Unique Interaction between Layered Black Phosphorus and Nitrogen Dioxide

Air pollution caused by acid gases (NO₂, SO₂) or greenhouse gases (CO₂) is an urgent environmental problem. Two-dimensional nanomaterials exhibit exciting application potential in air pollution control, among which layered black phosphorus (LBP) has superior performance and is environmentally friendly. However, the current interaction mechanism of LBP with hazardous gases is contradictory to experimental observations, largely impeding development of LBP-based air pollution control nanotechnologies. Here, interaction mechanisms between LBP and hazardous gases are unveiled based on density functional theory and experiments. Results show that NO₂ is different from other gases, as it can react with unsaturated defects of LBP, resulting in oxidation of LBP and reduction of NO₂. Computational results indicate that the redox is initiated by p orbital hybridization between one oxygen atom of NO₂ and the phosphorus atom carrying a dangling single electron in a defect's center. For NO, the interaction mechanism is chemisorption on unsaturated LBP defects, whereas for SO₂, NH₃, CO₂ or CO, the interaction is dominated by van der Waals forces (57-82% of the total interaction). Experiments confirmed that NO₂ can oxidize LBP, yet other gases such as CO₂ cannot. This study provides mechanistic understanding in advance for developing novel nanotechnologies for selectively monitoring or treating gas pollutants containing NO₂.

Photo- and Radiation-Induced One-Electron Oxidation of Methionine in Various Structural Environments Studied by Time-Resolved Techniques

Oxidation of methionine (Met) is an important reaction that plays a key role in protein modifications during oxidative stress and aging. The first steps of Met oxidation involve the creation of very reactive and short-lived transients. Application of complementary time-resolved radiation and photochemical techniques (pulse radiolysis and laser flash photolysis together with time-resolved CIDNP and ESR techniques) allowed comparing in detail the one-electron oxidation mechanisms initiated either by ●OH radicals and other one-electron oxidants or the excited triplet state of the sensitizers e.g., 4-,3-carboxybenzophenones. The main purpose of this review is to present various factors that influence the character of the forming intermediates. They are divided into two parts: those inextricably related to the structures of molecules containing Met and those related to external factors. The former include (i) the protection of terminal amine and carboxyl groups, (ii) the location of Met in the peptide molecule, (iii) the character of neighboring amino acid other than Met, (iv) the character of the peptide chain (open vs cyclic), (v) the number of Met residues in peptide and protein, and (vi) the optical isomerism of Met residues. External factors include the type of the oxidant, pH, and concentration of Met-containing compounds in the reaction environment. Particular attention is given to the neighboring group participation, which is an essential parameter controlling one-electron oxidation of Met. Mechanistic aspects of oxidation processes by various one-electron oxidants in various structural and pH environments are summarized and discussed. The importance of these studies for understanding oxidation of Met in real biological systems is also addressed.

Persistent 2c-3e σ-bonded heteronuclear radical cations centered on S/Se and P/As atoms

The two-center three-electron (2c-3e) bonded species are important in chemical and biological science. Reported isolable 2c-3e σ-bonded species are usually constructed in homoatomic radicals. The one-electron oxidation of main-group heteronuclear species Nap(SPh)(P(Mes)2) (1), Nap(SePh)(P(Mes)2) (2), Nap(SPh)(As(Mes)2) (3) and Nap(SePh)(As(Mes)2) (4) produced persistent radical cations 1˙+-4˙+ in solution. Large couplings of heteroatoms in EPR spectra of 1˙+-4˙+, shorter bond distances and bigger Wiberg bond orders of Ch-Pn in 1˙+-4˙+ than those in 1-4 in DFT calculations indicate large amounts of spin densities over heteroatoms and the formation of 2c-3e σ-bonds between chalcogen and pnicogen atoms. This work provides evidence of 2c-3e σ-bonds constructed between main-group heteronuclears and rare examples of radical cations involving three-electron σ-bonds between S/Se and P/As atoms.

Neither too Classic nor too Exotic: One-Electron Na⋅B Bond in NaBH3 - Cluster

It is here reported that the NaBH₃ ^- cluster exhibits a Na⋅B one-electron bond, a well-established type of electron-deficient bonding in the literature. The topological analysis of the electron localization function, at the correlated level, reveals that Na^- , when approaching the bonding distance, fairly distributes its valence electron pair between two lobes. One of these electrons is used to bond with BH₃ , which participates through its boron empty p-orbital. Furthermore, the bonding situation of LiBH₃ ^- , KBH₃ ^- , MgBH₃ , and CaBH₃ global minima structures are similar to that of NaBH₃ ^- , extending the family of these new one-electron bond systems with biradicaloid character.

Comment on "The 'Inverted Bonds' Revisited. Analysis of 'in Silico' Models and of [1.1.1]Propellane Using Orbital Forces"

Inverted bonds: In this Correspondence, the authors comment on the recent paper on inverted bonds in [1.1.1]propellane by Chaquin et al.

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.

Charge-Shift Bonding: A New and Unique Form of Bonding

Charge-shift bonds (CSBs) constitute a new class of bonds different than covalent/polar-covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence-bond pioneers and then demonstrates that the unique status of CSBs is not theory-dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis-à-vis covalent and ionic bonds. Furthermore, the covalent-ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g., benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity.

Analyzing Fluxional Molecules Using DORI

The Density Overlap Region Indicator (DORI) is a density-based scalar field that reveals covalent bonding patterns and noncovalent interactions in the same value range. This work goes beyond the traditional static quantum chemistry use of scalar fields and illustrates the suitability of DORI for analyzing geometrical and electronic signatures in highly fluxional molecular systems. Examples include a dithiocyclophane, which possesses multiple local minima with differing extents of π-stacking interactions and a temperature dependent rotation of a molecular rotor, where the descriptor is employed to capture fingerprints of CH-π and π-π interactions. Finally, DORI serves to examine the fluctuating π-conjugation pathway of a photochromic torsional switch (PTS). Attention is also placed on postprocessing the large amount of generated data and juxtaposing DORI with a data-driven low-dimensional representation of the structural landscape.

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 (RE_CS) of the three electrons, and the bond dissociation energies (D_e). 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 RE_CS values for 3-electron bonds are invariably larger than the corresponding bond energies. For the doubly-π-(3e)-bonded species, RE_CS 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.

Activation of C–H and B–H bonds through agostic bonding: an ELF/QTAIM insight

X–H agostic bonding: a topological insight.

Three-electron two-centred bonds and the stabilisation of cationic sulfur radicals

Spin stabilisation of conjugated motifs and heteroatoms suggests that the primary mechanism for cationic thioether stabilisation is not three-electron two-centred bonds.

Intramolecular hydrogen bonds: the QTAIM and ELF characteristics

B3LYP/aug-cc-pVTZ calculations were performed on the species with intramolecular O-H···O hydrogen bonds. The Quantum Theory of Atoms in Molecules (QTAIM) and the Electron Localization Function (ELF) method were applied to analyze these interactions. Numerous relationships between ELF and QTAIM parameters were found. It is interesting that the CVB index based on the ELF method as well as the total electron energy density at the bond critical point of the proton-acceptor distance (H(bcp)) may be treated as universal descriptors of the hydrogen bond strength, they are also useful to estimate the covalent character of this interaction. There are so-called resonance-assisted hydrogen bonds (RAHBs) among the species analyzed here. It was found that there are not any distinct differences between RAHBs and the other intramolecular hydrogen bonds.