Journal of Theoretical and Computational Chemistry

The time-dependent Schrödinger Equation (TDSE) is a parabolic partial differential equation (PDE) comparable to a diffusion equation but with imaginary time. Due to its first order time derivative, exponential integrators or propagators are natural methods to describe evolution in time of the TDSE, both for time-independent and time-dependent potentials. Two splitting methods based on Fer and/or Magnus expansions allow for developing unitary factorizations of exponentials with different accuracies in the time step △t. The unitary factorization of exponentials to high order accuracy depends on commutators of kinetic energy operators with potentials. Fourth-order accuracy propagators can involve negative or complex time steps, or real time steps only but with gradients of potentials, i.e. forces. Extending the propagators of TDSE's to imaginary time allows to also apply these methods to classical many-body dynamics, and quantum statistical mechanics of molecular systems.

Reactions of the carbon-chain radicals are of great importance in the combustion and astrophysical processes. The kinetics of the butadiynyl radical, C₄H, has received recent attention. While there has been sufficient knowledge concerning the oxidation of the ethynyl radical, C₂H, oxidation of the higher even-numbered members C_2nH (n > 1) is hardly known. In this paper, to enrich the C₄H-chemistry, we report the first study of the oxidation mechanism of C₄H. At the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(d,p)+ZPVE level, the potential energy surface (PES) survey is presented covering various product channels P₁(CO + HC₃O) (-152.7 kcal/mol), P₂(C₃H + CO₂) (-117.9), P₃(HCO + C₃O) (-108.5), P₄(HC₄O+³O) (-45.2), and P₅(OH + C₄O) (-33.2) accompanied by the master equation rate constant calculations. Despite the similarity in the PES, the kinetics of C₄H+³O₂ differs dramatically from that of the analogous C₂H+³O₂ reaction. For the C₄H+³O₂ reaction, the O-abstraction product P₄(HC₄O+³O) is almost the exclusive product, whereas the lowest C,O-exchange product P₁(CO + HC₃O) and other products have little importance. By contrast, the C₂H+³O₂ reaction favors the C,O-exchange product HCO + CO. Being overall barrierless and mainly associated with the molecular → atomic oxygen conversion, the C₄H+³O₂ reaction should play an important role in the soot formation and interstellar chemistry where C₄H is involved.

Some selected acceptor-Be hydrocarbon complexes have been considered for evaluation of second hyperpolarizability at different DFT functional. All the complexes have been found thermally stable and there is a significant increase of second hyperpolarizability compared to the ligands. Second hyperpolarizability has been explained in terms of charge transfer interaction. Co-operative interaction is required for maximization of second hyperpolarizability. Localized charge transfer in the vicinity of metal alone cannot increase second hyperpolarizability while charge delocalization over entire molecule is mandatory. Substitution by more electropositive metals e.g. Mg, Ca have greater enhancement in longitudinal component of γ. Basis set 6-311++G** is reasonable with respect to computational cost at aug-cc-pVnZ's (n = D, T, Q) for evaluation of second hyperpolarizability for the present complexes.

The multireference approach (CASSCF/CASPT2) combined with the contracted with atomic natural orbital (ANO-RCC-VTZP) basis set has been used to investigate systematically, the low-lying electronic states of (CH₃)₂CHS in C_s symmetry. The result of geometry optimization using CASSCF/ANO-RCC-VTZP shows that the theoretically determined geometric parameters and harmonic vibrational frequencies for the ground state X²A′ of (CH₃)₂CHS are in good agreement with previous studies. In addition, we also explored several cationic states adiabatically and found that the 1¹A′ state of (CH₃)₂CHS⁺ is unstable and converts to (CH₃)₂CSH⁺. The vertical and adiabatic ionization energies were obtained to compare with photoelectron spectroscopic data.

A model to describe the micro-structure of macromolecular microsphere composite (MMC) hydrogel is proposed in the framework of self-consistent mean field theory (SCMFT),¹ which is usually used to investigate copolymer. Based on the SCMFT approximation, a system of equations associated with the complex topology of MMC hydrogel is derived and solved by a new kind of relaxation algorithm successfully. From the numerical simulation of the model, we find that the two model parameters play important roles in describing the micro-structure of MMC hydrogel, the interactions between two species (polymer chains and MMS spheres) and the volume fraction of MMS spheres. The role of other model parameters on the structure of the MMC hydrogel is also discussed. The numerical results are consistent with the observation from the chemical experiments. Moreover, we also show some new micro-structures obtained by using the SCMFT model, but discovered in chemical experiments.

