Global Electron Density Research Articles

Properties of Plasmoids Observed in Saturn's Dayside and Nightside Magnetodisc

AbstractPlasmoid is a key structure for transferring magnetic flux and plasma in planetary magnetospheres. At Earth, plasmoids are key media which transfer energy and mass in the “Dungey Cycle.” For giant planets, plasmoids are primarily generated by the dynamic processes associated with “Vasyliunas cycle.” It is generally believed that planetary magnetotails are favorable for producing plasmoids. Nevertheless, recent studies reveal that magnetic field lines could be sufficiently stretched to allow magnetic reconnection in Saturn's dayside magnetodisc. In the study, we report direct observations of plasmoids in Saturn's dayside magnetodisc for the first time. Moreover, we perform a statistical investigation on the global plasmoid electron density distribution. The results show an inverse correlation between the nightside plasmoid electron density and local time, and the maximum plasmoid electron density around prenoon local time on the dayside. These results are consistent with the magnetospheric circulation picture associated with the “Vasyliunas cycle.”

Global‐Scale Ionospheric Tomography During the March 17, 2015 Geomagnetic Storm

AbstractThe correct representation of global‐scale electron density is crucial for monitoring and exploring the space weather. This study investigates whether the ground‐based Global Navigation Satellite System (GNSS) tomography can be used to reflect the global spatial and temporal responses of the ionosphere under storm conditions. A global tomography of the ionosphere electron density is constructed based on data from over 2,700 GNSS stations. In comparison to previous techniques, advances are made in spatial and temporal resolution, and in the assessment of results. To demonstrate the capabilities of the approach, the developed method is applied to the March 17, 2015 geomagnetic storm. The tomographic reconstructions show good agreement with electron density observations from worldwide ionosondes, Millstone Hill incoherent scatter radar and in‐situ measurements from satellite missions. Also, the results show that the tomographic technique is capable of reproducing plasma variabilities during geomagnetically disturbed periods including features such as equatorial ionization anomaly enhancements and depletion. Validation results of this brief study period show that the accuracy of our tomography is better than the Neustrelitz Electron Density Model, which is the model used as background, and physics‐based thermosphere‐ionosphere‐electrodynamics general circulation model. The results show that our tomography approach allows us to specify the global electron density from ground to ∼900 km accurately. Given the demonstrated quality, this global electron density reconstruction has potential for improving applications such as assessment of the effects of the electron density on radio signals, GNSS positioning, computation of ray tracing for radio‐signal transmission, and space weather monitoring.

Does the reaction of nitrone derivatives with allenoates proceed by an initial (3 + 2) cycloaddition or O-Nucleophilic addition? A quantum chemical investigation

The question of whether the reaction between nitrone derivatives and allenoates proceeds with initial (3 + 2) cycloaddition (32CA) or the oxygen-nucleophilic (O-nucleophilic) addition has been theoretically investigated using density functional theory (DFT) at the M06–2X/6-311G(d,p) level of theory. In all of the reactions considered, the initial 32CA route is kinetically and thermodynamically preferred over the initial O-nucleophilic addition. Increasing the steric bulk of the substituents on the reactants bridges the kinetic and thermodynamic gap between the initial 32CA and O-nucleophilic addition but the 32CA is still energetically preferred, showing that with increasing steric bulk, the O-nucleophilic addition may become competitive with the 32CA route. Within the 32CA reactions, analysis of electrophilic (Pk+) and nucleophilic (Pk−) Parr functions at the various reaction centers in the allenoate indicates that the three-atom-components (TACs) chemo-selectively add across the olefinic bond bearing the ester functionality (COOMe) with the largest Mulliken spin density coefficients and this observation agrees with the energetic trends and experimental outcomes. The global electron density transfer (GEDT) analysis reveals a high polar character associated with the 32CA reaction of methylbuta-2,3-dienoate with N-methyl-C-phenylnitrone while that of the 32CA reaction of methylpenta-2,3-dienoate with N-cyclohexenylnitrone has a low polar character.

