Nonbonding Electrons Research Articles

Uncovering a Vital Band Gap Mechanism of Pnictides

Pnictides are superior infrared (IR) nonlinear optical (NLO) material candidates, but the exploration of NLO pnictides is still tardy due to lack of rational material design strategies. An in‐depth understanding structure–performance relationship is urgent for designing novel and eminent pnictide NLO materials. Herein, this work unravels a vital band gap mechanism of pnictides, namely P atom with low coordination numbers (2 CN) will cause the decrease of band gap due to the delocalization of non‐bonding electron pairs. Accordingly, a general design paradigm for NLO pnictides, ionicity–covalency–metallicity regulation is proposed for designing wide‐band gap NLO pnictides with maintained SHG effect. Driven by this idea, millimeter‐level crystals of MgSiP2 are synthesized with a wide band gap (2.34 eV), a strong NLO performance (3.5 x AgGaS2), and a wide IR transparency range (0.53–10.3 µm). This work provides an essential guidance for the future design and synthesis of NLO pnictides, and also opens a new perspective at Zintl chemistry important for other material fields.

Experimental verification of increased electronic excitation energy of water in hydrate-melt water by attenuated total reflection-far-ultraviolet spectroscopy.

The demand for Li secondary batteries is increasing, with the need for batteries with a higher level of performance and improved safety features. The use of a highly concentrated aqueous electrolyte solution is an effective way to increase the safety of batteries because it is possible to use "water-in-salt" (WIS) and "hydrate-melt" (HM) electrolytes for practical applications. These electrolytes exhibit a potential window of >3.0V, which is attributed to the difference between the HOMO and the LUMO energies of the n orbital of the pure water molecules and that of the water molecules in the hydration shells of a metal ion, according to theoretical predictions. Thus, in the present study, the attenuated total reflectance (ATR)-far-ultraviolet (FUV) spectra of water and super-concentrated aqueous solutions, such as WIS and HM using a Li salt, were experimentally investigated. The effects of anions, cations, and deuteriums on the ATR-FUV spectra were examined. The ATR-FUV method is an excellent means of studying highly concentrated aqueous salt solutions. The results suggest that the transition energy of water molecules in an ultrahighly concentrated aqueous electrolyte containing HM and WIS increased by nearly 0.4eV (corresponding to an energy shift of over 10nm) compared to an aqueous electrolyte with a typical water concentration. It was also revealed that the transition energy of water changes depending on the environment of the non-bonding electron, which is directly connected with or affected by hydrogen bonding with other water molecules or directly connected with Li+.

Impact of the coordination of multiple Lewis acid functions on the electronic structure and vn configuration of a metal center.

The covalent bond classification (CBC) method represents a molecule as MLlXxZz by evaluating the total number of L, X and Z functions interacting with M. The CBC method is a simplistic approach that is based on the notion that the bonding of a ligating atom (or group of atoms) can be expressed in terms of the number of electrons it contributes to a 2-electron bond. In many cases, the bonding in a molecule of interest can be described in terms of a 2-center 2-electron bonding model and the MLlXxZz classification can be derived straightforwardly by considering each ligand independently. However, the bonding within a molecule cannot always be described satisfactorily by using a 2-center 2-electron model and, in such situations, the MLlXxZz classification requires a more detailed consideration than one in which each ligand is treated in an independent manner. The purpose of this article is to provide examples of how the MLlXxZz classification is obtained in the presence of multicenter bonding interactions. Specific emphasis is given to the treatment of multiple π-acceptor ligands and the impact on the vn configuration, i.e. the number of formally nonbonding electrons on an element of interest.

