Understanding chemical reactivity using the activation strain model

Stereoselectivity Control Interplay in Racemic Lactide Polymerization by Achiral Al-Salen Complexes

The origin of stereocontrol in ring opening polymerization (ROP) of racemic lactide (rac-LA) promoted by achiral aluminium-based catalysts has been explained through DFT calculations combined with a molecular descriptor (%VBur) and the activation strain model (ASM-NEDA) analysis. The proposed chain end control (CEC) model suggests that the ligand framework adopts a chiral configuration mimicking the enantiomorphic site control (ESC) while also incorporating control of the last inserted monomer unit. It is found that the ligand wrapping mode around the aluminium centre is dictated by the monomer configuration (R,R-LA and S,S-LA). A good correlation with experimental data is achieved only when accounting for the ligand dynamic features and its steric influences, as highlighted by %VBur steric maps and ASM-NEDA analysis. Understanding the ESC and CEC interplay is an important target for obtaining stereoselective ROP polymerization for the synthesis of biodegradable materials with tailored properties.

Reactivities of tertiary phosphines towards allenic, acetylenic, and vinylic Michael acceptors

The addition of phosphines (PR3) to Michael acceptors is a key step in many Lewis-base catalysed reactions. The kinetics of the reactions of ten phosphines with ethyl acrylate, ethyl allenoate, ethyl propiolate, ethenesulfonyl fluoride, and ethyl 2-butynoate in dichloromethane at 20 °C was followed by photometric and NMR spectroscopic methods. The experimentally determined second-order rate constants k 2 show that electronic effects in sterically unencumbered phosphines affect their nucleophilicity towards different classes of Michael acceptors in the same ordering. Michael acceptors with sp-hybridised electrophilic centres, however, are less susceptible to changes in the PR3 nucleophilicity than those with sp2-hybridised reactive sites. Linear correlations of lg k 2 from this work with published rate constants for SN2 and SN1 reactions as well as with Brønsted basicities and fugalities for PR3 demonstrate the generality of the detected reactivity trends. Computed reaction barriers (ΔG ‡ calc) as well as reaction energies (ΔG add) for Michael adduct formations show excellent correlations with experimentally obtained reaction barriers (ΔG ‡ exp) corroborating the interpretation of the kinetic data and revealing the philicity/fugality features of the reactants in phospha-Michael additions.

Mechanistic Insight of High-Valent First-Row Transition Metal Complexes for Dehydrogenation of Ammonia Borane

Designing an efficient and cost-effective catalyst for ammonia borane (AB) dehydrogenation remains a persistent challenge in advancing a hydrogen-based economy. Transition metal complexes, known for their C-H bond activation capabilities, have emerged as promising candidates for AB dehydrogenation. In this study, we investigated two recently synthesized C-H activation catalysts, 1 (CoIV-dinitrate complex) and 2 (NiIV-nitrate complex), and demonstrated their efficacy for AB dehydrogenation. Using density functional theory calculations and a detailed analysis, we elucidated the AB dehydrogenation mechanism of these complexes. Our results revealed that both complexes 1 and 2 can efficiently dehydrogenate AB at room temperature, although the abstraction of molecular H2 from these complexes requires slightly elevated temperatures. We utilized H2 binding free energy calculations to identify potentially active sites and observed that complex 2 can release two equivalents of H2 at a temperature slightly higher than room temperature. Furthermore, we investigated AB dehydrogenation kinetics and thermodynamics in iron (Fe)-substituted systems, complexes 3 and 4. Our results showed that the strategic alteration of the central metal atom, replacing Ni in complex 2 with Fe in complex 4, resulted in enhanced kinetics and thermodynamics for AB dehydrogenation in the initial cycle. These results underscore the potential of high-valent first-row transition metal complexes for facilitating AB dehydrogenation at room temperature. Additionally, our study highlights the beneficial impact of incorporating iron into such mononuclear systems, enhancing their catalytic activity.

"Hydridic Hydrogen-Bond Donors" Are Not Hydrogen-Bond Donors

Herein, we dismiss a recent proposal by Civiš, Hobza, and co-workers to modify the IUPAC definition of hydrogen bonds in order to expand the scope from protonic Y-Hδ+ to hydridic Y-Hδ- hydrogen-bond donor fragments [J. Am. Chem. Soc. 2023, 145, 8550]. Based on accurate Kohn-Sham molecular orbital (KS-MO) analyses, we falsify the conclusion that interactions involving protonic and hydridic hydrogens are both hydrogen bonds; they are not. Instead, our quantitative KS-MO, energy decomposition, and Voronoi deformation density analyses reveal two fundamentally different bonding mechanisms for protonic Y-Hδ+ and hydridic Y-Hδ- fragments which go with charge transfer in opposite directions. On one hand, we confirm the IUPAC definition for regular hydrogen bonds in the case of protonic Y-Hδ+ fragments. On the other hand, complexes involving Y-Hδ- fragments are, in fact, acceptors in other well-known families of Lewis-acid/base interactions, such as halogen bonds, chalcogen bonds, and pnictogen bonds. These mechanisms lead to the same spectroscopic phenomenon in both the Y-Hδ+ and Y-Hδ- fragments, that is, the redshift in the Y-H stretching frequency, which is, thus, not an exclusive indicator for hydrogen bonding.

Phthalazine as a Diene in Diels-Alder Reactions With P- and As-Containing Anionic Dienophiles: Comparison of Possible Reaction Channels

Phthalazine can behave as a diene in Diels-Alder (DA) cycloadditions, typically at the pyridazine ring, however, its application is somewhat limited because these reactions usually require harsh conditions or sophisticated catalysts. As an unconventional example, phthalazine was reported to undergo cycloaddition with the [PCO]- anion without any catalyst. In this computational study, we scrutinise the mechanism of the DA reactions between phthalazine and the so far known [ECX]- (E: P, As; X: O, S, Se) anions as dienophiles. In principle, the attack of an [ECX]- anion may occur at two different sites of phthalazine, either at the benzene or the pyridazine ring, and both of these possible reaction channels were juxtaposed on the basis of energetic aspects. In all of the investigated cases, the analysis of the energy profiles reveals a clear regioselectivity that favours the attack at the pyridazine ring. As a result, so far unprecedented 2-pnictanaphth-3-olate analogues seem achievable as final products. Comparing the characteristics of these pathways allowed us to clarify the source of this regioselectivity: The pyridazine ring of phthalazine exhibits lower aromaticity than the benzene subring; therefore, in the DA step, the former ring shows a higher affinity toward a dienophile than the latter, leading to lower activation barriers. To further map the electronic and structural features of the cycloaddition steps, the local interactions evolving in the transition states were analysed and compared using global and local descriptors. In most aspects, the characteristics of both pathways were found to be rather similar, in contrast to the markedly differing activation barriers on the two routes.