Exploring the chemo-, regio-, and stereoselectivities of the (3 + 2) cycloaddition reaction of 5,5-dimethyl-3-methylene-2-pyrrolidinone with C,N-diarylnitrones and nitrile oxide derivatives: a DFT study

The (3 + 2) cycloaddition (32CA) reaction is an efficient method for the synthesis of many biologically active heterocyclic compounds, but there are several regio- and stereochemical issues that must be fully understood to exploit the full utility of its synthetic power. We herein explored the chemo-, regio-, and stereoselectivities of the 32CA reaction of 5,5-dimethyl-3-methylene-2-pyrrolidinone (B1) to C,N-diarylnitrones (B2), and nitrile oxide derivatives (B3) with DFT at the M06/6-311G(d,p) level of theory. The reactions occur via an asynchronous one-step mechanism, with the chemoselective addition of the C,N-diarylnitrones, and nitrile oxide derivatives across the olefinic bond of 5,5-dimethyl-3-methylene-2-pyrrolidinone being the most preferred kinetically and thermodynamically. The regio- and stereoselectivities of the reactions are affected by the electronic and steric nature of substituents on B2 but they are not affected by the electronic and steric nature of substituents on B3. The C,N-nitrones and the nitrile oxide derivatives add across the atomic centers with the largest atomic spin densities on 5,5-dimethyl-3-methylene-2-pyrrolidinone as seen through the local electrophilic ([Formula: see text]) and nucleophilic ([Formula: see text]) Parr functions of the various reaction centers. Results from the global electron density transfer (GEDT) reveal the low polar nature of the reactions.

A projection-based reduced-order method for electron transport problems with long-range interactions.

Long-range interactions play a central role in electron transport. At the same time, they present a challenge for direct computer simulations since sufficiently large portions of the bath have to be included in the computation to accurately compute the Coulomb potential. This article presents a reduced-order approach by deriving an open quantum model for the reduced density matrix. To treat the transient dynamics, the problem is placed in a reduced-order framework. The dynamics described by the Liouville-von Neumann equation is projected to subspaces using a Petrov-Galerkin projection. In order to recover the global electron density profile as a vehicle to compute the Coulomb potential, we propose a domain decomposition approach, where the computational domain also includes segments of the bath that are selected using logarithmic grids. This approach leads to a multi-component self-energy that enters the effective Hamiltonian. We demonstrate the accuracy of the reduced model using a molecular junction built from lithium chains.

Unveiling the Intramolecular Ionic Diels–Alder Reactions within Molecular Electron Density Theory

The intramolecular ionic Diels–Alder (IIDA) reactions of two dieniminiums were studied within the Molecular Electron Density Theory (MEDT) at the ωB97XD/6-311G(d,p) computational level. Topological analysis of the electron localization function (ELF) of dieniminiums showed that their electronic structures can been seen as the sum of those of butadiene and ethaniminium. The superelectrophilic character of dieniminiums accounts for the high intramolecular global electron density transfer taking place from the diene framework to the iminium one at the transition state structures (TSs) of these IIDA reactions, which are classified as the forward electro density flux. The activation enthalpy associated with the IIDA reaction of the experimental dieniminium, 8.7 kcal·mol−1, was closer to that of the ionic Diels–Alder (I-DA) reaction between butadiene and ethaniminium, 9.3 kcal·mol−1. However, the activation Gibbs free energy of the IIDA reaction was 12.7 kcal·mol−1 lower than that of the intermolecular I-DA reaction. The strong exergonic character of the IIDA reaction, higher than 20.5 kcal·mol−1, makes the reaction irreversible. These IIDA reactions present a total re/exo and si/endo diastereoselectivity, which is controlled by the most favorable chair conformation of the tetramethylene chain. ELF topological analysis of the single bond formation indicated that these IIDA reactions take place through a non-concerted two-stage one-step mechanism. Finally, ELF and atoms-in-molecules (AIM) topological analyses of the TS associated with the inter and intramolecular processes showed the great similarity between them.

