Frontier Orbital Interactions Research Articles

The counterintuitive aromaticity of bent metallabenzenes: A theoretical exploration

Metallabenzenes, a type of aromatic compound that includes metal atoms, have opened up new avenues for creating materials with unique properties. A distinctive feature of metallabenzenes is the significant deviation of their metal atoms from the planar configuration of the C5 ring, a phenomenon that paradoxically enhances their aromatic character. In this investigation, we propose that this counterintuitive increase in aromaticity upon geometric distortion is governed by the interactions of frontier orbitals in the σ-space. This insight not only corroborates the previously suggested role of σ-space orbitals in inducing geometric non-planarity in metallabenzenes but also underscores their pivotal contribution to the compounds' enhanced aromaticity. As a result, this work broadens the scope of the σ-control mechanism, highlighting its usefulness for the rational design of functional metalla-aromatic materials.

Deciphering orbital hybridization in heterogeneous catalysis

AbstractThe catalytic coordinate is essentially the evolving frontier orbital interaction while feeding with catalytic materials and adsorbates under proper reaction conditions. The heterogeneous catalytic reaction mechanism involves the initial adsorption and activation of reactants, subsequent intermediate transformation, final target product desorption, and regeneration of catalytic materials. In these catalytic processes, interaction modulations in terms of orbital hybridization/coupling allow an intrinsic control on both thermodynamics and kinetics. Concerned charge transfer and redistribution, orbital splitting and rearrangement with specific orientation, and spin change and crossover pose a formidable challenge on mechanism elucidation; it is hard to precisely correlate the apparent activity and selectivity, let alone rational modulations on it. Therefore, deciphering the orbital couplings inside a catalytic round is highly desirable and the dependent descriptor further provides in‐depth insights into catalyst design at the molecule orbital level. This review hopes to provide a comprehensive understanding on orbital hybridizations, modulations, and correlated descriptors in heterogeneous catalysis.

(Invited) Computing the Newns-Anderson Chemisorption Function from First Principles

In analysing the interactions of adsorbates with extended surfaces, theory, and to a large degree also experiments, tend to rely on adsorption models such as the famous Newns-Anderson model. There, the interaction of adsorbate and substrate is represented by the interaction of the adsorbate's one-electron frontier orbitals with the delocalised Bloch states of the substrate. Together with its simplified cousin, the d-band model, such adsorption models yielded significant insight into the chemistry of extended surfaces.The Newns-Anderson Model gives rise to a so-called chemisorption function, a density of states weighted by the electronic coupling between adsorbate and substrate states, measuring the line broadening of an adsorbate frontier state upon adsorption. In my contribution, I will describe our efforts to compute the chemisorption function from first principles, employing a diabatization scheme to estimate electronic couplings between states. Recognizing the periodic nature of the substrate and the finite extent of computational supercells, we extend the original chemisorption function by a Brillouin zone integration to account for the (generally localized) adsorbate states' coupling to different substrate states at different points in reciprocal space. Including this integration, the chemisorption function not only converges well with both lateral simulation cell size and slab depth, but also compares well with experiments. We test our approach against highly accurate experimental measurements of ultrafast charge transfer of a probe Ar atom on various metal surfaces and find that the chemisorption function reproduces both absolute transfer rates and rate asymmetries due to spin on magnetic surfaces. Next to its predictiveness, though, the Newns Anderson chemisorption function allows us to investigate the charge transfer mechanism in terms of e.g. contributions of states with different angular momentum character, showing the importance of phase cancellation effects in interpreting charge transfer at surfaces.

Quantum DFT studies on the drug delivery of favipiravir using pristine and functionalized chitosan nanoparticles

Considering the spread of the COVID-19 pandemic, finding new drugs along with the development of effective drug delivery methods can help in the treatment of this disease. For this reason, in this research work, the possibility of drug-delivery of Favipiravir (FP), one of the drugs approved in the treatment of COVID-19, by pristine chitosan (Chit) nanoparticles (NP), and functionalized chitosan nanoparticles with N-acylate, N-methyl, O-acetyl, and Oxazoline functional groups was studied using quantum mechanical DFT methods at B3LYP-D3(BJ)/6-311 + g(d,p) theoretical level in water medium. The QTAIM, NBO, DOS, frontier orbital, conceptual-DFT indices, and non-covalent interaction analysis were further implemented to investigate the possible interactions between FP and Chit NPs. The results show that the adsorption of FP on Chit NPs is done through the creation of hydrogen bonds, and the highest absorption energy of − 18.15 kcal/mol between pristine chitosan and FP. In the case of all functionalized Chit NPs, a decrease in the absorption energy is observed, which is more noticeable in the case of N-acylated and O-acetyl functionalize Chit NPs, and indicates the weakening of the van der Waals interactions for these cases. Considering the compatibility of Chit NPs with the human body and their non-toxicity, as well as the fact that factors such as pH, solubility, the ionic strength, and so on can be adjusted to control the release rate using the functionalized Chit NPs, it seems that the results of this work can be a comprehensive guide to design the drug delivery methods of FP drug using Chit NPs, to reduce the symptoms of COVID-19 disease.

