Cycloaddition Reactivities Analyzed by Energy Decomposition Analyses and the Frontier Molecular Orbital Model

Comparative computational analysis of orthoconic antiferroelectric liquid crystals: DFT analysis

CONTEXT

The study undertakes a comparative analysis of four distinct semi-fluorinated chiral Organic Active Ferroelectric Liquid Crystals (OAFLCs). The comparative analysis of the compounds is done by using various parameters, including thermodynamic, non-linear optical, electrical, atomic charge distribution, and atomic orientations. We use optimization algorithms to look at chemical reactivity, electrical properties, intermolecular interactions, and static hyperpolarizability. Sample 4 is the best choice for a wide range of display applications. This research contributes to understanding the nuanced properties of semi-fluorinated chiral OAFLCs and highlights Sample 4's potential for novel applications in display technology, owing to its superior stability and optimized properties. This study helps to enhance our understanding of the comparative analysis of semi-fluorinated chiral OAFLCs for potential advancements in display technologies by incorporating findings from key studies.

METHOD

The simulations are performed using density functional theory (DFT) with the B3LYP functional for predicting molecular properties, and Vibrational Energy Distribution Analysis (VEDA) software is used to perform the vibrational analysis of the molecules.

Research and Analysis on Enhancement of Surface Flashover Performance of Epoxy Resin Based on Dielectric Barrier Discharge Plasma Fluorination Modification

To conquer the challenges of charge accumulation and surface flashover in epoxy resin under direct current (DC) electric fields, numerous efforts have been made to research dielectric barrier discharge (DBD) plasma treatments using CF4/Ar as the medium gas, which has proven effective in improving surface flashover voltage. However, despite being an efficient plasma etching medium, SF6/Ar has remained largely unexplored. In this work, we constructed a DBD plasma device with an SF6/Ar gas medium and explored the influence of processing times and gas flow rates on the morphology and surface flashover voltage of epoxy resin. The surface morphology observed by SEM indicates that the degree of plasma etching intensifies with processing time and gas flow rate, and the quantitative characterization of AFM indicates a maximum roughness of 144 nm after 3 min of treatment. Flashover test results show that at 2 min of processing time, the surface flashover voltage reached a maximum of 19.02 kV/mm, which is 25.49% higher than that of the untreated sample and previously reported works. In addition to the effect of surface roughness, charge trap distribution shows that fluorinated groups help to deepen the trap energy levels and density. The optimal modification was achieved at a gas flow rate of 3.5 slm coupled with 2 min of processing time. Furthermore, density functional theory (DFT) calculations reveal that fluorination introduces additional electron traps (0.29 eV) and hole traps (0.38 eV), enhancing the capture of charge carriers and suppressing surface flashover.

Carbene-Decorated Geometrically Constrained Borylenes for Bond Activations

While metal-ligand cooperativity is well-known, studies on element-ligand cooperativity involving main group species are comparatively much less explored. In this study, we computationally designed a few geometrically constrained borylenes supported by different carbenes. Our density functional theory studies indicate that they possess enhanced nucleophilicity as well as electrophilicity, thus rendering them promising candidates for exhibiting borylene-ligand cooperativity. The cooperation between the boron and adjacent carbene centers facilitates different bond activation processes, including the cycloaddition of acetylene across the boron-carbene bond as well as B-H/Si-H bond activation reactions, which have been analyzed in detail. To the best of our knowledge, the borylenes proposed in this study represent the first examples of theoretically proposed geometrically constrained bis(carbene)-stabilized borylenes capable of cooperative activation of enthalpically strong bonds.