Reactivity of Zinc Fingers in Oxidizing Environments: Insight from Molecular Models Through Activation Strain Analysis

The reactivity of Zn2+ tetrahedral complexes with H2O2 was investigated in silico, as a first step in their disruption process. The substrates were chosen to represent the cores of three different zinc finger protein motifs, i. e., a Zn2+ ion coordinated to four cysteines (CCCC), to three cysteines and one histidine (CCCH), and to two cysteines and two histidines (CCHH). The cysteine and histidine ligands were further simplified to methyl thiolate and imidazole, respectively. H2O2 was chosen as an oxidizing agent due to its biological role as a metabolic product and species involved in signaling processes. The mechanism of oxidation of a coordinated cysteinate to sulfenate-κS and the trends for the different substrates were rationalized through activation strain analysis and energy decomposition analysis in the framework of scalar relativistic Density Functional Theory (DFT) calculations at ZORA-M06/TZ2P ae // ZORA-BLYP-D3(BJ)/TZ2P. CCCC is oxidized most easily, an outcome explained considering both electrostatic and orbital interactions. The isomerization to sulfenate-κO was attempted to assess whether this step may affect the ligand dissociation; however, it was found to introduce a kinetic barrier without improving the energetics of the dissociation. Lastly, ligand exchange with free thiolates and selenolates was investigated as a trigger for ligand dissociation, possibly leading to metal ejection; molecular docking simulations also support this hypothesis.

Retro-Cope elimination of cyclic alkynes: reactivity trends and rational design of next-generation bioorthogonal reagents

The retro-Cope elimination reaction between dimethylhydroxylamine (DMHA) and various cyclic alkynes has been quantum chemically explored using DFT at ZORA-BP86/TZ2P. The purpose of this study is to understand the role of the following three unique activation modes on the overall reactivity, that is (i) additional cycloalkyne predistortion via fused cycles, (ii) exocyclic heteroatom substitution on the cycloalkyne, and (iii) endocyclic heteroatom substitution on the cycloalkyne. Trends in reactivity are analyzed and explained by using the activation strain model (ASM) of chemical reactivity. Based on our newly formulated design principles, we constructed a priori a suite of novel bioorthogonal reagents that are highly reactive towards the retro-Cope elimination reaction with DMHA. Our findings offer valuable insights into the design principles for highly reactive bioorthogonal reagents in chemical synthesis.

Structural Flexibility is a Decisive Factor in FLP Dihydrogen Cleavage with Tetrahedral Lewis Acids: A Silane Case Study

Dihydrogen activation is the paradigmatic reaction of frustrated Lewis pairs (FLPs). While trigonal-planar Lewis acids have been well established in this transformation, tetrahedral Lewis acids are surprisingly limited. Indeed, several cases were computed as thermodynamically and kinetically feasible but exhibit puzzling discrepancies with experimental results. In the present study, a computational investigation of the factors influencing dihydrogen activation are considered by large ensemble sampling of encounter complexes, deformation energies and the activation strain model for a silicon/nitrogen FLP and compared with a boron/phosphorous FLP. The analysis adds the previously missing dimension of Lewis acids' structural flexibility as a factor that influences preexponential terms beyond pure transition state energies. It sheds light on the origin of "overfrustration" (defined herein), indicates structural constraint in Lewis acids as a linchpin for activation of weak donor substrates, and allows drawing a more refined mechanistic picture of this emblematic reactivity.

Unraveling C-Selective Ring-Opening of Phosphiranes with Carboxylic Acids and Other Nucleophiles: A Mechanistically-Driven Approach

Phosphiranes are weak Lewis bases reacting with only a limited number of electrophiles to produce the corresponding phosphiranium ions. These salts are recognized for their propensity to undergo reactions with oxygen pronucleophiles at the phosphorus site, leading to the formation of phosphine oxide adducts. Building on a thorough mechanistic understanding, we have developed an unprecedented approach that enables the selective reaction of carboxylic acids, and other nucleophiles, at the carbon site of phosphiranes. This method involves the photochemical generation of highly reactive carbenes, which react with 1-mesitylphosphirane to yield ylides. The latter undergoes a stepwise reaction with carboxylic acids, resulting in the production of the desired phosphines. In addition to DFT calculations, we have successfully isolated and fully characterized the key intermediates involved in the reaction.

Origin of the Felkin-Anh(-Eisenstein) model: a quantitative rationalization of a seminal concept

Quantum chemical calculations were carried out to quantitatively understand the origin of the Felkin-Anh(-Eisenstein) model, widely used to rationalize the π-facial stereoselectivity in the nucleophilic addition reaction to carbonyl groups directly attached to a stereogenic center. To this end, the possible approaches of cyanide to both (S)-2-phenylpropanal and (S)-3-phenylbutan-2-one have been explored in detail. With the help of the activation strain model of reactivity and the energy decomposition analysis method, it is found that the preference for the Felkin-Anh addition is mainly dictated by steric factors which manifest in a less destabilizing strain-energy rather than, as traditionally considered, in a lower Pauli repulsion. In addition, other factors such as the more favorable electrostatic interactions also contribute to the preferred approach of the nucleophile. Our work, therefore, provides a different, more complete rationalization, based on quantitative analyses, of the origin of this seminal and highly useful concept in organic chemistry.