Xanthene-Separated 24 π-Electron Antiaromatic Rosarin Dimer.

Antiaromatic molecules have recently received attention because of their intrinsic properties, such as high reactivity and their narrow HOMO-LUMO gaps. Stacking of antiaromatic molecules has been predicted to induce three-dimensional aromaticity via frontier orbital interactions. Here, we report a covalently linked π-π stacked rosarin dimer that has been examined experimentally by steady-state absorption and transient absorption measurements and theoretically by quantum chemical calculations, including time-dependent density functional theory, anisotropy of induced current density, and nucleus-independent chemical shift calculations. Relative to the corresponding monomer, the dimer exhibits diminished antiaromaticity upon lowering the temperature to 77 K, a finding ascribed to intramolecular interactions between the macrocyclic rosarin subunits.

Multi-targeted fluorescent probes for detection of Zn(II) and Cu(II) ions based on ESIPT mechanism

With the rapid development of industry, it is pretty critical to detect the heavy metals. Recently, the 2-(2-hydroxyphenyl) benzothiazole derivatives with different numbers of rotatable phenyl m (m = 1,2,3) fluorescent probes HLm were synthesized. However, the theoretical analysis of the mechanism was still missing. In this work, we have systematically researched the mechanisms of excited state intramolecular proton transfer (ESIPT) and the detection of Cu(II), Zn(II) ions for the hydrogen-bond system HLm though quantum chemistry methods. By bond parameters and the minimum energy pathways analyses, the proton of this system was probed directly transfer without barrier. Bond parameters, real space function at bond critical point, the frontier molecular orbital, electron spectra and orbital interaction diagram were carried to elucidate response to Cu(II), Zn(II) ions. In addition, comparing the Gibbs free energy variation of the complexation reaction between fluorescent probes and ions, it can be proved that the number of rotatable benzene rings affects the response ability of the probe to target ions.

Non-metal atom modified SnS2 sheet for CO2 photoreduction with significant activity and selectivity improvements: A first-principles study

Photocatalytic CO2 reduction suffers from low activity and poor product selectivity, seriously limiting its further application. Here, by means of density functional theory, we investigated the structural and catalytic properties of non-metal atoms X (X = B, C, N, and O) anchored at S vacancy site of SnS2 monolayer denoted as X/Sv-SnS2. The HSE06 result shows that both the band gap and band edge positions of X/Sv-SnS2 substrates are quite suitable for CO2 photoreduction. Interestingly, the adsorption strength of CO2 can be linearly enhanced with the decreasing non-metal atoms X from O to B, and the corresponding value ranges from −0.25 to −1.23 eV. Particularly, the diversity of non-metal atoms X generates completely different preference of CO2 reduction products according to the Gibbs free energy change. Whereas O, C, and B/Sv-SnS2 separately favor to form CO, HCOOH, and CH4 products, with remarkably low energy barrier of 1.07 eV, 0.58 eV, and 1.04 eV, respectively. These peculiar activity and selectivity properties mainly stem from the unique adsorption behavior and frontier orbital interaction between non-metal atoms X and intermediates. Our findings strongly suggest that cooperation of non-metal atoms and intrinsic defect could offer an exciting opportunity to design high activity and selectivity photocatalysis for CO2 reduction.

P-block atom modified Sn(200) surface as a promising electrocatalyst for two-electron CO2 reduction: a first-principles study.

Low activity and poor product selectivity of CO2 reduction have seriously hampered its further practical application. Introducing p-block atoms to the catalyst is regarded as a promising strategy due to the versatility of p orbitals and diversity of p-block elements. Here, we systematically studied the influence of p-block atom X (X = C, N, O, S, and Se) on CO2 catalytic properties on a Sn(200) surface by first-principles calculation. Our work shows that all the p-block atoms are relative stable with Ef in the range of -5.11 to -3.59 eV. Further calculation demonstrates that the diversity of the p-block atoms results in unique CO2 electrocatalytic activity and product selectivity. Interestingly, the p-block C atom shows bi-functional activity to form two-electron products HCOOH and CO, with the corresponding energy barriers remarkably low at about 0.19 eV and 0.28 eV. In particular, the p-block S(Se) atom appears to have striking HCOOH selectivity, with the energy barrier to form HCOOH only a quarter of that to form the CO product. This unusual behavior is mainly attributed to the adsorption strength and frontier orbital interaction between the p-block atom and intermediates. These findings can effectively provide a valuable insight into the design of highly efficient CO2 electrocatalyst.