Woodward-Hoffmann or Hoffmann-Woodward? Cycloadditions and the Transformation of Roald Hoffmann from a "Calculator" to an "Explainer"

On May 1, 1965, Roald Hoffmann and R. B. Woodward published their second joint communication, Selection Rules for Concerted Cycloaddition Reactions, in the Journal of the American Chemical Society. Herein is presented a historical analysis of Woodward and Hoffmann's determination of the mechanism of cycloadditions. This analysis is based on thorough analyses with Roald Hoffmann of his 1964 and 1965 laboratory notebooks and his archived documents and on numerous in-person, video, and email interviews. This historical research pinpoints several seminal moments in chemistry and in the professional career of Hoffmann. For example, now documented is the fact that Woodward and Hoffmann had no anticipation that their collaboration would continue after the publication of their first 1965 communication on electrocyclizations. Also pinpointed is the moment in Hoffmann's professional and intellectual trajectories that he became a full-fledged, equal collaborator with Woodward and Hoffmann's transition from a "calculator" to an "explainer."

Copper-mediated cyclization of thiosemicarbazones leading to 1,3,4-thiadiazoles: Structural elucidation, DFT calculations, in vitro biological evaluation and in silico evaluation studies

Cancer's global impact necessitates innovative and less toxic treatments. Thiosemicarbazones (TSCs), adaptable metal chelators, offer such potential. In this study, we have synthesized N (4)-substituted heterocyclic TSCs from syringaldehyde (TSL1, TSL2), and also report the unexpected copper-mediated cyclization of the TSCs to form thiadiazoles (TSL3, TSL4), expanding research avenues. This work includes extensive characterization and studies such as DNA/protein binding, molecular docking, and theoretical analyses to demonstrate the potential of the as-prepared TSCs and thiadiazoles against different cancer cells. The DFT results depict that the thiadiazoles exhibit greater structural stability and reduced reactivity compared to the corresponding TSCs. The docking results suggest superior EGFR inhibition for TSL3 with a binding constant value of - 6.99 Kcal/mol. According to molecular dynamics studies, the TSL3-EGFR complex exhibits a lower average RMSD (1.39 nm) as compared to the TSL1-EGFR complex (3.29 nm) suggesting that both the thiadiazole and thiosemicarbazone examined here can be good inhibitors of EGFR protein, also that TSL3 can inhibit EGFR better than TSL1. ADME analysis indicates drug-likeness and oral availability of the thiadiazole-based drugs. The DNA binding experiment through absorption and emission spectroscopy discovered that TSL3 is more active towards DNA which is quantitatively calculated with a Kb value of 4.74 × 106 M-1, Kq value of 4.04 × 104 M-1and Kapp value of 5 × 106 M-1. Furthermore, the BSA binding studies carried out with fluorescence spectroscopy showed that TSL3 shows better binding capacity (1.64 × 105 M-1) with BSA protein. All the compounds show significant cytotoxicity against A459-lung, MCF-7-breast, and HepG2-liver cancer cell lines; TSL3 exhibits the best cytotoxicity, albeit less effective than cisplatin. Thiadiazoles demonstrate greater cytotoxicity than the TSCs. Overall, the promise of TSCs and thiadiazoles in cancer research is highlighted by this study. Furthermore, it unveils unexpected copper-mediated cyclization of the TSCs to thiadiazoles.

Computational Organic Chemistry: The Frontier for Understanding and Designing Bioorthogonal Cycloadditions

Computational organic chemistry has become a valuable tool in the field of bioorthogonal chemistry, offering insights and aiding in the progression of this branch of chemistry. In this review, I present an overview of computational work in this field, including an exploration of both the primary computational analysis methods used and their application in the main areas of bioorthogonal chemistry: (3 + 2) and [4 + 2] cycloadditions. In the context of (3 + 2) cycloadditions, detailed studies of electronic effects have informed the evolution of cycloalkyne/1,3-dipole cycloadditions. Through computational techniques, researchers have found ways to adjust the electronic structure via hyperconjugation to enhance reactions without compromising stability. For [4 + 2] cycloadditions, methods such as distortion/interaction analysis and energy decomposition analysis have been beneficial, leading to the development of bioorthogonal reactants with improved reactivity and the creation of orthogonal reaction pairs. To conclude, I touch upon the emerging fields of cheminformatics and machine learning, which promise to play a role in future reaction discovery and optimization.

