Bond Dissociation Mechanism Research Articles

Controlling Phase in Colloidal Synthesis.

A fundamental precept of chemistry is that properties are manifestations of the elements present and their arrangement in space. Controlling the arrangement of atoms in nanocrystals is not well understood in nanocrystal synthesis, especially in the transition metal chalcogenides and pnictides, which have rich phase spaces. This Perspective will cover some of the recent advances and current challenges. The perspective includes introductions to challenges particular to chalcogenide and pnictide chemistry, the often-convoluted roles of bond dissociation energies and mechanisms by which precursors break down, using very organized methods to map the synthetic phase space, a discussion of polytype control, and challenges in characterization, especially for solving novel structures on the nanoscale and time-resolved studies.

Mechanistic Insights into the N-Hydroxylations Catalyzed by the Binuclear Iron Domain of SznF Enzyme: Key Piece in the Synthesis of Streptozotocin.

SznF, a member of the emerging family of heme-oxygenase-like (HO-like) di-iron oxidases and oxygenases, employs two distinct domains to catalyze the conversion of Nω-methyl-L-arginine (L-NMA) into N-nitroso-containing product, which can subsequently be transformed into streptozotocin. Using unrestricted density functional theory (UDFT) with the hybrid functional B3LYP, we have mechanistically investigated the two sequential hydroxylations of L-NMA catalyzed by SznF's binuclear iron central domain. Mechanism B primarily involves the O-O bond dissociation, forming Fe(IV)=O, induced by the H+/e- introduction to the FeA side of μ-1,2-peroxo-Fe2(III/III), the substrate hydrogen abstraction by Fe(IV)=O, and the hydroxyl rebound to the substrate N radical. The stochastic addition of H+/e- to the FeB side (mechanism C) can transition to mechanism B, thereby preventing enzyme deactivation. Two other competing mechanisms, involving the direct O-O bond dissociation (mechanism A) and the addition of H2O as a co-substrate (mechanism D), have been ruled out.

Exploring the Enhanced Catalytic Activity of Pt Nanoparticles Generated on the Red Phosphorus/Graphitic Carbon Nitride Binary Heterojunctions in the Photo-assisted Hydrolysis of Ammonia Borane.

Ammonia borane (AB) holds great promise for chemical hydrogen storage, but its slow dehydrogenation kinetics under ambient conditions requires a suitable catalyst to facilitate hydrogen production from AB. Here, we fabricated binary red phosphorus/graphitic carbon nitride (RP/g-CN) heterojunctions decorated with Pt nanoparticles (NPs, denoted Pt/RP/g-CN) with a facile ultrasound-assisted two-step protocol as a photo-assisted catalyst for the hydrolysis of AB (HAB). The heterojunction established through intimate P-O-N bonds was proven to have improved photophysical properties such as a lower electron-hole recombination and enhanced visible light utilization compared to the pristine components. With the incorporation of Pt NPs, the optical properties of RP/g-CN heterojunctions were further improved through Schottky junction formation between semiconductors and Pt NPs, enabling a superb hydrogen gas (H2) generation rate of 142 mol H2·mol Pt-1·min-1 under visible light irradiation. Even though g-CN is a well-known host material for many metal NPs, here we discovered that the interaction of Pt NPs with RP in the ternary heterojunction structure is more favorable than that of g-CN, stressing the key role of RP as a support material in the designed ternary heterostructure. The band alignment of the ternary heterojunction catalyst along with the flow of charge carriers was also studied and shown to be a type-II heterojunction structure without hole migration, namely, a complex type-II heterojunction. Several scavenger experiments were also conducted to explain the mechanism of the photo-assisted HAB. To the best of our knowledge, this is the first example of a dual mechanism proposed for the visible light-assisted HAB. While the majority of the H2 was believed to be produced on the Pt NPs surface with the traditional B-N bond dissociation mechanism, the strong oxidizing action of OH• radicals formed by the heterojunction photocatalyst was also speculated to account for the 33% increase in the activity upon visible light irradiation through another mechanism.

Manipulating hydrogen bond dissociation rates and mechanisms in water dimer through vibrational strong coupling

The vibrational strong coupling (VSC) between molecular vibrations and cavity photon modes has recently emerged as a promising tool for influencing chemical reactivities. Despite numerous experimental and theoretical efforts, the underlying mechanism of VSC effects remains elusive. In this study, we combine state-of-art quantum cavity vibrational self-consistent field/configuration interaction theory (cav-VSCF/VCI), quasi-classical trajectory method, along with the quantum-chemical CCSD(T)-level machine learning potential, to simulate the hydrogen bond dissociation dynamics of water dimer under VSC. We observe that manipulating the light-matter coupling strength and cavity frequencies can either inhibit or accelerate the dissociation rate. Furthermore, we discover that the cavity surprisingly modifies the vibrational dissociation channels, with a pathway involving both water fragments in their ground vibrational states becoming the major channel, which is a minor one when the water dimer is outside the cavity. We elucidate the mechanisms behind these effects by investigating the critical role of the optical cavity in modifying the intramolecular and intermolecular coupling patterns. While our work focuses on single water dimer system, it provides direct and statistically significant evidence of VSC effects on molecular reaction dynamics.