Enlightening of the low-dimensional high-<italic>T</italic><sub>C</sub> superconductivity: Bond contraction and electron dual polarization

<p indent=0mm>Electron-phonon interaction that couples the carriers for the Bose-Einstein condensation and the nature and configuration of the O−Cu chains and O−Cu planes that accommodate carriers are key issues to the cuprite high-<italic>T</italic>_C superconductivity discovered in 1986. However, the new kind of high-<italic>T</italic>_C superconductivity is beyond the description of the conventional Bardeen–Cooper–Schrieffer (BCS) theory. Here we address these two issues based on the bond order-length-strength correlation and nonbonding electron polarization (BOLS-NEP) notion for the effect of atomic undercoordination and the hydrogen bond (O:H−O equivalent of O:Cu−O) cooperativity and polarizability (HBCP) premise for water ice with involvement of electron lone pairs “:” interactions. First, a Cu^p:O:Cu^p dipolar chain is formed by connecting a series of tetrahedrons made of two Cu⁺ ions and two Cu^p dipoles surrounding the center O^2–. The O−Cu(110) plane includes three sublayers. The first one is made of alternative rows of Cu⁰ vacancies and the Cu^p:O:Cu^p dipolar chain with dipoles pointing out the surface; the second sublayer is formed by O^2– whose lone pairs polarize the dipoles and squeeze out the missing rows of Cu atoms, and the third sublayer is made of Cu⁺. However, the O−Cu(001) plane shows a different manner of the sublayers. The first sublayer is made of Cu^p, Cu⁰, and Cu²⁺, and the third sublayer of Cu⁺ and Cu. The O^2– bonds to the Cu⁺ and polarizes the Cu to form the oppositely-coupled dipoles on the Cu(001) surface. The O−Cu bonding proceeds in four discrete stages: O² bonds to one Cu to form the Cu²⁺−2O^– and then each O^– bonds to its neighboring Cu in the second layer to form the twin tetrahedron with production of the lone pairs. The introduction of O to Cu host creates valence density features of the O−Cu bonding, lone pair nonbonding, the dipolar antibonding above <italic>E</italic>_F, and electron holes. Second, atomic undercoordination of the O−Cu chains and the O−Cu planes shortens and stiffens the local O−Cu bond and lengthens and weakens the O:Cu^p nonbonding interactions associated with further enhancement of the Cu^p dipoles polarization, this process is the same to the O:H−O bond relaxation and polarization occurred to the skins of water and ice. Last, the electron-phonon interaction is realized by the undercoordination-induced bond relaxation and the dual polarization by lone pairs and bond contraction. The dual polarization weakens the O:Cu^p interactions and lowers its vibration frequency of the Cu^p, reducing the effective mass of the Cu^P electrons with high group velocity for carrier transport between the adjacent O−Cu planes that are made of atomic vacancies and dipoles. The self-entrapment of the core and bonding electrons, and the localized polarization and O:Cu^p weakening may describe the effect electron-phonon coupling. The discovered “attraction force” between electrons along the Cu−O chain may fingerprint the effect of atomic undercoordination-induced bond contraction. The understandings of the dual polarization by oxidation and atomic undercoordination of carriers and the electron-phonon interaction may extend to devising the low-dimensional high-<italic>T</italic>_C superconductivity and the edge states polarization for the topological insulator superconductivity. The BOLS-NEP and HBCP regulations could be essential ingredients for the carrier generation and their interactions, which shall play certain substantial roles in the new types of superconductivity.

Single-Bonded Cubic AsN from High-Pressure and High-Temperature Chemical Reactivity of Arsenic and Nitrogen.

Chemical reactivity between As and N2, leading to the synthesis of crystalline arsenic nitride, is here reported under high pressure and high temperature conditions generated by laser heating in a diamond anvil cell. Single‐crystal synchrotron X‐ray diffraction at different pressures between 30 and 40 GPa provides evidence for the synthesis of a covalent compound of AsN stoichiometry, crystallizing in a cubic P213 space group, in which each of the two elements is single‐bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination. The identification of characteristic structural motifs highlights the key role played by the directional repulsive interactions between non‐bonding electron lone pairs in the formation of the AsN structure. Additional data indicate the existence of AsN at room temperature from 9.8 up to 50 GPa. Implications concern fundamental aspects of pnictogens chemistry and the synthesis of innovative advanced materials.

