Removing Electrons Can Increase the Electron Density: A Computational Study of Negative Fukui Functions

Extending Conceptual Density Functional Theory toward First-Order Reduced Density Matrices: An Open Subsystems Viewpoint on the Fukui Matrix

As a matrix extension of the Fukui function, a reactivity descriptor grounded within Conceptual Density Functional Theory, the Fukui matrix extends Frontier Molecular Orbital Theory to correlated regimes with its eigendecomposition in Fukui occupations and Fukui naturals. Despite successful applications, the questions remain as to whether replacing a quantity derived from a purely density-based framework by its matrix extension is theoretically well-founded and what chemical information is contained in the corresponding eigendecomposition. In this study, we show that the matrix extension of the Fukui function is only well-defined if one also generalizes the external potential to become nonlocal, leading to the introduction of Conceptual First-Order Reduced Density Matrix Functional Theory. By interpreting the Anderson impurity model from an interacting open subsystem perspective, we show how Fukui occupations and Fukui naturals reflect the influence of an increasing (static) correlation and which characteristic patterns we should expect within a molecular context. This study represents a step in generalizing Conceptual Density Functional Theory beyond its density-based perspective.

Kinetics, mechanism, and tautomerism in ametryn acid hydrolysis: From molecular structure to environmental impacts

The excessive use of pesticides and the demand for environmentally friendly compounds have driven the focus to detailed studies of the environmental destination of these compounds. Degradation by hydrolysis of pesticides, when released into the soil, can result in the formation of metabolites with potentially adverse effects on the environment. Moving in this direction, we investigated the mechanism of acid hydrolysis of the herbicide ametryn (AMT) and predicted the toxicities of metabolites through experimental and theoretical approaches. The formation of ionized hydroxyatrazine (HA) occurs with the release of the SCH₃- group and the addition of H₃O⁺ to the triazine ring. The tautomerization reactions privileged the conversion of AMT into HA. Furthermore, the ionized HA is stabilized by an intramolecular reaction that provides the molecule in two tautomeric states. Experimentally, the hydrolysis of AMT was obtained under acidic conditions and at room temperature with HA as the main product. HA was isolated in a solid state through its crystallization as organic counterions. The mechanism of conversion of AMT to HA and the experimental investigation of the reaction kinetics allowed us to determine the dissociation of CH₃SH as the rate-controlling step in the degradation process that culminates in a half-life between 7 and 24 months under typical acid soil conditions of the Brazilian Midwest - region with strong agricultural and livestock vocation. The keto and hydroxy metabolites showed substantial thermodynamic stability and a decrease in toxicity compared to AMT. We hope that this comprehensive study will support the understanding of the degradation of s-triazine-based pesticides.

Insights into chalcone analogues with potential as antioxidant additives in diesel-biodiesel blends

Biodiesel production is one of the promising strategies to reduce diesel consumption and an important contribution to climate change. However, biodiesel stability remains a challenging problem in biofuel use in the global energy matrix. In this context, organic additives have been investigated to minimize these problems and reduce harmful emissions to comply with fuel requirement standards. In this study, we discuss a comprehensive structural description, a behavior of B15 [85% volume of diesel and 15% volume of biodiesel (B100)] stability in the presence of antioxidants (chalcone analogues), and a theoretical calculation to pave the way for clarifying and expanding the potential of title compounds as an antioxidant additive for diesel-biodiesel blends. Finally, a systematic description of the oxidation stability was undertaken using a specialized machine learning computational pySIRC platform.

Dienophilic reactivity of 2-phosphaindolizines: a conceptual DFT investigation

The >C=P– or –N=P– functionality in 1,3-azaphospholo[1,5-a]pyridine, named as 2-phosphaindolizine and its 1- and 3-aza derivatives act as dienophiles and undergo Diels–Alder reactions with 1,3-dienes. However, the dienophilic reactivity is affected by the nature of the substituent groups on the two sides of the σ2,λ3-P atom and also by the presence of more nitrogen atom(s) in the five-membered ring. The conceptual density functional theory (DFT) calculations have been used in recent years to predict the reactivity of organic molecules in reactions. We calculated global hardness (η), global softness (S), electronic chemical potential (μ), electrophilicity (ω), and nucleophilicity (N) indices of four classes of 2-phosphaindolizines, on the basis of which their observed relative dienophilic reactivities could be rationalized. Besides, the Fukui functions of the carbon/nitrogen and phosphorus atoms of the >C=P– and –N=P– functionalities were also computed which revealed their hard electrophilic character and accorded well with the dienophilic reactivities observed experimentally. Furthermore, energies and symmetries of the lowest unoccupied molecular orbitals (LUMO) of 2-phosphaindolizines were found to be in conformity with their dienophilic reactivities.