Density functional theory-based analyses on selective gas separation by β-PVDF-supported ionic liquid membranes

Finding proper candidates for polymer-supported ionic liquid (IL)-based gas separating membranes is a challenge. The current article elucidates the quantum chemical perspective of the selective gas adsorption efficiency, from a mixture of CO2, CO, CH4, and H2, of α- and β-polyvinylidene fluoride (PVDF)-supported imidazolium- and pyridinium-based six ionic liquid membranes. Although IL-based membrane efficiency mainly depends on the gas solubility of ILs, IL/support binding and gas adsorption on the support material are also studied to describe the overall gas adsorption properties of the PVDF/IL complexes. β-PVDF exhibits better binding with the ILs, and better gas affinity, thus, qualified as a more suitable membrane component as compared to α-PVDF. Dispersion-corrected density functional calculations are performed to provide a detailed insight into the energetic interactions, nonbonding intermolecular interactions based on symmetry adapted perturbation theory (SAPT), natural bond orbitals (NBO), Bader's quantum theory of atoms in molecules (QTAIM), reduced density gradient (RDG), frontier orbital interactions, density of states (DOS), and thermochemical analyses of the gas-adsorbed systems. Gas molecules interact with the membrane components through weak hydrogen bonds and exhibit low interaction energies, indicating physisorption of the gases. Gas adsorption energies are more negative than the mutual interaction energies of the gas molecules, ensuring effective gas adsorption by the membrane components. All the β-PVDF/IL systems have shown the highest and lowest affinity for CO2 and H2, respectively, leading to effective separation of CO2 and H2 from the other gases.

Anionic Activation of CO2 via (Mn-CO2)- Complex on Magic-Numbered Anionic Coinage Metal Clusters Mn- (M = Cu, Ag, Au).

Given the immense challenge of excessive accumulation of carbon dioxide (CO2) in the earth's atmosphere, an extensive search is under way to convert atmospheric CO2 to compounds of more utility. With CO2 being thermodynamically extremely stable, activation of CO2 is the first and most important step toward its chemical conversion. Building upon our earlier model for the anionic activation of CO2 with azabenzene and inspired by the work of others on metal atom-CO2 complexes, we investigated the possibility of anionic activation of CO2 on small anionic metal clusters, which would have implications for catalytic conversion of CO2 on metal surfaces with atomic-scale structural irregularities. We carried out theoretical calculations using density functional theory to examine small anionic metal clusters of Cu, Ag, and Au to check whether they form a complex with CO2, with the sign of CO2 being chemically activated. We found that a class of anionic metal clusters Mn- with 1, 2, and 6 atoms consistently produced the activated complex (Mn-CO2)- for all three metals. There exists a strong interaction between the CO2 moiety and Mn- via a partially covalent M-C bond with a full delocalization of the electronic charge, as a result of electron transfer from the HOMO of Mn- to the LUMO of CO2 as in metal-CO2 π-backbonding. We examined the interaction of frontier orbitals from the viewpoints of the orbital geometry and orbital energetics and found that the above magic numbers are consistent with both aspects.

Unifying Conceptual Density Functional and Valence Bond Theory: The Hardness-Softness Conundrum Associated with Protonation Reactions and Uncovering Complementary Reactivity Modes.

In this study, we address the long-standing issue—arising prominently from conceptual density functional theory (CDFT)—of the relative importance of electrostatic, i.e., “hard–hard”, versus spin-pairing, i.e., “soft–soft”, interactions in determining regiochemical preferences. We do so from a valence bond (VB) perspective and demonstrate that VB theory readily enables a clear-cut resolution of both of these contributions to the bond formation/breaking process. Our calculations indicate that appropriate local reactivity descriptors can be used to gauge the magnitude of both interactions individually, e.g., Fukui functions or HOMO/LUMO orbitals for the spin-pairing/(frontier) orbital interactions and molecular electrostatic potentials (and/or partial charges) for the electrostatic interactions. In contrast to previous reports, we find that protonation reactions cannot generally be classified as either charge- or frontier orbital-controlled; instead, our results indicate that these two bonding contributions generally interplay in more subtle patterns, only giving the impression of a clear-cut dichotomy. Finally, we demonstrate that important covalent, i.e., spin pairing, reactivity modes can be missed when only a single spin-pairing/orbital interaction descriptor is considered. This study constitutes an important step in the unification of CDFT and VB theory.

