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.

We report re-optimization of a recently proposed long-range corrected (LC) hybrid density functional [J.-D. Chai and M. Head-Gordon, J. Chem. Phys., 2008, 128, 084106] to include empirical atom-atom dispersion corrections. The resulting functional, oB97X-D yields satisfactory accuracy for thermochemistry, kinetics, and non-covalent interactions. Tests show that for non-covalent systems, oB97X-D shows slight improvement over other empirical dispersion-corrected density functionals, while for covalent systems and kinetics it performs noticeably better. Relative to our previous functionals, such as oB97X, the new functional is significantly superior for non-bonded interactions, and very similar in performance for bonded interactions.

Ab initio calculations were used to analyze the interactions of arachno-B 5 H 11 with HF and LiH at the MP2/ 6-311++g(d,p) computational level. The B 5 H 11 interact with HF and LiH through both its terminal and bridged hydrogens. Interaction of H b with HF and LiH result to H b . . .F hydrogen and H b . . .H dihydrogen bonds, respectively. In contrast, H t . . .H and H t . . .Li bonds were obtained from interactions of H t of B 5 H 11 with HF and LiH. In the two complexes named as S4 and S6 the B 5 H 11 shows interesting ability for trapping the HF and LiH molecules in host-guest like interactions. The structures obtained have been analyzed using AIM methodology.

Relativistic and electron correlation effects play a important role in the electronic structure of molecules containing heavy elements (main group elements, transition metals, lanthanide and actinide complexes). It is therefore mandatory to account for them in quantum mechanical methods used in theoretical chemistry, when investigating for instance the properties of heavy atoms and molecules in their excited states. In this chapter we introduce the present state-ofthe-art ab initio spin-orbit configuration interaction methods for relativistic electronic structure calculations. These include the various types of relativistic effective core potentials in the scalar relativistic approximation, and several methods to treat electron correlation effects and spinorbit coupling. We discuss a selection of recent applications on the spectroscopy of gas-phase molecules and on embedded molecules in a crystal environment to outline the degree of maturity of quantum chemistry methods. This also illustrates the necessity for a strong interplay between theory and experiment.

The influence of various all-electron basis sets and effective core potentials employed along with several DFT functionals (B3LYP, B3PW91, BLYP, BP86 and M06) on the magnitude of nucleus independent chemical shift (NICS) values in different metallic nano-clusters and hydrocarbons is studied. In general, it is demonstrated that the NICS values are very sensitive to the applied method/basis set; however, the method/basis set dependence is more prominent for computed NICS values in transition metal clusters. In hydrocarbons, medium-size basis sets perform roughly similar to large basis sets in most cases. It is also found that NICS(0) values are more sensitive to the method/basis set variation compared to the NICS values computed at 1 or 2 Å above the ring plane. However, in many cases, no broad-spectrum regulation is found for the effect of basis set/method on the magnitude of NICS values. A detailed study showed that bond length alternation in a molecule has an insignificant effect on the magnitude of NICS values so the influence of method/basis sets on the magnitude of NICS values mostly arises from the different predicted ring current intensities at various computational levels.

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.

Singlet-triplet (S 0 -T 1 ) well-to-well (WWE S-T ) and adiabatic (AE S-T ) excitation energies of benzene and the linear polyacenes naphthalene through decacene were estimated using a range of density functional theory (DFT) methods and basis sets along with the assumption of a closed-shell singlet state. Via single exponential decay regression based extrapolations to the polymeric limit, significant variability in theoretically obtained WWE S-T /AE S-T predicted for longer polyacenes is evident that is primarily dependent on the model chemistry employed, with minor variations due to basis set incompleteness and zero-point energy (ZPE) corrections. With the exception of the B2PLYPD density functional (which, along with the mPW2PLYPD functional, combines exact HF exchange with an MP2-like correlation to the DFT calculation), all DFT methods investigated predict a negative WWE S-T /AE S-T (ground state triplet) at the polymer limit, with most functionals predicting a transition from a singlet to triplet ground state between octacene and decacene. Extrapolation of the B2PLYPD results predicts a vanishingly small singlet-triplet gap at the polymeric limit for an infinitely long homolog. Hartree-Fock calculations significantly underestimate the polyacene WWE S-T /AE S-T , whereas MPn methods overestimate the singlet-triplet gap but display a convergence toward experimental values with increasing truncation order and substitutions. The B2PLYPD and mPW2PLYPD functionals appear to balance the WWE S-T /AE S-T underestimating tendency of HF/DFT methods for longer polyacenes against the propensity for MPn methods to overestimate the WWE S-T /AE S-T for these compounds, and predict all acenes from benzene through decacene will be ground state singlets with positive singlet-triplet gaps.

