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 CH₄, CO₂, 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 CO₂ 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 CO₂–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 CO₂ 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_R in the range of ca. 0.2–0.5 ns; (ii) the rate of water exchange is not critical, but a τ_M < 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 (Gd₂L1 and Gd₂L2, respectively) were synthesized through a multistep synthesis. Chemical Exchange Saturation Transfer (CEST) experiments carried out on Eu₂L2 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 ¹H and ¹⁷O NMR relaxometric study was carried out in order to evaluate the parameters that govern the relaxivity associated with these complexes. The relaxivities of Gd₂L1 and Gd₂L2 (20 MHz, 298 K) are 8.7 and 9.5 mM⁻¹ s⁻¹, 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 Gd₂L2. MR phantom images of the dinuclear complexes compared to GdHPDO3A, recorded at 7 T, confirmed the superiority of Gd₂L2. 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 (SiO₂/Al₂O₃ = 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^–1, respectively) are involved in the MTBE adsorption process with the formation of H-bonded complexes and associated shifts (516 and 358 cm^–1, 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 CH₃O– group can be used to discriminate between H-bond and van der Waals MTBE–zeolite interactions (2843 and 2828 cm^–1, 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 (SiO₂/Al₂O₃ = 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^–1, respectively) are involved in the MTBE adsorption process with the formation of H-bonded complexes and associated shifts (516 and 358 cm^–1, 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 CH₃O– group can be used to discriminate between H-bond and van der Waals MTBE–zeolite interactions (2843 and 2828 cm^–1, 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 SiO₂/Al₂O₃ 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