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Base-catalyzed stereoselective hydrophenoxylation and hydrothiolation of hexafluorobutyne

Novel base-catalyzed hydrophenoxylation and hydrothiolation of hexafluorobutyne are described. By using an easily available base, a variety of vinyl ethers as well as sulfides are prepared at room temperature through two-phase nucleophilic addition reaction. Mechanistic study revealed that the stereoselectivity of vinyl ethers might depend on frontier orbital interaction under kinetic control.

Induced magnetic field in sp-hybridized carbon rings: analysis of double aromaticity and antiaromaticity in cyclo[2N]carbon allotropes.

The induced magnetic field of C2N (N = 3-14) carbon rings was dissected to contributions from out-of-plane and in-plane π orbitals revealing two concurrent long range shielding or deshielding cones as a manifestation of the dual aromatic and antiaromatic character of C4n+2 and of C4n rings respectively. Aromaticity based on the magnetic criterion was evaluated with regard to the bonding pattern and geometrical characteristics that elucidate the influence of bond length and bond angle alteration on out-of-plane and in-plane magnetic responses. Ground state polyynic geometries of C4n+2 rings exhibit comparable shielding cones to annulenes, decreasing the magnetic response with regard to the ring size and similar πout and πin diatropicity. Transition state cumulenic rings display increased aromaticity expressed by a very strong constant magnetic response and augmented πout diatropicity with regard to πin. The variations of the induced magnetic field are explained on the basis of frontier orbital interactions through rotational excitations, which enable further rationalization of the aromatic/antiaromatic behavior.

Chemical interactions and dynamics with femtosecond X-ray spectroscopy and the role of X-ray free-electron lasers.

X-ray free-electron lasers with intense, tuneable and short-pulse X-ray radiation are transformative tools for the investigation of transition-metal complexes and metalloproteins. This becomes apparent in particular when combining the experimental observables from X-ray spectroscopy with modern theoretical tools for calculations of electronic structures and X-ray spectra from first principles. The combination gives new insights into how charge and spin densities change in chemical reactions and how they determine reactivity. This is demonstrated for the investigations of structural dynamics with metal K-edge absorption spectroscopy, spin states in excited-state dynamics with metal 3p-3d exchange interactions, the frontier-orbital interactions in dissociation and substitution reactions with metal-specific X-ray spectroscopy, and studies of metal oxidation states with femtosecond pulses for ‘probe-before-destroy’ spectroscopy. The role of X-ray free-electron lasers is addressed with thoughts about how they enable ‘bringing back together’ different aspects of the same problem and this is thought to go beyond a conventional review paper where these aspects are formulated in italic font type in a prequel, an interlude and in a sequel.This article is part of the theme issue ‘Measurement of ultrafast electronic and structural dynamics with X-rays'.

Inhibitory regulation mechanism of naphthoquinone and its derivatives in radical polymerization

AbstractThe inhibitory regulation mechanism towards the methyl 2‐methylpropionate radical (a model of the poly(methyl methacrylate) [MMA] propagation radical) by naphthoquinone derivatives in radical polymerization in an aerobic or anaerobic atmosphere has been studied experimentally by liquid chromatography, mass spectrometry, and nuclear magnetic resonance (NMR) measurements of the isolated reaction products, which were purified by preparative thin‐layer chromatography. In addition, we have carried out a theoretical analysis (DFT B3LYP/6‐311+G**) of the frontier orbital interactions and reaction Gibbs free energies (ΔG) for the formation of the relevant reaction products and intermediates and activation free energies (ΔG#) and frontier electron densities (fA(R)) of the radical reactions. Consequently, the reaction mechanism where quinone products are formed by the attack of propagation radicals at the C2 carbon next to the carbonyl group in naphthoquinone was rationalized.

Spectroscopic and Electrochemical Properties of Electronically Modified Cycloplatinated Formazanate Complexes.