The QSPR Studies of [SO4 (H2O)n]2−, n = 1-4, 16 Clusters through Quantum-Chemical Descriptors

The quantitative structure property relationship (QSPR) is a powerful analytical approach that can link physicochemical properties of the molecular or ionic specimens mathematically with their 3D structures. The primary strategy of this predictive molecular based approach is to obtain optimum quantitative structure-property correlations by employing specific quantum-chemical descriptors (QCDs) that can encode structural features numerically. This theoretical insight is mainly aimed at computing HOMO and LUMO Eigen values and the associated QCDs such as HOMO-LUMO energy gap ( ), Ionization potential ( IP ), Electron affinity ( EA ), Electronegativity ( c ), Chemical hardness ( h ), Chemical softness ( s ), Electronic chemical potentials ( m ), and Electrophilicity index ( ω ) for the variably sized hydrated sulfate clusters [SO 4 (H 2 O) n ] 2− , n = 1-4, & 16 separately using DFT:B3LYP hybrid functional, and predicting their most significant physical properties plus kinetic stabilities quantum mechanically. The major structure-based physical properties assessed here are electronic localization & polarizability, electronegativity, electrophilicity, degree of chemical hardness & softness, and the extent of global electron density transfer. The general results show that [SO 4 (H 2 O) 4 ] 2− has more propensity to loose electrons among the ions with hydration number n < 4, and [SO 4 (H 2 O) 16 ] 2− is the most kinetically instable ion having significant electronic polarizability among other smaller clusters sampled in this study. It is believed that this QCD based QSPR studies for the biologically most important and abundant micronutrient SO 4 2 − ions would be highly useful to understand their physiological roles.

Quantum chemical investigation of the formation of spiroheterocyclic compounds via the (3 + 2) cycloaddition reaction of 1-methyl-3-(2,2,2-trifluoroethylidene) pyrrolidin-2-one with diazomethane and nitrone derivatives

The chemo-, regio- and stereoselectivities of the formation of spiroheterocycles via the (3 + 2) cycloaddition (32CA) reaction of 1-methyl-3-(2,2,2-trifluoroethylidene) pyrrolidin-2-one (A1) derivatives with diazomethane and nitrone derivative have been studied at the M06–2X/6-311G level of theory. The reactions of diazomethane (A2) and N-methyl-C-phenyl nitrone (A3) derivatives with 1-methyl-3-(2,2,2-trifluoroethylidene)pyrrolidin-2-one derivatives (A1) occurs chemoselectively along the olefinic bond of A1 via an asynchronous one-step mechanism. Substituents on A3 do not affect the selectivity of reaction whereas the electronic and steric nature of the substituent on A2 does. The title reaction proceeds via forward electron density flux (FEDF), where electron density fluxes from the three-atom components to A1. The global electron density transfer (GEDT) values suggest that the 32CA of A1 with diazomethane is a polar reaction while the 32CA reaction of A1 with N-methyl-C-phenyl nitrone is a non-polar reaction, and an inverse relationship has been established between the polar character of the reactions and activation barriers.

Unveiling the Ionic Diels-Alder Reactions within the Molecular Electron Density Theory.

The ionic Diels–Alder (I-DA) reactions of a series of six iminium cations with cyclopentadiene have been studied within the Molecular Electron Density Theory (MEDT). The superelectrophilic character of iminium cations, ω > 8.20 eV, accounts for the high reactivity of these species participating in I-DA reactions. The activation energies are found to be between 13 and 20 kcal·mol−1 lower in energy than those associated with the corresponding Diels–Alder (DA) reactions of neutral imines. These reactions are low endo selective as a consequence of the cationic character of the TSs, but highly regioselective. Solvents have poor effects on the relative energies, and an unappreciable effect on the geometries. In acetonitrile, the activation energies increase slightly as a consequence of the better solvation of the iminium cations than the cationic TSs. Electron localization function (ELF) topological analysis of the bonding changes along the I-DA reactions shows that they are very similar to those in polar DA reactions. The present MEDT study establishes that the global electron density transfer (GEDT) taking place at the TSs of I-DA reactions, and not steric (Pauli) repulsions such as have been recently proposed, are responsible for the features of these types of DA reactions.