A detailed quantum chemical calculation is performed at the MP2(full)/6-311G* level to explore the mechanism of calcium carbonate thermal decomposition. Four microscopic pathways are identified. The rate constants of rate-determining steps in four pathways are calculated over a temperature range 298–1200 K. The calculating results show that only path A (R(CaCO₃) → IM₁ → P(CaO + CO₂)) and path B (R(CaCO₃) → IM_B1 → IM_B2 → P(CaO + CO₂) have contributions to the CaCO₃ thermal decomposition, and path A may be more favorable than path B. The present theoretical studies may provide useful information in understanding reaction mechanism of metal carbonates at the molecular level.

Optical properties of 60 anthraquinone derivatives in ethanol solution were characterized using QSPR multivariance linear regression (MLR). Application of parameter set comprising molecular descriptors and aromaticity indices allowed for highly accurate prediction of λ_max values of considered anthraquinone dyes. Due to the nature of chromophore the main predictive power comes from hardness what obviously is related to π → π* excitation and consequently the LUMO and HOMO energies. However, due to the fact that substituent effect is much more pronounced on π-electron delocalization in central quinonoid ring rather than in vicinal aromatic ones, the aromaticity indices describing properties of this ring can be effectively used in MLR procedure. The highest predictive power of the final models was found for three parameters regression equation comprising hardness, NICS(1)_zz and normalized Pozharsky geometry based index. The values of standard deviation were equal to 12.6 nm and square of adjusted correlation coefficient reached 0.97. This confirmed validity of the hypothesis that in cases in which molecular orbitals responsible for UV-Vis excitation belong, at least partly, to delocalized ring orbitals the UV-Vis imposed π → π* excitations can be related to the extent of π-electron delocalization expressed in term of geometric and magnetic indices.

To evaluate shear viscosity of ethylene glycol oligomers (EGO)/water binary mixture by means of coarse-grained molecular dynamics (CG-MD) simulations, we proposed the self-diffusion-coefficient-based parameterization of non-bonded interactions among CG particles. Our parameterization procedure consists of three steps: (1) determination of bonded potentials, (2) scaling for time and solvent diffusivity and (3) optimization of Lennard-Jones parameters to reproduce experimental self-diffusion coefficient and density data. With the determined parameters and the scaling relations, we evaluated shear viscosities of aqueous solutions of EGOs with various molecular weights and concentrations. Our simulation results are in close agreement with the experimental data. The largest simulation in this article corresponds to a 1.2 μs atomistic simulation for 100,000 atoms. Our CG model with the parameterization scheme for CG particles may be useful to study the dynamic properties of a liquid which contains relatively low molecular weight polymers or oligomers.

Density functional calculations have been employed to elucidate the structures of some six coordinated complexes of alloxan monohydrate with some d- and f-block metals. Alloxan monohydrate may exist in the mono-ionized or di-ionized form in its complexes, and both states were investigated. It is found that when the metal ion is coordinated to three bidentate ligands, the structures are nearly trigonal prismatic, but replacement of a bidendate ligand by two monovalent ligands changes the geometry to deformed octahedral. The metal-alloxanate bonding is largely ionic for the lanthanoids. The calculated vibrational frequencies are in agreement with the experimentally determined ones.

The feasibility of using cascade neural networks to correlate the bubble points of ternary mixtures containing ionic liquids (ILs) and classical solvents was investigated. The systems investigated consisted of the ILs from the alkylimidazolium family with either the chloride, tetrafluoroborate, methylsulfate, or ethyl sulfate anions. The classical solvents investigated included 1-propanol, 2-propanol, ethanol, ethyl ethanoate, and water. A total of 272 bubble points were used in the training (205 data points) and testing (67 data points) stages. The optimized network comprised of nine neurons in the hidden layer and used the tangent-sigmoid and purelin functions as the activation functions in the hidden and output layers, respectively. This proposed network was able to correlate the ternary bubble points of both the training (AARD % = 0.13% and R² = 0.9921) and the testing (AARD % = 0.15% and R² = 0.9873) subsets with good accuracy, revealing the good correlative capability of the network. The statistical error analysis of all the data indicated an overall absolute average relative deviation (AARD %), mean square error (MSE) and correlation coefficient (R²) of 0.13%, 0.44% and 0.9909%, respectively.