Density functional theory calculations have been performed to quantify the aromaticity of 15,16dimethyldihydropyrene relative to benzene through the dimethyldihydropyrene probe. Moreover, the strain energies in [e]-fused bis dihydropyrenes 6a-d are also calculated by comparison with a suitable reference system. The geometric analysis reveals that all isomers of the bis dihydropyrene 6 (6a-d) are strained, and the strain energies range from 5.87-11.31 kcal mol À1 . The dihydropyrenes 6b and 6d show only splitting type distortion (S-) whereas 6a and 6c have additional arching type distortion (A-) which results in higher strain energies for 6a and 6c. A planar non-strained analog 7 has been proposed, and investigated to elucidate the aromaticity of dimethyldihydropyrene relative to benzene, through magnetic ( 1 H NMR and NICS) and geometric (bond fixation) criteria. The aromaticity of the dimethyldihydropyrene is estimated comparable to benzene. Moreover, magnetic and geometric parameters of 6a-d are also discussed within the context of their inability to estimate the aromaticity of dimethyldihydropyrene.

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.

We have obtained the electric quadrupole and hexadecapole moment, dipole polarizability and hyperpolarizability of copper tetramer from finite-field ab initio and density functional theory (DFT) calculations performed with a systematic sequence of basis sets built upon the Stevens/Basch/Krauss effective core potential triple-split basis set CEP-121G. Our present ab initio values show significant discrepancies with previous all-electron results [G. Maroulis, A. Haskopoulos, J. Comput. Theor. Nanosci. 6 (2009) 418]. Representative calculations yield self-consistent field (SCF) values (second-order Møller-Plesset results in parentheses) for (hyper)polarizability invariants: mean polarizability ⇑ Corresponding author.

Swarm intelligence has becoming a powerful technique in solving design and scheduling tasks. Metaheuristic algorithms are an integrated part of this paradigm, and particle swarm optimization is often viewed as an important landmark. The outstanding performance and efficiency of swarm-based algorithms inspired many new developments, though mathematical understanding of metaheuristics remains partly a mystery. In contrast to the classic deterministic algorithms, metaheuristics such as PSO always use some form of randomness, and such randomization now employs various techniques. This paper intends to review and analyze some of the convergence and efficiency associated with metaheuristics such as firefly algorithm, random walks, and Lévy flights. We will discuss how these techniques are used and their implications for further research.

Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr 3 ) and cerium-activated lanthanum tribromide, their commercial availability and application are limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. This investigation employed aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca 2þ ) was used as a dopant to strengthen CeBr 3 without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca 2þ dopant were grown, i.e., 1.9% of the CeBr 3 molecules were replaced by CaBr 2 molecules, to match our target replacement of 1 out of 54 cerium atoms be replaced by a calcium atom. Precisely the mixture was composed of 2.26 g of CaBr 2 added to 222.14 g of CeBr 3 . Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca 2þ -doped CeBr 3 exhibited little or no change in the peak fluorescence emission for 371 nm optical excitation for CeBr 3 . The structural, electronic, and optical properties of CeBr 3 crystals were studied using the density functional theory within the generalized gradient approximation. Calculated lattice parameters are in agreement with the experimental data. The energy band structures and density of states were obtained. The optical properties of CeBr 3 , including the dielectric function, were calculated. V C 2014 AIP Publishing LLC.