Additively Manufactured Carbon-Fiber Composite Electrodes for Structural Batteries and Grid Storage

Additively-Manufactured Electroactive Carbon-fiber/Phenolic Anodes for Structural Batteries and Grid Storage Craig Milroy, Tim Phillips, Joseph Beaman (Department of Mechanical Engineering, University of Texas, Austin).This presentation describes the electrochemical energy-storage properties of additively -manufactured anodes fabricated from carbon fibers and an electroactive phenolic resin, and demonstrates their utility for three important energy-storage applications:(1) additively-manufactured batteries(2) grid storage(3) structural batteriesPhenolic polymer resins are polymerized by reacting phenols with formaldehyde, and are used in a broad range of applications such as coatings, adhesives, and molded products. Although phenolic resins are technically thermosets, the melting point of most phenolics is lower than their cure (crosslinking) temperature; as a result, many phenolics are melt-processable and may therefore be used as thermoplastic binders for indirect selective laser sintering (SLS) additive manufacturing (AM) processes.Although the uncured phenolic compound is soft and melts at low temperature, the cured polymer has excellent strength/hardness, thermal stability, and chemical resistance/imperviousness. As such, phenolic resins are frequently used in building materials, and are therefore an excellent choice for structural battery applications.In addition, the phenol monomer that forms the basis of phenolic resins is an aromatic molecule with delocalized non-bonding electrons, which allows the hydroxyl moiety on the phenol to undergo reversible hydroxyl/quinine redox conversion. In addition, the polymerization process of phenolic monomers creates methylene bridges between phenol molecules, which preserves the electroactivity of the phenol molecule in its polymerized form. These properties have led to many reports of phenols as “natural” energy storage materials, since there are numerous biological sources of phenolic compounds.We observed the electroactivity of the cured phenolic (GP 5520) when using it as a thermoplastic binder in an indirect SLS fabrication process to produce additively-manufactured graphite anodes. To unambiguously measure the electroactivity of the cured phenolic, we fabricated electrodes by mixing phenolic powder in solid or dissolved form with non-electroactive conductive carbon fibers. The electrodes were fabricated via SLS and then cured to affix the electroactive phenol to the carbon fibers, which provided mechanical strength and served as a current collector for charge injection/extraction . The Figure depicts representative voltage profiles during galvanostatic discharge/charge testing of an SLS-fabricated composite electrode containing carbon fibers and phenolic, and indicates that the material is predisposed to function well as an anode, since the discharge occurs exclusively in a relatively narrow potential range below 1.0 V relative to the Li / Li+ equilibrium potential.The Figure also presents lgalvanostatic cycle performance of the SLS-fabricated electrodes at different rates. As expected, electrodes that were discharged at lower current density produced higher capacity. However, all electrodes exhibited a gradual maturation of capacity that is commonly observed in other electroactive polymers such as polypyrrole and polyanaline. The total number of cycles required to reach stable capacity was inversely proportional to the current density. Overall, the electrodes displayed consistent cycle performance once their discharge capacity stabilized; therefore, to address the observed electrode maturation process, and to definitively evaluate the rate-dependent cycle performance, we utilized rate-capability testing, beginning with low current for a few cycles at the beginning of each cell test, followed by a traditional rate-capability ladder to assess the dependence of capacity on previous cycling history. As shown in the Figure, these tests found that the electrodes produced 200 mAh/g of phenolic at 15 mA/g , and 100 mAh/g phenolic at 75 mA/g. Figure 1

Diradicals or Zwitterions: The Chemical States of m -Benzoquinone and Structural Variation after Storage of Li Ions

Diradicals or Zwitterions: The Chemical States of <i>m</i> -Benzoquinone and Structural Variation after Storage of Li Ions

Exploration of Gas-Liquid Interfaces for Liquid Water and Methanol Using Extreme Ultraviolet Laser Photoemission Spectroscopy.