Aqueous picloram degradation by hydroxyl radicals: Unveiling mechanism, kinetics, and ecotoxicity through experimental and theoretical approaches

Pesticides are chemical compounds widely used to combat pests in crops, and they thus play a key role in agricultural production. However, due to their persistence in aquatic environments, even at low concentrations, their use has been considered an environmental problem and caused concern regarding the adverse effects on human health. This paper reports, for the first time, the mechanisms, kinetics, and an evaluation of the toxicity of picloram degradation initiated by OH radicals in the aqueous environment using quantum chemistry and computational toxicology calculations. The rate constants are calculated using a combination of formulations derived from the Transition State Theory in a realistic temperature range (250-310 K). The results indicate that the two favorable pathways (R1 and R5) of OH -based reactions occur by addition to the pyridine ring. The calculated rate constant at 298 K is compared with the overall second-order reaction rate constant, quantified herein experimentally via the competition kinetics method and data available in the literature showing an excellent agreement. The toxicity assessment and a photolysis study provide important information: i) picloram and the majority of degradation products are estimated as harmful; however, ii) these compounds can suffer photolysis in sunlight. The results of the present study can help understand the mechanism of picloram, also providing important clues regarding risk assessment in aquatic environments as well as novel experimental information.

Conceptual density functional theory based electronic structure principles

In this review article, we intend to highlight the basic electronic structure principles and various reactivity descriptors as defined within the premise of conceptual density functional theory (CDFT). Over the past several decades, CDFT has proven its worth in providing valuable insights into various static as well as time-dependent physicochemical problems. Herein, having briefly outlined the basics of CDFT, we describe various situations where CDFT based reactivity theory could be employed in order to gain insights into the underlying mechanism of several chemical processes.

High regioselectivity in the amination reaction of isoquinolinequinone derivatives using conceptual DFT and NCI analysis

DFT-derived reactivity descriptors and Non-Covalent interaction (NCI) analysis were performed to rationalize the regioselectivity in the amination reaction of some isoquinolinequinone derivatives. Statistical analysis was performed to assess robustness of atomic charges to the basis set. Various electronic population schemes including Mulliken population analysis (MPA), electrostatic method (ChelpG), Hirshfeld population analysis (HPA) and Natural population analysis (NPA) have been considered. The results revealed that NPA was the most efficient for this purpose. NCI study using the reduced density gradient (RDG) was performed for revealing weak interactions. Domains defined by isosurfaces of RDG have been integrated to quantitatively study the strength of weak interactions and their stabilities have also been examined. Steric hindrance caused by coordination of ethanol with the neighboring carbonyl prevents the nucleophilic attack on C-6 and therefore leads to preferential C-7 substitution. The quantitative study of NCI clearly demonstrates that the hydrogen bond of carbonyl (2) is more stabilizing. Consequently, the high polarity of hydrogen bond on this carbonyl may explain the high electrophilicity of C-5 compared to C-8. Our work proved that the difference in local reactivity, as well as the steric hindrance are the key elements explaining the high regioselectivity exhibited by the amination reaction.

Electrochemical Synthesis of Hindered Primary and Secondary Amines via Proton-Coupled Electron Transfer

Accessing hindered amines, particularly primary amines α to a fully substituted carbon center, is synthetically challenging. We report an electrochemical method to access such hindered amines starting from benchtop-stable iminium salts and cyanoheteroarenes. A wide variety of substituted heterocycles (pyridine, pyrimidine, pyrazine, purine, azaindole) can be utilized in the cross-coupling reaction, including those substituted with a halide, trifluoromethyl, ester, amide, or ether group, a heterocycle, or an unprotected alcohol or alkyne. Mechanistic insight based on DFT data, as well as cyclic voltammetry and NMR spectroscopy, suggests that a proton-coupled electron-transfer mechanism is operational as part of a hetero-biradical cross-coupling of α-amino radicals and radicals derived from cyanoheteroarenes.

Experimental and Theoretical Investigation of the Reaction of Secondary Amines with Maleic Anhydride

The reaction of secondary amines, namely 1-methylpiperazine, pyrrolidine, morpholine, 2-methylpiperidine, and diethylamine, with maleic anhydride has been investigated experimentally and theoretically at the DFT level. Under kinetic control, i.e. at −78°C or −15°C, amines add across the C=O functionality exclusively and the initially formed addition products isomerize to the corresponding N-substituted maleimic acid derivatives. In contrast to the acyclic α,β-unsaturated carbonyl compounds, amine does not add across the C=C functionality in maleic anhydride even under thermodynamic control. This behaviour of maleic anhydride can be rationalized on the basis of the local condensed Fukui functions, which reveal that the carbonyl carbon atoms in maleic anhydride are much harder than in an acyclic α,β-unsaturated carbonyl compound, such as acrolein. This prompts the amines to attack the carbonyl group in maleic anhydride exclusively.