Transition-Metal-Free Approach for Z-Vinyl Fluorides by Hydrofluorination of Alkynes bearing SF4 and SF5 Groups

The synthesis of vinyl fluorides plays a crucial role in various scientific disciplines, including pharmaceutical and materials sciences. Herein, we present a direct and stereoselective hydrofluorination method for the synthesis of Z isomers of vinyl fluorides from alkynes containing unexplored SF5 and SF4 groups. Our strategy employed tetrabutylammonium fluoride (TBAF) as a fluorine source. It demonstrates high compatibility with aryls, biaryls, heteroaryls, and tert-alkyl groups, allowing facile incorporation of SF5 and SF4 groups across the triple bond without any transition-metal catalysts. This approach avoids the potential decomposition of the SF5 or SF4 units via coordination with transition metals or acidic protic sources. Remarkably, this transformation proceeded at room temperature without any additional additives, providing the Z isomer of vinyl fluorides in excellent yield and high selectivity. The presence of a water molecule as a hydrate in TBAF is essential for efficient conversion. This methodology opens new avenues for the synthesis of enchanting SF5 - and SF4 -containing fluorinated vinylic scaffolds, thereby providing advanced opportunities for novel drug discovery and fluorinated polymers.

Directionality of Halogen-Bonds: Insights from 2D Energy Decomposition Analysis

Halogen bonds are typically observed to have a linear arrangement with a 180° angle between the nucleophile and the halogen bond acceptor X-R. This linearity is commonly explained using the σ-hole model, although there have been alternative explanations involving exchange repulsion forces. We employ two-dimensional Distortion/Interaction and Energy Decomposition Analysis to examine the archetypal H3 N⋯X2 halogen bond systems. Our results indicate that although halogen bonds are predominantly electrostatic, their directionality is largely due to decreased Pauli repulsion in linear configurations as opposed to angled ones in the I2 and Br2 systems. As we move to the smaller halogens, Cl2 and F2 , the influence of Pauli repulsion diminishes, and the energy surface is shaped by orbital interactions and electrostatic forces. These results support the role of exchange repulsion forces in influencing the directionality of strong halogen bonds. Additionally, we demonstrate that the 2D Energy Decomposition Analysis is a useful tool for enhancing our understanding of the nature of potential energy surfaces in noncovalent interactions.

Discovery of new tetrazines for bioorthogonal reactions with strained alkenes via computational chemistry

Tetrazines are widely employed reagents in bioorthogonal chemistry, as they react readily with strained alkenes in inverse electron demand Diels-Alder reactions, allowing for selective labeling of biomacromolecules. For optimal performance, tetrazine reagents have to react readily with strained alkenes, while remaining inert against nucleophiles like thiols. Balancing these conditions is a challenge, as reactivity towards strained alkenes and nucleophiles is governed by the same factor - the energy of unoccupied orbitals of tetrazine. Herein, we utilize computational chemistry to screen a set of tetrazine derivatives, aiming to identify structural elements responsible for a better ratio of reactivity with strained alkenes vs. stability against nucleophiles. This advantageous trait is present in sulfone- and sulfoxide-substituted tetrazines. In the end, the distortion/interaction model helped us to identify that the reason behind this enhanced reactivity profile is a secondary orbital interaction between the strained alkene and sulfone-/sulfoxide-substituted tetrazine. This insight can be used to design new tetrazines for bioorthogonal chemistry with improved reactivity/stability profiles.

