Ferroelectric Alignment of Organic Cations Inhibits Nonradiative Electron–Hole Recombination in Hybrid Perovskites: Ab Initio Nonadiabatic Molecular Dynamics

The Journal of Physical Chemistry Letters
Hybrid organic–inorganic perovskites show impressive potential for photovoltaic applications and currently give rise to one of the most vibrant research areas in the field. Until recently, the electrostatic interactions between their organic and inorganic components were considered mostly for stabilization of the fragile perovskite structure. We study the effect of local interactions of polar C–N bonds in the organic layer on the nonradiative electron–hole recombination in the recently reported room-temperature ferroelectric hybrid perovskite, (benzylammonium)2PbCl4. Using nonadiabatic molecular dynamics and real-time time-dependent density functional theory, we show that ferroelectric alignment of the polar groups weakens the electron–phonon nonadiabatic coupling and inhibits the nonradiative charge recombination. The effect is attributed to suppression of contributions of higher frequency phonons to the electron–phonon coupling. The coupling is...  Read more

Comparison of tetrel bonds in neutral and protonated complexes of pyridineTF3 and furanTF3 (T = C, Si, and Ge) with NH3

Physical Chemistry Chemical Physics
Ab initio calculations have been performed for the complexes H+–PyTX3⋯NH3 and H+–furanTF3⋯NH3 (T = C, Si, and Ge; X = F and Cl) with focus on geometries, energies, orbital interactions, and electron densities to study the influence of protonation on the strength of tetrel bonding. The primary interaction mode between α/β-furanCF3/p-PyCF3 and NH3 changes from an F⋯H hydrogen bond to a C⋯N tetrel bond as a result of protonation. Importantly, the protonation has a prominent enhancing effect on the strength of tetrel bonding with an increase in binding energy from 14 to 30 kcal mol−1. The tetrel bonding becomes stronger in the order...  Read more

Driving Forces in the Sharpless Epoxidation Reaction: A Coupled AIMD/QTAIM Study

Inorganic Chemistry
In order to better understand the epoxide-formation step of the Sharpless epoxidation process, a set of 263 oxygen-transfer reactions reflecting the complexity of the Sharpless epoxidation process were studied using density functional theory (DFT) and Bader’s quantum theory of atoms in molecules (QTAIM). The diversity within these reactions reflects the different ligands in the coordination sphere of vanadium and also different substrates (alkene and an allylic alcohol both free and in the form of an alcoxo ligand). The transition states for 76 of these reactions were also characterized using DFT and QTAIM, allowing for an estimation of the impact of the different ligands and substrates on the activation barriers. A smaller subset of the latter was further subjected to an ab initio molecular dynamics (AIMD) simulation coupled to QTAIM analysis. The results show that the type of active catalyst plays an important role in the thermodinamic outcome of these reactions, with vanadium(V)...  Read more

Electrochemical reduction of CO2 into CO on Cu(100): a new insight into the C–O bond breaking mechanism

Chemical Communications
Using ab initio molecular dynamics simulations and an aqueous interfacial model with explicit water molecules, we firstly identified a new C–O bond breaking mechanism in the electrochemical conversion of CO2 to CO on Cu(100) via proton–electron transfer, which is different from the traditional surface catalytic mechanism.Read more

Mode specific THz spectra of solvated amino acids using the AMOEBA polarizable force field

Physical Chemistry Chemical Physics
We have used the AMOEBA model to simulate the THz spectra of two zwitterionic amino acids in aqueous solution, which is compared to the results on these same systems using ab initio molecular dynamics (AIMD) simulations. Overall we find that the polarizable force field shows promising agreement with AIMD data for both glycine and valine in water. This includes the THz spectral assignments and the mode-specific spectral decomposition into intramolecular solute motions as well as distinct solute–water cross-correlation modes some of which cannot be captured by non-polarizable force fields that rely on fixed partial charges. This bodes well for future studies for simulating and decomposing the THz spectra for larger solutes such as proteins or polymers for which AIMD studies are presently intractable. Furthermore, we believe that the current study on rather simple aqueous solutions offers a way to systematically investigate the importance of charge transfer, nuclear quantum...  Read more