The nature of metallophilic interactions in closed-shell d8-d8 metal complexes

We have quantum chemically analyzed the closed-shell d8-d8 metallophilic interaction in dimers of square planar [M(CO)2X2] complexes (M = Ni, Pd, Pt; X = Cl, Br, I) using dispersion-corrected density functional theory at ZORA-BLYP-D3(BJ)/TZ2P level of theory. Our purpose is to reveal the nature of the [X2(CO)2M]⋯[M(CO)2X2] bonding mechanism by analyzing trends upon variations in M and X. Our analyses reveal that the formation of the [M(CO)2X2]2 dimers is favored by an increasingly stabilizing electrostatic interaction when the M increases in size and by more stabilizing dispersion interactions promoted by the larger X. In addition, there is an overlooked covalent component stemming from metal-metal and ligand-ligand donor-acceptor interactions. Thus, at variance with the currently accepted picture, the d8-d8 metallophilicity is attractive, and the formation of [M(CO)2X2]2 dimers is not a purely dispersion-driven phenomenon.

Untangling the catalytic importance of the Se oxidation state in organoselenium-mediated oxygen-transfer reactions: the conversion of aniline to nitrobenzene

Seleninic acids and their precursors are well-known oxygen-transfer agents that can catalyze several oxidations with H2O2 as the final oxidant. Until very recently, the Se(iv) "peroxyseleninic" acid species has been considered the only plausible catalytic oxidant. Conversely, in 2020, the involvement of Se(vi) "peroxyselenonic" acid has been proposed for the selenium mediated epoxidation of alkenes. In this work, we theoretically probe different mechanisms of H2O2 activation and of Se(iv) to Se(vi) interconversion. In addition, we investigate through a combined theoretical (DFT) and experimental approach the mechanistic steps leading to the oxidation of aniline to nitrobenzene, when Se(iv) seleninic acid or Se(vi) selenonic acids are used as catalysts and H2O2 as the oxidant. This process encompasses three subsequent organoselenium mediated oxidations by H2O2. These results provide a mechanistic explanation of the advantages and disadvantages of both oxidation states (iv and vi) in the different stages of catalytic oxygen-transfer reactions: hydrogen peroxide activation and actual substrate oxidation. While the Se(vi) "peroxyselenonic" acid is found to be a better oxidant, the privileged role of "peroxyseleninic" acid as the main active species is assessed and its origin is identified in the lower catalyst-distortion that seleninic acid undergoes when activating H2O2. Conversely, the higher catalyst-distortion that characterizes the reaction of selenonic acid with H2O2 supports an inactivating role of Se(iv) to Se(vi) interconversion.

Chalcogen Noncovalent Interactions between Diazines and Sulfur Oxides in Supramolecular Circular Chains

The noncovalent chalcogen interaction between SO2/SO3 and diazines was studied through a dispersion-corrected DFT Kohn-Sham molecular orbital together with quantitative energy decomposition analyses. For this, supramolecular circular chains of up to 12 molecules were built with the aim of checking the capability of diazine molecules to detect SO2/SO3 compounds within the atmosphere. Trends in the interaction energies with the increasing number of molecules are mainly determined by the Pauli steric repulsion involved in these σ-hole/π-hole interactions. But more importantly, despite the assumed electrostatic nature of the involved interactions, the covalent component also plays a determinant role in its strength in the involved chalcogen bonds. Noticeably, π-hole interactions are supported by the charge transfer from diazines to SO2/SO3 molecules. Interaction energies in these supramolecular complexes are not only determined by the S···N bond lengths but attractive electrostatic and orbital interactions also determine the trends. These results should allow us to establish the fundamental characteristics of chalcogen bonding based on its strength and nature, which is of relevance for the capture of sulfur oxides.

Competing C and N as Reactive Centers for Microsolvated Ambident Nucleophiles CN-(H2O)n=0-3: A Theoretical Study of E2/SN2 Reactions with CH3CH2X (X = Cl, Br, I)

As an ambident nucleophile, CN- has both C and N atoms that can act as the reactive center to facilitate substitution reactions. We investigate in detail the potential energy profiles of CN-(H2O)0-3 with CH3CH2X (X = Cl, Br, I) to explore the influence of solvent molecules on competition between the different nucleophilic atoms C and N involving the SN2 and E2 pathways. The energy barrier sequence for the transition states follows C@inv-SN2 < N@inv-SN2 < C@anti-E2 < N@anti-E2. When two different atoms act as nucleophilic atoms, the SN2 reaction is always preferred over the E2 reaction, and this preference increases with microsolvation. For the ambident nucleophiles CN-(H2O)0-3, C as the reactive center always has stronger nucleophilicity and basicity than N acting as the reactive center. Regarding the leaving group, the height of the energy barrier is positively correlated with the acidity of the CH3CH2X substrate for the E2 pathway and with X-heterolysis for the SN2 pathway. Furthermore, we found that in the gas phase, the energy barrier for different leaving group systems decreases gradually in the order Cl > Br > I, while in the SMD solution, the energy barrier and product energy increase slightly in the system from X = Cl to Br; this change may be due to the significantly weakened transition-state interaction for the X = Br system. Our activation strain, quantitative molecular orbital, and charge analyses reveal the physical mechanisms underlying the various computed trends. In addition, we also demonstrate the two points recently proposed by Vermeeren, P. . Chem. Eur. J. 2020, 26, 15538-15548.

Modular Divergent Synthesis of Indole Alkaloid Derivatives by an Atypical Ugi Multicomponent Reaction

We present an Ugi multicomponent approach to explore the chemical space around Aspidosperma-type monoterpene indole alkaloids. By variation of the isocyanide and carboxylic acid inputs we demonstrate the rapid generation of molecular diversity and the possibility to introduce handles for further modification. The key Ugi three-component reaction showed full diastereoselectivity towards the cis-fused ring system, which can be rationalized by DFT calculations that moreover indicate that the reaction proceeds via a Passerini-type hydrogen bonding mechanism. Several post-Ugi modifications were also performed, including Pictet-Spengler cyclization to highly complex nonacyclic natural product hybrid scaffolds.

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.