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.

Pseudopotential calculations within density-functional theory for a few selected solids ͑Si, GaAs, and Al͒ are used to assess the validity of two generalized-gradient approximations ͑GGA's͒, the one proposed by Becke and Perdew ͑BP͒ and the more recent one proposed by Perdew and Wang ͑PW͒ in comparison with the currently used local-density approximation ͑LDA͒. The GGA's give total energies of atoms and cohesive energies of solids that are closer to experiment than the LDA results. Lattice constants are reproduced with the same accuracy as in LDA, while bulk moduli and zone-center phonon frequencies are underestimated with respect to both the LDA and the experimental values. Comparison to all-electron results shows that in both GGA schemes the validity of the pseudopotential approach is as good as in LDA. The predictions of the two GGA's are similar, the PW functional yielding results marginally, but systematically, closer to experiment. The calculated values of the transition pressure in Si between the diamond and the ␤-tin structure are 72 kbar ͑LDA͒, 164 kbar ͑BP͒, and 135 kbar ͑PW͒, to be compared with the two available experimental values of 103 and 125 kbar.

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.

Quantum-chemical calculation with the B3LYP functional indicates that the modeling of the iron Fe 2+ and Fe 3+ ions in Fenton reactions by the aquacomplexes Fe 2+ (H 2 O) 6 and Fe 3+ (H 2 O) 6 in the system FeSO 4 + -HOOH does not correspond to reality. The calculated value of the Gibbs energy for the Fenton reaction in a ''pure'' aquacomplex model is positive (D r G o 298 = 765 kJ/mol); thus the reaction is thermodynamically forbidden. Oxidation of ferrous ion in aqueous solutions of sulfuric acid is facilitated by an introduction of sulfate ligands into the first coordination sphere of the iron aquacomplexes. It is shown that the most favorable thermodynamic condition for the catalytic Fenton process corresponds to the presence of one SO 2À 4 ion in the first coordination sphere of the aquacomplex. This new mechanism of the Fenton reaction provides opportunity for the controlled acceleration of the Haber-Weiss process through the single-minded addition of another catalytic metal ion.

Please cite this article as: E. Eliav, H. Yakobi, U. Kaldor, Impurity effects on the electronic structure and spectra of spherical quantum dots by the 4-component relativistic coupled cluster method, Computational & Theoretical Chemistry (2014), doi: http://dx.

The relationship between the electron delocalization and the total electronic energy of a number of neutral
poly-nitrogen clusters are studied at B3LYP/aug-cc-pVDZ computational level. It is shown that
increasing the electron delocalization is accompanied with decrease of the total electronic energy. However,
the geometry of clusters affects the amount of energy decrease. The energy decrease in more prominent
in planar and linear clusters compared with the three dimensional, cage like, clusters with the same
degree of the electron delocalization. Aromaticity of clusters whose relative electron delocalizations are
higher/lower than other clusters is studied by means of the Nucleus Independent Chemical Shift (NICS). It
is shown that predicted aromaticity trend based on electron delocalization in these species is in line with
that emerged from NICS study, i.e. magnetic aromaticity

We have quantum chemically analyzed the ring-opening reaction of the model non-symmetrical epoxide 2,2-dimethyloxirane under basic and acidic conditions using density functional theory at OLYP/TZ2P. For the first time, our combined activation strain and Kohn–Sham molecular orbital analysis approach have revealed the interplay of physical factors that control the regioselectivity of these chemical reactions. Ring-opening under basic conditions occurs in a regime of strong interaction between the nucleophile (OH–) and the epoxide and the interaction is governed by the steric (Pauli) repulsion. The latter steers the attack preferentially towards the sterically less encumbered Cβ. Under acidic conditions, the interaction between the nucleophile (H2O) and the epoxide is weak and, now, the regioselectivity is governed by the activation strain. Protonation of the epoxide induces elongation of the weaker (CH3)2Cα–O bond, and effectively predistorts the substrate for the attack at the sterically more hindered side, which goes with a less destabilizing overall strain energy. Our quantitative analysis significantly builds on the widely accepted rationales behind the regioselectivity of these ring-opening reactions and provide a concrete framework for understanding these indispensable textbook reactions.