Unveiling the Chemo‐ and Regioselectivity of the [3+2] Cycloaddition Reaction between 4‐Chlorobenzonitrile Oxide and β‐Aminocinnamonitrile with a MEDT Perspective**

AbstractA molecular electron density theory (MEDT) study for the [3+2] cycloaddition (32CA) reaction of 4‐chlorobenzonitrile oxide (CBNO) and β‐aminocinnamonitrile (ACN) has been performed at the MPWB1K/6‐311G(d,p) computational level. This 32CA reaction, characterised as the zwitterionic type (zw‐type), presents an activation enthalpy of 11.8, 14.2 and 16.6 kcal mol−1, in gas phase, benzene and methanol. The formation of 3‐aryl‐5‐(β‐aminostyryl)‐1,2,4‐oxadiazole is predicted as the energetically feasible one following a non‐concertedtwo‐stage one‐stepmechanism. Thiszw‐type32CA reaction shows a total chemo and regioselectivity. The global electron density transfer at the most favourable transition state structure (TS), higher than 0.14 e, suggests some polar character. The most energetically favoured TS shows an intramolecular hydrogen bond between the amine −NH2of ACN and the oxygen of CBNO, which favours thiszw‐type32CA reaction. This behaviour is enhanced with the inclusion of a methanol molecule hydrogen bonded to this oxygen atom. The present MEDT study suggests that the best experimental conditions are to use a non‐polar solvent such as benzene containing catalytic amounts of a protic solvent such as methanol.

Validation and Improvement of NeQuick Topside Ionospheric Formulation Using COSMIC/FORMOSAT‐3 Data

AbstractWe examine systematic differences between topside electron density measurements and different topside model formulations including α‐Chapman, NeQuick, and an improved NeQuick (NeQuick‐corr) topside formulation, recently proposed. The global topside electron density data set used, was extracted from Radio Occultation (RO) topside electron density profiles on board Low‐Earth Orbit satellites from the COSMIC/FORMOSAT‐3 (Constellation Observing System for Meteorology, Ionosphere, and Climate and Formosa Satellite) mission. By using RO topside electron density measurements collocated with digisonde stations, we ensure that our investigation is based on two independent data sets (RO and digisondes). A subset of these profiles, with matched (within 5%) peak RO‐digisonde characteristics (foF2 and hmF2) is also exploited. This subset is exploited to extend the investigation on the basis of a higher quality validation data set. The comparison demonstrates that α‐Chapman and NeQuick‐corr underestimate, whereas NeQuick overestimates COSMIC topside electron density observations. The main outcome of this study is the significant NeQuick topside representation improvement that can be achieved near the F region peak, if the key parameter g, which controls the change of scale height with respect to altitude, is optimized to a value of 0.15 (compared to a currently adopted value of 0.125). The NeQuick‐corr topside formulation using the optimized g value of 0.15 outperforms all other topside formulations.

Unveiling the high regioselectivity and stereoselectivity within the synthesis of spirooxindolenitropyrrolidine: A molecular electron density theory perspective