Formazanates are a class of monoanionic, chromophoric, redox-active chelating ligands, and until recently, the coordination chemistry of formazanates with third-row transition metals was unexplored. Described here is a series of heteroleptic cyclometalated platinum formazanate complexes of the type Pt(C^N)(Fz), where "C^N" is the cyclometalating ligand and "Fz" is the formazanate. This work includes a set of 14 complexes derived from four different cyclometalating ligands and five different formazans, greatly expanding the structural and electronic diversity of this class of complexes. The formazans fall into two major categories: triarylformazans with either electron-donating (Me, OMe) or electron-withdrawing (CN) para substituents, and even more electron-deficient diarylformazans where the center position of the backbone is a cyano group. Seven of the new compounds are characterized by single-crystal X-ray diffraction, and all are characterized by cyclic voltammetry and UV-vis absorption spectroscopy to provide insight into the frontier orbitals. The complexes can be electrochemically reduced by three electrons, with the first two electrons populating formazanate-centered frontier orbitals and the third being added to the Pt-C^N fragment. The formazanate-based reduction potentials are highly sensitive to the substitution pattern of the formazanate and can be tuned over a wide range of ca. 0.8 V by altering substituents. The formazanates also impart these complexes with very strong, low-energy visible absorption bands assigned to HOMO → LUMO transitions, which are greatly perturbed from the free ligand due to interactions of the formazanate frontier orbitals with platinum-centered orbitals. Unlike the redox properties, the formazanate-derived visible absorption band is relatively insensitive to substituent effects. Additional visible absorption bands, attributed to Pt(d) → C^N(π*) MLCT transitions, can be tuned over a wide range by changing the structure of the C^N ligand, giving rise to strong panchromatic absorption for some members of the series.

Understanding the Nonplanarity in Aromatic Metallabenzenes: A σ-Control Mechanism.

Metallabenzenes, the organometallic counterparts of benzene with one of the C atoms being replaced by a metal atom, expand the family of aromatics and further create prospective candidates for novel applications as functional materials. One intriguing feature of these complexes is that their MC5 rings do not always constrict into a planar configuration as in the C6 ring of benzene. Such a deviation has often been attributed to the unfavorable antibonding interactions between an occupied metal d orbital and the π orbitals of the C5 moiety. We herein scrutinize the frontier orbital interactions in both σ and π spaces in a plethora of metallabenzene complexes using extensive density functional theory calculations. Unexpectedly, the nonplanarity in metallabenzenes is found to be hardly related to the π orbitals. It is the antibonding interaction between an occupied metal d orbital and the σ orbitals of the C5 moiety that dominates the observed distortion. Such a σ-control mechanism not only provides an explanation for the commonly observed nonplanarity in metallabenzenes but also points out a novel direction toward the rational design of functional materials with enhanced metalla-aromaticity.

Stable Molecular Diodes Based on π-π Interactions of the Molecular Frontier Orbitals with Graphene Electrodes.

In molecular electronics, it is important to control the strength of the molecule-electrode interaction to balance the trade-off between electronic coupling strength and broadening of the molecular frontier orbitals: too strong coupling results in severe broadening of the molecular orbitals while the molecular orbitals cannot follow the changes in the Fermi levels under applied bias when the coupling is too weak. Here, a platform based on graphene bottom electrodes to which molecules can bind via π-π interactions is reported. These interactions are strong enough to induce electronic function (rectification) while minimizing broadening of the molecular frontier orbitals. Molecular tunnel junctions are fabricated based on self-assembled monolayers (SAMs) of Fc(CH2 )11 X (Fc = ferrocenyl, X = NH2 , Br, or H) on graphene bottom electrodes contacted to eutectic alloy of gallium and indium top electrodes. The Fc units interact more strongly with graphene than the X units resulting in SAMs with the Fc at the bottom of the SAM. The molecular diodes perform well with rectification ratios of 30-40, and they are stable against bias stressing under ambient conditions. Thus, tunnel junctions based on graphene with π-π molecule-electrode coupling are promising platforms to fabricate stable and well-performing molecular diodes.

Stacked antiaromatic porphyrins

Aromaticity is a key concept in organic chemistry. Even though this concept has already been theoretically extrapolated to three dimensions, it usually still remains restricted to planar molecules in organic chemistry textbooks. Stacking of antiaromatic π-systems has been proposed to induce three-dimensional aromaticity as a result of strong frontier orbital interactions. However, experimental evidence to support this prediction still remains elusive so far. Here we report that close stacking of antiaromatic porphyrins diminishes their inherent antiaromaticity in the solid state as well as in solution. The antiaromatic stacking furthermore allows a delocalization of the π-electrons, which enhances the two-photon absorption cross-section values of the antiaromatic porphyrins. This feature enables the dynamic switching of the non-linear optical properties by controlling the arrangement of antiaromatic π-systems on the basis of intermolecular orbital interactions.