We present a study using extreme UV (EUV) photoemission spectroscopy of the valence electronic structures of aqueous and methanol solutions using a 10 kHz EUV light source based on high-order harmonic generation and a magnetic bottle time-of-flight electron spectrometer. Two aspects of the observed spectra are highlighted in this study. One is variation of the vertical ionization energy (VIE) for liquids as a function of the solute concentration, which is closely related to surface dipoles at the gas-liquid interface. The experimental results show that the VIE of liquid water increases slightly with increasing concentrations of NaCl and NaI and decreases with NaOH. The VIE of liquid methanol was also found to change slightly with NaI. On the other hand, tetrabutylammonium iodide (TBAI) and butylamine (BA) clearly reduce the VIE for liquid water, which is attributed to the formation of an electric double layer (EDL) by segregated solutes at the gas-liquid interface. As evidence for this, when the pH of an aqueous BA solution is reduced to protonate BA, the VIE shift gradually decreases because the protonated BA moves into the bulk to suppress the influence of the EDL. We computed the surface potentials for these solutions using molecular dynamics simulations, and the results supported our interpretation of the experimental results. Another observation is the variation of the relative energy and shape of individual photoelectron bands for solvents, which is related to alteration of the structure and constituents of the first solvation shell of ionized solvent molecules. All of the solutes cause changes in the photoelectron spectra at high concentration, one of the most prominent of which is the degree of splitting of the 3a1 band for liquid water and the 7a' band for liquid methanol, which are sensitive to hydrogen bonding in the liquids. The 3a1 splitting decreases with the increasing concentration of NaI, NaCl, and NaOH, indicating that Na+ penetrates into the hydrogen-bonding network to coordinate to a nonbonding electron of a water molecule. On the other hand, TBAI and BA cause smaller changes in the 3a1 splitting. Full interpretation of these spectroscopic features awaits extensive quantum chemical calculations and is beyond the scope of this study. However, these results illustrate the strong potential of EUV laser photoemission spectroscopy of liquids for exploration of interfacial and solution chemistry.

Directed Gas-Phase Formation of Aminosilylene (HSiNH2; X1A'): The Simplest Silicon Analogue of an Aminocarbene, under Single-Collision Conditions.

The aminosilylene molecule (HSiNH2, X1A')-the simplest representative of an unsaturated nitrogen-silylene-has been formed under single collision conditions via the gas phase elementary reaction involving the silylidyne radical (SiH) and ammonia (NH3). The reaction is initiated by the barrierless addition of the silylidyne radical to the nonbonding electron pair of nitrogen forming an HSiNH3 collision complex, which then undergoes unimolecular decomposition to aminosilylene (HSiNH2) via atomic hydrogen loss from the nitrogen atom. Compared to the isovalent aminomethylene carbene (HCNH2, X1A'), by replacing a single carbon atom with silicon, a profound effect on the stability and chemical bonding of the isovalent methanimine (H2CNH)-aminomethylene (HNCH2) and aminosilylene (HSiNH2)-silanimine (H2SiNH) isomer pairs is shown; i.e., thermodynamical stabilities of the carbene versus silylene are reversed by 220 kJ mol-1. Hence, the isovalency of the main group XIV element silicon was found to exhibit little similarities with the atomic carbon revealing a remarkable effect not only on the reactivity but also on the thermochemistry and chemical bonding.

Coulombic repulsion between O1-oxygen and non-bonding 3d electrons of M3+ ion in C2/c NaM3+Si2O6 clinopyroxene, where M = V, Fe and Cr, at high pressures.

Coulombic repulsion between O1-oxygen and non-bonding 3d electrons of M3+ ion in C2/c NaM3+Si2O6 clinopyroxene, where M = V, Fe and Cr, at high pressures.

Linear Group 13 E≡E Triple Bonds in E2 Li6 2.

The triply bonded heavier main-group compounds have a textbook trans-bent geometry, in contrast to a familiar linear form found for the lightest analogues. Strikingly, the unexpected linear group 13 E≡E triple bonds were herein found in the D4h -symmetry E2 Li6 2+ clusters, and they possess a large barrier (>18.0 kcal/mol) towards the dissociation of Li+ . The perfectly surrounded Li4 motifs and two linear coordinated Li atoms strongly suppress the increasing nonbonded electron density of heavier E atoms, making two degenerate π bonds and one multi-center σ bond in linear heavier main-group triple bonds. The surrounding Li6 motifs not only creates an effective electronic structure to form a linear E≡E triple bond, but the resulting electrostatic interactions account for the highly stable global E2 Li6 2+ clusters.

