Quantum chemical calculations are applied to study the complexes between X2TO (X=H, F, Cl, Br, CH3; T=C, Si, Ge, Sn) and CO2. The carbon atom of CO2 as a Lewis acid participates in the O•••C carbon bond, whereas its oxygen atom as a base engages in the O•••T tetrel bond with X2TO. Most of complexes are stabilized by a combination of both O•••C and O•••T interactions. The interaction energies are dependent on the nature of T and X atoms/groups. Both the electron-withdrawing halogen group and the electron-donating methyl group increase the interaction energy, up to 51 kJ/mol in F2SiO•••CO2. One F2SiO molecule can bind with different number of CO2 molecules from one to four; as the number of CO2 increases, the average interaction energy for each CO2 is decreased but it can contribute at least 27 kJ/mol stabilization energy. Therefore, silicon-containing molecules are good absorbents for CO2.
Boroxol (B3O3) rings and relevant hexagonal B3S3 structural blocks are ubiquitous in boron oxide/sulfide glasses, crystals, and high temperature liquids. However, the isolation of an ultimate heterocyclic B3O3 or B3S3 cluster in the free-standing form, with as few as six atoms, has been unsuccessful so far. We report on computational design of the simplest case of such a system: highly symmetric D3h B3S3+ (1A1) cluster. It is the well-defined global minimum on the potential energy surface, following global searches and electronic structure calculations at the B3LYP and single-point CCSD(T) levels. Chemical bonding analysis reveals an ideal system with skeleton Lewis B3S3 single bonds and unique double 6π/2σ aromaticity, which underlies its stability. The cluster turns out to be an inorganic analog of the 3,5-dehydrophenyl cation, a typical double π/σ aromatic system. It offers an example for chemical analogy between boron-based heterocyclic clusters and aromatic hydrocarbons. Double π/σ aromaticity is also a new concept in heterocyclic boron clusters. Prior systems such as borazine, boroxine, and boronyl boroxine only deal with π aromaticity as in benzene.
Experimental studies on the speciation of berkelium in carbonate media have shown that complexation of berkelium(III) by carbonate results in spontaneous oxidation to berkelium(IV) and that multiple species can be present in solution. We studied two proposed structures present in solution based on theoretical comparisons with spectroscopic data previously reported for Bk(IV) carbonate solutions. The multiconfigurational character of the ground and low-lying excited states in both complexes is demonstrated to result from the strong spin-orbit coupling. Although bonding in berkelium(IV) carbonate and carbonate-hydroxide complexes is dominated by strong Coulombic forces, the presence of the non-negligible covalent character is supported by ligand-field theory, natural localized orbitals and topological studies of the electron density. Bond orders based in natural localized molecular orbitals (NLMOs) show that Bk–OH bonds possess enhanced orbital overlap that is reflected in the bond strength.
Hydroxyl derivatives of cinnamic acid, both natural and synthetic, are well-known antioxidants. However, not all of them feature the same radical-scavenging propensity. Establishing the relation between structure and reactivity towards radical of those species plays a crucial role in the design of novel antioxidant pharmaceuticals founded on the same parent structure. The study aims at clarifying the relationship between topology, geometry, electron and spin density distribution and the radical-scavenging activity. Different mechanisms are discussed based on the enthalpies of the possible structures generated in the process of dissociation of the OH-bonds. All structures are modelled utilizing first principles methods and accounting for the polar medium at neutral pH (B3LYP/6-311++G**/PCM). A hybrid mechanism is suggested applicable not only to hydroxylated cinnamic acids but to phenolic acids in polar environment in general.
Models of surface enhancement of molecular electronic response properties are challenging for two reasons: (1) molecule-surface interactions require the simultaneous solution of the molecular and the surface dynamic response (a daunting task); (2) when solving for the electronic structure of the combined molecule+surface system, it is not trivial to single out the particular physical effects responsible for enhancement. To attack this problem, in this work we apply a formally exact decomposition of the system’s response function into subsystem contributions by employing subsystem DFT which grants access to dynamic polarizabilities and optical spectra. In order to access information about the interactions between the subsystems, we extend a previously developed subsystem-based adiabatic connection fluctuation-dissipation theorem of DFT to separate the additive from the nonadditive correlation energy and identify the nonadditive correlation as the van der Waals interactions. As an example, we choose benzene adsorbed on monolayer MoS2. We isolate the contributions to the benzene’s dynamic response arising from the interaction with the surface and for the first time, we evaluate the enhancements to the effective C6 coefficients as a function of benzene-MoS2 distance and adsorption site. We also quantify the spectral broadening of the benzene’s electronic excited states due to their interaction with the surface. We find that the broadening has a similar decay law with the molecule-surface distance as the leading van der Waals interactions (i.e., R-6) and that the surface enhancement of dispersion interactions between benzene molecules is less than 5\%, but still large enough (0.5 kcal/mol) to likely play a role in the prediction of interface morphologies.
