Photovoltaic properties of the natural dyes of chlorophylls consist of Chl a, Chl b, Chl c2, Chl d, Phe a, Phe y and Mg-Phe a, were studied in the gas phases and water. The extension of the π-conjugated system, the substitution of the central Mg2+ and proper functional groups in the chlorophyll structures can amplify the charge transfer and photovoltaic performance. Chl a shows more favorable dynamics of charge transfer than other studied chlorophylls. Chl d, Phe a, Phe y and Mg-Phe a, have a greater rate of the exciton dissociation in comparison with Chl a, Chl b, and Chl c2 originated from a lower electronic chemical hardness, a lower exciton binding energy, and a bigger electron-hole radius. As a result, better efficiencies of the light-harvesting and energy conversion of the chlorophylls mainly appear in the Soret band. The LHE values of the chlorophylls in water show that solvent favorably affects the ability of light-harvesting of the photosensitizers. Finally, based on the energy conversion efficiency, Chl a, Phe a, and Mg-Phe a, are proposed as the best candidates for using in the dye-sensitized solar cells.
A systematic benchmark of phosphorus and fluorine NMR chemical shifts predictions at six different density functional theory (DFT) / the gauge-including atomic orbital (GIAO) methods was conducted. Two databases were compiled: one consists of 35 phosphorus-containing molecules, which cover the most common intra-molecular bonding environments of trivalent and pentavalent phosphorus atoms; the other is composed of 46 fluorine-containing molecules. The characteristics of each DFT/GIAO method with different solvent models were demonstrated in details. The application of linear regression between the calculated isotropic shielding constants and experimental chemical shifts was applicable to improve the prediction accuracy. And, the best methods with the SMD and CPCM implicit solvent models for 31P chemical shifts predictions, are able to yield a root-mean-square deviation (RMSDs) of 5.58 ppm and 5.42 ppm, respectively; for 19F, the corresponding lowest prediction errors with these two applied solvent models are 4.43 ppm and 4.12 ppm. The developed scaling factors fitted from linear regression are applicable to enhance the chance of successful structural elucidations of phosphorus or fluorine-containing compounds, as an efficient complement to 13C, 1H, 11B and 15N chemical shifts predictions.
Density functional theory (DFT) calculations were conducted to investigate mechanistic details of ethanol-to-butadiene conversion reaction over MgO or ZnO catalyst. We evaluated the Lewis acidity and basicity of MgO and ZnO and found that ZnO had the stronger Lewis acidity and basicity compared with those of MgO. Potential energy surfaces (PESs) of ethanol-to-butadiene conversion, which included relevant transition states (TSs) and intermediates, were computed in detail following the generally accepted mechanism reported in the literature, where such mechanism included ethanol dehydrogenation, aldol condensation, Meerwein-Pondorf-Verley (MPV) reduction and crotyl alcohol dehydration. DFT results showed that ethanol dehydrogenation was the rate limiting step of overall reaction when the reaction was catalyzed by MgO. Also, DFT results showed that ethanol dehydrogenation occurred more easily on ZnO compared with MgO where such a result correlated with the stronger Lewis acidity of ZnO. In addition, we computed ethanol dehydration which generates ethylene, one of the major undesired side reaction products for butadiene formation. DFT results showed that ZnO favored dehydrogenation over dehydration while MgO favored dehydration.
This work presents analytical and numerical results for the position- and momentum-space information entropies, of the 1s2-state of helium-like ions, using different interaction potentials. The potentials that we used are the Yukawa potential (YP), and the exponential-cosine-screened Coulomb potential (ECSCP). The investigated studies allow us to relate the position-space information with the momentum-space information of Shannon and Fisher, as well as Shannon entropy power, and the Fisher-Shannon information product, through different famous relations. The calculation is done using the one-electron charge density of entangled two-parameter wave function. On one hand, the results that are presented for ten members in the helium isoelectronic sequence demonstrate with precision the effect of correlation on bare charge distributions. On the other hand, it leads to some very important results for both the correlated and uncorrelated values of the informatic entropies. Analytical formula for the momentum-space information entropies are given. The effect of the nuclear charge and the screening parameter on the information expressions has been studied for both potentials. Detailed computational and numerical values and characteristics of these information quantities, as a function of the screening parameter, are reported here for the ﬁrst time. New inequality has been proposed with Fisher’s total value to measure the correlation of two electrons.