On-Surface Synthesis and Characterization of 9-Atom Wide Armchair Graphene Nanoribbons

ACS Nano
The bottom-up approach to synthesize graphene nanoribbons strives not only to introduce a band gap into the electronic structure of graphene but also to accurately tune its value by designing both the width and edge structure of the ribbons with atomic precision. We report the synthesis of an armchair graphene nanoribbon with a width of nine carbon atoms on Au(111) through surface-assisted aryl–aryl coupling and subsequent cyclodehydrogenation of a properly chosen molecular precursor. By combining high-resolution atomic force microscopy, scanning tunneling microscopy, and Raman spectroscopy, we demonstrate that the atomic structure of the fabricated ribbons is exactly as designed. Angle-resolved photoemission spectroscopy and Fourier-transformed scanning tunneling spectroscopy reveal an electronic band gap of 1.4 eV and effective masses of ≈0.1 me for both electrons and holes, constituting a substantial improvement over previous efforts toward the development of...  Read more

Diverging effects of isotopic fractionation upon molecular diffusion of noble gases in water: mechanistic insights through ab initio molecular dynamics simulations

Environmental Science: Processes & Impacts
Atmospheric noble gases are routinely used as natural tracers to analyze gas transfer processes in aquatic systems. Their isotopic ratios can be employed to discriminate between different physical transport mechanisms by comparison to the unfractionated atmospheric isotope composition. In many applications of aquatic systems molecular diffusion was thought to cause a mass dependent fractionation of noble gases and their isotopes according to the square root ratio of their masses. However, recent experiments focusing on isotopic fractionation within a single element challenged this broadly accepted assumption. The determined fractionation factors of Ne, Ar, Kr and Xe isotopes revealed that only Ar follows the prediction of the so-called square root relation, whereas within the Ne, Kr and Xe elements no mass-dependence was found. The reason for this unexpected divergence of Ar is not yet understood. The aim of our computational exercise is to establish the molecular-resolved mechanisms...  Read more

Structural Rearrangement of Au–Pd Nanoparticles under Reaction Conditions: An ab Initio Molecular Dynamics Study

ACS Nano
The structure, composition, and atomic distribution of nanoalloys under operating conditions are of significant importance for their catalytic activity. In the present work, we use ab initio molecular dynamics simulations to understand the structural behavior of Au–Pd nanoalloys supported on rutile TiO2 under different conditions. We find that the Au–Pd structure is strongly dependent on the redox properties of the support, originating from strong metal–support interactions. Under reducing conditions, Pd atoms are inclined to move toward the metal/oxide interface, as indicated by a significant increase of Pd–Ti bonds. This could be attributed to the charge localization at the interface that leads to Coulomb attractions to positively charged Pd atoms. In contrast, under oxidizing conditions, Pd atoms would rather stay inside or on the exterior of the nanoparticle. Moreover, Pd atoms on the alloy surface can be stabilized by hydrogen adsorption, forming Pd–H...  Read more

Vibrational branching ratios and radiative lifetimes in the laser cooling of AlBr

Physical Chemistry Chemical Physics
The feasibility of laser cooling of the AlBr molecule is investigated using ab initio quantum chemistry. Potential energy curves, permanent dipole moments, and transition dipole moments for the ground state X1Σ+ and the first two excited states (a3Π and A1Π) are calculated using the multi-reference configuration interaction plus Davidson corrections (MRCI+Q) method with the ACVQZ basis set; the spin–orbit coupling effects are also taken into account in electronic structure calculations at the MRCI level. Based on the acquired potential energy curves and transition dipole moments, highly diagonally distributed Franck–Condon factors (f00 = 0.9540, f11 = 0.8172) and vibrational branching ratios (R00 = 0.9708, R11 = 0.8420) for the...  Read more

The ability of a fibrous titanium oxophosphate for nitrogen-adsorption above room temperature

Chemical Communications
Ti2O(PO4)2·2H2O (1), a three-dimensional titanium oxophosphate, thermally transforms to Ti2O(PO4)2 (2), a fibrous novel compound. The crystal structure of 2 was solved ab initio using powder X-ray diffraction data (triclinic, P[1 with combining macron], a = 5.0843(1) Å, b = 8.6121(2) Å, c = 9.6766(2) Å, α = 74.501(2)°, β = 76.146(2)°, γ = 74.488(3)°, Z = 2). Compound 2, containing both 4- and 6-fold coordinated titanium atoms, shows measurable thermally activated nitrogen-adsorption. To our knowledge, the process described here constitutes the first example of...  Read more