Cationic surfactants are widely used in biological and industrial processes. Notably, surfactants with pyridinium salts, such as cetylpyridinium bromide (CPB), have diverse applications. The cetylpyridium cation has a quaternary nitrogen in the aromatic heterocyclic ring of the headgroup and 16 carbons in the hydrocarbon tail. At present and in the past, it has been widely used in germicides. Recently, several interesting applications of CPB have been explored, including its use in protein folding, polymerization, enzyme studies, and gene delivery as well as in pharmaceuticals as a drug delivery tool. A molecular-level understanding of CPB and its micelle in solution can enhance its development in such applications. Herein, we have proposed the first united-atom force field for CPB that yields stable micellar aggregates in molecular dynamics (MD) simulations. The force field is validated through classical MD simulations of the CPB monomer in pure water and 1-octanol as well as in an aqueous CPB micelle. We have performed principal component analysis (PCA) and calculated the translational and rotational diffusion coefficients, spatial distribution of solvent, counterion distribution, and rotational correlation time of CPB molecule in solutions and in micelle, comparing these data to previous experimental and theoretical results for a strong validation of the force field. PCA confirms that the pyridinium ring remains planar, whereas the movement of the hydrophobic tail region leads to conformational changes during the simulations. The collective modes of the pyridinum ring were identical for CPB molecule in solution and micelle, but conformational dynamics of the CPB tail were restricted in the micelle relative to motions in water and 1-octanol. Using this force field, a spherical CPB micelle was shown to be stable throughout the course of simulation, and its solvation and structural properties are characterized.

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.

High-accuracy results for energy levels of heavy and superheavy atoms are presented. The projected (or no-virtual-pair) Dirac-Coulomb-Breit Hamiltonian serves as the starting point and defines the physical framework. One-electron four-component Dirac-Fock-Breit functions, similar in spirit to Hartree-Fock orbitals in the nonrelativistic formulation, are calculated first, followed by treatment of electron correlation. Correlation is included by the Fock-space coupled cluster method. The recent intermediate Hamiltonian approach makes it possible to use larger and more flexible P (model) spaces, thereby extending the range of applicability to states not accessible before. Accuracy is greatly improved for systems which can be treated by both methods, and the model space structure can be studied and pushed to convergence for the first time. Applications address mostly transition energies (ionization potentials, excitation energies, electron affinities) in various atoms. Very large basis sets, going up to I = 8, are used. High-/orbitals are particularly important for transitions involving f electrons. The outer 20-40 electrons of the atom are correlated. The Breit term is required for fine-structure splittings and for f transitions. Representative applications are described, including electron affinities of alkali atoms, obtained within 5 meV of known experimental values and providing the best estimate of the experimentally unknown EA of francium; the gold atom, with relativistic effects of 3-4 eV on transition energies; and Pr 3+, where the many f2 levels are reproduced with great precision. The most exciting aspect of the high accuracy provided by the method is the ability to obtain reliable predictions for superheavy elements, where level ordering (and therefore chemistry) may differ from that of the lighter homologues. Thus, the ground state of eka-gold (El11) is 6d97s 2, rather than the 6dl~ 82 expected from other group-ll elements; in Rf (El04), opposite effects of relativity and correlation lead finally to a 7s26d 2 ground state, ~0.3 eV below the 7s26dTp predicted by MCDF; eka-lead (Ell4), a potential member of the "island of stability" forecast by nuclear physics, is predicted to have ionization potentials higher than all other group-14 atoms except carbon; and eka-radon (Ell8) has a unique property for a rare gas, binding an electron with an affinity of-0.064(2) eV. QED corrections are calculated for the latter property, reducing the binding energy of the 8s electron by 0.0059(5) eV or 9%, the largest relative QED effect reported for a neutral or weakly ionized species.

