Computational & Theoretical Chemistry

In this study, we have investigated possible role as iron corrosion inhibitor of four benzotriazole derivatives, BTA, BTA-C1, BTA-C4, BTA-C6, using DFT calculations at B3LYP/6-311G** level of theory. For this purpose, we have determined some structural and electronic parameters such as HOMO and LUMO orbital energies, energy gap, electron affinity, ionization potential, hardness, softness, absolute electronega-tivity, chemical potential, electrophilicity index, fractions of electrons transferred and back donation, logP, molecular surface area, polar surface area, molecular volume, molar refractivity and have compared with experimental literature results. We have also computed and discussed the interaction energy of the inhibitors with iron surface. The calculated parameters are closely related to the inhibition efficiencies, and have compared with experimental literature values using linear regression analysis to determine the most effective parameters on inhibition efficiency.

In the present account the nature of aromaticity/antiaromaticity of fourteen metallic complexes/clusters are reexamined. These species were classified as aromatic by means of different nucleus independent chemical shift- (NICS) based approaches, previously. Visualization of the current density and magnetizability of atomic basins reveals that none of the studied systems are magnetic aromatic, i.e. sustain diamagnetic ring current. It is demonstrated that negative NICS values near the ring plane of the studied molecules originates from remarkably strong local paramagnetic current around their transition metal atom nuclei. This phenomenon has been observed only for Sc3– all-metal cluster but current study demonstrates that the influence of the local paramagnetic currents around transition metal atoms on NICS is a general phenomenon that must be carefully considered prior to classification of the metallic systems as aromatic. Furthermore, this study suggests that NICS is not a reliable aromaticity index for transition-metal clusters/molecules.

In the present account we investigate a theoretical link between bond length, electron sharing, and bond energy within the context of quantum chemical topology theories. The aromatic stabilization energy, ASE, was estimated from this theoretical link without using isodesmic reactions for the first time. The ASE values obtained from our method show a meaningful correlation with the number of electrons contributing in the aromaticity. This theoretical link demonstrates that structural, electronic, and energetic criteria of aromaticity –ground-state aromaticity– belong to the same class and guarantees that they assess the same property as aromaticity. Theory suggests that interatomic exchange-correlation potential, obtained from the theory of Interacting Quantum Atoms (IQA), is linearly connected to the delocalization index of Quantum Theory of Atoms in Molecules (QTAIM) and bond length through a first order approximation. Our study shows that the relationship between energy-structure-electron sharing marginally deviates from the ideal linear form expected from the first order approximation. The observed deviation from linearity was attributed to a different contribution of exchange-correlation into the bond energy for the σ- and π-frameworks. Finally, we proposed two-dimensional energy-structure-based aromaticity indices in analogy to the electron sharing indices of aromaticity.

Quantum chemical calculations using density functional theory and correlated ab initio methods of the 10 p-electron systems (N 6 H 6) 2 + and C 2 N 4 H 6 show that the planar forms are no minima on the potential energy surfaces. The twisted ring structures of the two species are energy minima, but acyclic isomers are much lower in energy. The planar geometries sustain strong diamagnetic ring current comparable with that of benzene. In contrast, the calculated multicenter normalized Giambiagi electron delocalization index I NG suggests that p-delocalization in planar (N 6 H 6) 2 + and C 2 N 4 H 6 is much weaker than in benzene. Since aromaticity is synonymous for a particular stability of cyclic delocalized systems, it may be stated that calculation or measurement of magnetic chemical shifts due to induced ring currents is not a reliable method to ascertain the aromatic character of a molecule. Aromatic compounds exhibit ring current induced magnetic shielding, but the reverse conclusion that ring current induced magnetic shielding identifies aroma-ticity is not justified. Furthermore, the 4n + 2 rule as indicator of aromatic stabilization should only be used in conjunction with the ring size; the nature of the occupied p orbitals must always be examined.

Interatomic magnetizability provides insight into the
extent of electronic current density between two adjacent atomic
basins. By studying a number of well-known aromatic, nonaromatic,
and antiaromatic molecules, it is demonstrated that interatomic
magnetizability (bond magnetizability) not only is able to verify the
exact nature of aromaticity/antiaromaticity among different molecules,
but also can distinguish the correct aromaticity order among
sets of aromatic/antiaromatic molecules. The interatomic magnetizability
is a direct measure of the current flux between two adjacent
atomic basins and is the first QTAIM-derived index that evaluates
aromaticity based on a response property, that is, magnetizability. Bond magnetizability is easy to compute, straightforward to
interpret, and can be employed to evaluate the pure π- or σ-orbital contributions to magnetic aromaticity.

