Three-electron Bond Research Articles

Modulating dual carrier-transfer channels and band structure in carbon nitride to amplify ROS storm for enhanced cancer photodynamic therapy

Graphite carbon nitride (CN) eliminates cancer cells by converting H2O2 to highly toxic •OH under visible light. However, its in vivo applications are constrained by insufficient endogenous H2O2, accumulation of OH− and finite photocarriers. We designed Fe/NV-CN, co-modified CN with nitrogen vacancies (NV) and ferric ions (Fe3+). NV and Fe3+, not only adjust the band structure of CN through quantum confinement effect and the altered coupled oscillations of atomic orbitals to facilitates •OH production by oxidizing OH−, but also construct dual carrier-transfer channels for electrons and holes to respective active sites by introducing stepped electrostatic potential and shortening three-electron bonds, thereby involving more carriers in •OH production. Fe/NV-CN, the novel reactor, effectually produces vast •OH under illumination by expanding OH− as the raw material of •OH and augmenting carriers at active sites, which induces cancer cell apoptosis by disrupting mitochondrial function for significant shrinkage of Cal27 cell-induced tumor under illumination. This work provides not only an effective photosensitizer avoiding the accumulation of OH− for cancer therapy but also a novel strategy by constructing dual carrier-transfer channels on semiconductor photosensitizers for improving the therapeutic effect of photodynamic therapy.

One-Electron (2c/1e) Tin···Tin Bond Stabilized by ortho-Phenylenediamido Ligands.

Odd-electron bonds, i.e., the two-center, three-electron (2c/3e), or one-electron (2c/1e) bonds, have attracted tremendous interest owing to their novel bonding nature and radical properties. Herein, complex [K(THF)6][LSn:···Sn:L] (1), featuring the first and unsupported 2c/1e Sn···Sn σ-bond with a long distance (3.2155(9) Å), was synthesized by reduction of stannylene [LSn:] (L = N,N-dpp-o-phenylene diamide) with KC8. The one-electron Sn-Sn bond in 1 was confirmed by the crystal structure, DFT calculations, EPR spectroscopy, and reactivity studies. This compound can be viewed as a stabilized radical by delocalizing to two metal centers and can readily mediate radical reactions such as C-C coupling of benzaldehyde.

Three-Electron Two-Centered Bond and Single-Electron Transfer Mechanism of Water Splitting via a Copper–Bipyridine Complex

We report the atomistic and electronic details of the mechanistic pathway of the oxygen-oxygen bond formation catalyzed by a copper-2,2'-bipyridine complex. Density functional theory-based molecular dynamics simulations and enhanced sampling methods were employed for this study. The thermodynamics and electronic structure of the oxygen-oxygen bond formation are presented in this study by considering the cis-bishydroxo, [CuIII(bpy)(OH)2]+, and cis-(hydroxo)oxo, [CuIV(bpy)(OH)(═O)]+, complexes as active catalysts. In the cis-bishydroxo complex, the hydroxide transfer requires a higher kinetic barrier than the proton transfer process. In the case of [CuIV(bpy)(OH)(═O)]+, the proton transfer requires a higher free energy than the hydroxide one. The peroxide bond formation is thermodynamically favorable for the [CuIV(bpy)(OH)(═O)]+ complex compared with the other. The hydroxide ion is transferred to one of the Cu-OH moieties, and the proton is transferred to the solvent. The free energy barrier for this migration is higher than that for the former transfer. From the analysis of molecular orbitals, it is found that the electron density is primarily present on the water molecules near the active sites in the highest occupied molecular orbital (HOMO) state and lowest unoccupied molecular orbital (LUMO) of the ligands. Natural bond orbital (NBO) analysis reveals the electron transfer process during the oxygen-oxygen bond formation. The σ*Cu(dxz)-O(p) orbitals are involved in the oxygen-oxygen bond formation. During the bond formation, three-electron two-centered (3e--2C) bonds are observed in [CuIII(bpy)(OH)2]+ during the transfer of the hydroxide before the formation of the oxygen-oxygen bond.

Three-Electron Bond - Interesting Facts and Birthday.

Three-Electron Bond - Interesting Facts and Birthday.