Many-body interactions in soft jammed materials

Soft Matter
In jammed packings of soft frictionless particles such as foams or emulsions, stress is transmitted via a network of mechanical contacts between neighbors. In generic simplified models of such materials, particle interaction energies are assumed to be pairwise additive. We report ab initio simulations of foam microstructures, showing that in general, this fundamental assumption is not justified: the conservation of bubble volumes introduces a many-body coupling between all the contacts of a given particle. It strongly modifies the relation between forces and displacements at individual contacts, in a way that cannot be captured by an effective two-body interaction. We report the impact of this effect on the linear and nonlinear elastic response of ordered bubble packings with coordination numbers ranging from 6 to 12, used as simple model systems, and we present an analytical model without free parameters which is valid as long as bubbles have an approximately...  Read more

Predicting a graphene-like WB4 nanosheet with a double Dirac cone, an ultra-high Fermi velocity and significant gap opening by spin–orbit coupling

Physical Chemistry Chemical Physics
The zero-band gap nature of graphene prevents it from performing as a semi-conductor in modern electronics. Although various graphene modification strategies have been developed to address this limitation, the very small band gap of these materials and the suppressed charge carrier mobility of the devices developed still significantly hinder graphene's applications. In this work, a two dimensional (2D) WB4 monolayer, which exhibits a double Dirac cone, was conceived and assessed using density functional theory (DFT) methods, which would provide a sizable band gap while maintaining higher charge mobility with a Fermi velocity of 1.099 × 106 m s−1. Strong spin–orbit-coupling can generate an observable band gap of up to 0.27 eV that primarily originates from the d-orbit of the heavy metal atom W; therefore a 2D WB4 nanosheet would be operable at room temperature (T = 300...  Read more

Puzzling Lack of Temperature Dependence of the PuO2 Magnetic Susceptibility Explained According to Ab Initio Wave Function Calculations

The Journal of Physical Chemistry Letters
The electronic structure and the magnetic properties of solid PuO2 are investigated by wave function theory calculations, using a relativistic complete active space (CAS) approach including spin–orbit coupling. The experimental magnetic susceptibility is well reproduced by calculations for an embedded PuO812– cluster model. The calculations indicate that the surprising lack of temperature dependence of the magnetic susceptibility χ of solid PuO2 can be rationalized based on the properties of a single Pu4+ ion in the cubic ligand field of the surrounding oxygen ions. Below ∼300 K, the only populated state is the nonmagnetic ground state, leading to standard temperature-independent paramagnetism (TIP). Above 300 K, there is an almost perfect cancellation of temperature-dependent contributions to χ that depends delicately on the mixing of ion levels in the electronic states, their relative energies, and the magnetic coupling...  Read more

Nucleophilic Deoxyfluorination of Phenols via Aryl Fluorosulfonate Intermediates

Journal of the American Chemical Society
This report describes a method for the deoxyfluorination of phenols with sulfuryl fluoride (SO2F2) and tetramethylammonium fluoride (NMe4F) via aryl fluorosulfonate (ArOFs) intermediates. We first demonstrate that the reaction of ArOFs with NMe4F proceeds under mild conditions (often at room temperature) to afford a broad range of electronically diverse and functional group-rich aryl fluoride products. This transformation was then translated to a one-pot conversion of phenols to aryl fluorides using the combination of SO2F2 and NMe4F. Ab initio calculations suggest that carbon–fluorine bond formation proceeds via a concerted transition state rather than a discrete Meisenheimer intermediate.Read more

Freezing and Melting of Silver Nanoparticles on Silica Substrate Using a Simple Interatomic Potential for Ag–SiO2 Interaction on the Basis of ab Initio Calculations and Experimental Data