The mechanisms of Cp*Rh(OAc)2-catalyzed coupling reaction of N-methoxybenzamide with alkyl-terminated enyne have been investigated by density functional theory (DFT) calculations. With the addition of NaOAc and changing solvent, the product transforms from lactam P1 in reaction A to iminolactone P2 in reaction B, due to the formed stable OAc- coordinated intermediate. The electronic effect and steric effect account for the observed regioselectivity in reaction B collectively.
The transformation mechanism and kinetics of 2-chloro-1,1,2-trifluoroethyl-difluoromethyl-ether (CTDE, CHF2OCF2CHFCl) triggered by OH radicals were researched by DFT methods and canonical variational transition state theory. The computational rate constant including small-curvature tunneling correction was in commendable agreement with the experimental data. Two hydrogen abstraction channels to form the alkyl radicals of C·F2OCF2CHFCl and CHF2OCF2C·FCl were observed, and the formation of CHF2OCF2C·FCl was more favorable than C·F2OCF2CHFCl in kinetics and thermodynamics. Subsequent evolution of CHF2OCF2C·FCl in the presence of NO and O2 indicated that the organic nitrate (CHF2OCF2CONO2FCl) was the stable product. The dechlorinate of alkoxy radical (CHF2OCF2C(O·)FCl) was the most favorable degradation channel and the estimated ozone depletion potential for CTDE relative to CFC-11 was 0.0204, which could lead to a consequence of ozone depletion. Computed atmospheric lifetime for CTDE was 3.69 years by considering the combined contributions from OH radicals and Cl atoms. The total radiative forcing and global warming potential of CTDE were respectively 0.547 W m-2 ppbv and 628.58 (100 years) at 298 K, suggesting that the contribution of CTDE to the greenhouse effect is moderate.
Introduction of a heterocyclic ring and an amino-ethyl-amino group to D-A type photosensitive dyes can modulate the lifetime of the charge separation generated in the D-A dyes as well as their electronic and UV-Vis absorption properties. Here we performed DFT and TDDFT calculations to study eleven derivatives of a triphenylamine-pyrimidine, MTPA-Pyc, in order to improve the performance of MTPA-Pyc as solar cell sensitizers. Five heterocyclic rings and an amino-ethyl-amino group were introduced on the styryl moiety of MTPA-Pyc. The results show that introduction of heterocyclic rings generally causes an absorption red-shift, but absorption intensity is reduced due to the increase of dihedral angle between the donor and acceptor. Further introduction of an amino-ethyl-amino group to these dyes with a heterocyclic ring modification disrupts the conjugation between donor and acceptor, which does not benefit the absorption but may have potential to increase the lifetime of charge separation of the dyes. This work identified two out of eleven dyes that have the best potential for solar cell applications.
Time-dependent density functional theory approach implemented at hybrid-B3LYP, GGA-PBE and DFTB levels of theory was used to model photoinjection in organic-dye/TiO2 quantum-dot to explore the prospects of improvement of DSSC. The photosensitizer used in this study consisted of six carbazole based organic dyes having acceptor as cyanoacrilic acid group and oligothiophene π-bridge spacer. The modifications were made in the dyes by increasing length of the spacer by adding thiophene and oxadiazole rings at different positions of the donor-acceptor bridge. The structural variations appeared to alter the electronic and optical properties of dyes studied via energy levels and excitation spectra. The UV-Vis spectra calculated for all the dyes in solvents exhibited a red shift in spectral peaks with increase in polarity of the solvents. The findings of the study pointed towards photoinjection of indirect nature studied in dye-(TiO2)96 complex for six different dyes. The substitution of oxadiazole ring in center and addition of a thiophene ring at the edge of the spacer produced two dyes which exhibited lowest injection energies of 0.11eV and 0.17 eV along with the regeneration energies of 1.18 eV and 1.12 eV respectively. The dyes reported herein may have promising applications in photoanode for enhancing the performance of DSSC.
In this paper, the Shannon entropy and Fisher information are studied for the screened Kratzer potential model (SKP). We calculated the position and momentum entropies for the screened Kratzer potential for its ground states as well as the first excited state. Our result shows that the sum of the position and momentum entropies satisfies the lower bound Berkner, Bialynicki–Birula and Mycieslki (BBM) inequality. Also, our results showed that decreasing Shannon entropy in the position space was complemented with an increasing Shannon entropy in the momentum space. Similarly, we evaluated for Fisher information and show that the Stam, Cramer-Rao inequalities are satisfied. The squeezing phenomena were also observed for certain values of the screening parameter α.