Atom and Bond Fukui Functions and Matrices: A Hirshfeld-I Atoms-in-Molecule Approach

The Fukui function is often used in its atom-condensed form by isolating it from the molecular Fukui function using a chosen weight function for the atom in the molecule. Recently, Fukui functions and matrices for both atoms and bonds separately were introduced for semiempirical and ab initio levels of theory using Hückel and Mulliken atoms-in-molecule models. In this work, a double partitioning method of the Fukui matrix is proposed within the Hirshfeld-I atoms-in-molecule framework. Diagonalizing the resulting atomic and bond matrices gives eigenvalues and eigenvectors (Fukui orbitals) describing the reactivity of atoms and bonds. The Fukui function is the diagonal element of the Fukui matrix and may be resolved in atom and bond contributions. The extra information contained in the atom and bond resolution of the Fukui matrices and functions is highlighted. The effect of the choice of weight function arising from the Hirshfeld-I approach to obtain atom- and bond-condensed Fukui functions is studied. A comparison of the results with those generated by using the Mulliken atoms-in-molecule approach shows low correlation between the two partitioning schemes.

Structural and electronic determinants of flavonoid binding to human serum albumin: an extensive ligand-based study

The most prominent features responsible for binding of flavonoid aglycones to the IIA region of human serum albumin (HSA) were determined based onin vitrofluorescence measurements and density functional theory calculations.

Reaction mechanisms and kinetics of the iminovinylidene radical with NO: ab initio study

The nitric oxide (NO) is a notorious compound for polluting environment. Recent year, removing nitric oxide from the atmosphere becomes a focus of the investigation. In our work, we study the iminovinylidene (HNCC) radical reacted with NO molecule. The mechanism and kinetic for reaction of the HNCC radical with the NO molecule is investigated via considering the possible channels of the N and O atoms of NO attacking the N and C atoms of the HNCC based on the high level ab initio molecular orbital calculations in conjunction with variational TST and RRKM calculations. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level and their single-point energies are refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) method. The calculated potential energy surfaces indicated that energetically the most favorable channel for the HNCC + NO reaction was predicted to be the formation of HNC+CNO (P8) product via the addition reaction of the C atom of HNCC radical and the N atom of NO with the head to head orientation. To rationalize the scenario of the calculated results, we also employ the Fukui functions and HSAB theory to seek for a possible explanation. In addition, the reaction rate constants were calculated using VariFlex code, and the results show that the total rate coefficient, ktotal, at Ar pressure 760 Torr can be represented with an equation: ktotal = 6.433 × 10(-11) T (0.100) exp(0.275 kcal mol(-1)/RT) at T = 298-3000 K, in units of cm(3) molecule(-1) s(-1).

An information-theoretic resolution of the ambiguity in the local hardness

A definition of the local hardness, suitable for application in the local hard/soft acid/base principle, is derived by applying information theory.

Revisiting electroaccepting and electrodonating powers: proposals for local electrophilicity and local nucleophilicity descriptors

It is shown that the electrophilicity index is also a rational choice for measuring nucleophilicity.

Regioselective synthesis of 3,4,5-trisubstituted 2-aminofurans

Three series of methyl 5-substituted 2-aminofuran-4-keto-3-carboxylates have been prepared following a multicomponent reaction strategy by the addition of an isocyanide to 4-oxo-2-butynoate in the presence of an aldehyde.

Hybrid Ag@TiO2 core-shell nanostructures with highly enhanced photocatalytic performance

A new synthetic approach has been developed to prepare silver@titanium dioxide (Ag@TiO2) core-shell nanostructures with controllable size, shape, crystal phase and function at ambient conditions (e.g. in water, ≤100 ° C). This approach shows a few unique features, including short reaction time (a few minutes) for forming core-shell nanostructures, no requirement of high temperature calcinations for generating TiO2 (e.g. at ~100 ° C in our case), tunable TiO2 shell thickness, high yield and good reproducibility. The experimental results show that the Ag@TiO2 core-shell nanostructures exhibit excellent photocatalytic activity compared to the commercial TiO2 (P25) and Ag-doped TiO2 nanocomposite in the degradation of organic dye molecules (e.g. methyl orange) with ultraviolet (UV) irradiation. This could be attributed to the large surface area of TiO2 nanoparticles for maximum harvesting of UV light, mixed anatase and rutile crystalline phases in the TiO2 shell and the effective charge separation between Ag and TiO2 that can reduce the possible recombination of electron-hole (e(-)-h(+)) pairs within TiO2 generated under UV radiation. To further understand the charge separation situation within Ag-TiO2 composites, theoretical simulation (e.g. density functional theory, DFT) was employed in this study. The DFT simulation results indicate that for the Ag@TiO2 core-shell nanostructures, photo-generated electrons transfer readily from the external TiO2 layer to the internal Ag layer with heavy accumulation compared to those doping Ag on TiO2 surfaces, which may reduce the recombination of e(-)-h(+) pairs and thus enhance the photocatalytic efficiency. The findings may open a new strategy to synthesize TiO2-based photocatalysts with highly enhanced efficiency for environmental remediation applications.