Developing a comprehensive method to compute bond orders is a problem that has eluded chemists since Lewis's pioneering work on chemical bonding a century ago. Here, a computationally efficient method solving this problem is introduced and demonstrated for diverse materials including elements from each chemical group and period. The method is applied to non-magnetic, collinear magnetic, and non-collinear magnetic materials with localized or delocalized bonding electrons. Examples studied include the stretched O 2 molecule, 26 diatomic molecules, 3d and 5d transition metal solids, periodic materials with 1 to 8748 atoms per unit cell, a biomolecule, a hypercoordinate molecule, an electron deficient molecule, hydrogen bound systems, transition states, Lewis acid–base complexes, aromatic compounds, magnetic systems, ionic materials, dispersion bound systems, nanostructures, and other materials. From near-zero to high-order bonds were studied. Both the bond orders and the sum of bond orders for each atom are accurate across various bonding types: metallic, covalent, polar-covalent, ionic, aromatic, dative, hypercoordinate, electron deficient multi-centered, agostic, and hydrogen bonding. The method yields similar results for correlated wavefunction and density functional theory inputs and for different S Z values of a spin multiplet. The method requires only the electron and spin magnetization density distributions as input and has a computational cost scaling linearly with increasing number of atoms in the unit cell. No prior approach is as general. The method does not apply to electrides, highly time-dependent states, some extremely high-energy excited states, and nuclear reactions.

Bond order quantifies the number of electrons dressed-exchanged between two atoms in a material and is important for understanding many chemical properties. Diatomic molecules are the smallest molecules possessing chemical bonds and play key roles in atmospheric chemistry, biochemistry, lab chemistry, and chemical manufacturing. Here we quantum-mechanically calculate bond orders for 288 diatomic molecules and ions. For homodiatomics, we show bond orders correlate to bond energies for elements within the same chemical group. We quantify and discuss how semicore electrons weaken bond orders for elements having diffuse semicore electrons. Lots of chemistry is effected by this. We introduce a first-principles method to represent orbital-independent bond order as a sum of orbital-dependent bond order components. This bond order component analysis (BOCA) applies to any spin-orbitals that are unitary transformations of the natural spin-orbitals, with or without periodic boundary conditions, and to non-magnetic and (collinear or non-collinear) magnetic materials. We use this BOCA to study all period 2 homodiatomics plus Mo 2 , Cr 2 , ClO, ClO À , and Mo 2 (acetate) 4. Using Manz's bond order equation with DDEC6 partitioning, the Mo-Mo bond order was 4.12 in Mo 2 and 1.46 in Mo 2 (acetate) 4 with a sum of bond orders for each Mo atom of $4. Our study informs both chemistry research and education. As a learning aid, we introduce an analogy between bond orders in materials and message transmission in computer networks. We also introduce the first working quantitative heuristic model for all period 2 homodiatomic bond orders. This heuristic model incorporates s-p mixing to give heuristic bond orders of 3 4 (Be 2), 1 3 4 (B 2), 2 3 4 (C 2), and whole number bond orders for the remaining period 2 homodiatomics.

The DDEC6 method is one of the most accurate and broadly applicable atomic population analysis methods. It works for a broad range of periodic and non-periodic materials with no magnetism, collinear magnetism, and non-collinear magnetism irrespective of the basis set type. First, we show DDEC6 charge partitioning to assign net atomic charges corresponds to solving a series of 14 Lagrangians in order. Then, we provide flow diagrams for overall DDEC6 analysis, spin partitioning, and bond order calculations. We wrote an OpenMP parallelized Fortran code to provide efficient computations. We show that by storing large arrays as shared variables in cache line friendly order, memory requirements are independent of the number of parallel computing cores and false sharing is minimized. We show that both total memory required and the computational time scale linearly with increasing numbers of atoms in the unit cell. Using the presently chosen uniform grids, computational times of ~9 to 94 seconds per atom were required to perform DDEC6 analysis on a single computing core in an Intel Xeon E5 multi-processor unit. Parallelization efficiencies were usually >50% for computations performed on 2 to 16 cores of a cache coherent node. As examples we study a B-DNA decamer, nickel metal, supercells of hexagonal ice crystals, six X@C60 endohedral fullerene complexes, a water dimer, a Mn 12-acetate single molecule magnet exhibiting collinear magnetism, a Fe4O12N4C40H52 single molecule magnet exhibiting non-collinear magnetism, and several spin states of an ozone molecule. Efficient parallel computation was achieved for systems containing as few as one and as many as >8000 atoms in a unit cell. We varied many calculation factors (e.g., grid spacing, code design, thread arrangement, etc.) and report their effects on calculation speed and precision. We make recommendations for excellent performance.