Revisiting Stereoselective Propene Polymerization Mechanisms: Insights through the Activation Strain Model

The stereoelectronic factors responsible for stereoselectivity in propene polymerization with several metallocene and post-metallocene transition metal catalysts have been revisited using a combined approach of DFT calculations, the Activation Strain Model, Natural Energy Decomposition Analysis and a molecular descriptor (%VBur). There are in most cases two different paths leading to the formation of stereoerrors (SE), and the classical model does not suffice to fully understand stereoregulation. Improving stereoselectivity requires raising the energies of both SE insertion transition states. Our analyses show that the degrees of deformation of the active site (catalyst+chain) and the prochiral monomer differ for these two paths, and between different catalyst classes. Based on such analyses we discuss: a) the subtle differences in SE formation between stereoselective catalysts with different ligand frameworks; b) the reason for exceptional stereoselectivity reported for a special ansa-metallocene catalyst; c) the (double) stereocontrol origin for isoselective catalysts; d) the electronic contribution for isoselective catalysts generating SE by a modification of the ligand wrapping mode during the polymerization. Although this study will not immediately suggest new catalyst structures, we believe that understanding stereoregulation in great detail will increase our chances of success.

Backside versus Frontside SN2 Reactions of Alkyl Triflates and Alcohols

Nucleophilic substitution reactions are elementary reactions in organic chemistry that are used in many synthetic routes. By quantum chemical methods, we have investigated the intrinsic competition between the backside SN2 (SN2-b) and frontside SN2 (SN2-f) pathways using a set of simple alkyl triflates as the electrophile in combination with a systematic series of phenols and partially fluorinated ethanol nucleophiles. It is revealed how and why the well-established mechanistic preference for the SN2-b pathway slowly erodes and can even be overruled by the unusual SN2-f substitution mechanism going from strong to weak alcohol nucleophiles. Activation strain analyses disclose that the SN2-b pathway is favored for strong alcohol nucleophiles because of the well-known intrinsically more efficient approach to the electrophile resulting in a more stabilizing nucleophile-electrophile interaction. In contrast, the preference of weaker alcohol nucleophiles shifts to the SN2-f pathway, benefiting from a stabilizing hydrogen bond interaction between the incoming alcohol and the leaving group. This hydrogen bond interaction is strengthened by the increased acidity of the weaker alcohol nucleophiles, thereby steering the mechanistic preference toward the frontside SN2 pathway.

Solvent-induced dual nucleophiles and the α-effect in the SN2 versus E2 competition

We have quantum chemically investigated how microsolvation affects the various E2 and SN2 pathways, their mutual competition, and the α-effect of the model reaction system HOO-(H2O)n + CH3CH2Cl, at the CCSD(T) level. Interestingly, we identify the dual nature of the α-nucleophile HOO- which, upon solvation, is in equilibrium with HO-. This solvent-induced dual appearance gives rise to a rich network of competing reaction channels. Among both nucleophiles, SN2 is always favored over E2, and this preference increases upon increasing microsolvation. Furthermore, we found a pronounced α-effect, not only for SN2 substitution but also for E2 elimination, i.e., HOO- is more reactive than HO- in both cases. Our activation strain and quantitative molecular orbital analyses reveal the physical mechanisms behind the various computed trends. In particular, we demonstrate that two recently proposed criteria, required for solvent-free nucleophiles to display the α-effect, must also be satisfied by microsolvated HOO-(H2O)n nucleophiles.

Cobalt-Catalyzed Enantio- and Regioselective C(sp3 )-H Alkenylation of Thioamides

An enantioselective cobalt-catalyzed C(sp3 )-H alkenylation of thioamides with but-2-ynoate ester coupling partners employing thioamide directing groups is presented. The method is operationally simple and requires only mild reaction conditions, while providing alkenylated products as single regioisomers in excellent yields (up to 85 %) and high enantiomeric excess [up to 91 : 9 enantiomeric ratio (er), or up to >99 : 1 er after a single recrystallization]. Diverse downstream derivatizations of the products are demonstrated, delivering a range of enantioenriched constructs. Extensive computational studies using density functional theory provide insight into the detailed reaction mechanism, origin of enantiocontrol, and the unusual regioselectivity of the alkenylation reaction.

Mapping the Catalytic-Space for the Reactivity of Metal-free Boron Nitride with O2 for H2O-Mediated Conversion of Methane to HCHO and CO

Transition-metal based catalysts have been widely employed to catalyze partial oxidation of light alkanes. Recently, metal-free hexagonal-boron nitride (h-BN) has emerged as a promising catalyst for the oxidation of CH4 to HCHO and CO; however, the intricate catalytic surface of h-BN at molecular and electronic levels remains inadequately understood. Key questions include how electron-deficient boron atoms in h-BN reduce O2, and whether the partial oxidation of methane over h-BN exhibits similarities to traditional transition-metal catalysts. In our study, we computationally-mapped in-detail the surface catalytic-space of h-BN for methane oxidation. We considered different structures of h-BN and show that these structures contain numerous sites for O2 binding and therefore various routes for methane oxidation are possible. The activation barriers for methane oxidation via various paths varies from ~83 to ~123 kcal mol-1. To comprehend the differences in activation barriers, we employed geometrical, orbital and distortion/interaction analysis (DIA). Orbital analysis reveals that methane activation over h-BN in presence of dioxygen follows a standard hydrogen atom transfer mechanism. It is also shown that water plays an intriguing role in reducing the barrier for HCHO and CO formation by acting as a bridge.

What defines electrophilicity in carbonyl compounds

The origin of the electrophilicity of a series of cyclohexanones and benzaldehydes is investigated using the activation strain model and quantitative Kohn-Sham molecular orbital (MO) theory. We find that this electrophilicity is mainly determined by the electrostatic attractions between the carbonyl compound and the nucleophile (cyanide) along the entire reaction coordinate. Donor-acceptor frontier molecular orbital interactions, on which the current rationale behind electrophilicity trends is based, appear to have little or no significant influence on the reactivity of these carbonyl compounds.

The Cambridge Structural Database and structural dynamics

With the availability of the computer readable information in the Cambridge Structural Database (CSD), wide ranging, largely automated comparisons of fragment, molecular, and crystal structures have become possible. They show that the distributions of interatomic distances, angles, and torsion angles for a given structural fragment occurring in different environments are highly correlated among themselves and with other observables such as spectroscopic signals, reaction and activation energies. The correlations often extend continuously over large ranges of parameter values. They are reminiscent of bond breaking and forming reactions, polyhedral rearrangements, and conformational changes. They map-qualitatively-the regions of the structural parameter space in which molecular dynamics take place, namely, the low energy regions of the respective (free) energy surfaces. The extension and continuous nature of the correlations provides an organizing principle of large groups of structural data and suggests a reconsideration of traditional definitions and descriptions of bonds, "nonbonded" and "noncovalent" interactions in terms of Lewis acids interacting with Lewis bases. These aspects are illustrated with selected examples of historic importance and with some later developments. It seems that the amount of information in the CSD (and other structural databases) and the knowledge on the nature of, and the correlations within, this body of information should allow one-in the near future-to make credible interpolations and possibly predictions of structures and their properties with machine learning methods.