Metal Influence on Cyaphide-Azide 1,3-Dipolar Cycloaddition Reactions: Aromaticity and Activation Strain

The factors governing 1,3-dipolar cycloaddition reactions involving C≡P-containing compounds are computationally explored in detail using quantum chemical tools. To this end, the parent process involving tBuN3 and tBuCP is analyzed and compared to the analogous reaction involving organometallic cyaphide complexes (metal=Au, Pt, Ge, Mg), in order to understand the role of the metal fragment in such transformations. It is found that while the metal fragment does not significantly influence the aromaticity of the corresponding concerted transition states or the regioselectivity of the transformation, it may modify the reactivity of the cyaphide complexes (i. e. Ge and Mg cyaphide complexes are comparatively more reactive). The computed reactivity trends and the factors behind the regioselectivity of the cycloaddition reaction are quantitatively analyzed with the help of the activation strain model in combination with the energy decomposition analysis method.

Mechanistic Insight into Lewis Acid-Catalyzed Cycloaddition of Bicyclo[1.1.0]butanes with Ketene: Bicyclo[1.1.0]butanes Serving as an Electrophile

Lewis acid-catalyzed cycloaddition between bicyclo[1.1.0]butanes (BCBs) and unsaturated substrates has recently been demonstrated to be a powerful strategy for synthesizing bicyclo[2.1.1]hexanes. However, their reaction mechanisms remain elusive. This computational work explored the recently developed TMSOTf-catalyzed cycloaddition of BCB ketone to ketene and determined the rate-determining step as the activation of BCB ketone. Contrary to the previous proposal of BCB enolate as the active species, this work instead identified the catalytically active species to be a partially Lewis acid-activated BCB cation, which shows a greater electrophilicity and larger orbital interactions with ketene compared to those of the pristine BCB. The most favorable reaction pathway uniquely utilizes this activated BCB species as an electrophile to react with ketene as a nucleophile, while the previously proposed enolate is relatively inactive. Moreover, the in situ-generated TfO anion is revealed to be non-innocent, and its coordination mode and orientation could affect the reaction kinetics.

Frequency-dependent nonlinear optical response and refractive index investigation of lactone-derived thermochromic compounds

Nonlinear optical (NLO) switchable materials play a crucial role in the fields of electronics and optoelectronics. The selection of an appropriate switching approach is vital in designing such materials to enhance their NLO response. Among various approaches, thermos-switching materials have shown a 4-fold increase in NLO response compared to other photo-switching materials. In this study, we computationally investigated the geometric, electronic, and nonlinear optical properties of reversible lactone-based thermochromic compounds using the ωB97XD/6-311+G (d,p) level of theory. Molecular orbital studies are employed to analyze the electronic properties of the close and open isomers of these compounds, while time-dependent density functional theory (TD-DFT) analysis is utilized to evaluate their molecular absorption. Our findings reveal that the π-electronic conjugation-induced delocalization significantly influences the ON-OFF switchable nonlinear optical response of the lactone-based thermochromic compounds. Notably, among all compounds, the open isomer of lactone 2 exhibits the highest hyperpolarizability value (6596.69 au). Furthermore, we extended our analysis to investigate the frequency-dependent second and third-order hyperpolarizabilities. The most pronounced frequency-dependent NLO response is observed at 532 nm. Additionally, we calculated the refractive index of these thermochromic compounds to further assess their nonlinear optical response. The open isomer of lactone 1 demonstrates the highest refractive index value (3.99 × 10-14 cm2/W). Overall, our study highlights the excellent potential of reversible thermochromic compounds as NLO molecular thermos-switches for future applications.

Modular and diverse synthesis of amino acids via asymmetric decarboxylative protonation of aminomalonic acids

Stereoselective protonation is a challenge in asymmetric catalysis. The small size and high rate of transfer of protons mean that face-selective delivery to planar intermediates is hard to control, but it can unlock previously obscure asymmetric transformations. Particularly, when coupled with a preceding decarboxylation, enantioselective protonation can convert the abundant acid feedstocks into structurally diverse chiral molecules. Here an anchoring group strategy is demonstrated as a potential alternative and supplement to the conventional structural modification of catalysts by creating additional catalyst-substrate interactions. We show that a tailored benzamide group in aminomalonic acids can help build a coordinated network of non-covalent interactions, including hydrogen bonds, π-π interactions and dispersion forces, with a chiral acid catalyst. This allows enantioselective decarboxylative protonation to give α-amino acids. The malonate-based synthesis introduces side chains via a facile substitution of aminomalonic esters and thus can access structurally and functionally diverse amino acids.

