Journal of Theoretical and Computational Chemistry

Organic nonlinear optical co-crystals 2-aminopyrimidine benzoic acid (2APB) and 2-aminopyrimidine succinic acid (2APS) have been successfully grown by slow solvent evaporation method at room temperature. The structural characterization of the grown crystals was carried out by single crystal X-ray diffraction. Fourier transform infrared spectroscopy (FT-IR) and FT-Raman spectra of the grown crystals are recorded and the observed vibrational frequencies are assigned. The hybrid computational calculations are carried out by Hartree–Fock (HF) and density functional theory (DFT) (3-parameter, Lee-Yang-Parr (B3LYP)) methods with 6-311$++$G(d,p) basis sets and the corresponding results are tabulated. The geometrical parameters of the molecules also have been analyzed. The computed vibrational spectra were compared with experimental result which shows appreciable agreement. The chemical hardness, electronegativity, chemical potential and electrophilicity index were determined by highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) plot. The grown crystals were subjected to population analysis, thermal, linear and nonlinear optical studies of materials. The calculated second-order hyperpolarizability values of these materials are nearly two to four times that of urea.

The interatomic interactions in a diatomic molecule can be fairly modeled by the Morse potential. Short range interactions of the molecule with the neighboring environment can be analyzed by modifying this potential by delta functions. Energy spectra and radial matrix elements have been calculated using an accurate nine-point finite-difference method for such an interacting homonuclear diatomic molecule. The effect of the strength and position of a single delta function interaction on the alignment of this molecule has been studied. The dependence of alignment on the strength of applied field has also been analyzed.

The equilibrium geometry of the 2,5-bis-[2-(pyridin-2-yl)-vinyl]-1$H$-pyrrole calculated at the MP2/6-311$++$G($d$,$p$) level of theory evidences the breaking of one of the components in the three-centered intramolecular hydrogen bond due to the steric strain. For this reason, the three-centered intramolecular hydrogen bonding turns out to be asymmetric interaction involving the major and minor components. However, the reversible switching between these components under an external impact is also possible. Two different stable states with unequal geometric and electronic structure are observed in the derivatives of the 2,5-bis-[2-(pyridin-2-yl)-vinyl]-1$H$-pyrrole. These molecules represent novel molecular switches operating due to the pendulum-like transition between the nonequivalent two-centered components of the overcrowded three-centered intramolecular hydrogen bond. Implantation of hydrogen bond as a unit of the molecular scale device enhances potential of molecular electronics and could serve as a step towards the construction of artificial biological ensembles.

Pair correlation methods are able to achieve highly accurate solutions for chemical problems. Unfortunately, their applicability is generally restricted to medium-sized molecules due to storage requirements and computational costs. These restrictions can be partly overcome by local correlation methods. These methods use physical and mathematical criteria to decide which interactions are of such a long range that they do not have to be computed and saved. In our new ansatz, we define an alternative way towards local correlation. The range of interactions is strictly bound to the decay of integrals over Gaussian type geminals in the atomic orbital basis. The number of variables is reduced by orders of magnitude applying an efficient contraction scheme, leading to a naturally local representation of correlation effects. This scheme is extended by orbital optimization to describe multi-reference problems and explicit correlation to improve the basis set convergence.

The dynamic amphiphilic behavior of N-octyl-N-quaternized chitosan derivatives in aqueous solution is investigated using molecular dynamics (MD) simulations. It is found that quaternization decreases the intra-chain hydrogen bond formation which leads to reduced rigidity of the chitosan backbone. The effect of octyl substitution is much less pronounced. Analysis of hydrogen bonding reveals the presence of a hydrogen bond within the quaternized glucosamine unit, which causes the distortion of the usual chair conformation. Also, H-bond formation with the solvent water molecules was found to stabilize the intra-chain HO3-O5 hydrogen bond. Additionally, an aqueous solution containing the 10%-N-octyl-50%-N-quaternized chitosan derivative (1O5QCS) and the anti-cancer drug 10-hydroxycamptothecin (10-HCPT) was also investigated using MD simulations. It was found that van der Waals and electrostatic forces have virtually equal contributions to the nonbonded interactions responsible for complexation. Furthermore, H-bond formation between drug and drug carrier contributes to lactone ring stability and subsequent bioavailability.

