Many squaraines have been observed to exhibit two‐photon absorption at transition energies close to those of the lowest energy one‐photon electronic transitions. Here, the electronic and vibronic ...contributions to these low‐energy two‐photon absorptions are elucidated by performing correlated quantum‐chemical calculations on model chromophores that differ in their terminal donor groups (diarylaminothienyl, indolenylidenemethyl, dimethylaminopolyenyl, or 4‐(dimethylamino)phenylpolyenyl). For squaraines with diarylaminothienyl and dimethylaminopolyenyl donors and for the longer examples of 4‐(dimethylamino)phenylpolyenyl donors, the calculated energies of the lowest two‐photon active states approach those of the lowest energy one‐photon active (1Bu) states. This is consistent with the existence of purely electronic channels for low‐energy two‐photon absorption (TPA) in these types of chromophores. On the other hand, for all squaraines containing indolinylidenemethyl donors, the calculations indicate that there are no low‐lying electronic states of appropriate symmetry for TPA. Actually, we find that the lowest energy TPA transitions can be explained through coupling of the one‐photon absorption (OPA) active 1Bu state with bu vibrational modes. Through implementation of Herzberg–Teller theory, we are able to identify the vibrational modes responsible for the low‐energy TPA peak and to reproduce, at least qualitatively, the experimental TPA spectra of several squaraines of this type.
Squaring up! Many squaraines have been observed to exhibit two‐photon absorption at transition energies close to those of the lowest energy one‐photon electronic transitions (see scheme). Here, the electronic and vibronic contributions to these low‐energy two‐photon absorptions are elucidated by performing correlated quantum‐chemical calculations on model chromophores that differ in their terminal donor groups.
The predictive simulation of molecular liquids requires potential energy surface (PES) models that are not only accurate but also computationally efficient enough to handle the large systems and long ...time scales required for reliable prediction of macroscopic properties. We present a new approach to the systematic approximation of the first-principles PES of molecular liquids using the GAP (Gaussian Approximation Potential) framework. The approach allows us to create potentials at several different levels of accuracy in reproducing the true PES and thus to determine the level of quantum chemistry that is necessary to accurately predict macroscopic properties. We test the approach by building a series of many-body potentials for liquid methane (CH4), which is difficult to model from first principles because its behavior is dominated by weak dispersion interactions with a significant many-body component. The increasing accuracy of the potentials in predicting the bulk density correlates with their fidelity to the true PES, whereas the trend with the empirical potentials tested is surprisingly the opposite. We conclude that an accurate, consistent prediction of its bulk density across wide ranges of temperature and pressure requires not only many-body dispersion but also quantum nuclear effects to be modeled accurately.
Ultrathin amorphous silica membranes with embedded organic molecular wires (oligo(p-phenylenevinylene), three aryl units) provide chemical separation of incompatible catalytic environments of CO2 ...reduction and H2O oxidation while maintaining electronic and protonic coupling between them. For an efficient nanoscale artificial photosystem, important performance criteria are high rate and directionality of charge flow. Here, the visible-light-induced charge flow from an anchored Ru bipyridyl light absorber across the silica nanomembrane to Co3O4 water oxidation catalyst is quantitatively evaluated by photocurrent measurements. Charge transfer rates increase linearly with wire density, with 5 nm–2 identified as an optimal target. Accurate measurement of wire and light absorber densities is accomplished by the polarized FT-IRRAS method. Guided by density functional theory (DFT) calculations, four wire derivatives featuring electron-donating (methoxy) and -withdrawing groups (sulfonate, perfluorophenyl) with highest occupied molecular orbital (HOMO) potentials ranging from 1.48 to 0.64 V vs NHE were synthesized and photocurrents evaluated. Charge transfer rates increase sharply with increasing driving force for hole transfer from the excited light absorber to the embedded wire, followed by a decrease as the HOMO potential of the wire moves beyond the Co3O4 valence band level toward more negative values, pointing to an optimal wire HOMO potential around 1.3 V vs NHE. Comparison with photocurrents of samples without nanomembrane indicates that silica layers with optimized wires are able to approach undiminished electron flux at typical solar intensities. Combined with the established high proton conductivity and small-molecule blocking property, the charge transfer measurements demonstrate that oxidation and reduction catalysis can be efficiently integrated on the nanoscale under separation by an ultrathin silica membrane.