Mechanistic Insights into the DABCO-Catalyzed Cloke-Wilson Rearrangement: A DFT Perspective

The mechanism and selectivity patterns of the DABCO-catalyzed Cloke-Wilson rearrangement were computationally studied in detail using density functional theory calculations. Our computations suggest that the process occurs stepwise involving the initial ring opening of the cyclopropane promoted by a DABCO molecule followed by a ring-closure reaction of the readily formed zwitterionic intermediate. The regioselectivity of the initial nucleophilic ring-opening step strongly depends on the nature of the substituent attached to the cyclopropane moiety. The physical factors governing the preference for the more sterically hindered C2 (tertiary) position have been quantitatively analyzed by applying the combined activation strain model-energy decomposition analysis method. In addition, our calculations revealed a new mechanism for the analogous transformation involving vinylcyclopropanes consisting of an initial SN2' ring-opening process followed by a 5-exo-trig cyclization step, which proceeds without facial selectivity.

Designing of banana shaped chromophores via molecular engineering of terminal groups to probe photovoltaic behavior of organic solar cell materials

To meet the rising requirement of photovoltaic compounds for modernized hi-tech purpose, we designed six new molecules (DTPD1-DTPD6) from banana shaped small fullerene free chromophore (DTPR) by structural tailoring at terminal acceptors. Frontier molecular orbitals (FMOs), density of states (DOS), open circuit voltage (Voc), transition density matrix (TDM) analysis, optical properties, reorganization energy value of hole and electron were determined utilizing density function theory (DFT) and time-dependent density function theory (TD-DFT) approaches, to analyze photovoltaic properties of said compounds. Band gap contraction (∆E = 2.717-2.167 eV) accompanied by larger bathochromic shift (λmax = 585.490-709.693 nm) was observed in derivatives contrary to DTPR. The FMOs, DOS and TDMs investigations explored that central acceptor moiety played significant role for charge transformation. The minimum binding energy values for DTPD1-DTPD6 demonstrated the higher exciton dissociation rate with greater charge transferal rate than DTPR, which was further endorsed by TDM and DOS analyses. A comparable Voc (1.49-2.535 V) with respect to the HOMOPBDBT-LUMOacceptor for entitled compounds was investigated. In a nutshell, all the tailored chromophores can be considered as highly efficient compounds for promising OSCs with a good Voc response.

Orthogonal Inverse-Electron-Demand Cycloaddition Reactions Controlled by Frontier Molecular Orbital Interactions

Chemoselective pairs of bioorthogonal reactants enable the simultaneous labeling of several biomolecules. Here, we access orthogonal click reactions by exploiting differences in frontier molecular orbital interaction energies in transition states. We establish that five-membered cyclic dienes are inert to isonitriles but readily react with strained alkynes, while tetrazines with bulky substituents readily react with isonitriles. Strained alkynes show an opposite reactivity pattern. The approach was demonstrated by orthogonally labeling two proteins with different fluorophores.

Lewis Acid-Catalyzed Carbonyl-Ene Reaction: Interplay between Aromaticity, Synchronicity, and Pauli Repulsion

The physical factors governing the catalysis in Lewis acid-promoted carbonyl-ene reactions have been explored in detail quantum chemically. It is found that the binding of a Lewis acid to the carbonyl group directly involved in the transformation greatly accelerates the reaction by decreasing the corresponding activation barrier up to 25 kcal/mol. The Lewis acid makes the process much more asynchronous and the corresponding transition state less in-plane aromatic. The remarkable acceleration induced by the catalyst is ascribed, by means of the activation strain model and the energy decomposition analysis methods, mainly to a significant reduction of the Pauli repulsion between the key occupied π-molecular orbitals of the reactants and not to the widely accepted stabilization of the LUMO of the enophile.