The Ab Initio Valence Bond program TURTLE has been under development for about 12 years and is now becoming useful for the non-specialist computational chemist as is exemplified by its incorporation in the GAMESS-UK program. We describe here the principles of the matrix evaluation and orbital optimisation algorithms and the extensions required to use the Valence Bond wavefunctions in analytical (nuclear) gradient calculations. For the applications, the emphasis is on the selective use of restrictions on the orbitals in the Valence Bond wavefunctions, to investigate chemical concepts, in particular resonance in aromatic systems.

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 domino reactions of imidazole carbene 9, ketenimine 10 and DMAD 5 or MP 7 to yield the formal [3+2] cycloadducts 12 or 14 have been investigated using DFT methods at the B3LYP/6-31G* level. These domino reactions are initialized by the nucleophilic attack of imidazole 9 on ketenimine 10 to yield the zwitterionic intermediate 11. The second reaction is a formal [3+2] cycloaddition (32CA) reaction between the zwitterionic intermediate 11 and acetylene carboxylates DMAD 5 or MP 7 yielding spiro-pyrroles 12 and 14. These formal 32CA reactions are initialized by the nucleophilic attack of the N3 nitrogen of intermediate 11 on the most electrophilic centres of these acetylene derivatives yielding zwitterionic intermediates, which by a fast ring-closure afford spiro-pyrroles 12 and 14. The first step of these formal 32CA reactions is completely chemoselective, while the reaction with MP 7 is completely regioselective. Analysis of DFT reactivity indices allows one to explain the behaviour of these domino reactions. The nucleophilic addition of imidazole 9 to ketenimine 10 generates a strong nucleophilic intermediate 11 able to attack electrophilic acetylenes DMAD 5 or MP 7 in the subsequent reaction.

We discuss the role of the coordinates for the convergence of quantum dynamical simulations involving motions of large amplitude. We put special emphasis on the MCTDH algorithm and discuss several illustrative examples that are parametrized by polyspherical coordinates. Finally, new sets of curvilinear coordinates for malonaldehyde and benzopyran are proposed.

A new family of stereo–electronically promoted aluminum and scandium super Lewis acids is introduced based on the state–of–the–art computations. Structures of these molecules are designed to minimize resonance electron donation to central metal atoms in the Lewis acids. Acidity of these species is evaluated based on their fluoride–ion affinities in comparison with antimony pentafluoride reference system. It is demonstrated that introduced changes in the stereochemistry of the designed ligands increase acidity considerably compared with Al and Sc complexes with analogous monodentate ligands. High stability of fluoride complexes of these species makes them ideal candidates to be used as weakly coordinating anions in combination with highly reactive cations instead of conventional Lewis acid – fluoride complexes. Further, interaction of all designed molecules with methane is investigated. All studied acids form stable pentavalent–carbon complexes with methane. Besides, interactions of the strongest acid of this family with very weak bases, namely, H2, N2, carbon oxides, and noble gases were investigated; it is demonstrated that this compound can form considerably stable complexes with the aforementioned molecules. To the best of our knowledge, carbonyl and nitrogen complexes of this species are the first hypothetical four–coordinated carbonyl and nitrogen complexes of aluminum. The nature of bonding in these systems is studied in detail by various bonding analysis approaches.