Nucleophilicity Index Based on Atomic Natural Orbitals

A simple method of evaluating a semilocal (regional) nucleophilicity is introduced. The concept involves use of the natural orbitals for atomic populations to identify the most “reactive population” of electrons on particular atom in molecule. The results of test calculations considering the regioselectivity problem in electrophilic aromatic substitution to the benzene derivatives are presented and briefly discussed.

An intermediate level of approximation for computing the dual descriptor

At present, there are two levels of approximation to compute the dual descriptor (DD). The first uses the total electronic density of the original system along with the electronic densities of the system with one more electron and one less electron, but this procedure is time consuming and normal termination of computation of total electronic densities is not guaranteed. The second level of approximation uses only the electronic densities of frontier molecular orbitals, HOMO and LUMO, to avoid the former approximation; however, the orbital relaxation implicitly included in the first level of approximation is absent in the second, thus risking an incorrect interpretation of local reactivity. Between the lowest occupied molecular orbital (LOMO) and the highest unoccupied molecular orbital (HUMO), a framework to provide an expression of the DD in terms of the electronic densities of all molecular orbitals (except HUMO and LOMO) has been proposed to be implemented by programmers as a computational code. This methodology implies another level of approximation located between the conventional approximation methods mentioned above. In this study, working equations have been oriented toward molecular closed- and open-shell systems. In addition, the mathematical expression for a closed-shell system was applied to acetylene in order to assess the capability of this approach to generate the DD.

In pursuit of negative Fukui functions: examples where the highest occupied molecular orbital fails to dominate the chemical reactivity

In our quest to explore molecules with chemically significant regions where the Fukui function is negative, we explored reactions where the frontier orbital that indicates the sites for electrophilic attack is not the highest occupied molecular orbital. The highest occupied molecular orbital (HOMO) controls the location of the regions where the Fukui function is negative, supporting the postulate that negative values of the Fukui function are associated with orbital relaxation effects and nodal surfaces of the frontier orbitals. Significant negative values for the condensed Fukui function, however, were not observed.

EFFECT OF LEWIS ACID CATALYSTS ON THE POSITIONAL SELECTIVITY OF THE ELECTROPHILIC AROMATIC SUBSTITUTION ON α-SUBSTITUTED THIOPHENES: A CONCEPTUAL DFT INVESTIGATION

The α′/β regioselectivity of the electrophilic aromatic substitution of some thiophene α-substituted derivatives ( R = CHO, COMe or CO2Me ), catalyzed and not catalyzed by the Lewis acid, AlCl3 , has been investigated by means of the local nucleophilicity index, recently proposed by Pérez et al. [J Mol Struct: Theochem895: 86, 2009]. The quantum chemistry calculations, carried out at the B3LYP/6-311G(d,p) level of theory, show that the α′-substitution is preferred in absence of the catalyst, while the β-substitution is more favored in the presence of the catalyst. The theoretical results, predicted using DFT-based reactivity indices, are in good agreement with the experimental outcomes.

Computational study on reaction mechanisms and kinetics of RNCN (R = H, F, Cl, Br, CH3) radicals with NO

We carried out a computational study of radical reactions of RNCN (R = H, F, Cl, Br, CH(3)) + NO to investigate how the substitution can influence their corresponding energy barriers and rate coefficients. The preferable reactive sites of RNCN radicals with various substituents are calculated by employing the Fukui functions and hard-and-soft acid-and-base theory, which were generally proved to be successful in the prediction and interpretation of regioselectivity in various types of electrophilic and nucleophilic reactions. Our calculated results clearly show that if the substituted RNCN radical has electron-donating substituent (for R = CH(3)), its corresponding barrier heights for transition states will be substantially decreased. The possible explanations of the observed increase and/or decrease in the energy barriers for the varied substituted RNCN radicals are also analyzed in this article.