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.

Stabilization of Diborynes versus Destabilization of Diborenes by Coordination of Lewis Bases: Unravelling the Dichotomy

We have quantum chemically investigated the boron-boron bonds in B2 , diborynes B2 L2 , and diborenes B2 H2 L2 (L=none, OH2 , NH3 ) using dispersion-corrected relativistic density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. B2 has effectively a single B-B bond provided by two half π bonds, whereas B2 H2 has effectively a double B=B bond provided by two half π bonds and one σ 2p-2p bond. This different electronic structure causes B2 and B2 H2 to react differently to the addition of ligands. Thus, in B2 L2 , electron-donating ligands shorten and strengthen the boron-boron bond whereas, in B2 H2 L2 , they lengthen and weaken the boron-boron bond. The aforementioned variations in boron-boron bond length and strength become more pronounced as the Lewis basicity of the ligands L increases.

Nature and strength of group-14 A-A' bonds

We have quantum chemically investigated the nature and stability of C-C and Si-Si bonds in R3A-AR3 (A = C, Si; R3 = H3, Me3, Me2Ph, MePh2, Ph3, t-Bu3) using density functional theory (DFT). Systematic increase of steric bulk of the substituents R has opposite effects on C-C and Si-Si bonds: the former becomes weaker whereas the latter becomes stronger. Only upon going further, from R = Ph to the bulkiest R = t-Bu, the R3Si-SiR3 bond begins to weaken. Our bonding analyses show how different behavior upon increasing the steric bulk of the substituents stems from the interplay of (Pauli) repulsive and (dispersion) attractive steric mechanisms. Extension of our analyses to other model systems shows that C-Si bonds display behavior that is in between that of C-C and Si-Si bonds. Further increasing the size of the group-14 atoms from C-C and Si-Si to Ge-Ge, Sn-Sn and Pb-Pb leads to a further decrease in the sensitivity of the bond strength with respect to the substituents' bulkiness. Our findings can be used as design principles for tuning A-A and A-A' bond strengths.

Exploring borderline SN1-SN2 mechanisms: the role of explicit solvation protocols in the DFT investigation of isopropyl chloride

Nucleophilic substitution at saturated carbon is a crucial class of organic reactions, playing a pivotal role in various chemical transformations that yield valuable compounds for society. Despite the well-established SN1 and SN2 mechanisms, secondary substrates, particularly in solvolysis reactions, often exhibit a borderline pathway. A molecular-level understanding of these processes is fundamental for developing more efficient chemical transformations. Typically, quantum-chemical simulations of the solvent medium combine explicit and implicit solvation methods. The configuration of explicit molecules can be defined through top-down approaches, such as Monte Carlo (MC) calculations for generating initial configurations, and bottom-up methods that involve user-dependent protocols to add solvent molecules around the substrate. Herein, we investigated the borderline mechanism of the hydrolysis of a secondary substrate, isopropyl chloride (iPrCl), at DFT-M06-2X/aug-cc-pVDZ level, employing explicit and explicit + implicit protocols. Top-down and bottom-up approaches were employed to generate substrate-solvent complexes of varying number (n = 1, 3, 5, 7, 9, and 12) and configurations of H2O molecules. Our findings consistently reveal that regardless of the solvation approach, the hydrolysis of iPrCl follows a loose-SN2-like mechanism with nucleophilic solvent assistance. Increasing the water cluster around the substrate in most cases led to reaction barriers of ΔH‡ ≈ 21 kcal mol-1, with nine water molecules from MC configurations sufficient to describe the reaction. The More O'Ferrall-Jencks plot demonstrates an SN1-like character for all transition state structures, showing a clear merged profile. The fragmentation activation strain analyses indicate that energy barriers are predominantly controlled by solvent-substrate interactions, supported by the leaving group stabilization assessed through CHELPG atomic charges.

Cooperative small molecule activation by apolar and weakly polar bonds through the lens of a suitable computational protocol

Small molecule activation processes are central in chemical research and cooperativity is a valuable tool for the fine-tuning of the efficiency of these reactions. In this contribution, we discuss recent and remarkable examples in which activation processes are mediated by bimetallic compounds featuring apolar or weakly polar metal-metal bonds. Relevant experimental breakthroughs are thoroughly analyzed from a computational perspective. We highlight how the rational and non-trivial application of selected computational approaches not only allows rationalization of the observed reactivities but also inferring of general principles applicable to activation processes, such as the breakdown of the structure-reactivity relationship in carbon dioxide activation in a cooperative framework. We finally provide a simple yet unbiased computational protocol to study these reactions, which can support experimental advances aimed at expanding the range of applications of apolar and weakly polar bonds as catalysts for small molecule activation.

Photoexcitation of Distinct Divalent Palladium Complexes in Cross-Coupling Amination Under Air

The development of metal complexes that function as both photocatalyst and cross-coupling catalyst remains a challenging research topic. So far, progress has been shown in palladium(0) excited-state transition metal catalysis for the construction of carbon-carbon bonds where the oxidative addition of alkyl/aryl halides to zero-valent palladium (Pd0 ) is achievable at room temperature. In contrast, the analogous process with divalent palladium (PdII ) is uphill and endothermic. For the first time, we report that divalent palladium can act as a light-absorbing species that undergoes double excitation to realize carbon-nitrogen (C-N) cross-couplings under air. Differently substituted aryl halides can be applied in the mild, and selective cross-coupling amination using palladium acetate as both photocatalyst and cross-coupling catalyst at room temperature. Density functional theory studies supported by mechanistic investigations provide insight into the reaction mechanism.