Abstract: The inhibition activity of three imine compounds with sulphanilic acid was done using density
functional theory (DFT) method at the B3LYP and O3LYP/6-31+G(d,p) basis set level in order to obtain
the different inhibition efficiencies and reactive sites of corresponding compounds as corrosion inhibitors.
Some structural and electronic properties such as the frontier molecular orbital (HOMO and LUMO)
energies, energy gap, charge distribution, electron affinity, ionization potential, dipole moment, hardness,
softness, the absolute electronegativity, the electrophilicity index and the fractions of electrons transferred
from imine molecules to Fe were obtained from the calculation results at B3LYP and O3LYP/6-31+G(d,p)
level of theory. These parameters are closely related to the inhibition efficiency. The calculated these
parameters are compared with experimental inhibition efficiency using linear regression analysis to
determine the most effective parameter on inhibition efficiency.

Quantum  calculations  of  the  physical  properties  (electronic  and  vibrational),  based  on  density  functional  theory  (DFT) method at B3LYP/6-31G** level of theory, were performed, by means of the Gaussian 09 set of programs, to investigate the effect of the addition of the radical CN on pyridazine molecules. The best geometry, the total energy, frontier molecular orbital energies  (HOMO and HUMO),  energy  gap,  ionization  potential,  electron  affinity,  electronegativity,  chemical  hardness,  chemical  softness,  electrophilicity, dipole moment and the harmonic vibration frequencies were calculated and discussed for the study molecules. The electronic properties are computed by two different methods, a finite difference approximation and Koopman’s theorem. The study clearly shows that adding the radicals CN cause decreased the energy gap and the chemical hardness, and increase the electrophilicity. Therefore, the p resence of these  radicals  improves  the  conductivities  and  enhances  the  solubility  and  reactivity.  All  the  results  indicate  that  the  molecule 2,3-(CH) ₄N₂ (CN)2 is the best option for n-type organic semiconductors.

The influence of presence of counter ions and π-complexation with benzene on the bonding and
magnetic properties of Al4
2, the most studied all-metal cluster, is studied here. It is shown that
complexation by either counter ions or benzene decreases the delocalization index between
Al atoms and the magnitude of bond magnetizability, that is a Quantum Theory of Atoms in
Molecules, QTAIM, -based magnetic index of aromaticity. Benzene forms two types of π-complexes
with the Al4 framework; CH–π (T-shaped) complexes and parallel π–π stacking (PPS) complexes.
It is shown that variation in the p-charge of the Al4 framework affects the relative stability of the
T-shaped/PPS complexes. Free Al4
2 forms a stable T-shaped anion–π complex with benzene but
in the presence of cations, formation of PPS complexes is more favourable, energetically. It is
suggested that this property could be used for designing molecular switches and tuneable anion
sensors.

In an attempt to eradicate the challenges of fungi infections confronting Nigeria health sector by identification of novel anti-fungi agents, this study was embarked on to evaluate the antifungal activity of Garcinia kola seed against Candida albican, Aspergillus flaws, Fusarium oxysperium and Aspergilus niger using experimental and theoretical approaches. Garcinia kola seeds were bought from Okitipupa local market in Ondo State. It was authenticated, dehusked, sliced and air dried. The air dried sample was pulverized and extracted with methanol. The antifungal activity of the Garcinia kola seed extract at different concentrations were investigated using standard method. The zones of inhibition of the extract against test fungi ranges from 6 to 23mm while that of the control (amphotericin B) ranges from 17 to 29 mm. The broad spectrum of inhibition against test fungi exhibited by Garcinia kola seed revealed Garcinia kola seed as an important ingredient needed in pharmaceutical industry for the design of novel antifungal agent. The order of potency of Garcinia kola seed extract against test fungal at 75mg/ml was Candida albican, >Aspergilus niger > Aspergillus flaws > Fusarium oxysperium. Judging from the broad spectrum of inhibition and potency of Garcinia kola seed against test fungi, this study therefore ascertain Int. 42 that Garcinia kola seed could be used as a good therapeutic agent for the prevention or cure of fungi infections.