The multiple non-covalent interactions in alkenylthioquinolines-diiodine complexes with conformationally flexible S-alkenyl fragments are studied from the viewpoint of the mutual influence of closelylocated N and S donor sites. Despite several electron donor sites in an alkenylthioquinoline molecule (N, S and p-systems), which are capable of forming halogen bonds, the most stable complexes are observed if sulfur atom is a single donor center only. It corresponds to the most favorable mutual arrangement of the electron lone pairs of donor atoms and the depleted areas in electron localization function, which reflects the reduced shielding of iodine nucleus charge and coincides with r-holes in the molecular electrostatic potential. Quinolines substituted at position 2 prefer to form the complexes with three-atomic non-covalent interactions: NÁÁÁI, SÁÁÁI and IÁÁÁH. The bifurcated halogen bond with two-atomic interactions NÁÁÁI and SÁÁÁI turns out to be preferable for quinolines substituted at position 8. We have concluded that iodine participation in the bifurcated halogen bonding simultaneously with N and S atoms leads to reducing the binding strength in the complexes. It is found that the dipole polarization of an iodine atom provides the quantitative estimation of interaction energy in the complexes with multiple non-covalent interactions. The possibility to distinguish between the halogen bond SÁÁÁI and van der Waals interaction SÁÁÁI is also discussed.

A theoretical study on a non-native amino acid, 2-fluoro-L-histidine at DFT-B3LYP/6-311++G** level has been carried out. The effect of substitution is discussed and it is found that all the changes can be accredited to the inductive effects of fluorine. AIM's topological analysis is performed for a confirmation of hydrogen bonding within the molecule. NBO analysis is used in order to understand various intra-molecular interactions and charge transfer due to hydrogen bonding interaction. Substituent effect is taken into account to discuss atomic charge distribution as well as electronic properties. Biological activity of the molecule is also calculated and discussed.

The geometry of tetracyanoplatinate(II) (TCP) has been optimized with density functional theory (DFT) calculations in order to compare different computational strategies. Two approximate scalar relativistic methods, i.e. the scalar zeroth-order regular approximation (ZORA) and non-relativistic calculations with relativistic effective core potentials (ECPs), were benchmarked against the four-component fully relativistic approach using the Dirac-Coulomb Hamiltonian and all-electron non-relativistic calculations. We find that the 5% contraction of the platinum-carbon bond due to relativistic effects is almost quantitatively reproduced in the ZORA and ECP calculations. In addition, the effect of the exchange-correlation functional and one-electron basis set was studied by employing the two generalized gradient approximation (GGA) functionals, BLYP and PBE, as well as their hybrid version B3LYP and PBE0 in combination with both correlation consistent and Ahlrichs type basis sets. The platinum-carbon bond length (relativistic or non-relativistic) is approximately 1% shorter on using the PBE exchange-correlation functional compared to the BLYP functional but including exact exchange has no significant effect. For the C-N bond these trends are reversed and an order of magnitude smaller. With respect to the basis set dependence we observed that a triple zeta basis set with polarization functions gives in general sufficiently converged results, but while for the Pt-C bond it is advantageous to include extra diffuse functions, this did not turn out to be important for the C-N bond.

The α-effect is a term used to explain the dramatically enhanced reactivity of α-nucleophiles (R−Y−X:−) compared to their parent normal nucleophile (R−X:−) by deviating from the classical Brønsted-type reactivity-basicity relationship. The exact origin of this effect is, however, still heavily under debate. In this work, we have quantum chemically analyzed the α-effect of a set of anionic nucleophiles, including O-, N- and S-based normal and α-nucleophiles, participating in an SN2 reaction with ethyl chloride using relativistic density functional theory at ZORA-OLYP/QZ4P. Our activation strain and Kohn–Sham molecular orbital analyses identified two criteria an α-nucleophile needs to fulfill in order to show α-effect: (i) a small HOMO lobe on the nucleophilic center, pointing towards the substrate, to reduce the repulsive occupied–occupied orbital overlap and hence (steric) Pauli repulsion with the substrate; and (ii) a sufficiently high energy HOMO to overcome the loss of favorable HOMO–LUMO orbital overlap with the substrate, as a consequence of the first criterion, by reducing the HOMO–LUMO orbital energy gap. If one of these two criteria is not fulfilled, one can expect no α-effect or inverse α-effect.

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...