPDO3A complexes. A significant contribution of second sphere water molecules was accounted for the strong relaxivity enhancement of Gd<sub>2</sub>L2. MR phantom images of the dinuclear complexes compared to GdHPDO3A, recorded at 7 T, confirmed the superiority of Gd<sub>2</sub>L2. Finally, ab initio (DFT) calculations were performed to obtain information about the solution structure of the dinuclear complexes.</p
The Role of Silanols in the Interactions between Methyl <i>tert</i>-Butyl Ether and High-Silica Faujasite Y: An Infrared Spectroscopy and Computational Model Study
It is generally believed that the retention of methyl <i>tert</i>-butyl ether (MTBE) by zeolites is positively correlated
to the silica
content of these materials. Nevertheless, highly dealuminated zeolites
can contain relevant amounts of silanol groups. In this study, the
effect of these point defects in a zeolite Y (SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> = 200) on the adsorption of MTBE was evaluated
by means of infrared spectroscopy, supported by DFT calculations.
The adsorption process is found to occur in two steps, involving isolated
silanol sites and the siloxane network of the zeolite, respectively,
with an average loading of 1.3 molecules per cage in the first and
1.3 molecules in the second stage. Both external and internal isolated
silanol groups (stretching at 3746 and 3738 cm<sup>–1</sup>, respectively) are involved in the MTBE adsorption process with
the formation of H-bonded complexes and associated shifts (516 and
358 cm<sup>–1</sup>, respectively), consistent with a H-bonding
strength higher for external than for the internal ones. However,
MTBE is more tightly adsorbed on the internal silanols as a result
of the cage confinement effect. The band assigned to the methyl symmetric
stretching of the CH<sub>3</sub>O– group can be used to discriminate
between H-bond and van der Waals MTBE–zeolite interactions
(2843 and 2828 cm<sup>–1</sup>, respectively). Ab initio models
were used to compute the harmonic frequencies of different MTBE–zeolite
models and to simulate the cage confinement of one and three ether
molecules
The Role of Silanols in the Interactions between Methyl <i>tert</i>-Butyl Ether and High-Silica Faujasite Y: An Infrared Spectroscopy and Computational Model Study
It is generally believed that the retention of methyl <i>tert</i>-butyl ether (MTBE) by zeolites is positively correlated
to the silica
content of these materials. Nevertheless, highly dealuminated zeolites
can contain relevant amounts of silanol groups. In this study, the
effect of these point defects in a zeolite Y (SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> = 200) on the adsorption of MTBE was evaluated
by means of infrared spectroscopy, supported by DFT calculations.
The adsorption process is found to occur in two steps, involving isolated
silanol sites and the siloxane network of the zeolite, respectively,
with an average loading of 1.3 molecules per cage in the first and
1.3 molecules in the second stage. Both external and internal isolated
silanol groups (stretching at 3746 and 3738 cm<sup>–1</sup>, respectively) are involved in the MTBE adsorption process with
the formation of H-bonded complexes and associated shifts (516 and
358 cm<sup>–1</sup>, respectively), consistent with a H-bonding
strength higher for external than for the internal ones. However,
MTBE is more tightly adsorbed on the internal silanols as a result
of the cage confinement effect. The band assigned to the methyl symmetric
stretching of the CH<sub>3</sub>O– group can be used to discriminate
between H-bond and van der Waals MTBE–zeolite interactions
(2843 and 2828 cm<sup>–1</sup>, respectively). Ab initio models
were used to compute the harmonic frequencies of different MTBE–zeolite
models and to simulate the cage confinement of one and three ether
molecules
Interactions of Toluene and <i>n</i>‑Hexane on High Silica Zeolites: An Experimental and Computational Model Study.
The knowledge of host–guest
interactions occurring in confined
space between porous solids and embedded molecules of different origin
is an important task to improve adsorption properties of materials,
thus extending their application fields. In this work, the interactions
of toluene and <i>n</i>-hexane molecules (selected as models
of organic pollutants coming from industrial waste of oil refineries
and gas stations) on different dealuminated high silica zeolites were
studied by means of both experimental and computational approaches.
Zeolites with different textural and surface features were selected
as adsorbents and the effect of their physicochemical properties (i.e.,
pore size architecture and type and amount of surface OH sites) on
sorption capacity were studied. High silica Y and ZSM-5 zeolites (with
a SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> ratio of 200 and 280,
respectively) were selected as model sorbents. FTIR and SS-NMR spectroscopy
were used to study the type and strength of the host−guest
interactions between the molecules and the zeolite surface. Gravimetric
analysis allowed the determination of the sorption capacity of both
zeolites and their affinity to pollutants. The interactions between
the silica surfaces and pollutants molecules computed at the DFT level,
and supplemented by empirical formulation of dispersion energies,
led to estimate the intensity of hydrogen bonding and cooperative
van der Waals interactions