Organosulfur silanes grafted on an aluminum current collector have been proposed and demonstrated to function as a sulfur source in the cathode for a lithium–sulfur (Li–S) battery. ...Bis3-(triethoxysilyl)propyldisulfide silane (TESPD) and bis3-(triethoxysilyl)propyltetrasulfide silane (TESPT) are typical examples of organosulfur complexes used for the study. These organosulfur silanes act as an insulator. Formation of polysulfides (Li2S x ), which is a major bottleneck in the case of elemental sulfur, can be eliminated using this novel cathode. In the absence of charge-carrying polysulfide species, the role of insulating TESPD/TESPT in the charge conduction pathway is an open question. Insight into the interface between the Al current collector and grafted TESPD/TESPT at an atomic level is a prerequisite for addressing the charge conduction pathway. The systematic theoretical methodology is developed based on electronic structure calculations and ab initio molecular dynamics simulations to propose the realistic cathode model (hydration environment) for the Li–S battery. A cluster model is developed to predict the reduction potentials of TESPD/TESPT disclosing the reduction reaction with Li, resulting in the intramolecular S–S bond breaking which is validated by experimental cyclic voltammetry measurements. A realistic cathode model between the aluminum current collector and TESPD/TESPT is also proposed to mimic the experimental conditions where the Al surface was exposed to O2 and H2O. The top few layers of Al are transformed into α-Al2O3 and covered with H2O molecules in the vicinity of grafted TESPD/TESPT. The structural models are further validated by comparing simulated S 2p binding energies with experimental X-ray photoelectron spectroscopy studies.
Wax and water mixtures are the main products of the Gas-To-Liquids (GTL) process through the Fischer-Tropsch synthesis (FTS), along with relatively small quantities of oxygenated compounds such as ...alcohols. It has been established that water production in high concentrations leads to phase segregation inside the catalyst support pores. Excess water conditions are considered responsible for sintering of catalytic nanoparticles, which reduces catalyst lifetime and increases GTL operational cost. Apart from the effects of increased water concentration in wax – water FTS mixtures, the presence of oxygenates has not been studied at the molecular level. Moreover, the effect of the surface support chemistry on the wax – water mixture inside catalyst pores is essential in understanding mixture phase behavior. All these open questions are of significant interest for the petrochemical industry. The present study focuses on simulating by means of Molecular Dynamics (MD) the n-octacosane (n-C28) – water mixture inside titanium dioxide (TiO2) and silicon carbide (SiC) mesopores at low-temperature FTS conditions (473.15 K). The presence of small alcohols such as methanol and ethanol on the n-C28 – H2O mixture's phase behavior was also evaluated. The united atom TraPPE (TraPPE-UA) force field was used for n-C28 and the alcohols, while the SPC/E was used for water. Catalyst support atoms were modeled as Lennard-Jones spheres (charged or neutral) at fixed positions. Our simulations show that inside TiO2, water molecules organize into two discrete layers on the TiO2 surface, with n-C28 occupying the pore center; mixture phase separation is completely reversed inside the SiC pore. In both cases, alcohol addition does not influence either n-C28 or H2O distribution inside the pores. Furthermore, we examine the hydrogen bonds formed between water molecules and how these are influenced upon alcohol addition and confinement. Finally, through the calculation of lateral diffusion coefficients, we provide insights on the mobility of the individual n-C28 – H2O mixture components inside the pores considered.