Due to it is potential application in the field of high energy density materials, how to stabilize cyclopentazolate anion (cyclo-N5-) has attracted many interests theoretically and experimentally. Therefore, a series of ion salts containing [cyclo-N5]- were synthesized and studied. The instability of [cyclo-N5]- is caused by the five lone pairs of electrons localized on five neighbored N atoms. In this work, we expect if the [cyclo-N5]- can be stabilized by the coordination with acidic ligands, by weakening the multi repulsion from the lone pairs to stabilize the [cyclo-N5]-. The two compounds of [N5(BH3)5]-, and [N5(AgCN)5]- have been designed and compared based on the Lewis acid-base theory. [N5(H2O)5]- is designed to evaluate the effect of hydrogen bond in the stabilization. For all the structures, we study the bonding properties and thermal stabilities based on the analysis of electronic structures and Car-Parrinello molecular dynamics (CPMD) simulations. The results indicate it is a effective method to stabilize [cyclo-N5]- by introducing the Lewis acid. Our insights on [cyclo-N5]- compounds with high thermal stability under ambient conditions will provide a new idea for the research and synthesis of new high energetic [cyclo-N5]- series compounds.
Transition metal porphyrazines are a widely used class of compounds with applications in catalysis, organic solar cells, photodynamic therapy and nonlinear optics. The most prominent members of that family of compounds are metallophtalocyanines that have been subject of numerous spectroscopic and theoretical studies. In this work, the electronic structure and X-ray absorption characteristics of three Cu-porphyrazine derivatives are investigated by means of modern electronic structure theory. More precisely, the experimentally observed N K-edge and Cu L-edge features are presented and reproduced by time-dependent density functional theory, restricted open-shell configuration interaction and a restricted active space approach. Where possible, the calculations are used to interpret the observed spectroscopic features in terms of electronic transitions and furthermore connect spectral differences to chemical variations. Part of the discussion of the computational results concerns the impact of various parameters and approximations that enter the calculations, e.g. the choice of active space.
Li‐rich layered Mn‐based oxide (LMO) cathode materials, with the formation of Li2MnO3, have attracted much attention due to their potential in various applications with high energy density. However, these cathode materials for Lithium‐ion batteries still suffer from drawbacks such as poor rate capability and voltage decay, which makes further investigation vital and rational. Here, the doping strategy is employed to investigate the effect of TM = Ti, Cu, and Zn on Li2Mn0.5TM0.5O3 cathode materials for improving electrochemical performances of Li2MnO3. Electrochemical properties such as voltage, electrical conductivity, safety, structural stability, and kinetics and mechanism of Li‐ion diffusion are evaluated and compared. All doped cathodes decrease the voltage reduction and improve the electrical conductivity coefficient in comparison with LMO. Doping Cu notably increases the electrical conductivity of LMO by 77%. Ti doping exhibits the potential to increase the maximum voltage of LMO and structural stability. Doping Zn and Cu elements can delay the oxygen loss significantly, which leads to a higher life cycle and safety. In addition, doping Zn is expected to have a higher Li‐ion diffusion coefficient due to its low energy barrier and partial charge of oxygen atoms in its cathode structure. This first‐principle study of doping effects of TM = Ti, Cu, and Zn with α = 0.5 in Li2Mn0.5TMαO3 may be a useful leading study for further investigation into the synthesis of lithium‐rich materials with enhanced electrochemical performance.