Alanine is a transfer standard dosimeter using in gamma-ray and electron beam calibration. One of the important factor affecting its dosimetric response is amount of humidity which can deviate the dosimetry expert from the exact value of absorbed doses. Ab initio molecular dynamics calculations were performed to determine the environmental effects on the EPR parameters of L-α-Alanine radicals in acidic and alkaline solutions. Similar to the closed-shell amino acid molecule alanine, the zwitterionic form of alanine radical is the stable form in the gas phase while the non-zwitterionic neutral alanine radical is not a stable structure. Geometric and EPR parameters of radicals in both gas and solution phases are found to be dependent on hydrogen bonding of water molecules with the polar groups and by dynamic solvation. Calculations on the optimized free radicals in the gas phase revealed that for neutral radical, hydrogen bonding to water molecules drives a decrease in the magnitudes of g-tensor components gxx and gyy without affecting neither gzz component nor the HFCCs. For the transfer from the gas to solution phase of the alanine radical anion is accompanied with an increase in the spin density on the carboxylic group’s oxygen atoms. However, for the neutral radical, this transfer from gas to solution phase is accompanied with the decrease in the spin density on oxygen atoms. Calculated isotropic HFCCs and g-tensor of all radicals were in good agreement with their experimental counterparts in both acidic and alkaline solutions, which enhances the confidence in our calculated results.
The existence and stability of MNg42+(Sb2F11−1)2 (Ng=Ar,Ne,He,M=Au, Ag, Cu) salt compounds are theoretically investigated in this study. This undertaking is carried out to address the following challenges: (1) synthesizing a bulk salt compound containing a noble gas lighter than krypton and (2) synthesizing the congeners of AuXe42+(Sb2F11−1)2 containing noble gases other than Xe. The reliability of our calculations on the MNg42+(Sb2F11−1)2 (Ng=Ar,Ne,He,M=Au, Ag, Cu) systems is assessed by benchmark calculations of the well-known AuXe42+(Sb2F11−1)2 salt. In the benchmark calculations, a two-pronged evaluation strategy, including direct and indirect evaluation methods, is used to theoretically investigate the spectroscopic constants of AuXe42+and the existence and stability of the AuXe42+(Sb2F11−1)2 salt. The validity of the theoretical calculation methods in the benchmark calculations of AuXe42+(Sb2F11−1)2 allows us to adopt a similar methodology to effectively predict the existence and stability of MNg42+(Sb2F11−1)2 (Ng=Ar,Ne,He,M=Au, Ag, Cu) salt compounds. Calculations based on the Born–Haber cycle using estimated lattice energies and some necessary ancillary thermochemical data show that MAr42+(Sb2F11−1)2 (M=Au, Ag, Cu) salt compounds can be synthesized. The upper-limit stable temperatures are estimated to be −224.43, −146.21, and −80.39 °C. The CuAr42+(Sb2F11−1)2salt compound is a promising candidate. Our calculations also show that the MNg42+(Sb2F11−1)2 (Ng=Ne,He,M=Au, Ag, Cu) salt compounds cannot be stabilized.
Carbazole (Cz) dimers in various cofacial conformations, including staggered (Stg), anti, and syn, were explored by means of ab initio calculations at SOS-MP2, SOS-CIS(D0), and additional coupled cluster calculation levels. As in other π-conjugated molecules, strong Cz excimers form in the syn conformation in both the S1 and T1 states, leading to significantly reduced optical excitation energies, whereas the dimers in the Stg and anti conformations, upon excitation, remain as simple excited dimers, showing similar optical energy gaps to that of the monomer. Being far more stable in the ground state, however, the Stg dimer turned out to be nearly isoenergetic to the syn dimer in the S1 state, and even more stable in the T1 state. In addition, a considerable potential energy barrier between the syn and Stg dimers was found in the calculated S1-state potential energy surface. Given that the ground-state intermolecular interactions are expected to govern the dimer conformations of Cz-based materials in the solid-state films of organic electronics, these results strongly demonstrate that the electronic excitation of Cz dimers do not necessarily lead to the strong excimer formation, unless Cz molecules were forced to be arranged in the syn conformation.