Display omitted
We study the Heisenberg exchange couplings in polynuclear transition-metal clusters with strong spin frustration using a variety of theoretical techniques. We present results for a trinuclear CrIII ...molecule, a tetranuclear FeIII complex, and an octanuclear FeIII molecular magnet. We explore the physics of the exchange couplings in these systems using standard broken-symmetry (BS) techniques and a more recently developed constrained density functional theory (C-DFT) approach. The calculations show that the expected picture of localized spin moments on the metal centers is appropriate, and in each case C-DFT predicts coupling constant values in good agreement with experiment. Furthermore, we demonstrate that all of the C-DFT spin states for a given cluster can be reasonably described by a single Heisenberg Hamiltonian. These findings are significant in part because standard BS calculations are in conflict with the experiments on a number of key points. For example, BS-DFT predicts a doublet (rather than quartet) ground state for the CrIII cluster while for the FeIII complexes BS-DFT predicts some of the exchange couplings to be ferromagnetic whereas the experimentally derived couplings are all antiferromagnetic. Furthermore, for BS-DFT the best-fit exchange parameters can depend significantly on the set of spin configurations chosen. For example, by choosing configurations with M s closer to the BS-DFT couplings can typically be made somewhat closer to the C-DFT and experimental results. Thus, in these cases, our results consistently support the experimental findings.
Fuels are subjected to extreme conditions inside a fuel injector. In modern common rail diesel engines, fuel temperatures can reach 150°C and pressures can exceed 2500 bar inside the rail. Under such ...conditions the fluid physical properties of the fuel can differ substantially from ambient pressure and temperature and can impact the spray behavior and characteristics. Moreover, experimental determination of the fuel physical properties at these extreme conditions can be very difficult.
Previously it has been shown that for pure components, all atom molecular simulations offer a reliable means to calculate the key physical properties (including transport properties, e.g., viscosity) at FIE representative conditions. In this study we extend the approach to calculate these properties of binary mixtures using atomistic molecular simulations. We modelled the diesel fuel using two diesel surrogates: a modern “GTL-like” diesel fuel surrogate, n-hexadecane, and a “conventional” diesel fuel surrogate n-decylbenzene and calculated viscosity and density for the respective pure components and their various binary mixtures.
The molecular dynamics simulations provide insights into the mechanisms at an atomic level and can reliably calculate density and viscosity of the fuel at extreme conditions commonly found inside a fuel injector.
The transport properties of wax and water mixtures under confinement and particularly inside catalyst nanopores is a topic of significant interest for the petrochemical industry. These mixtures are ...the products of the Gas-To-Liquids (GTL) process through the Fischer–Tropsch (FT) route, which experienced an increasing number of commercially viable applications over the past decades. Under reaction conditions, water is produced in high concentrations, leading to phase segregation inside the catalyst nanopores and water-assisted sintering of catalytic nanoparticles, reducing catalyst lifetime and increasing GTL operational cost. It is thus important to understand the wax–water liquid–liquid equilibrium (LLE) at reaction conditions, as it determines the maximum allowable amount of water in the FT wax. Furthermore, elucidating the phase behavior of wax–water mixture inside the nanopores, by explicit incorporation of wall effects, is essential in revealing the role of confinement on mixture phase behavior. The present study focuses on simulating the phase behavior of the n-octacosane (n-C28)–water mixture inside TiO2 nanopores. Molecular Dynamics (MD) simulations with realistic molecular models were employed, highlighting the importance of confinement on the mixture transport properties, particularly in the excess water regime. Even though phase segregated mixtures retain their structural properties compared to their bulk counterparts, significant deviations arise in terms of density profiles inside the nanopore. Water molecules organize into two discrete layers on the TiO2 surface, shielding n-C28 from the nanopore walls. Octacosane’s self-diffusion is not influenced by confinement; water on the other hand is severely hindered by the TiO2 nanopore surface, with its diffusivity bearing a strong dependence on the distance from the nanopore center.
Atomistic molecular dynamics simulations were carried out at equilibrium to calculate the shear viscosity of n-decane and n-hexadecane under ambient and high temperature-high pressure conditions. Two ...force fields, one using a computationally efficient united-atom (TrAPPE-UA) approach and another an all-atom (AA) approach (Tobias, Tu and Klein), were employed. Quantitative agreement with experimental data is obtained with the AA force field, whereas the UA model predicts the viscosity within 20-30% of the experiment. The intra- and inter-molecular structure of the fluid obtained from these two models is similar, pointing to the role of differences in their dynamical characteristics to the observed difference in the calculated viscosities.
PDF