Anisotropic to Isotropic Transition in Monolayer Group-IV Tellurides.

Monolayer group-IV tellurides with phosphorene-derived structures are attracting increasing research interest because of their unique properties. Here, we systematically studied the quasiparticle electronic and optical properties of two-dimensional group-IV tellurides (SiTe, GeTe, SnTe, PbTe) using the GW and Bethe–Salpeter equation method. The calculations revealed that all group-IV tellurides are indirect bandgap semiconductors except for monolayer PbTe with a direct gap of 1.742 eV, while all of them are predicted to have prominent carrier transport ability. We further found that the excitonic effect has a significant impact on the optical properties for monolayer group-IV tellurides, and the predicted exciton binding energy is up to 0.598 eV for SiTe. Interestingly, the physical properties of monolayer group-IV tellurides were subject to an increasingly isotropic trend: from SiTe to PbTe, the differences of the calculated quasiparticle band gap, optical gap, and further exciton binding energy along different directions tended to decrease. We demonstrated that these anisotropic electronic and optical properties originate from the structural anisotropy, which in turn is the result of Coulomb repulsion between non-bonding electron pairs. Our theoretical results provide a deeper understanding of the anisotropic properties of group-IV telluride monolayers.

Hg3P2S8: A New Promising Infrared Nonlinear Optical Material with a Large Second-Harmonic Generation and a High Laser-Induced Damage Threshold

The development of new infrared (IR) nonlinear optical (NLO) materials with strong second-harmonic generation (SHG) responses and high laser-induced damage thresholds (LIDTs) is urgent but challenging. Herein, the defective diamond-like chalcogenide Hg3P2S8 (HPS) was rationally designed and fabricated by a flexible and rigid tetrahedral motif combined strategy. HPS exhibits a phase-matching SHG response of similar to 3.6 x AGS (the largest one in the reported ternary sulfides), high LIDT (similar to 3 x AGS), wide IR transparent region (0.43-16.3 mu m), moderate birefringence (0.043 at 1064 nm), and good physicochemical stability and crystal growth habits. Theoretical analyses confirm that the large SHG effect originates from the synergistic effects between distorted HgS4, PS4, and vacancy-induced nonbonding electrons. The results indicate that HPS is a promising NLO candidate for high-power, high-efficiency laser output in the mid-IR region, which provides an insight into the exploration of new IR NLO materials.

Understanding the molecular mechanism of the chlorine atom transfer between ammonia and hypochlorous acid with electron localisation function (ELF)

The nature of the chlorine atom transferred between ammonia and hypochlorous acid was investigated using the bonding evolution theory (BET). The application of topological analysis of electron localisation function (ELF) and Thom's catastrophe theory enabled a detailed description of the evolution of chemical bonds and non-bonding electron density. The BET analysis was performed at the DFT(MN15)/def2-TZVPPD computational level. The Hausdorff distance was used as a qualitative view on the evolution of the ELF field. The molecular mechanism of the chlorination reaction consisted of 11 structural stability domains (SSDs) and 10 ELF catastrophes. Four SSDs were observed before the transition state (TS). The TS was found in SSD V, and six SSDs were distinguished after the TS. The energy barriers were analysed with six different DFT functionals (B3LYP, CAM-B3LYP, LC-ωPBE, M11, MN15, ωB97XD) and def2-TZVPPD basis set. The energy barrier between transition state and reagents was smaller than transition state and products, thus the chlorination of ammonia is thermodynamically favourable. Alongside BET analysis, NPA (natural population analysis), AIM (atoms in molecules) and APT (atomic polar tensor) charge analysis was also applied to the transferred chlorine atom.