Net atomic charges (NACs) are widely used in all chemical sciences to concisely summarize key information about the partitioning of electrons among atoms in materials. The objective of this article is to develop an atomic population analysis method that is suitable to be used as a default method in quantum chemistry programs irrespective of the kind of basis sets employed. To address this challenge, we introduce a new atoms-in-materials method with the following nine properties: (1) exactly one electron distribution is assigned to each atom, (2) core electrons are assigned to the correct host atom, (3) NACs are formally independent of the basis set type because they are functionals of the total electron distribution, (4) the assigned atomic electron distributions give an efficiently converging polyatomic multipole expansion, (5) the assigned NACs usually follow Pauling scale electronegativity trends, (6) NACs for a particular element have good transferability among different conformations that are equivalently bonded, (7) the assigned NACs are chemically consistent with the assigned atomic spin moments, (8) the method has predictably rapid and robust convergence to a unique solution, and (9) the computational cost of charge partitioning scales linearly with increasing system size. We study numerous materials as examples: (a) a series of endohedral C 60 complexes, (b) high-pressure compressed sodium chloride crystals with unusual stoichiometries, (c) metal–organic frameworks, (d) large and small molecules, (e) organometallic complexes, (f) various solids, and (g) solid surfaces. Due to non-nuclear attractors, Bader's quantum chemical topology could not assign NACs for some of these materials. We show for the first time that the Iterative Hirshfeld and DDEC3 methods do not always converge to a unique solution independent of the initial guess, and this sometimes causes those methods to assign dramatically different NACs on symmetry-equivalent atoms. By using a fixed number of charge partitioning steps with well-defined reference ion charges, the DDEC6 method avoids this problem by always converging to a unique solution. These characteristics make the DDEC6 method ideally suited for use as a default charge assignment method in quantum chemistry programs.

Polarizabilities and London dispersion forces are important to many chemical processes. Force fields for classical atomistic simulations can be constructed using atom-in-material polarizabilities and C n (n ¼ 6, 8, 9, 10.) dispersion coefficients. This article addresses the key question of how to efficiently assign these parameters to constituent atoms in a material so that properties of the whole material are better reproduced. We develop a new set of scaling laws and computational algorithms (called MCLF) to do this in an accurate and computationally efficient manner across diverse material types. We introduce a conduction limit upper bound and m-scaling to describe the different behaviors of surface and buried atoms. We validate MCLF by comparing results to high-level benchmarks for isolated neutral and charged atoms, diverse diatomic molecules, various polyatomic molecules (e.g., polyacenes, fullerenes, and small organic and inorganic molecules), and dense solids (including metallic, covalent, and ionic). We also present results for the HIV reverse transcriptase enzyme complexed with an inhibitor molecule. MCLF provides the non-directionally screened polarizabilities required to construct force fields, the directionally-screened static polarizability tensor components and eigenvalues, and environmentally screened C 6 coefficients. Overall, MCLF has improved accuracy compared to the TS-SCS method. For TS-SCS, we compared charge partitioning methods and show DDEC6 partitioning yields more accurate results than Hirshfeld partitioning. MCLF also gives approximations for C 8 , C 9 , and C 10 dispersion coefficients and quantum Drude oscillator parameters. This method should find widespread applications to parameterize classical force fields and density functional theory (DFT) + dispersion methods.