In order to control the structure and size distribution of silica inclusions in high purity steel, it is necessary to understand the nucleation mechanism of solid silica in molten steel. In high temperature reactions, crystallization begins with nucleation, which plays a crucial role in determining the structure and size of solid products. Nucleation originates in the formation of product intermediates. The structure and thermodynamic properties of silica clusters as the intermediate of solid silica products during nucleation were calculated by density functional theory. Comparison of thermodynamic properties of silica clusters and silicon deoxidation equilibrium experiment in liquid iron results shows the silica clusters with most of the dissolved silicon and oxygen in equilibrium; the molten iron silicon deoxidation reaction ([Si] $+$ [O] $={\mathrm{\text{SiO}}}_{2(s)})$ cannot reach thermodynamic equilibrium state, and some deoxidation products could only exist in the form of silica clusters but not the solid silica. Therefore, the nucleation process of solid silica in Fe-O-Si melt can be considered as a two-step process with silica clusters as intermediates. Finally, there are two paths of solid silica inclusion formation: one is that in the molten iron, the dissolved silicon reacts with the dissolved oxygen to form silica clusters, and clusters further nucleate and grow up; the second one is that silica clusters directly crystallized during the cooling process of the melt.

$N$-tert-butylmethanimine $N$-oxide is a potent spin-trapping probe for biologically important radicals, and this nitrone undergoes complete regioselective cycloadditions to less electron-deficient monosubstituted olefins. In the present study, solvent effects on the cycloaddition of this nitrone to styrene have been theoretically studied in terms of the global properties of the reactants, electrophilic and nucleophilic Parr function analysis and the activation and reaction energies of located transition states and products. Formation energies of optimized radical adducts were computed to determine their stabilities in different solvents. The cycloaddition is predicted to be completely ortho regioselective which is in complete agreement with experiments and involved earlier C–C bond formation owing to the insufficient depopulation of $\beta$-conjugated carbon atom in styrene and shows varying asymmetry indices in different solvents. Decrease in activation parameters and increase in stability of cycloadducts are predicted with decreasing solvent polarity. Aqueous media destabilize the radical adducts as predicted from the calculated formation energy, enthalpy and free energy of reaction. Hydroxyl as well as methyl radical adducts are predicted to be more stable than superoxide anion radical adducts. These predictions are in complete agreement with the experiments.

Density-function-theory calculations were performed to find the performance of a series of 2,2’-(1,2,4,5-tetrazine-3,6diyl) dihydrazinecarboxamide-based nitrogen-rich macrocyclic compounds as an energetic plasticizer. Reliable methods have been used to predict energetic properties such as gas-phase and solid-phase heat of formation (HOF), density, detonation velocity, detonation pressure, explosive power, heat of combustion, heat of detonation, specific impulse, flame temperature, brisance, and sensitivity. All the designed macrocycles possess a nitrogen content of over 48%. The designed compounds show positive HOFs and high predicted densities ranging from 1.81g/cm³ to 1.86g/cm³. The predicted properties were compared with GAP, polyGLYN and their monomers which establish the designed macrocycles of interest for further investigations concerning their suitability as plasticizers in energetic formulations.

Chlorobenzene is one of the Persistent Organic Pollutants (POPs) threatening human health. It is significant to study the degradation mechanism under external electric fields. Based on the density functional theory, the physical and dissociation properties including C–Cl bond length, total energy, dipole moment, frontier orbital energy, energy gap, IR spectrum, UV-vis absorption spectrum and potential energy curve are studied under external electric fields. According to these results, it is found that the C–Cl bond length becomes longer and tends to break with the increase of external electric field and the energy gap decreases with the increase of positive as well as negative external electric field. Moreover, the dissociation barrier in potential energy curve decreases and equilibrium bond length increases with increase of positive external electric field. And when external electric field reaches 0.040 atomic units ($\approx 2.057\times 10^{10}\mathrm{\text{V}}\cdot {\mathrm{\text{m}}}^{-1}$, 1 atomic $\mathrm{\text{unit}}=5.142\times 10^{11}\mathrm{\text{V}}\cdot {\mathrm{\text{m}}}^{-1})$, the dissociation barrier disappears which means that degradation of chlorobenzene occurs under strong external electric field due to the breakage of C–Cl bond. These results provide important references for studying the degradation mechanism of chlorobenzene under strong external electric fields.