Densities, heat capacities, viscosities, 1H- and 13C-NMR spectra, and solvatochromic parameters of binary mixtures of 1,3-dimethyl-1,3-diazinan-2-one (DMPU) and water

The compound 1,3-dimethyl-1,3-diazinan-2-one (N,N’-dimethylpropyleneurea, DMPU) is a dipolar aprotic solvent with very strong electron-donating abilities but which is much less toxic and less carcinogenic than similar solvents such as the widely used HMPA (hexamethylphosphorictriamide). This paper reports densities and viscosities of the binary mixtures of (DMPU + water) in the temperature range (293.15 to 353.15) K at 5 K intervals, over the whole composition range. Isobaric heat capacities of these mixtures were measured by flow calorimetry at 298.15 K. The 1H- and 13C-NMR spectra as well as the solvatochromic parameters (polarizability, π*, hydrogen-bond acceptor basicity, β, and hydrogen-bond donor acidity, α) of the mixtures were also determined. The densities and viscosities were sensitive to traces of water in the organic-rich region while the heat capacities were not. Excess molar volumes were strongly negative, showing an extremum at mole fraction xDMPU ≈ 0.40. The variation of viscosity with mixture composition was similar to the density with a sharp maximum at xDMPU ≈ 0.35. Apparent molar volumes, Vϕ, of the mixtures show that the water-DMPU interactions have a larger effect on Vϕ of water than on DMPU. While the standard isobaric molar heat capacity of DMPU in water is much higher than that of DMPU itself, the corresponding value for water in DMPU was not much influenced by the presence of DMPU. The 1H-NMR spectra show that the chemical shift of the water protons is more influenced by mixture composition than those of the DMPU protons. A de-shielding of the DMPU carbonyl carbon resonance was observed in the 13C-NMR spectra at low xDMPU, indicative of H-bonding between the water protons and the non-bonding electrons of the carbonyl oxygen.

Full Variation of Site Substitution in Ni-Mn-Ga by Ferromagnetic Transition Metals

Systematic doping by transition elements Fe, Co and Ni on each site of Ni2MnGa alloy reveal that in bulk material the increase in martensitic transformation temperature is usually accompanied by the decrease in ferromagnetic Curie temperature, and vice versa. The highest martensitic transformation temperature (571 K) was found for Ni50.0Mn25.4(Ga20.3Ni4.3) with the result of a reduction in Curie temperature by 55 K. The highest Curie point (444 K) was found in alloy (Ni44.9Co5.1)Mn25.1Ga24.9; however, the transition temperature was reduced to 77 K. The dependence of transition temperature is better scaled with the Ne/a parameter (number of non-bonding electrons per atom) compared to usual e/a (valence electrons per atom). Ne/a dependence predicts a disappearance of martensitic transformation in (Ni45.3Fe5.3)Mn23.8Ga25.6, in agreement with our experiment. Although Curie temperature usually slightly decreases while the martensitic transition increases, there is no significant correlation of Curie temperature with e/a or Ne/a parameters. The doping effect of the same element is different for each compositional site. The cascade substitution is discussed and related to the experimental data.

Frosting behavior of louvered-fin and tube heat exchanger after surface treatment: Experimental analysis

In heat pump systems, frost creates insulation, restricts airflow, and induces stress that can have serious negative effects on the heat exchanger. Hydrophobic surface treatment is a low-cost, efficient, passive defrosting method that has attracted widespread research attention. In this study, the hydrophobic-modification surfaces of louvered-fin heat exchanger (HEX) in heat pump systems were investigated at microscopic and macroscopic scales, respectively. Frosting and defrosting experiments were conducted on evaporators with hydrophobic surfaces on an automotive performance test bench to provide novel information regarding the hydrophobic louvered-fin HEX. The nucleation growth process of modified fins in the microscopic state was studied under classical nucleation theory and electron polarization theory. The experiments show that the refrigerant flow rate of the bare HEX is drastically reduced at 35 min, leading to a sharp decay in heat pump system performance. The air pressure drop in the HEX at this time is 140 Pa. The time taken to reach this air pressure drop for the coated HEX is 153 min, which indicates that the hydrophobic surface has excellent anti-frost performance. We also find that subcooling acts as an intrinsic driving force for nucleation. The nucleation work increases by 77.8% as the bare HEX moves from the surface temperature region of −20 °C to the −15 °C region. The nucleation work on the hydrophobic surface increases by 255% at −20 °C compared to the bare surface. The theoretical analyses of surface elemental composition, non-bonded electron polarization, and crystal perturbation are consistent with the experimental results.