Blueshift in Trifurcated Hydrogen Bonds: A Tradeoff between Tetrel Bonding and Steric Repulsion

We have quantum chemically investigated the origin of the atypical blueshift of the H-C bond stretching frequency in the hydrogen-bonded complex X- •••H3 C-Y (X, Y=F, Cl, Br, I), as compared to the corresponding redshift occurring in Cl- •••H3 N and Cl- •••H3 C-H, using relativistic density functional theory (DFT) at ZORA-BLYP-D3(BJ)/QZ4P. Previously, this blueshift was attributed, among others, to the contraction of the H-C bonds as the H3 C moiety becomes less pyramidal. Herein, we provide quantitative evidence that, instead, the blueshift arises from a direct and strong X- •••C interaction of the HOMO of A- with the backside lobe on carbon of the low-lying C-Y antibonding σ* LUMO of the H3 C-Y fragment. This X- •••C bond, in essence a tetrel bond, pushes the H atoms towards a shorter H-C distance and makes the H3 C moiety more planar. The blueshift may, therefore, serve as a diagnostic for tetrel bonding.

More Electropositive is More Electronegative: Atom Size Determines C=X Group Electronegativity

Opposite to what one might expect, we find that the C=X group can become effectively more, not less, electronegative when the Pauling electronegativity of atom X decreases down Groups 16, 15, and 14 of the Periodic Table. Our quantum-chemical analyses, show that, and why, this phenomenon is a direct consequence of the increasing size of atom X down a group. These findings can be applied to tuning and improving the hydrogen-bond donor strength of amides H2 NC(=X)R by increasingly withdrawing density from the NH2 group. A striking example is that H2 NC(=SiR2 )R is a stronger hydrogen-bond donor than H2 NC(=CR2 )R.

Carbon dioxide activation by discandium dioxide cations in the gas phase: a combined investigation of infrared photodissociation spectroscopy and DFT calculations

We present a combined computational and experimental study of CO2 activation at the Sc2O2+ metal oxide ion center in the gas phase. Density functional theory calculations on the structures of [Sc2O2(CO2)n]+ (n = 1-4) ion-molecule complexes reveal a typical end-on binding motif as well as bidentate and tridentate carbonate-containing configurations. As the number of attached CO2 molecules increases, activated forms tend to dominate the isomeric populations. Distortion energies are unveiled to account for the conversion barriers from molecularly bound isomers to carbonate structures, and show a monotonically decreasing trend with successive CO2 ligand addition. The infrared photodissociation spectra of target ion-molecule complexes were recorded in the 2100-2500 cm-1 frequency region and interpreted by comparison with simulated IR spectra of low-lying isomers representing distinct configurations, demonstrating a high possibility of carbonate structure formation in current experiments.

Atropisomerism about aryl-C(sp3) bonds: chemically driven rotational pathway in cannabidiol derivatives

The conformational behaviour arising from the restricted C(sp2)-C(sp3) axis in ortho O-substituted naphthylcyclohexane and naphthylcyclohexene oxide derivatives of cannabidiol was examined by means of VT-NMR experiments and DFT calculations. Atropisomeric compounds with barriers in the range of 91.1 to 95.1 kJ mol-1 were obtained at 298 K. Two possible transition states (TS1 and TS2) were located, one is more stable depending on the chemical modification made on the monoterpene ring close to the pivot bond. Extended analysis of TS structures to previously reported phenyl derivatives bearing the same O-substituent led to similar rotational pathways according to the series: through TS1 in arylcylohexenes and TS2 in arylcyclohexanes. Likewise, conversion of arylcyclohexenes into both series affects the rotation speed by decelerating it, and the nature of the aryl ring seems to have a very minor effect on this phenomenon.

Strengthened cooperativity of DNA-based cyclic hydrogen-bonded rosettes by subtle functionalization

Cooperative effects cause extra stabilization of hydrogen-bonded supramolecular systems. In this work we have designed hydrogen-bonded rosettes derived from a guanine-cytosine Janus-type motif with the aim of finding a monomer that enhances the synergy of supramolecular systems. For this, relativistic dispersion-corrected density functional theory computations have been performed. Our proposal involves a monomer with three hydrogen-bonds pointing in the same direction, which translates into shorter bonds, stronger donor-acceptor interactions, and more attractive electrostatic interactions, thus giving rise to rosettes with strengthened cooperativity. This newly designed rosette has triple the cooperativity found for the naturally occurring guanine quadruplex.

Unraveling the Bürgi-Dunitz Angle with Precision: The Power of a Two-Dimensional Energy Decomposition Analysis

Understanding the geometrical preferences in chemical reactions is crucial for advancing the field of organic chemistry and improving synthetic strategies. One such preference, the Bürgi-Dunitz angle, is central to nucleophilic addition reactions involving carbonyl groups. This study successfully employs a novel two-dimensional Distortion-Interaction/Activation-Strain Model in combination with a two-dimensional Energy Decomposition Analysis to investigate the origins of the Bürgi-Dunitz angle in the addition reaction of CN- to (CH3)2C═O. We constructed a 2D potential energy surface defined by the distance between the nucleophile and carbonylic carbon atom and by the attack angle, followed by an in-depth exploration of energy components, including strain and interaction energy. Our analysis reveals that the Bürgi-Dunitz angle emerges from a delicate balance between two key factors: strain energy and interaction energy. High strain energy, as a result of the carbonyl compound distorting to avoid Pauli repulsion, is encountered at high angles, thus setting the upper bound. On the other hand, interaction energy is shaped by a dominant Pauli repulsion when the angles are lower. This work emphasizes the value of the 2D Energy Decomposition Analysis as a refined tool, offering both quantitative and qualitative insights into chemical reactivity and selectivity.

Role of Group 12 Metals in the Reduction of H2O2 by Santi's Reagent: A Computational Mechanistic Investigation

PhSeZnCl, which is also known as Santi's reagent, can catalyze the reduction of hydrogen peroxide by thiols with a GPx-like mechanism. In this work, the first step of this catalytic cycle, i.e., the reduction of H2O2 by PhSeZnCl, is investigated in silico using state-of-the-art density functional theory calculations. Then, the role of the metal is evaluated by replacing Zn with its group 12 siblings (Cd and Hg). The thermodynamic and kinetic factors favoring Zn are elucidated. Furthermore, the role of the halogen is considered by replacing Cl with Br in all three metal compounds, and this turns out to be negligible. Finally, the overall GPx-like mechanism of PhSeZnCl and PhSeZnBr is discussed by evaluating the energetics of the mechanistic path leading to the disulfide product.