Ab initio calculations on systems involving singlet molecular oxygen (O2 (1g)) are challenging due to signicant multi-reference character arising from the degeneracy of the HOMO and LUMO orbitals in singlet oxygen. Here we investigate the stragegy of bypassing singlet oxygen’s multi-reference character by simply adding the experimen- tally determined singlet/triplet splitting (22.5 kcal/mol) to the triplet ground state of molecular oxygen. This method is tested by calculating rate constants for the reac- tions of singlet molecular oxygen with furan, 2-methylfuran, 2,5-dimethylfuran, pyrrole, 2-methylpyrrole, 2,5-dimethylpyrrole, and cyclopentadiene. The calculated rate con- stants are within a factor of 15 compared to experimentally determined rate constants. The results show that energy renement at the CCSD(T)-F12 level of theory is cru- cial to achieving accurate results. The reasonable agreement with experimental values validates the bypassing approach which can be used for other systems involving the 1,4-cyclo-addition of singlet oxygen. 2
Hydrogen peroxide (H2O2), as clean oxidant, has long suffered from low efficiency and selectivity for the oxidation of olefins. In the present paper, the redox important ferrate anion (FeO42-) has been anchored into a silanol-decorated polyoxometalates (POM) to form single–site supported Fe-POM catalyst. And possible reaction mechanism for the epoxidation of propylene with hydrogen peroxide (H2O2) catalyzed by the Fe-POM catalyst have been investigated based on density functional theory with M06L functional. The study of molecular geometry, electronic structure, and bonding feature shows that the Fe-POM complex can be viewed as a high-valent Fe-oxo (Fe=O) species. The propylene molecule was activated by the Fe-POM catalyst via an effective electron transfer from propylene to the Fe-POM catalyst to form a cation propylene radical. Due to the high reactivity of radical species, the calculated activation energy barrier is only 4.50 kcal mol-1 for epoxidation of propylene to epoxypropane catalyzed by the Fe-POM catalyst. Subsequently, the calculated free energy profiles show that H2O2 was decomposed into a H2O molecule and a surface O species over the Fe-POM catalyst, and the remaining O atom attaches to the exposed the Fe center, resulting in the replenishing of Fe-POM catalyst via a two-state reaction pathway. The calculated activation energy barrier for this process is 23.42 kcal mol–1, and thus decomposition of H2O2 is the rate-determining step for the whole reaction. The Fe center serves as an electron acceptor, accepting electrons from the binding propylene molecule to form radical species in the first half of the reaction, and acts as the role of electron donor in the rest reaction steps to eliminate the radical feature, reduce the reactivity, and stop the reaction at the stage of the desired epoxypropane product.
The mechanisms of rhodium-catalyzed coupling reaction of ketoxime and 1,3-enynes were investigated by employing the density functional theory (DFT) calculations. Different 1,3-enynes would lead to different annulation products. Reaction A undergoes five sequential steps (C-H activation, 1,3-enyne migratory insertion, 1,4-Rh migration, cyclization, and deprotonation) to lead to [4 + 1] annulation product. Whereas, due to the electronic effect, the process generating [4 + 2] product in reaction A is restricted. In contrast, the electron-withdrawing group of N(Me)2 group in 1,3-enyne would bring about the [4 + 2] annulation product in reaction B. Our calculated results indicate that no [4 + 1] annulation product could be obtained in reaction C, in agreement with the experimental observation that the cis-allyl hydrogen in 1,3-enyne is crucial for the [4 + 1] annulation reaction.
Based on the combination of novel carbon material graphynes (GYs) and superalkalis (OM3), a class of graphyne superalkali complexes, OM3+@(GY/GDY/GTY)– (M = Li, Na, and K), have been designed and investigated by density functional theory method. Computational results reveal that these complexes with high stability can be regarded as novel superalkali salts of graphynes due to electron transfer from OM3 to GYs. For second order nonlinear optical response, these superalkali salts exhibit large first hyperpolarizabilities (β0). Two important effects on β0 values are found, namely the atomic number of alkali atom in superalkali and the pore size of graphyne. Integrating the two effects, the selected combination of OLi3 with large pore size GTY can bring the considerable β0 value (6.5×105 au), which is a new record for superatom-doped graphynes. In the resulting complex, the OLi3 molecule is located at the center of the pore of GTY, forming a planar structure with the highest stability among these salts. Besides large β0 values, these superalkali salts of graphynes have deep-ultraviolet working region, hence can be considered as a new kind of high-performance deep-ultraviolet NLO molecules.