Effect of delocalization of nonbonding electron density on the stability of the M–Ccarbene bond in main group metal-imidazol-2-ylidene complexes: a computational and structural database study

Abstract The M–Ccarbene bond in metal (M) complexes involving the imidazol-2-ylidene (Im) ligand has largely been described using the σ-donor only model with donation of σ electrons from the sp-hybridized orbital of the carbene carbon into vacant orbitals on the metal centre. Analyses of the M–Ccarbene bond in a series of group IA, IIA and IIIA main group metal complexes show that the M-Im interactions are mostly electrostatic with the M–Ccarbene bond distances greater than the sum of the respective covalent radii. Estimation of the binding energies of a series of metal hydride/fluoride/chloride imidazol-2-ylidene complexes revealed that the stability of the M–Ccarbene bond in these complexes is not always commensurate with the σ-only electrostatic model. Further natural bond orbital (NBO) analyses at the DFT/B3LYP level of theory revealed substantial covalency in the M–Ccarbene bond with minor delocalization of electron density from the lone pair electrons on the halide ligands into antibonding molecular orbitals on the Im ligand. Calculation of the thermodynamic stability of the M–Ccarbene bond showed that these interactions are mostly endothermic in the gas phase with reduced entropies giving an overall ΔG &gt; 0.

The Nature of Azo-Substituted Carbocations: N-N π-Electron Stabilization versus Nitrogen Nonbonding Electron Stabilization.

Computational and experimental studies reveal two different modes of cation stabilization by the phenylazo group. The first mode involves a relatively weak conjugative interaction with the azo π-bond, while the second mode involves an interaction with the nitrogen nonbonding electrons. The 4-phenylazo group is slightly rate-retarding in the solvolysis of cumyl chloride and benzyl mesylate derivatives but rate-enhancing in the solvolysis of α-CF3 benzylic analogs. The phenylazo group can become a potent electron-donating group in cations such as [Me2C─N═N─Ph]+. Nonbonding electron stabilization can be strong enough to offset the very powerful γ-silyl stabilization. In aromatic cyclopropenium and tropylium cations, the demand for stabilization is quite low, and the mode of phenylazo stabilization reverts back to the less-effective π-stabilization. The solvolysis of cis-4-phenylazo benzyl mesylate is faster than that of trans-4-phenylazo benzyl mesylate. Products formed suggest a stepwise ionization, cation isomerization, and nucleophile capture mechanism. Computational studies indicate a vanishingly small barrier for the isomerization of the cis-cation intermediate to the trans-cation.

Non-Adiabatic Reaction Dynamics in the Gas-Phase Formation of Phosphinidenesilylene, the Isovalent Counterpart of Hydrogen Isocyanide, under Single-Collision Conditions.

The phosphinidenesilylene (HPSi; X1A') molecule was prepared via a directed gas-phase synthesis in the bimolecular reaction of ground-state atomic silicon (Si; 3P) with phosphine (PH3; X1A1) under single-collision conditions. The chemical dynamics are initiated on the triplet surface via addition of a silicon atom to the non-bonding electron pair of phosphine, followed by non-adiabatic dynamics and surface hopping to the singlet manifold, accompanied by isomerization via atomic hydrogen shift and decomposition to phosphinidenesilylene (HPSi, X1A') along with molecular hydrogen. Statistical calculations predict that silylidynephosphine (HSiP, X1Σ+) is also formed, albeit with lower yields. The barrier-less route to phosphinidenesilylene opens up a multipurpose mechanism to access the hitherto obscure class of phosphasilenylidenes through silicon-phosphorus coupling via reactions of atomic silicon with alkylphosphines under single-collision conditions in the absence of successive reactions of the reaction products, which are not feasible to prepare by traditional synthetic routes.