AbstractA molecular electron density theory (MEDT) study for the [3 + 2] cycloaddition (32CA) reaction of indoledione and N‐methyl glycine with (E)‐1‐bromonitrostyrene leading to the spirooxindole‐pyrrolidine adduct is presented. Electron localisation function (ELF) study of the in situ generated azomethine ylide classifies a pseudodiradical electronic structure associated with low activation energies. This 32CA reaction is a polar process with electronic flux from the strongly nucleophilic azomethine ylide to the strongly electrophilic nitrostyrene, confirmed from the global electron density transfer (GEDT) above 0.20 e at the transition states (TSs). Parr function analysis predicts the correct regioselectivity from the two‐centre interactions. The reaction is kinetically controlled with negative free energy of reaction which makes it irreversible. The activation enthalpy of the favoured TS is lowered by 3.4, 6.3 and 9.3 kcal mol−1 in methanol relative to the other feasible reaction paths, consistent with the experimentally observed complete regioselectivity and stereoselectivity. ELF topological characterisation along the reaction path predicts a two‐stage one‐step molecular mechanism, and bond formation is controlled by the nucleophilic character of the azomethine ylide. ELF study, Laplacian of electron density and independent gradient model (IGM) analysis predict non‐covalent interactions at the TSs, where the new C–C bond formation has not been started.

Perfluorobicyclo[2.2.0]hex-1(4)-ene as unique partner for Diels\u2013Alder reactions with benzene: a density functional theory study

The mechanism of the Diels–Alder reactions between perfluorobicyclo[2.2.0]hex-1(4)-ene (1a) and bicyclo[2.2.0]hex-1(4)-ene (1b) with benzene (2a) and naphthalene (2b) has been studied within the density functional theory at the MPWB1K/6-311G(d,p) level. The bonding pattern in these reactions is analyzed in the topology of the electron localization function within the bonding evolution theory perspective. The bonding electron density changes along the reaction paths reveal that the C–C bond formation takes place through a synchronous and non-concerted one-step mechanism and proceeds with a moderate activation energy. The reactivity order with 1a is 2a–2b. The reactions begin by the rupture of the double bond in the strained 1a-b molecules, and then two pseudoradical centers at the 1a-b fragments are created. Finally, at the same time, two new single bonds are formed in the cycloaddition products. The TSs proceed with high global electron density transfer providing a polar character at these reactions.

Unveiling the Unexpected Reactivity of Electrophilic Diazoalkanes in [3+2] Cycloaddition Reactions within Molecular Electron Density Theory

The [3+2] cycloaddition (32CA) reactions of strongly nucleophilic norbornadiene (NBD), with simplest diazoalkane (DAA) and three DAAs of increased electrophilicity, have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G (d,p) computational level. These pmr-type 32CA reactions follow an asynchronous one-step mechanism with activation enthalpies ranging from 17.7 to 27.9 kcal·mol−1 in acetonitrile. The high exergonic character of these reactions makes them irreversible. The presence of electron-withdrawing (EW) substituents in the DAA increases the activation enthalpies, in complete agreement with the experimental slowing-down of the reactions, but contrary to the Conceptual DFT prediction. Despite the nucleophilic and electrophilic character of the reagents, the global electron density transfer at the TSs indicates rather non-polar 32CA reactions. The present MEDT study establishes the depopulation of the N–N–C core in this series of DAAs with the increase of the EW character of the substituents present at the carbon center is responsible for the experimentally found deceleration.

Understanding the Reactivity of C-Cyclopropyl-N-Methylnitrone Participating in [3+2] Cycloaddition Reactions Towards Styrene with a Molecular Electron Density Theory Perspective

Abstract. The [3+2] cycloaddition (32CA) reactions of C-cyclopropyl-N-methylnitrone 1 with styrene 2 have been studied within molecular electron density theory (MEDT) at the B3LYP/6-311++G(d,p) level of theory. These zwitterionic type 32CA reactions occur through a one-step mechanism. The 32CA reactions undergo four stereo- and regioisomeric reaction paths to form four different products, 3, 4, 5 and 6. Analysis of the conceptual density functional theory (CDFT) indices predict the global electronic flux from the strong nucleophilic nitrone 1 to the styrene 2. These 32CA reactions are endergonic with reactions Gibbs free energies between 2.83 and 7.39 kcal.mol-1 in the gas phase. The 32CA reaction leading to the formation of cycloadduct 3 presents the lowest activation enthalpy than the other paths due to a slightly increase in polar character evident from the global electron density transfer (GEDT) at the transition states and along the reaction path. The bonding evolution theory (BET) study suggests that these 32CA reactions occur through the coupling of pseudoradical centers and the formation of new C-C and C-O covalent bonds has not been started in the transition states.&#x0D; &#x0D; Resumen.