We systematically investigate the binding nature of CB towards 20 amino acids in both neutral (AAs) and protonated (AAs+) states by quantum chemistry methods. The result indicates molecular recognition process are enthalpy-driven. Among AAs, Arg and Asn shows the largest binding strength to CB, and for AAs+, Gln+ and Asn+ bind to CB the strongest. The binding strength of protonated CB/AA+ is much stronger than that of neutral CB/AA counterpart, due to the introduction of ion-dipole interaction and the increase number and strength of hydrogen bonds. Energy decomposition analysis (EDA) indicates that electrostatic interactions play major roles in both CB/AAs and CB/AAs+ complexes. Moreover, we analyzed the dependence of binding strength on single AA volume and dipole moment. This study is benefit for providing valuable information in predicting the recognition sites for sequence-based peptide or protein by CB and rationally designing synthetic host molecule for specific peptide or protein recognition.
Ternary metal hydrides play an essential role in the search for conventional high-temperature superconductors because they can be synthesized under mild condition and recovered at ambient pressure. It has been widely accepted that the electronic structure, metallization pressure and superconducting behavior of binary hydrides can be adjusted effectively by doping, replacing or introducing a new element. In this work, yttrium hydrides were chosen as parent hydrides while scandium was considered as the doped element to perform systematical crystal structure searches on the Sc-Y-H system under pressure. A new ternary hydride ScYH6 was found according to PSO calculations, and it presents high symmetric character below 150 GPa with a Pm-3 structure (cP8), then a P4/mmm phase (tP8) becomes favorable from 150 GPa. Importantly, cP8-ScYH6 is dynamically stable under pressure as low as 0.01 GPa with a Tc of 32.110 K for Coulomb pseudopotential μ∗=0.13, indicating ternary hydrides are promising candidates in the search for superconductors which can be synthesized under mild conditions in hydrogen-rich materials. The analysis through “triangle straight-line method” (TSLM) compared with enthalpy difference calculations showed the most reasonable synthesis pathway of ScYH6 is in the whole studied pressure range. The Tc of ScYH6 takes a linear relationship with pressure up to 52.907 K under 200 GPa. The lattice dynamical calculations demonstrate the H atoms in both cP8 and tP8 structures make crucial contributions to the superconducting behavior of ScYH6. These findings can further reveal the influence of doping, replacing and introducing new element on superconducting behavior of binary hydrides.
Numerous materials are employed for treating wastewaters, e.g., for the removal of dyes from wastewater in the textile industry. However, the regeneration/reuse of these materials is still seldom practiced. Quantitative insights into intermolecular forces between the contaminants and the functional surfaces might aid the rational design of reusable materials. Here, we compare the efficacies of aliphatic (C8H18), aromatic (C6H6), and aromatic perfluorinated (C6F6) moieties at removing methylene blue (MB+), as a surrogate dye, from water. We employed DFT with an implicit water model (PCM) to accurately determine the contributions of the solvent’s electrostatics in the adsorption process. These calculations pinpointed the relative contributions of π-π stacking, van der Waals complexation, hydrogen bonding, and cation-π interactions. QM predicted that MB+ would bind the strongest with C6F6 due to hydrogen bonding and the weakest with C8H18. Laboratory experiments revealed that despite the similar hydrophobicity of silica beads functionalized with Si-C8H17, Si-C6H5, and Si-C6F5, as characterized by similar water contact angles, the relative uptake of aqueous MB+ varied as Si-C6F5 (95%) > Si-C6H5 (35%) > Si-C8H17 (3%). This first-principles-led experimental approach can be extended to other classes of dyes, and it should advance the rational design of adsorbents for treating wastewaters.
The mechanisms of C−C activation of 1-Benzylcyclopropan-1-ol to produce 1,6-diketone have been investigated by density functional theory (DFT) calculations. The catalyst [Cp*RhCl2]2 and additive Ag2CO3 play an important role in controlling the selectivity. By employing [Cp*RhCl2]2 as catalyst and Ag2CO3 as additive, the product is 1,6-diketone, whereas the β-hydride elimination product could not be obtained. The product would become monoketone in the absence of [Cp*RhCl2]2. In addition, the combination of catalyst [Cp*RhCl2]2 and additive AgOAc would also lead to monoketone. The observed selectivity could be attributed to the electronic effect.
