Structure-property relationships to direct the dynamic properties of acylsemicarbazide-based materials.

Secondary interactions, such as hydrogen bonding or phase separation, can enhance the stability of dynamic covalent materials without compromising on desired dynamic properties. Here, we investigate the combination of multiple secondary interactions in dynamic covalent materials based on acylsemicarbazides (ASCs), with the aim of achieving tunable material properties. The effects of different ASC substituents on the dynamic covalent and hydrogen bonding capabilities were investigated in a small molecule study using a combined experimental and theoretical approach, and revealed the presence of cooperative hydrogen-bonding interactions in 2 directions in one of the derivatives.

"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-H to hydridic Y-H hydrogen-bond donor fragments [ , , 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-H and hydridic Y-H fragments which go with charge transfer in opposite directions.

Synthesis, Reactivity, and Bonding Analysis of a Tetracoordinated Nickel Carbene.

Nickel carbenes are key reactive intermediates in the catalytic cyclopropanation of olefins and other reactions, but isolated examples are scarce and generally rely on low coordination numbers (≤3) to stabilize the metal-ligand multiple bond. Here we report the isolation and characterization of a stable tetracoordinated nickel carbene bearing a triphosphine pincer ligand. Its nucleophilic character is evidenced by reaction with acids, and it can transfer the carbene fragment to CO to form a ketene.

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

Correction for 'Not antiaromaticity gain, but increased asynchronicity enhances the Diels-Alder reactivity of tropone' by Eveline H. Tiekink , , 2023, , 3703-3706, https://doi.org/10.

Correction: Pericyclic reaction benchmarks: hierarchical computations targeting CCSDT(Q)/CBS and analysis of DFT performance.

Correction for 'Pericyclic reaction benchmarks: hierarchical computations targeting CCSDT(Q)/CBS and analysis of DFT performance' by Pascal Vermeeren , , 2022, , 18028-18042, https://doi.org/10.1039/D2CP02234F.

Understanding chemistry with the symmetry-decomposed Voronoi deformation density charge analysis.

The symmetry-decomposed Voronoi deformation density (VDD) charge analysis is an insightful and robust computational tool to aid the understanding of chemical bonding throughout all fields of chemistry. This method quantifies the atomic charge flow associated with chemical-bond formation and enables decomposition of this charge flow into contributions of (1) orbital interaction types, that is, Pauli repulsive or bonding orbital interactions; (2) per irreducible representation (irrep) of any point-group symmetry of interacting closed-shell molecular fragments; and now also (3) interacting open-shell (i.e.

S 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 S 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 , H CO , HS , H 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 S 2 pathway sharply decreases when the ring strain of the system is increased, thus on going from large to small cyclic ethers, the S 2 reactivity increases.

The Origin of Catalysis and Regioselectivity of Lewis Acid-Catalyzed Diels-Alder Reactions with Tropone.

We have studied the uncatalyzed and Lewis acid (LA)-catalyzed cycloaddition reaction between tropone and 1,1-dimethoxyethene using dispersion-corrected relativistic density functional theory (DFT). The LA catalysts BF , B(C H ) , and B(C F ) efficiently accelerate both the competing [4+2] and [8+2] cycloaddition reactions by lowering the activation barrier up to 12 kcal mol compared to the uncatalyzed reaction. Our study reveals that the LA catalyst promotes both cycloaddition reaction pathways by LUMO-lowering catalysis and demonstrates that Pauli-lowering catalysis is not always the operative catalytic mechanism in cycloaddition reactions.

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 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.

How Bases Catalyze Diels-Alder Reactions.

We have quantum chemically studied the base-catalyzed Diels-Alder (DA) reaction between 3-hydroxy-2-pyrone and N-methylmaleimide using dispersion-corrected density functional theory. The uncatalyzed reaction is slow and is preceded by the extrusion of CO via a retro-DA reaction. Base catalysis, for example, by triethylamine, lowers the reaction barrier up to 10 kcal mol , causing the reaction to proceed smoothly at low temperature, which quenches the expulsion of CO , yielding efficient access to polyoxygenated natural compounds.

