Mechanistic Insights into the DABCO-Catalyzed Cloke-Wilson Rearrangement: A DFT Perspective.

The mechanism and selectivity patterns of the DABCO-catalyzed Cloke-Wilson rearrangement were computationally studied in detail using density functional theory calculations. Our computations suggest that the process occurs stepwise involving the initial ring opening of the cyclopropane promoted by a DABCO molecule followed by a ring-closure reaction of the readily formed zwitterionic intermediate. The regioselectivity of the initial nucleophilic ring-opening step strongly depends on the nature of the substituent attached to the cyclopropane moiety. The physical factors governing the preference for the more sterically hindered C2 (tertiary) position have been quantitatively analyzed by applying the combined activation strain model-energy decomposition analysis method. In addition, our calculations revealed a new mechanism for the analogous transformation involving vinylcyclopropanes consisting of an initial SN2' ring-opening process followed by a 5-exo-trig cyclization step, which proceeds without facial selectivity.

Hydrogen Bonds in Lead Halide Perovskites: Insights from Ab Initio Molecular Dynamics.

Hydrogen bonds (HBs) play an important role in the rotational dynamics of organic cations in hybrid organic/inorganic halide perovskites, thus affecting the structural and electronic properties of the perovskites. However, the properties and even the existence of HBs in these perovskites are not well established. In this study, we investigate HBs in perovskites MAPbBr3 (MA+ = CH3NH3+), FAPbI3 (FA+ = CH(NH2)2+), and their solid solution with composition (FAPbI3)7/8(MAPbBr3)1/8, using ab initio molecular dynamics and electronic structure calculations. We consider HBs donated by X-H fragments (X = N and C) of the organic cations and accepted by the halides (Y = Br and I) and characterize their properties based on pair distribution functions and on a combined distribution function of the hydrogen-acceptor distance with the donor-hydrogen-acceptor angle. By analyzing these functions, we establish geometrical criteria for HB existence based on the hydrogen-acceptor (H-Y) distance and donor-hydrogen-acceptor angle (X-H-Y). The distance condition is defined as d(H - Y) < 3 Å for N-H-donated HBs and d(H - Y) < 4 Å for C-H-donated HBs. The angular condition is 135° < (X - H - Y) < 180° for both types of HBs. A HB is considered to be formed when both angular and distance conditions are simultaneously satisfied. At the simulated temperature (350 K), the HBs dynamically break and form. We compute the time correlation functions of HB existence and HB lifetimes, which range between 0.1 and 0.3 ps at that temperature. The analysis of HB lifetimes indicates that N-H-Br bonds are relatively stronger than N-H-I bonds, while C-H-Y bonds are weaker, with a minimal influence from the halide and cation. To evaluate the impact of HBs on the vibrational spectra, we present the power spectrum in the region of N-H and C-H stretching modes, comparing them with the normal mode frequencies of isolated cations. We show that the peaks associated with N-H stretching modes in perovskites are redshifted and asymmetrically deformed, while the C-H peaks do not exhibit these effects.

Selective reduction of nitroarenes using Ru/C and CaH2.

Herein, we report an efficient and highly selective method for the reduction of aromatic, heteroaromatic and halonitro compounds using the readily available and cost-effective Ru/C as a catalyst along with unconventional CaH2 as a source of hydride. In most cases the corresponding anilines can be obtained by simple filtration without further purification. The use of 2-MeTHF and the simple operational work-up constitute a valid alternative to previous methodologies.

Functionalization of Gold Nanostars with Cationic ß-Cyclodextrin-Based Polymer for Drug Co-Loading and SERS Monitoring.

Gold nanostars (AuNSs) exhibit modulated plasmon resonance and have a high SERS enhancement factor. However, their low colloidal stability limits their biomedical application as a nanomaterial. Cationic ß-cyclodextrin-based polymer (CCD/P) has low cytotoxicity, can load and transport drugs more efficiently than the corresponding monomeric form, and has an appropriate cationic group to stabilize gold nanoparticles. In this work, we functionalized AuNSs with CCD/P to load phenylethylamine (PhEA) and piperine (PIP) and evaluated SERS-based applications of the products. PhEA and PIP were included in the polymer and used to functionalize AuNSs, forming a new AuNS-CCD/P-PhEA-PIP nanosystem. The system was characterized by UV-VIS, IR, and NMR spectroscopy, TGA, SPR, DLS, zeta potential analysis, FE-SEM, and TEM. Additionally, Raman optical activity, SERS analysis and complementary theoretical studies were used for characterization. Minor adjustments increased the colloidal stability of AuNSs. The loading capacity of the CCD/P with PhEA-PIP was 95 ± 7%. The physicochemical parameters of the AuNS-CCD/P-PhEA-PIP system, such as size and Z potential, are suitable for potential biomedical applications Raman and SERS studies were used to monitor PhEA and PIP loading and their preferential orientation upon interaction with the surface of AuNSs. This unique nanomaterial could be used for simultaneous drug loading and SERS-based detection.