This work investigates possible improvements in the accuracy of semiempirical quantum chemistry (SQC) methods for the prediction of standard enthalpy of formation (Δ_f H^o) through the use of artificial neural network (ANN) with molecular descriptors. A total number of 142 organic compounds with enough structural diversity has been considered in the training set. Standard enthalpy of formation for the selected compounds at the semiempirical PM3 and PM6 quantum chemistry methods is collected from literature, and is calculated by using semiempirical PM7 method in this work. The multiple stepwise regression is first employed to screen effective molecular descriptors, which are highly correlated with the error terms of the standard enthalpy of formation compared with experimental values. The obtained 7 effective molecular descriptors are then used as input set to establish three 7-11-1 neural network-based correction models to improve the accuracy of SQC methods. By using the developed correction models, the mean absolute errors (MAE) for Δ_f H^oof PM3, PM6, and PM7 methods are reduced from 22.36, 18.60, 17.27to 9.86, 9.83, 8.95, respectively in kJ/mol. Meanwhile, the results of the test set show that the neural network does not have the problem of over-fitting. Detailed analysis of the 7 effective molecular descriptors indicates that the major source to the correction models is from the electron withdrawing effect. The developed ANN models for the three selected SQC methods provide an efficient method for the quick and accurate prediction of thermodynamic properties.
The structure, energetic and quantum chemical descriptors of Cs+ complexes of calixarene-crown-6 (C4C6) and substituted C4C6 i.e 1,3 alternate-diethoxy C4C6 are reported here based on the analysis of results using density functional theory. Substitution of benzo group to both C4C6 and 1,3 alternate-diethoxy C4C6 resulted in reduction of binding energy (BE). Further substitution on benzo group with methyl, methoxy and amino groups leads to increase in BE and nitro substitution leads to decrease in BE for C4C6, whereas in the case of 1,3 alternate-diethoxy calixarenebenzocrown-6, methoxy substitution leads to highest BE compared to other complexes. The calculated Gibbs free energy, ΔGgas also followed the same order as BE in the case of 1,3 alternate-diethoxy C4C6 and their substituted ligands. Furthermore, the ΔG of complexation were computed using thermodynamic cycle with conductor like screening model (COSMO) in different solvents: toluene, chloroform, octanol and nitrobenzene. The values of ΔGext are found to be increased with increase in the dielectric constant of the solvent and found to be highest in the nitrobenzene. The Atoms in Molecule (AIM) analysis reveals partial ionic character in Cs-O bond. Among all the studied complexes, 1,3 alternate-diethoxy calixarene 3’-methoxy benzo crown-6 displays highest ΔGext in nitrobenzene. The calculated value of ∆∆Gext (∆∆G= ∆GCs+-∆GNa+) is found to be -41.82 kcal/mol with 1,3 alternate-diethoxy calixarene 3’-methoxy benzocrown-6 which is higher than that obtained with calix  bis-crown-6 (-5.24 kcal/mol). The newly designed ligand might be suitable for the selective extraction of Cs+ over Na+ in the reprocessing of nuclear waste and thus invites the experimentalists for testing this DFT finding in the laboratory.
To cope with the energy crisis and global warming issues, researcher are rendering their efforts and paying their attentions to analyze and fabricate hydrogen storage devices. In this regard, we report a comprehensive study on the structural, vibrational, and optoelectronic properties of Lithium Borohydride (LiBH4), a hydrogen storage material. For this purpose, calculations of structural properties have been made using the local, non-local and hybrid functionals within the framework of density functional theory (DFT). The lattice constants for the orthorhombic phase are determined by applying LDA, PBE and HSE06 density functionals and their results are compared with available experimental and theoretical studies. In order to determine IR and Raman active modes of vibrations, vibrational spectroscopy has been utilized through Density Functional Perturbation Theory (DFPT) approach. Li, B and H atoms are noticed to be contributing in the modes of vibrations between different ranges of frequencies, i.e., 0 to 400 cm-1, 1100 to 1300 cm-1 and 2250 -2400 cm-1. The respective values of band gaps are found to be 6.35 eV, 6.81 eV and 7.58 eV for LDA, PBE and HSE06 functionals, respectively, leading to indicate insulating nature of LiBH4 which makes it a promising candidate for applications in optoelectronic devices. The mechanical analysis reveals that LiBH4 is a brittle material. The optical properties such as dielectric constant, refractive index, reflectivity, absorptivity, conductivity and loss function are also calculated with the aid of well-recognized relation of Kramer-Kronig. The plasma frequency is noted at the highest peak (13.7 eV) of the energy loss function.