Stability of alkyl carbocations.

The traditional and widespread rationale behind the stability trend of alkyl-substituted carbocations is incomplete. Through state-of-the-art quantum chemical analyses, we quantitatively established a generally overlooked driving force behind the stability of carbocations, namely, that the parent substrates are substantially destabilized by the introduction of substituents, often playing a dominant role in solution. This stems from the repulsion between the substituents and the C-X bond.

Pericyclic reaction benchmarks: hierarchical computations targeting CCSDT(Q)/CBS and analysis of DFT performance.

Hierarchical, convergent benchmark computations were performed followed by a systematic analysis of DFT performance for five pericyclic reactions comprising Diels-Alder, 1,3-dipolar cycloaddition, electrocyclic rearrangement, sigmatropic rearrangement, and double group transfer prototypes. Focal point analyses (FPA) extrapolating to the limit were executed explicit quantum chemical computations with electron correlation treatments through CCSDT(Q) and correlation-consistent Gaussian basis sets up to aug'-cc-pV5Z. Optimized geometric structures and vibrational frequencies of all stationary points were obtained at the CCSD(T)/cc-pVTZ level of theory.

The 1,3-Dipolar Cycloaddition: From Conception to Quantum Chemical Design.

The 1,3-dipolar cycloaddition (1,3-DCA) reaction, conceptualized by Rolf Huisgen in 1960, has proven immensely useful in organic, material, and biological chemistry. The uncatalyzed, thermal transformation is generally sluggish and unselective, but the reactivity can be enhanced by means of metal catalysis or by the introduction of either predistortion or electronic tuning of the dipolarophile. These promoted reactions generally go with a much higher reactivity, selectivity, and yields, often at ambient temperatures.

S2 versus S2' Competition.

We have quantum chemically explored the competition between the S2 and S2' pathways for X + HC═CHCHY (X, Y = F, Cl, Br, I) using a combined relativistic density functional theory and coupled-cluster theory approach. Bimolecular nucleophilic substitution reactions at allylic systems, i.e.

How Ionization Catalyzes Diels-Alder Reactions.

Invited for the cover of this issue are Pascal Vermeeren, Trevor A. Hamlin, and F. Matthias Bickelhaupt of the TheoCheM group at the Vrije Universiteit Amsterdam.

How Ionization Catalyzes Diels-Alder Reactions.

The catalytic effect of ionization on the Diels-Alder reaction between 1,3-butadiene and acrylaldehyde has been studied using relativistic density functional theory (DFT). Removal of an electron from the dienophile, acrylaldehyde, significantly accelerates the Diels-Alder reaction and shifts the reaction mechanism from concerted asynchronous for the neutral Diels-Alder reaction to stepwise for the radical-cation Diels-Alder reaction. Our detailed activation strain and Kohn-Sham molecular orbital analyses reveal how ionization of the dienophile enhances the Diels-Alder reactivity via two mechanisms: (i) by amplifying the asymmetry in the dienophile's occupied π-orbitals to such an extent that the reaction goes from concerted asynchronous to stepwise and thus with substantially less steric (Pauli) repulsion per reaction step; (ii) by enhancing the stabilizing orbital interactions that result from the ability of the singly occupied molecular orbital of the radical-cation dienophile to engage in an additional three-electron bonding interaction with the highest occupied molecular orbital of the diene.

σ-Electrons Responsible for Cooperativity and Ring Equalization in Hydrogen-Bonded Supramolecular Polymers.

Invited for this month's cover are collaborators from the TheoCheM group of the Vrije Universiteit Amsterdam and the University of Perugia. The cover picture shows a σ-electron traveling through a hydrogen-bonded squaramide linear chain. The charge transfer within the σ-electronic system is the cause for the cooperativity in the investigated urea, deltamide, and squaramide polymers.

σ-Electrons Responsible for Cooperativity and Ring Equalization in Hydrogen-Bonded Supramolecular Polymers.