Substituent Effects in Bioorthogonal Diels-Alder Reactions of 1,2,4,5-Tetrazines

1,2,4,5-Tetrazines are increasingly used as reactants in bioorthogonal chemistry due to their high reactivity in Diels-Alder reactions with various dienophiles. Substituents in the 3- and 6-positions of the tetrazine scaffold are known to have a significant impact on the rate of cycloadditions; this is commonly explained on the basis of frontier molecular orbital theory. In contrast, we show that reactivity differences between commonly used classes of tetrazines are not controlled by frontier molecular orbital interactions. In particular, we demonstrate that mono-substituted tetrazines show high reactivity due to decreased Pauli repulsion, which leads to a more asynchronous approach associated with reduced distortion energy. This follows the recent Vermeeren-Hamlin-Bickelhaupt model of reactivity increase in asymmetric Diels-Alder reactions. In addition, we reveal that ethylene is not a good model compound for other alkenes in Diels-Alder reactions.

Regiodivergent (3 + 2) annulation reactions of oxyallyl cations

We report a new method for the regiodivergent dearomative (3 + 2) reaction between 3-substituted indoles and oxyallyl cations. Access to both regioisomeric products is possible and is contingent on the presence or absence of a bromine atom on the substituted oxyallyl cation. In this way, we are able to prepare molecules that contain highly-hindered, stereodefined, vicinal, quaternary centers. Detailed computational studies employing energy decomposition analysis (EDA) at the DFT level establishes that regiochemical control arises from either reactant distortion energy or orbital mixing and dispersive forces, depending on the oxyallyl cation. Examination of the Natural Orbitals for Chemical Valence (NOCV) confirms that indole acts as the nucleophilic partner in the annulation reaction.

Additivity of Diene Substituent Gibbs Free Energy Contributions for Diels-Alder Reactions between Me2C=CMe2 and Substituted Cyclopentadienes

Systematic computational studies of pericyclic Diels-Alder reactions between (H₃C)₂C=C(CH₃)₂, 1, and all permutations of substituted cyclopentadienes c-C₅R¹R²R³R⁴R^5aR^5b (R = H, CH₃, CF₃, F) allowed isolation of substitutional effects on Gibbs free energy barrier heights and reaction Gibbs free energies. "Average Substitution Gibbs Free Energy Correction" ΔG _ASC# ^‡/ΔG _ASC# values for each substituent in each position appeared to be additive. Substituent effects on barriers showed interesting contrasts. Methyl substitution at positions 5a and 5b increased barriers significantly, while substitution at all other positions had essentially no impact. In contrast, fluoro substitution at positions 5a and 5b lowered barriers more than substitution at other positions. Trifluoromethyl substitution mixed these effects, in that substitution at positions 5a and 5b increased barriers, but substitution at other positions lowered them. Despite the variances, ΔG _ASC# ^‡/ΔG _ASC# values allowed reliable prediction of barriers and exergonicities for reactions between 1 and highly substituted cyclopentadienes, and between 1 and cyclopentadienes with random mixtures of CH₃/CF₃/F substituents. ΔG _ASC# ^‡/ΔG _ASC# values were correlated with steric considerations and quantum theory of atoms in molecules (QTAIM) calculations. Overall, the ASC values provide a resource for predicting which Diels-Alder reactions of this type should occur at rapid rates and/or give stable bicyclic products.