The catalytic effects of a thiazolium salt in the oxa-Diels-Alder reaction between benzaldehyde and Danishefsky's diene: a molecular electron density theory study.

The oxa-Diels-Alder (ODA) reaction of benzaldehyde with Danishefsky's diene in the presence of a [thiazolium][Cl] salt, as a model of an ionic liquid, has been studied within Molecular Electron Density Theory (MEDT) at the M06-2X/6-311G(d,p) computational level. The formation of two hydrogen bonds (HBs) between the thiazolium cation and the carbonyl oxygen of benzaldehyde modifies neither the electrophilic character of benzaldehyde nor its electronic structure substantially but accelerates the reaction considerably. This ODA reaction presents an activation energy of 4.5 kcal mol-1; the formation of the only observed dihydropyranone is strongly exothermic by -28.8 kcal mol-1. The presence of the [thiazolium][Cl] salt decreases the Gibbs free energy of activation of the ODA reaction between benzaldehyde and Danishefsky's diene by 5.9 kcal mol-1. This ODA reaction presents total para regioselectivity and high endo stereoselectivity. This ODA reaction takes place through a highly asynchronous polar transition state structure (TS) associated with a non-concerted two-stage one-step mechanism. ELF analysis of para/endo TSs associated with the ODA reactions in the absence and presence of the [thiazolium][Cl] salt shows that the formation of the HBs at the TSs does not modify their electronic structure substantially. This MEDT study makes it possible to conclude that the acceleration found in the ODA reaction of benzaldehyde with Danishefsky's diene in ILs is a consequence of an increase of the global electron density transfer at TS3-pn, resulting from HB formation, and the greater strength of the HBs at the polar TS3-pn compared to that at the benzaldehyde : [thiazolium][Cl] complex, and that the strength in the HB formed is more relevant that than an increase of the electrophilic character of the interaction between reagent.

A DFT Study on Adsorption of Alanine on Pristine, Functionalized and Boron and/or Nitrogen Doped Functionalized C60 Fullerenes

In this study, the adsorption of alanine on pristine, functionalized, and boron and/or nitrogen doped functionalized C60 fullerenes was studied by theoretical methods. For this purpose, the structures of alanine, C60 fullerenes derivatives and complexes (C60-alanine) were optimized by using M062X/6-31G* level of theory. Then, it was calculated the adsorption energies, global DFT reactivity indices, the atomic charges and the global electron density transfer (GEDT). The results showed that the maximum adsorption energy occurs for the adsorption of alanine on C60H-OH derivative in both gaseous and aqueous phases. In addition, the doping three nitrogen/boron atoms with the functionalized carbon atom in C60H-OH increases the adsorption energy significantly. The results were confirmed by global DFT reactivity indices such as chemical potential and electrophilicity indices. In addition, analysis of the GEDT values showed that the charge transfer occurs from alanine toward C60H-OH fullerene in both phases upon adsorption. AIM results indicated that the interaction between alanine and C60H-OH in both gaseous and aqueous phases is non-covalent and hydrogen interaction.

The 1,3-dipolar cycloaddition of adamantine-derived nitrones with maleimides: A computational study