Two Paths to Oxidative C-H Amination Under Basic Conditions: A Theoretical Case Study Reveals Hidden Opportunities Provided by Electron Upconversion.

Traditionally, cross-dehydrogenative coupling (CDC) leads to C-N bond formation under basic and oxidative conditions and is proposed to proceed via a two-electron bond formation mediated by carbenium ions. However, the formation of such high-energy intermediates is only possible in the presence of strong oxidants, which may lead to undesired side reactions and poor functional group tolerance. In this work we explore if oxidation under basic conditions allows the formation of three-electron bonds (resulting in "upconverted" highly-reducing radical-anions). The benefit of this "upconversion" process is in the ability to use milder oxidants (e. g., O2 ) and to avoid high-energy intermediates. Comparison of the two- and three-electron pathways using quantum mechanical calculations reveals that not only does the absence of a strong oxidant shut down two-electron pathways in favor of a three-electron path but, paradoxically, weaker oxidants react faster with the upconverted reductants by avoiding the inverted Marcus region for electron transfer.

Revealing a Decisive Role for Secondary Coordination Sphere Nucleophiles on Methane Activation

Density functional theory and ab initio calculations indicate that nucleophiles can significantly reduce enthalpic barriers to methane C-H bond activation. Valence bond analysis suggests the formation of a two-center three-electron bond as the origin for the catalytic nucleophile effect. A predictive model for methane activation catalysis follows, which suggests that strongly electron-attracting and electron-rich radicals, together with both a negatively charged and strongly electron-donating outer sphere nucleophile, result in the lowest reaction barriers. It is corroborated by the sensitivity of the calculated C-H activation barriers to the external nucleophile and to continuum solvent polarity. More generally, from the present studies, one may propose proteins with hydrophobic active sites, available strong nucleophiles, and hydrogen bond donors as attractive targets for engineering novel methane functionalizing enzymes.

Interaction of Radicals with σ-Holes.

The noncovalent interactions of the CH3•, Cl•, CF3•, and C(CH3)3• radicals with CF3X molecules (X = Cl, Br, I) have been investigated using CCSD(T)/aug-cc-pVTZ energy calculations on MP2/aug-cc-pVTZ-optimized geometries. The electrophilic chlorine atom prefers to complex with the equatorial belt of the heavier halogen X, whereas the nucleophilic radicals CH3• and C(CH3)3• coordinate preferentially with the σ-hole of X to give C3 v complexes. Complexation energies are generally small, ranging up to 5 kcal mol-1 for CF3I with either the chlorine atom or the tBu radical. Within the continuum from pure one-electron (SOMO-LUMO) to pure three-electron (SOMO-HOMO) bonds, the results can be interpreted in terms of predominantly one-electron bonds for nucleophilic radicals but closer to three-electron bonds for the chlorine atom. The trifluoromethyl radical binds very weakly via interhalogen interactions; the carbon radical center plays no role in the bonding.

Upconversion of Reductants.

The many applications of photon upconversion-conversion of low-energy photons into high-energy photons-raises the question of the possibility of "electron upconversion". In this Review, we illustrate how the reduction potential can be increased by using the free energy of exergonic chemical reactions. Electron (reductant) upconversion can produce up to 20-25 kcal mol-1 of additional redox potential, thus creating powerful reductants under mild conditions. We will present the two common types of electron-upconverting systems-dissociative (based on unimolecular fragmentations) and associative (based on the bimolecular formation of three-electron bonds). The possible utility of reductant upconversion encompasses redox chain reactions in electrocatalytic processes, photoredox cascades, design of peroxide-based medicines, firefly luminescence, and reductive repair of DNA photodamage.

Unraveling the regioselectivity of odd electron halogen bond formation using electrophilicity index and chemical hardness parameters.