Pyrazolidines are very important compounds that widely exist in many natural products. Herein, we have employed high-level DFT calculations to systematically investigate the underlying mechanism of Cu(OTf)2 catalyzed [3+2] cycloaddition reactions that synthesis CF3substituted pyrazolidines. About eight possible initial configurations of the [3+2] reaction is considered and all relevant reactants, transition states and products are optimized. Based on these structures, IRC paths and the wavefunction analysis, we concluded that the Cu(OTf)2 catalyzed [3+2] cycloaddition follow a concerted asynchronous mechanism. The CN bond forms immediately after the formation of the CC bond. Among all eight reaction paths, the energy barrier for the [3+2] reaction that lead to the CF3substituted synpyrazolidine is the lowest one, ca. 3.2 kcal/mol, which might result in the diastereoselectivity that observed in experiment. We have also investigated the reaction processes that without Cu(OTf)2 molecule. The computational results indicate that the energy barriers that form the diastereoisomers are much closer and also larger than the Cu(OTf)2 catalyzed one. Therefore, Cu(OTf)2 catalyst plays an important role for the diastereoselectivity of the [3+2] cycloaddition reaction. Our present work not only gives the detail mechanism of the Cu(OTf)2 catalyzed [3+2] cycloaddition, but can also be helpful for the future designation of Cu(OTf)2 based cycloaddition processes.
Calcium carbonate (CaCO3) is an inorganic compound which is widely used in industry, chemistry, construction, ocean acidification and biomineralization due to its rich constituent on earth and excellent performance, in which calcium carbonate hydrates are important systems. In Z.Y. Zou et al’s work (Science, 2019, 363, 396–400), they found a novel calcium carbonate hemihydrate phase, but the structure stability, optical and mechanical properties has not been studied. In this work, the stability, electronic, optical, and mechanical properties of novel calcium carbonate hydrates were investigated by using the first-principles calculations within density functional theory (DFT). CaCO3·xH2O (x=1/2, 1 and 6) are determined dynamically stable phases by phonon spectrum, but the Gibbs energy of reaction of CaCO3·1/2H2O is higher than other calcium carbonate hydrates. That’s why the CaCO3·1/2H2O is hard to synthesize in the experiments. In addition, the optical and mechanical properties of CaCO3·xH2O (x=1/2, 1 and 6) are expounded in detail. It shows that the CaCO3·1/2H2O has the largest bulk modulus, shear modulus, Young’s modulus with the values 60.51, 36.56 and 91.28 GPa with respect to other two calcium carbonate hydrates investigated in this paper. This work will provide guidance for experiments and its applications, such as biomineralization, geology, and industrial processes.
On the example of forty ion pairs, the study demonstrates how the core-level binding energy values can be calculated and used to plot theoretical spectra at a low computational cost using density functional theory methods. Three approaches for obtaining the binding energy values are based on delta Kohn–Sham (ΔKS) calculations, 1s Kohn–Sham orbital energies, and atomic charges. The ΔKS results show a good agreement between the available experimental X-ray photoelectron data. 1s Kohn–Sham orbital energies and atomic charges also correlate with the ΔKS results.
We investigate the ability of mechanical and electronic density functional theory (DFT)-based embedding approaches to describe the solvent effects on nuclear magnetic resonance (NMR) shielding constants of the ⁹⁵Mo nucleus in the molybdate ion in aqueous solution. From the description obtained from calculations with two- and four-component relativistic Hamiltonians, we find that for such systems spin-orbit coupling effects are clearly important for absolute shielding values, but for relative quantities a scalar relativistic treatment provides a sufficient estimation of the solvent effects. We find that the electronic contributions to the solvent effects are relatively modest yet decisive to provide a more accurate magnetic response of the system, when compared to reference supermolecular calculations. We analyze the errors in the embedding calculations by statistical methods as well as through a real-space representation of NMR shielding densities, which are shown to provide a clear picture of the physical processes at play.
Local correlation methods rely on the assumption that electronic correlation is nearsighted. In this work, we develop a method to alleviate this assumption. The first step is to approximately decompose the electron correlation to the nearsighted and farsighted components based on the wavelength decomposition of electron correlation by Langreth and Perdew. The nearsighted component is then calculated using the recently developed embedded cluster density approximation (ECDA) which is a local correlation method formulated in the context of density functional theory. The farsighted component is calculated based on the system’s Kohn-Sham orbitals. The accuracy of this new method depends on the quality of the decomposition. We examined the method’s accuracy by patching the random phase approximation (RPA) correlation energy in a H₂₄ chain in which the electron correlation is highly nonlocal. This new method predicts bond stretching energies, RPA correlation potential, and Kohn-Sham eigenvalues in good agreement with the benchmarks. Our results demonstrate the importance of including the farsighted part of electron correlation for studying systems having nonlocal correlations.