We have quantum chemically analyzed the cooperative effects and structural deformations of hydrogen-bonded urea, deltamide, and squaramide linear chains using dispersion-corrected density functional theory at BLYP-D3(BJ)/TZ2P level of theory. Our purpose is twofold: (i) reveal the bonding mechanism of the studied systems that lead to their self-assembly in linear chains; and (ii) rationalize the C-C bond equalization in the ring moieties of deltamide and squaramide upon polymerization. Our energy decomposition and Kohn-Sham molecular orbital analyses reveal cooperativity in all studied systems, stemming from the charge separation within the σ-electronic system by charge transfer from the carbonyl oxygen lone pair donor orbital of one monomer towards the σ* N-H antibonding acceptor orbital of the neighboring monomer.

Origin of asynchronicity in Diels-Alder reactions.

Asynchronicity in Diels-Alder reactions plays a crucial role in determining the height of the reaction barrier. Currently, the origin of asynchronicity is ascribed to the stronger orbital interaction between the diene and the terminal carbon of an asymmetric dienophile, which shortens the corresponding newly formed C-C bond and hence induces asynchronicity in the reaction. Here, we show, using the activation strain model and Kohn-Sham molecular orbital theory at ZORA-BP86/TZ2P, that this rationale behind asynchronicity is incorrect.

How metallylenes activate small molecules.

We have studied the activation of dihydrogen by metallylenes using relativistic density functional theory (DFT). Our detailed activation strain and Kohn-Sham molecular orbital analyses have quantified the physical factors behind the decreased reactivity of the metallylene on going down Group 14, from carbenes to stannylenes. Along this series, the reactivity decreases due to a worsening of the back-donation interaction between the filled lone-pair orbital of the metallylene and the σ*-orbital of H, which, therefore, reduces the metallylene-substrate interaction and increases the reaction barrier.

Origin of rate enhancement and asynchronicity in iminium catalyzed Diels-Alder reactions.

The Diels-Alder reactions between cyclopentadiene and various α,β-unsaturated aldehyde, imine, and iminium dienophiles were quantum chemically studied using a combined density functional theory and coupled-cluster theory approach. Simple iminium catalysts accelerate the Diels-Alder reactions by lowering the reaction barrier up to 20 kcal mol compared to the parent aldehyde and imine reactions. Our detailed activation strain and Kohn-Sham molecular orbital analyses reveal that the iminium catalysts enhance the reactivity by reducing the steric (Pauli) repulsion between the diene and dienophile, which originates from both a more asynchronous reaction mode and a more significant polarization of the π-system away from the incoming diene compared to aldehyde and imine analogs.

Origin of the α-Effect in S 2 Reactions.

The α-effect is a term used to explain the dramatically enhanced reactivity of α-nucleophiles (R-Y-X: ) compared to their parent normal nucleophile (R-X: ) by deviating from the classical Brønsted-type reactivity-basicity relationship. The exact origin of this effect is, however, still heavily under debate. In this work, we have quantum chemically analyzed the α-effect of a set of anionic nucleophiles, including O-, N- and S-based normal and α-nucleophiles, participating in an S 2 reaction with ethyl chloride using relativistic density functional theory at ZORA-OLYP/QZ4P.

Chemical reactivity from an activation strain perspective.

Chemical reactions are ubiquitous in the universe, they are at the core of life, and they are essential for industrial processes. The drive for a deep understanding of how something occurs, in this case, the mechanism of a chemical reaction and the factors controlling its reactivity, is intrinsically valuable and an innate quality of humans. The level of insight and degree of understanding afforded by computational chemistry cannot be understated.

Cooperative Self-Assembly in Linear Chains Based on Halogen Bonds.

Cooperative properties of halogen bonds were investigated with computational experiments based on dispersion-corrected relativistic density functional theory. The bonding mechanism in linear chains of cyanogen halide (X-CN), halocyanoacetylene (X-CC-CN), and 4-halobenzonitrile (X-C H -CN) were examined for X = H, Cl, Br, and I. Our energy decomposition and Kohn-Sham molecular-orbital analyses revealed the bonding mechanism of the studied systems.