Carbon-carbon bond activation by Mg, Al, and Zn complexes

Examples of carbon-carbon bond activation reactions at Mg, Al, and Zn are described in this review. Several distinct mechanisms for C-C bond activation at these metals have been proposed, with the key C-C bond activation step occurring by (i) α-alkyl elimination, (ii) β-alkyl elimination, (iii) oxidative addition, or (iv) an electrocyclic reaction. Many of the known pathways involve an overall 2-electron redox process. Despite this, the direct oxidative addition of C-C bonds to these metals is relatively rare, instead most reactions occur through initial installation of the metal on a hydrocarbon scaffold (e.g. by a cycloaddition reaction or hydrometallation) followed by an α-alkyl or β-alkyl elimination step. Emerging applications of Mg, Al, and Zn complexes as catalysts for the functionalisation of C-C bonds are also discussed.

Understanding the reactivity and selectivity of Diels-Alder reactions involving furans

The reactivity and endo/exo selectivity of the Diels-Alder cycloaddition reactions involving furan and substituted furans as dienes have been computationally explored. In comparison to cyclopentadiene, it is found that furan is comparatively less reactive and also less endo-selective in the reaction with maleic anhydride as the dienophile. Despite that, both the reactivity and the selectivity can be successfully modified by the presence of substituents at either 2- or 3-positions of the heterocycle. In this sense, it is found that the presence of strong electron-donor groups significantly increases the reactivity of the system while the opposite is found in the presence of electron-withdrawing groups. The observed trends in both the reactivity and selectivity are analyzed quantitatively in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis methods.

Decoding Chemical Bonds: Assessment of the Basis Set Effect on Overlap Electron Density Descriptors and Topological Properties in Comparison to QTAIM

Quantum chemical bonding descriptors based on the total and overlap density can provide valuable information about chemical interactions in different systems. However, these descriptors can be sensitive to the basis set used. To address this, different numerical treatments of electron density have been proposed to reduce the basis set dependency. In this work, we introduce overlap properties (OPs) obtained through numerical treatment of the electron density and present the topology of overlap density (TOP) for the first time. We compare the basis set dependency of numerical OP and TOP descriptors with their quantum theory of atoms in molecules (QTAIM) counterparts, considering the total electron density. Three single (C-C, C-O, and C-F) bonds in ethane, methanol, and fluoromethane and two double (C═C and C═O) bonds in ethene and formaldehyde were analyzed. Diatomic molecules Li-X with X = F, Cl, and Br were also analyzed. Eight parameters, including QTAIM descriptors and OP/TOP descriptors, are used to assess the basis dependency at the ωB97X-D level of theory using 28 basis sets from three classes: Pople, Ahlrichs, and Dunning. The study revealed that the topological overlap electron density properties exhibit comparatively lesser dependence on the basis set compared to their total electron density counterparts. Remarkably, these properties retain their chemical significance even with reduced basis set dependency. Similarly, numerical OP descriptors show less basis set dependency than their QTAIM counterparts. The excess of polarization functions increases charge concentration in the interatomic region and influences both QTAIM and OP descriptors. The basis sets Def2TZVP, 6-31++G(d,p), 6-311++G(d,p), cc-pVDZ, cc-pVTZ, and cc-pVQZ demonstrate reduced variability for the tested bond classes in this study, with particular emphasis on the triple-ζ quality Ahlrichs' basis set. We recommend against using basis sets with numerous polarization functions, such as augmented Dunning's and Ahlrichs' quadruple-ζ.

Beryllium compounds for the carbon-halogen bond activation of phenyl halides: the role of non-innocent ligands

Beryllium is a metallomimetic main-group element, i.e., it behaves similarly to transition metals (TMs) in some bond activation processes. To investigate the ability of Be compounds to activate C-X bonds (X = F-I), we have computationally investigated, using DFT methods, the reaction of (CAAC)2Be (CAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) and a series of five-membered heterocyclic beryllium bidentate ligands with phenyl halides. We have analysed all plausible reaction mechanisms and our results show that, after the initial C-X oxidative addition, migration of the phenyl group occurs towards the less electronegative heteroatom. Our theoretical study highlights the important role of bidentate non-innocent ligands in providing the required electrons for the initial Ph-X oxidative addition. In contrast, the monodentate ligand, CAAC, does not favour this oxidative addition.

Correlating Metal Redox Potentials to Co(III)K(I) Catalyst Performances in Carbon Dioxide and Propene Oxide Ring Opening Copolymerization

Carbon dioxide copolymerization is a front-runner CO2 utilization strategy but its viability depends on improving the catalysis. So far, catalyst structure-performance correlations have not been straightforward, limiting the ability to predict how to improve both catalytic activity and selectivity. Here, a simple measure of a catalyst ground-state parameter, metal reduction potential, directly correlates with both polymerization activity and selectivity. It is applied to compare performances of 6 new heterodinuclear Co(III)K(I) catalysts for propene oxide (PO)/CO2 ring opening copolymerization (ROCOP) producing poly(propene carbonate) (PPC). The best catalyst shows an excellent turnover frequency of 389 h-1 and high PPC selectivity of >99 % (50 °C, 20 bar, 0.025 mol% catalyst). As demonstration of its utility, neither DFT calculations nor ligand Hammett parameter analyses are viable predictors. It is proposed that the cobalt redox potential informs upon the active site electron density with a more electron rich cobalt centre showing better performances. The method may be widely applicable and is recommended to guide future catalyst discovery for other (co)polymerizations and carbon dioxide utilizations.