Initial-state-selected time-dependent wave packet dynamics studies have been performed for the H₂ + NH₂ → H + NH₃ reaction with a seven-dimensional model on a new interpolated ab initio potential energy surface (PES). The PES is constructed using modified Shepard interpolation Scheme and contains 1967 data points with ab initio calculations carried out on UCCSD(T)/aug-cc-pVTZ level. In the seven-dimensional model, NH₂ group keeps C_2v symmetry and two NH bonds are fixed at their equilibrium values. The total reaction probabilities are calculated when (1) the two reactants are initially at their ground states; (2) NH₂ bending mode is excited, and (3) H₂ is on its first vibrational excited state. The integral cross sections are also reported for these initial states with centrifugal-sudden approximation. Thermal rate constants are calculated for the temperature range of 200–2000 K and compared with the previous calculated values and available experimental data. Good agreements between theory and experiments for the rate constants at intermediate temperature are achieved on this PES.

The ground-state structures and absorption spectra of three dyes, carbazole, phenothiazine and diphenylamine, were studied by the density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The strong absorption peak, electron transition as well as the energy levels of molecular orbitals were obtained and compared in the gas and solvent phase. Furthermore, we further calculated the effect of the expanding conjugated bridge on the absorption spectra and the energy levels of molecular orbitals. Visualized method of charge difference density (CDD) was used to show the direction of charge transfer in the molecules of interest during photo-excitation.

Ab initio studies of the small (AlSb, InSb, GaSb) clusters is performed to investigate the changes in structural, vibrational and electronic properties. The calculated results clearly demonstrate that any change in the electronic configuration of the neutral clusters gives rise to noticeable changes in the structure. The structures of the neutral, positively and negatively charged clusters are fully optimized, taking the importance of the symmetry into account in all the cases. The changes in vibrational properties are explained keeping in mind the change in the interatomic distances due to any change in the electronic configuration.

Dielectric constants and Seebeck coefficients for semiconductor materials are studied by thermodynamic method plus ab initio quantum density functional theory (DFT). A single molecule which is formed in semiconductor material is treated in gas phase with molecular boundary condition and then electronic polarizability is directly calculated through Mulliken and atomic polar tensor (APT) density charges in the presence of the external electric field. This electronic polarizability can be converted to dielectric constant for solid material through the Clausius–Mossotti formula. Seebeck coefficient is first simulated in gas phase by thermodynamic method and then its value divided by its dielectric constant is regarded as Seebeck coefficient for solid materials. Furthermore, unit cell of semiconductor material is calculated with periodic boundary condition and its solid structure properties such as lattice constant and band gap are obtained. In this way, proper DFT function and basis set are selected to simulate electronic polarizability directly and Seebeck coefficient through chemical potential. Three semiconductor materials Mg₂Si, β-FeSi₂ and SiGe are extensively tested by DFT method with B3LYP, BLYP and M05 functionals, and dielectric constants simulated by the present method are in good agreement with experimental values. Seebeck coefficients simulated by the present method are in reasonable good agreement with experiments and temperature dependence of Seebeck coefficients basically follows experimental results as well. The present method works much better than the conventional energy band structure theory for Seebeck coefficients of three semiconductors mentioned above. Simulation with periodic boundary condition can be generalized directly to treat with doped semiconductor in near future.

In this work, the antioxidant properties of the series of 10 aminothiazol hydroxyl coumarin derivatives have been investigated with DFT/B3LYP method. For these antioxidants all reaction enthalpies related to HAT, SPLET, SET-PT mechanisms were calculated in the gas phase and polar solvents. Based on calculated reaction enthalpies (BDE, IP and PA values) the derivations 2, 3 and 4 have the highest antioxidant activity among the studied compounds. Calculated results show that derivations 7, 5 and 6 have the lowest antioxidant activity. The observed theoretical trends for antioxidant activities of studied compounds were similar to trends of previous experimental studies that OH50, TAC50, IC50, CE50 values have been used as a benchmark for measuring the antioxidant properties of these compounds. These results can be useful in synthesis of novel aminothiazol hydroxycoumarin derivatives with high antioxidant activity. Calculated results show that BDE, PA and IP values of studied derivations have linear dependence with structure parameters such as R(O–H), q(O) and E_HOMO. These observed linear dependences can be useful in synthesis of novel aminothiazol hydroxyl coumarin derivatives with high antioxidant activity. For studied compounds, results indicated that the SPLET and HAT mechanisms represent the thermodynamically preferred mechanism both, in solvent and the gas phase.