The Journal of Physical Chemistry C
We have devised a Ag–SiO2 potential for Monte Carlo and molecular dynamics studies of systems comprising silver nanoparticles in contact with silica surfaces. The potential is determined based on density functional theory (DFT) calculations on β-cristobalite, a silica crystal, as well as theoretical and experimental data on α-quartz and amorphous silica found in literature. The interactions between Ag and silica species are fitted with the simple Lennard-Jones (12, 6) potential with parameters σAg–O = 0.278 nm, σAg–Si = 0.329 nm, εAg–O = 0.012 eV, and εAg–Si = 0.002 eV. The nature of the substrate is taken into account at a phenomenological level as an extra multiplicative factor η for the total interaction between silver and silica: for α-quartz ηquartz = 1, for hydroxylated β-cristobalite ηcrist = 5.0, and for amorphous silica ηamorph = 2.5. This potential is thought to be...  Read more

High-Dimensional Atomistic Neural Network Potentials for Molecule–Surface Interactions: HCl Scattering from Au(111)

The Journal of Physical Chemistry Letters
Ab initio molecular dynamics (AIMD) simulations of molecule–surface scattering allow first-principles characterization of the dynamics. However, the large number of density functional theory calculations along the trajectories is very costly, limiting simulations of long-time events and giving rise to poor statistics. To avoid this computational bottleneck, we report here the development of a high-dimensional molecule–surface interaction potential energy surface (PES) with movable surface atoms, using a machine learning approach. With 60 degrees of freedom, this PES allows energy transfer between the energetic impinging molecule and thermal surface atoms. Classical trajectory calculations for the scattering of DCl from Au(111) on this PES are found to agree well with AIMD simulations, with ∼105-fold acceleration. Scattering of HCl from Au(111) is further investigated and compared with available experimental results.Read more

Decomposition mechanisms in metal borohydrides and their ammoniates

Journal of Materials Chemistry A
Ammoniation in metal borohydrides (MBs) with the form M(BH4)x has been shown to lower their decomposition temperatures with M of low electronegativity (χp ≲ 1.6), but raise it for high-χp MBs (χp ≳ 1.6). Although this behavior is just as desired, an understanding of the mechanisms that cause it is still lacking. Using ab initio methods, we elucidate those mechanisms and find that ammoniation always causes thermodynamic destabilization, explaining the observed lower decomposition temperatures for low-χp MBs. For high-χp MBs, we find that ammoniation blocks...  Read more

GW100: A Plane Wave Perspective for Small Molecules

Journal of Chemical Theory and Computation
In a recent work, van Setten and co-workers have presented a carefully converged G0W0 study of 100 closed shell molecules [ J. Chem. Theory Comput.2015, 11, 5665−5687]. For two different codes they found excellent agreement to within a few 10 meV if identical Gaussian basis sets were used. We inspect the same set of molecules using...  Read more

Thiol-Michael addition in polar aprotic solvents: nucleophilic initiation or base catalysis?

Polymer Chemistry
The thiol-Michael addition of ethanethiol to ethyl acrylate, methyl vinylsulfone and maleimide initiated by ethyl-, diethyl-, triethylamine and triethylphosphine in tetrahydrofuran (THF) is investigated at room temperature for concentrations ranging from 0.5 to 2 mol L−1 for the reactants and 0.03 to 0.3 mol L−1 for the initiators. Rate coefficients for all elementary steps in a reaction scheme consisting of both the base catalyzed and the nucleophile initiated mechanism are calculated using CBS-QB3 corrected for solvation with COSMO-RS. Diffusional limitations are taken into account using the coupled encounter pair model. The ab initio apparent kinetic parameters are used in a microkinetic model and simulated conversions agree well with experimental data. Competition with the aza-Michael addition is shown to be insignificant. Regardless of the choice of ene or catalyst, conversion is governed by an anionic cycle in...  Read more

Titanium(III) Member of the Family of Trigonal Building Blocks with Scorpionate and Cyanide Ligands

Inorganic Chemistry
The titanium(III) cyanide compound [Et4N][Tp*Ti(CN)3] ([Et4N] = tetraethylamonium; Tp* = 3,5-dimethyltrispyrazolylhydroborate) is reported, which exhibits a trigonally distorted geometry. Magnetic data and ab initio calculations verified that the molecule is an S = 1/2 paramagnet and that it exhibits significant temperature-independent paramagnetism.Read more