SN 2 versus E2 Competition of Cyclic Ethers

We have quantum chemically studied the influence of ring strain on the competition between the two mechanistically different SN 2 and E2 pathways using a series of archetypal ethers as substrate in combination with a diverse set of Lewis bases (F- , Cl- , Br- , HO- , H3 CO- , HS- , H3 CS- ), using relativistic density functional theory at ZORA-OLYP/QZ4P. The ring strain in the substrate is systematically increased on going from a model acyclic ether to a 6- to 5- to 4- to 3-membered ether ring. We have found that the activation energy of the SN 2 pathway sharply decreases when the ring strain of the system is increased, thus on going from large to small cyclic ethers, the SN 2 reactivity increases. In contrast, the activation energy of the E2 pathway generally rises along this same series, that is, from large to small cyclic ethers. The opposing reactivity trends induce a mechanistic switch in the preferred reaction pathway for strong Lewis bases from E2, for large cyclic substrates, to SN 2, for small cyclic substrates. Weak Lewis bases are unable to overcome the higher intrinsic distortivity of the E2 pathway and, therefore, always favor the less distortive SN 2 reaction.

Origin of the Bürgi-Dunitz Angle

The Bürgi-Dunitz (BD) angle plays a pivotal role in organic chemistry to rationalize the nucleophilic addition to carbonyl groups. Yet, the origin of the obtuse trajectory of the nucleophile remains incompletely understood. Herein, we quantify the importance of the underlying physical factors quantum chemically. The obtuse BD angle appears to originate from the concerted action of a reduced Pauli repulsion between the nucleophile HOMO and carbonyl π bond, a more stabilizing HOMO-π*-LUMO(C=O) interaction, as well as a more favorable electrostatic attraction.

Analyzing Fluoride Binding by Group 15 Lewis Acids: Pnictogen Bonding in the Pentavalent State

We report the results of a computational investigation into fluoride binding by a series of pentavalent pnictogen Lewis acids: pnictogen pentahalides (PnX5), tetraphenyl pnictogeniums (PnPh4+), and triphenyl pnictogen tetrachlorocatecholates (PnPh3Cat). Activation strain and energy decomposition analyses of the Lewis adducts not only clearly delineate the electrostatic and orbital contributions to these acid-base interactions but also highlight the importance of Pauli repulsion and molecular flexibility in determining relative Lewis acidity among the pnictogens.

A Computational Evaluation of the Steric and Electronic Contributions in Stereoselective Olefin Polymerization with Pyridylamido-Type Catalysts

A density functional theory (DFT) study combined with the steric maps of buried volume (%V_Bur) as molecular descriptors and an energy decomposition analysis through the ASM (activation strain model)-NEDA (natural energy decomposition analysis) approach were applied to investigate the origins of stereoselectivity for propene polymerization promoted by pyridylamido-type nonmetallocene systems. The relationships between the fine tuning of the ligand and the propene stereoregularity were rationalized (e.g., the metallacycle size, chemical nature of the bridge, and substituents at the ortho-position on the aniline moieties). The DFT calculations and %V_Bur steric maps reproduced the experimental trend: substituents on the bridge and on the ortho-positions of aniline fragments enhance the stereoselectivity. The ASM-NEDA analysis enabled the separation of the steric and electronic effects and revealed how subtle ligand modification may affect the stereoselectivity of the process.

Not antiaromaticity gain, but increased asynchronicity enhances the Diels-Alder reactivity of tropone

Tropone is an unreactive diene in normal electron demand Diels-Alder reactions, but it can be activated via carbonyl umpolung by using hydrazone ion analogs. Recently, the higher reactivity of hydrazone ion analogs was ascribed to a raised HOMO energy induced by antiaromaticity (L. J. Karas, A. T. Campbell, I. V. Alabugin and J. I. Wu, Org. Lett., 2020, 22, 7083). We show that this is incorrect, and that the activation barrier is lowered by increased asynchronicity.

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.

Influence of Group 15 elements on the [3 + 2] cycloaddition reactivity of G15 = G15-G15-based 1,3-dipoles with cyclooctyne

The influence of Group 15 elements (G15s) on the reactivity of the cycloaddition reactions of inorganic 1,3-dipolar analogs with cyclooctyne was computationally explored with density functional theory. To this end, the G15G15G15'-based 1,3-dipole, an interestingly representative model molecule of 1,3-dipole chemistry, was selected. The present computational investigations suggest that all NNG15-Rea and G15G15P-Rea molecules can be energetically feasible to undergo 1,3-dipolar cycloaddition reactions with cyclic alkynes, except for only the NNN-Rea 1,3-dipole molecule. The key factor, which can greatly affect the activation barriers, is quantitatively analyzed in detail through the frontier molecular orbital (FMO) theory, the activation strain model (ASM), and the energy decomposition analysis (EDA) approach. Our theoretical findings based on the FMO theory and the EDA suggest that two types of bonding interactions occur in such 1,3-dipolar cycloaddition reactions: forward bonding (filled p-π orbital (cyclooctyne) → the empty p-π* orbital (G15 = G15)) and back bonding (empty p-π* orbital (cyclooctyne) ← the lone pair (G15')). Notably, the bonding interactions of the former mechanism are much stronger than those of the latter mechanism. Evidence from the ASM reveals that the activation barriers of G15G15G15'-based 1,3-dipolar cycloaddition reactions strongly depend on the atomic radius of the G15 element. These theoretical conclusions allow a number of predictions to be made.

Intermolecular Covalent Interactions: Nature and Directionality

Quantum chemical methods were employed to analyze the nature and the origin of the directionality of pnictogen (PnB), chalcogen (ChB), and halogen bonds (XB) in archetypal F_m Z⋅⋅⋅F^- complexes (Z=Pn, Ch, X), using relativistic density functional theory (DFT) at ZORA-M06/QZ4P. Quantitative Kohn-Sham MO and energy decomposition analyses (EDA) show that all these intermolecular interactions have in common that covalence, that is, HOMO-LUMO interactions, provide a crucial contribution to the bond energy, besides electrostatic attraction. Strikingly, all these bonds are directional (i.e., F-Z⋅⋅⋅F^- is approximately linear) despite, and not because of, the electrostatic interactions which, in fact, favor bending. This constitutes a breakdown of the σ-hole model. It was shown how the σ-hole model fails by neglecting both, the essential physics behind the electrostatic interaction and that behind the directionality of electron-rich intermolecular interactions. Our findings are general and extend to the neutral, weaker ClI⋅⋅⋅NH₃ , HClTe⋅⋅⋅NH₃ , and H₂ ClSb⋅⋅⋅NH₃ complexes.