Odd electron halogen bonding of freons with five different free radicals have been investigated using M06-2X/6-311G++(d,p). Conceptual density functional theory parameters such as Fukui functions, electrophilicity index and chemical hardness have been utilised to examine the regioselectivity and strength of free radical induced halogen bonding. The strong electrophilic radical Cl˙ forms three-electron bonds upon interacting with the nucleophilic ring surrounding the sigma hole or fk- regions. This behavior is also observed for moderately electrophilic radicals OH˙ and OCl˙ for complexation with CHFCl2 only. In other complexes, these radicals along with least nucleophilic radicals ˙NO and ˙Ph form one-electron bond with sigma hole or fk+ regions of freons. This regioselectivity of radicals and the interaction energy upon complexation have been explained in terms of local electrophilicity index and chemical hardness. We hope that this finding will shed light on the understanding of non-covalent interactions in terms of conceptual density functional theory for halogen bonding as well as in pnictogen, chalcogen and tetrel bonding systems. This result may find applications in understanding the radical mediated reactions and ozone depletion in the stratosphere.

Nature of the Three-Electron Bond.

We analyze the properties of 15 3-electron bonds, which include σ-3-electron-bonds, such as dihalide radical anions and di-noble gas radical cations, π-3-electron-bonds as in hydrazine radical cations, and doubly-π-(3e)-bonded species such as O2, FeO+, S2, etc. The primary analytical tool is the breathing-orbital valence-bond (BOVB) method, which enables us to quantify the charge shift resonance energy (RECS) of the three electrons, and the bond dissociation energies (De). BOVB is tested reliable against MRCI calculations. Our findings show that in all 3-electron bonds, none of the VB structures have by themselves any bonding. In fact, in each VB structure, the three electrons maintain Pauli repulsion, while the entire bonding energy arises from resonance due to the charge shift between the two or more constituent VB structures. Hence, 3e-bonds are charge shift bonds (CSBs). The CSB character is probed by calculating the Laplacian (L) of the 3e-bond. Thus, much like the CSBs in electron-pair bonds, such as F2 or the central bond in [1.1.1]propellane, here too L is positive, thus showing the excess kinetic energy of the shared density due to the Pauli repulsion in the 3-electron VB structures. The RECS values for 3-electron bonds are invariably larger than the corresponding bond energies. For the doubly-π-(3e)-bonded species, RECS is very large, exceeding 100 kcal mol-1. As such, it is fitting to conclude that σ- and π-3-electron-bonds find their natural place in the CSB family along with two-electron CSBs, with which they share identical energetic and topological characteristics. Experimental manifestations/tests of 3e-CSBs are proposed.

Experimental Confirmation of the Existence of the Three-Electron Bond and Theoretical Basis of Its Existence

Analysis of images made in techniques of atomic force microscopy (AFM) of high resolution in pentacene and other aromatic systems shows that according to predictions, aromatic three-electron bond is deflected to the centre of aromatic nuclei, which clearly confirms the fact of existence of three-electron bond in benzene, pentacene and other aromatic systems. It also confirms the existence of this bond in carboxylate anions and other similar ions and molecules.

Structure of the Benzene Molecule on the Basis of the Three-Electron Bond

Using the concept of three-electron bond one can represent the actual electron structure of benzene, explain specificity of the aromatic bond and calculate the delocalization energy. It was shown, that functional relation y = a + b/x + c/x2 fully describes dependence of energy and multiplicity of chemical bond on bond distance. In this article carbon-to-carbon bonds are reviewed. Using these dependences it is possible to calculate chemical bound energy by different bond distance or different multiplicity of chemical bond, that makes possible to calculate delocalization energy of benzene molecule.

Theory of Three-Electrone Bond in the Four Works with Brief Comments (Notes)

Note 1. Three-electrone bond it is an existing particle (object). Note 2. Chemical bond - it is the interaction of fermions. Note 3. Alternative description of the chemical bond. Note 4. Urea molecule. Note 5. Oxygen. Note 6. Chemical bond - is a new indivisible particles. Note 7. Model of the Interfering Universe.

Review (127 Pages, Full Version). Benzene on the Basis of the Three-Electron Bond. (The Pauli Exclusion Principle, Heisenberg's Uncertainty Principle and Chemical Bond)

The present work shows the inapplicability of the Pauli principle to chemical bond, and a new theoretical model of the chemical bond is proposed based on the Heisenberg uncertainty principle. See pp. 88 - 104. Using the concept of three-electron bond we can represent the actual electron structure of benzene and other molecules, explain specificity of the aromatic bond and calculate the delocalization energy. Gives theoretical justification and experimental confirmation of existence of the three-electron bond. It was shown, that functional relation y = a + b/x + c/x^2 fully describes dependence of energy and multiplicity of chemical bond from bond distance.