ABSTRACT We present an implementation of the Polarizable Continuum Model (PCM) in combination with the Second-Order Polarization Propagator Approximation (SOPPA) electronic structure method. In analogy with the most common way of designing ground state calculations based on a Second-Order Møller–Plesset (MP2) wave function coupled to PCM, we introduce dynamical PCM solvent effects only in the Random Phase Approximation (RPA) part of the SOPPA response equations while the static solvent contribution is kept in both the RPA terms as well as in the higher order correlation matrix components of the SOPPA response equations. By dynamic terms, we refer to contributions that describe a change in environmental polarization which, in turn, reflects a change in the core molecular charge distribution upon an electronic excitation. This new combination of methods is termed PCM–SOPPA/RPA. We apply this newly defined method to the challenging cases of solvent effects on the lowest and intense electronic transitions in o-, m- and p-nitroaniline and o-, m- and p-nitrophenol and compare the performance of PCM–SOPPA/RPA with more conventional approaches. Compared to calculations based on time-dependent density functional theory employing a range-separated exchange–correlation functional, we find the PCM–SOPPA/RPA approach to be slightly superior with respect to systematicity. On the other hand, the absolute values of the predicted excitation energies are largely underestimated. This – however – is a well-know feature of the SOPPA model itself and is not connected to its combination with the PCM.

We have quantum chemically investigated how solvation influences the competition between the SN2 and E2 pathways of the model F– + C2H5Cl reaction. The system is solvated in a stepwise manner by going from the gas phase, then via microsolvation of one to three explicit solvent molecules, then last to bulk solvation using relativistic density functional theory at (COSMO)-ZORA-OLYP/QZ4P. We explain how and why the mechanistic pathway of the system shifts from E2 in the gas phase to SN2 upon strong solvation of the Lewis base (i.e., nucleophile/protophile). The E2 pathway is preferred under weak solvation of the system by dichloromethane, whereas a switch in reactivity from E2 to SN2 is observed under strong solvation by water. Our activation strain and Kohn–Sham molecular orbital analyses reveal that solvation of the Lewis base has a significant impact on the strength of the Lewis base. We show how strong solvation furnishes a weaker Lewis base that is unable to overcome the high characteristic distortivity associated with the E2 pathway, and thus the SN2 pathway becomes viable.

A host of important performance properties for metal-organic frameworks (MOFs) and other complex materials can be calculated by modeling statistical ensembles. The principle challenge is to develop accurate and computationally efficient interaction models for these simulations. Two major approaches are (i) ab initio molecular dynamics in which the interaction model is provided by an exchange-correlation theory (e.g., DFT + dispersion functional) and (ii) molecular mechanics in which the interaction model is a parameterized classical force field. The first approach requires further development to improve computational speed. The second approach requires further development to automate accurate forcefield parameterization. Because of the extreme chemical diversity across thousands of MOF structures, this problem is still mostly unsolved today. For example, here we show structures in the 2014 CoRE MOF database contain more than 8 thousand different atom types based on first and second neighbors. Our results showed that atom types based on both first and second neighbors adequately capture the chemical environment, but atom types based on only first neighbors do not. For 3056 MOFs, we used density functional theory (DFT) followed by DDEC6 atomic population analysis to extract a host of important forcefield precursors: partial atomic charges; atom-in-material (AIM) C 6 , C 8 , and C 10 dispersion coefficients; AIM dipole and quadrupole moments; various AIM polarizabilities; quantum Drude oscillator parameters; AIM electron cloud parameters; etc. Electrostatic parameters were validated through comparisons to the DFT-computed electrostatic potential. These forcefield precursors should find widespread applications to developing MOF force fields.

The molecular structures, conformational mobilities, harmonic vibrational frequencies and thermochemistry of the FC(O)OOO(O)CF, FS(O2)OOO(O2)SF and FC(O)OOO(O2)SF trioxides were studied by ab initio and density functional methods. The potential energy curves for the internal rotations were calculated using the B3LYP hybrid functional with the 6-311+G(3df) basis set. The equilibrium conformations are characterized by skew structure with dihedral angles COOO and SOOO of about 90° and 95°, respectively. The most stable structures were also calculated using the G3(MP2)B3 and G4(MP2) ab initio methods and with the functional M06-2X/6-311+G(3df). Average standard enthalpies of formation at 298 K derived for FC(O)OOO(O)CF, FS(O2)OOO(O2)SF and FC(O)OOO(O2)SF from isodesmic reactions energies calculated at the G3(MP2)//B3LYP/6-311++G(3df,3pd) and G4(MP2) levels of theory are predicted to be −195.4, −255.6, and −226.5 kcal mol−1. From these values, OO bond dissociation enthalpies of 25–33 kca...