This work investigated computationally at the DFT M06-2X/6-311++G(d,p) level the stereo- and enantio-selectivity of the reactions of adamantine-derived nitrones (aldonitrone and ketonitrone) with maleimides, towards the synthesis of compounds containing both isoxazolidine and adamantane units, which are privileged structures in medicinal chemistry. The results show that the thermal 1,3-dipolar cycloaddition (1,3-DCs) reaction of ketonitrone with maleimide leads to the formation of endo and exo-stereoisomers with activation barriers of 2.8 and 4.5 kcal/mol and rate constants of 5.5034 × 1010 and 3.1217 × 109 M−1 s−1 respectively, while the reaction of the aldonitrone with maleimide leads to trans and cis-stereoisomers with activation barriers of 10.3 and 8.0 kcal/mol and rate constants of 1.7480 × 105 and 8.4843 × 106 M−1 s−1 respectively, making the endo-isomer the kinetically preferred product. The endo-isomer is 5.3 kcal/mol more stable than the exo-isomer while the trans and cis-stereoisomers are thermodynamically similar. The calculated global reactivity indices classify the ketonitrone and aldonitrone as, respectively, a strong electrophile and a moderate electrophile, and decreasing or increasing electrophilicity index has no correlation with the activation energies. Global Electron Density Transfer (GEDT) analysis shows that the reaction of ketonitrone with maleimide is more polar than the reaction of aldonitrone with maleimide. The reaction of ketonitrone with maleimide, which has the lowest barrier and the highest rate constant, has the highest polar character.

A MEDT computational study of the mechanism, reactivity and selectivity of non-polar [3+2] cycloaddition between quinazoline-3-oxide and methyl 3-methoxyacrylate.

The Molecular Electron Density Theory (MEDT) was used for the study of the mechanism and the selectivity of the [3+2] cycloaddition reaction between quinazoline-3-oxide and methyl 3-methoxyacrylate, using the B3LYP/6-31G(d,p) DFT method. In gas phase, this [3+2] cycloaddition reaction is characterized by a completely ortho regioselectivity and a moderate exo stereoselectivity. Dichloroethane solvent did not modify the selectivities obtained in gas phase but increase the activation energies and decrease the exothermic character. Analysis of thermodynamic characters indicates that by the inclusion of the experimental conditions, the reaction becomes endergonic and thereby under thermodynamic control favouring the formation of the most stable product as observed experimentally, explaining the exo stereoselectivity. The analysis of the global electron density transfer (GEDT) at the transition states and bond order (BO) show that this reaction takes place via a very slightly synchronous and non-polar one-step mechanism. Conceptual DFT reactivity indices analysis accounts for the electrophilic character of the reagents, explaining the high obtained free activation energies, while local Parr functions analysis allows us to explain the ortho regioselectivity observed experimentally. ELF topological analysis of the most favoured reactive pathways indicates that mechanism of this 32CA reaction is one stage, one step, synchronous and non-concerted. The stability of the favourable cycloadduct is attributed to the presence of different non-conventional hydrogen bonds interactions as indicated by NCI and QTAIM analyses. Graphical Abstract.

A molecular electron density theory study for [3 + 2] cycloaddition reactions of1‐pyrroline‐1‐oxide with disubstituted acetylenes leading to bicyclic 4‐isoxazolines

AbstractThe [3 + 2] cycloaddition (32CA) reactions of 1‐pyrroline‐1‐oxide with acetylene and with a series of four symmetrically disubstituted acetylenes (XC ≡ CX, X = CH3, NH2, OH, F), leading to 4,5‐disubstituted bicyclic 4‐isoxazolines, have been studied within molecular electron density theory at the B3LYP‐D3/6‐311++G(d,p) computational level. These 32CA reactions take place through aone‐stepmechanism, with activation enthalpies in toluene between 24.4 (X = NH2) and 12.9 (X = F) kcal mol−1. Due to the strong exergonic character of these 32CA reactions, between −12.0 and −47.6 kcal mol−1, they are irreversible. The 32CA reaction of difluoroacetylene, involving fluorine as the most electronegative atom of this series attached to the acetylene carbons, presents the lowest activation enthalpy due to the increase of the polar character of this 32CA reaction, evidenced by the analysis of the global electron density transfer at the corresponding transition state structure. The topological analysis of the electron localization function along the reaction paths of these 32CA reactions allows us to characterize them as the zwitterionic‐type ones. The formation of the new CC and CO single bonds takes place at the end of the reactions through the coupling ofpseudoradicalcenters.