On the role of water in the hydrogen bond network in DESs: an ab initio molecular dynamics and quantum mechanical study on the urea-betaine system.

We herein report an ab initio molecular dynamics study on a natural DES composed of urea and betaine in a 3 : 2 ratio, as a test case for evaluating the water effect. The article deals with a theoretical study using both ab initio molecular dynamics and quantum chemistry computations in order to unravel the role of water in the nanostructure of a urea-betaine mixture. Preliminary molecular dynamics outcomes (both radial and spatial distribution functions) suggest that water promotes the association between urea and betaine by increasing the hydrogen bond network and precluding the aggregation of urea molecules. In other words, the presence of water allows a less restrictive hydrogen bond network, presenting a regimen where the strong hydrogen bond interactions are replaced by a wide variety of weaker hydrogen bond interactions. On the other hand, in a water free DES there is a regimen where strong urea-betaine interactions are dominant. It is shown that second order perturbation theory energy analysis provides cogent insights into charge spreading and hydrogen bond patterns. A vibrational analysis (both IR and power spectrum) over the ab initio molecular dynamics trajectories in the water free DES as well as in the urea-betaine-water systems reveals that our results are consistent with the second order perturbation theory analysis and with the hydrogen bond network pattern.

Methodological Issues in First-Principle Calculations of CH3NH3PbI3 Perovskite Surfaces: Quantum Confinement and Thermal Motion.

Characterization and control of surfaces and interfaces are critical for photovoltaic and photocatalytic applications. In this work, we propose CH3NH3PbI3 (MAPI) perovskite slab models whose energy levels, free of quantum confinement, explicitly consider the spin-orbit coupling and thermal motion. We detail methodological tools based on the density functional theory that allow achieving these models at an affordable computational cost, and analytical corrections are proposed to correct these effects in other systems. The electronic state energies with respect to the vacuum of the static MAPI surface models, terminated in PbI2 and MAI atomic layers, are in agreement with the experimental data. The PbI2-terminated slab has in-gap surface states, which are independent of the thickness of the slab and also of the orientation of the cation on the surface. The surface states are not useful for alignments in photovoltaic devices, while they could be useful for photocatalytic reactions. The energy levels calculated for the MAI-terminated surface coincide with the widely used values to estimate the MAPI alignment with the charge transport materials, i.e., -5.4 and -3.9 eV for valence band maximum and conduction band minimum, respectively. Our study offers these slab models to provide guidelines for optimal interface engineering.

Atomic-Scale Model and Electronic Structure of Cu2O/CH3NH3PbI3 Interfaces in Perovskite Solar Cells.

Cuprous oxide has been conceived as a potential alternative to traditional organic hole-transport layers in hybrid halide perovskite-based solar cells. Device simulations predict record efficiencies using this semiconductor, but experimental results do not yet show this trend. More detailed knowledge about the Cu2O/perovskite interface is mandatory to improve the photoconversion efficiency. Using density functional theory calculations, here, we study the interfaces of CH3NH3PbI3 with Cu2O to assess their influence on device performance. Several atomistic models of these interfaces are provided for the first time, considering different compositions of the interface atomic planes. The interface electronic properties are discussed on the basis of the optimal theoretical situation, but in connection with the experimental realizations and device simulations. It is shown that the formation of vacancies in the Cu2O terminating planes is essential to eliminate dangling bonds and trap states. The four interface models that fulfill this condition present a band alignment favorable for photovoltaic conversion. Energy of adhesion and charge transfer across the interfaces are also studied. The termination of CH3NH3PbI3 in PbI2 atomic planes seems optimal to maximize the photoconversion efficiency.

How Meaningful Is the Halogen Bonding in 1-Ethyl-3-methyl Imidazolium-Based Ionic Liquids for CO2 Capture?

We report on several parameters that can be used to describe the 1-ethyl-3-methyl-4,5-(X2)imidazolium cations (where X = H, Br, and I) within the Canongia-Lopez and Padua Force Field (CL&P) framework. Geometrical parameters like intramolecular distances and radial distribution functions are close to the experimental structure. Density values obtained with our force field are within the expected ones from CL&P calculations in related systems. This information is used to simulate through molecular dynamics the solubilization of CO2 by these ILs. For pure ILs, the addition of halides in position 4 and 5 promotes an enhanced hydrogen bond interaction at position 2 with the oxygen atoms in the anion. It is found that CO2 should be in the interstices of the anion-cation 3D network with longer distances than those found in other reports at ab initio levels, suggesting that halogen bond, if present, may be not the driving force interaction in these systems. Therefore, it seems that CO2 interacts linearly via an oxygen atom with the cation and with the anion through a π-stacking or hydrogen-bonded fashions. Solvation enthalpies compare well with the experimental data, thereby suggesting that halogenated ILs dissolve more efficiently in CO2 than C2C1Im+ derivatives. This result suggests that halogenated ILs can be considered as reliable candidates for CO2 capture.