One-Bond-Nucleophilicity and -Electrophilicity Parameters: An Efficient Ordering System for 1,3-Dipolar Cycloadditions

Diazoalkanes are ambiphilic 1,3-dipoles that undergo fast Huisgen cycloadditions with both electron-rich and electron-poor dipolarophiles but react slowly with alkenes of low polarity. Frontier molecular orbital (FMO) theory considering the 3-center-4-electron π-system of the propargyl fragment of diazoalkanes is commonly applied to rationalize these reactivity trends. However, we recently found that a change in the mechanism from cycloadditions to azo couplings takes place due to the existence of a previously overlooked lower-lying unoccupied molecular orbital. We now propose an alternative approach to analyze 1,3-dipolar cycloaddition reactions, which relies on the linear free energy relationship lg k₂(20 °C) = s_N(N + E) (eq 1) with two solvent-dependent parameters (N, s_N) to characterize nucleophiles and one parameter (E) for electrophiles. Rate constants for the cycloadditions of diazoalkanes with dipolarophiles were measured and compared with those calculated for the formation of zwitterions by eq 1. The difference between experimental and predicted Gibbs energies of activation is interpreted as the energy of concert, i.e., the stabilization of the transition states by the concerted formation of two new bonds. By linking the plot of lg k₂ vs N for nucleophilic dipolarophiles with that of lg k₂ vs E for electrophilic dipolarophiles, one obtains V-shaped plots which provide absolute rate constants for the stepwise reactions on the borderlines. These plots furthermore predict relative reactivities of dipolarophiles in concerted, highly asynchronous cycloadditions more precisely than the classical correlations of rate constants with FMO energies or ionization potentials. DFT calculations using the SMD solvent model confirm these interpretations.

Origins of Regioselectivity in CuH-Catalyzed Hydrofunctionalization of Alkenes

Factors controlling the regioselectivity in alkene hydrocupration were computationally investigated using energy decomposition analysis. The results demonstrate that the Markovnikov-selective hydrocupration with electronically activated mono-substituted olefins is mostly affected by the destabilizing Pauli repulsion, which is due to the electron delocalization effect. The anti-Markovnikov-selective hydrocupration with 1,1-dialkyl-substituted terminal olefins is dominated by the repulsive electrostatic interactions, which is because of the unequal π electron distribution caused by the induction effect of alkyl substituents.

Exploring the Relationship between Reactivity and Electronic Structure in Isorhodanine Derivatives Using Computer Simulations

The electronic structure and reactivity of 22 isorhodanine (IsRd) derivatives in the Diels-Alder reaction with dimethyl maleate (DMm) were investigated under two different environments (gas phase and continuous solvent CH3COOH), using free Gibbs activation energy, free Gibbs reaction energy, and frontier molecular orbitals to analyze their reactivity. The results revealed both inverse electronic demand (IED) and normal electronic demand (NED) characteristics in the Diels-Alder reaction and also provided insights into the aromaticity of the IsRd ring by employing HOMA values. Additionally, the electronic structure of the IsRd core was analyzed through topological examination of the electron density and electron localization function (ELF). Specifically, the study demonstrated that ELF was able to successfully capture chemical reactivity, highlighting the potential of this method to provide valuable insights into the electronic structure and reactivity of molecules.

Understanding chemistry: from "heuristic (soft) explanations and reasoning by analogy" to "quantum chemistry"

"Soft theories," i.e., "heuristic models based on reasoning by analogy" largely drove chemistry understanding for 150 years or more. But soft theories have their limitations and with the expansion of chemistry in the mid-20th century, more and more inexplicable (by soft theory) experimental results were being obtained. In the past 50 years, quantum chemistry, most often in the guise of applied theoretical chemistry including computational chemistry, has provided (a) the underlying "hard evidence" for many soft theories and (b) the explanations for chemical phenomena that were unavailable by soft theories. In this publication, we define "hard theories" as "theories derived from quantum chemistry." Both soft and hard theories can be qualitative and quantitative, and the "Houk quadrant" is proposed as a helpful categorization tool. Furthermore, the language of soft theories is often used appropriately to describe quantum chemical results. A valid and useful way of doing science is the appropriate use and application of both soft and hard theories along with the best nomenclature available for successful communication of results and ideas.