Roald Hoffmann

Roald Hoffmann

Odd-Electron Bonds.

The history and theory of one- and three-electron bonds are presented and discussed. It is shown that the dependence of the odd-electron bond energy on the difference in ionization potentials of the bonding partners must be corrected to include the ion-pairing energy for neutral odd-electron bonded complexes.

Theoretical study of the S…Cl interaction between sulfides and atomic chlorine. Nature and strength of the two-center three-electron bond

The interaction between sulfides (H2S, CH3SH, CH3CH2SH, CH3SCH3 and CH3SCH2F) and atomic chlorine is investigated using DFT based LC-BLYP and CCSD(T) methods in conjugation with the aug-cc-pVTZ basis set. The intermolecular S…Cl distances in the complexes range between 2.5922 Å (H2S…Cl) and 2.5273 Å (CH3SCH3…Cl). The interaction energies ranging between −29.1 and −77.5 kJ mol−1 reveal the formation of relatively strong 2c-3e S…Cl bonds. The binding energies are linearly related to the proton affinity of the sulfides, to the charge transfer taking place from the sulfides to the Cl atom and inversely proportional to the difference between the ionization potential of the sulfides and the electron affinity of the Cl atom. The spin density analysis suggests that almost 48% of the spin is transferred from the Cl to the S atom in the strongest CH3SCH3…Cl complex. The AIM analysis shows that the S…Cl interaction possess a significant covalent character which decreases with decreasing binding strength of the complexes. All the data indicate that the S…Cl 2c-3e bonds are stronger than the O…Cl or N…Cl bonds and more covalent in nature. The variation of the CH distances and the blue shifts of the ν(CH) vibration resulting from the interaction with Cl are discussed.

Theory of Three-Electron Bond in the Four Works with Brief Comments

Using the concept of three-electron bond we can represent the actual electron structure of benzene and other molecules, explain specificity of the aromatic bond and calculate the delocalization energy. Gives theoretical justification and experimental confirmation of existence of the three-electron bond. It was shown, that functional relation y=a+b/x+c/ x2 fully describes dependence of energy and multiplicity of chemical bond from bond distance.

Oxidation studies of a novel peptide model N-acetyl-3-(methylthio)propylamine

Abstract The OH radical induced oxidation of a selected methionine analogue − N-acetyl-3-(methylthio)propylamine (N-acetylated methionine without its carboxylic group) was investigated in aqueous solution. The oxidized intermediate (>S +) can be stabilized by the formation of two-centered three-electron bonds with electron-rich atoms (S, N, O), or it can undergo deprotonation yielding the long-lived, carbon-centered α-(alkylthio)alkyl radical (αS). The paths and kinetics of these reactions were followed using the pulse radiolysis technique with spectrophotometric detection. The reaction mechanism is shown to be different from the mechanism previously reported for 3-(methylthio)propylamine (3-MTPA), and depends strongly on the structural nature of the methionine substituents. The oxidation of 3-AcMTPA by OH radicals in anaerobic conditions yielded a sulfoxide (>S O) and a stable dimer (αS–αS) formed from the combination of two α-(alkylthio)alkyl radicals as major stable products. The proposed mechanism is further supported by the stable product analysis carried out using a high-resolution LC–MS/MS technique. A detailed mechanism involving characterizations of the transients and the stable products is discussed in qualitative terms. It is clearly demonstrated that the results from the time-resolved investigations are consistent with the findings from the stable product analysis.

Review. Benzene on the Basis of the Three-Electron Bond. Theory of Three-Electron Bond in the Four Works with Brief Comments

Using the concept of three-electron bond we can represent the actual electron structure of benzene and other molecules, explain specificity of the aromatic bond and calculate the delocalization energy. Gives theoretical justification and experimental confirmation of existence of the three-electron bond. It was shown, that functional relation y = a + b/x + c/x2 fully describes dependence of energy and multiplicity of chemical bond from bond distance.