Computational Screening of Supported Metal Oxide Nanoclusters for Methane Activation: Insights into Homolytic versus Heterolytic C–H Bond Dissociation

Since its discovery in zeolites, the [CuOCu]2+ motif has played an important role in our understanding of selective methane activation over supported metal oxide nanoclusters. Although there are two known C-H bond dissociation mechanisms, namely, homolytic and heterolytic cleavage, most computational studies on optimizing metal oxide nanoclusters for improved methane activation reactivity have focused only on the homolytic mechanism. In this work, both mechanisms were examined for a set of 21 mixed metal oxide complexes of the form of [M1OM2]2+ (M1 and M2 = Mn, Fe, Co, Ni, Cu, and Zn). Except for pure copper, heterolytic cleavage was found to be the dominant C-H bond activation pathway for all systems. Furthermore, mixed systems including [CuOMn]2+, [CuONi]2+, and [CuOZn]2+ are predicted to possess methane activation activity similar to pure [CuOCu]2+. These results suggest that both homolytic and heterolytic mechanisms should be considered in computing methane activation energies on supported metal oxide nanoclusters.

Manipulating hydrogen bond dissociation rates and mechanisms in water dimer through polaritonic vibrational strong coupling

Manipulating hydrogen bond dissociation rates and mechanisms in water dimer through polaritonic vibrational strong coupling

Towards the ionizing radiation induced bond dissociation mechanism in oxygen, water, guanine and DNA fragmentation: a density functional theory simulation

The radiation-induced damages in bio-molecules are ubiquitous processes in radiotherapy and radio-biology, and critical to space projects. In this study, we present a precise quantification of the fragmentation mechanisms of deoxyribonucleic acid (DNA) and the molecules surrounding DNA such as oxygen and water under non-equilibrium conditions using the first-principle calculations based on density functional theory (DFT). Our results reveal the structural stability of DNA bases and backbone that withstand up to a combined threshold of charge and hydrogen abstraction owing to simultaneously direct and indirect ionization processes. We show the hydrogen contents of the molecules significantly control the stability in the presence of radiation. This study provides comprehensive information on the impact of the direct and indirect induced bond dissociations and DNA damage and introduces a systematic methodology for fine-tuning the input parameters necessary for the large-scale Monte Carlo simulations of radio-biological responses and mitigation of detrimental effects of ionizing radiation.

Initial Thermal Decomposition Mechanism of (NH2)2C=C(NO2)(ONO) Revealed by Double-Hybrid Density Functional Calculations.

This work employs double-hybrid density functionals to re-examine the CO–NO bond dissociation mechanism of nitrite isomer of 1,1-diamino-2,2-dinitro-ethylene (DADNE) into (NH2)2C=C(NO2)O and nitric monoxide (NO). The calculated results confirm that an activated barrier is present in the CO–NO bond dissociation process of (NH2)2C=C(NO2)(ONO). Furthermore, it is proposed that a radical–radical adduct is involved in the exit dissociation path with subsequent dissociation to separate (NH2)2C=C(NO2)O and NO radicals. The activation and reaction enthalpies at 298.15 K for the nitrite isomer dissociation are predicted to be 43.6 and 5.4 kJ mol–1 at the B2PLYP/6-31G(d,p) level, respectively. Employing the B2PLYP/6-31G(d,p) reaction energetics, gradient, Hessian, and geometries, the kinetic model for the CO–NO bond dissociation of (NH2)2C=C(NO2)(ONO) is obtained by a fitting to the modified Arrhenius form 1.05 × 1013(T/300)0.39 exp[−27.80(T + 205.32)/R(T2 + 205.322)] in units of per second over the temperature range 200–3000 K based on the canonical variational transition-state theory with multidimensional small-curvature tunneling.

The Early Steps of Molecule-to-Material Conversion in Chemical Vapor Deposition (CVD): A Case Study.

Transition metal complexes with β-diketonate and diamine ligands are valuable precursors for chemical vapor deposition (CVD) of metal oxide nanomaterials, but the metal-ligand bond dissociation mechanism on the growth surface is not yet clarified in detail. We address this question by density functional theory (DFT) and ab initio molecular dynamics (AIMD) in combination with the Blue Moon (BM) statistical sampling approach. AIMD simulations of the Zn β-diketonate-diamine complex Zn(hfa)2TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine), an amenable precursor for the CVD of ZnO nanosystems, show that rolling diffusion of this precursor at 500 K on a hydroxylated silica slab leads to an octahedral-to-square pyramidal rearrangement of its molecular geometry. The free energy profile of the octahedral-to-square pyramidal conversion indicates that the process barrier (5.8 kcal/mol) is of the order of magnitude of the thermal energy at the operating temperature. The formation of hydrogen bonds with surface hydroxyl groups plays a key role in aiding the dissociation of a Zn-O bond. In the square-pyramidal complex, the Zn center has a free coordination position, which might promote the interaction with incoming reagents on the deposition surface. These results provide a valuable atomistic insight on the molecule-to-material conversion process which, in perspective, might help to tailor by design the first nucleation stages of the target ZnO-based nanostructures.

A tight binding model of the bond dissociation mechanism in thermochromic leuco dyes

It has long been known that heat, UV light, or protonization will cause the bond in Leuco dye's to break. The underlying microscopic mechanism is a still a matter of discussion. It stands to reason that a local model should correctly predict the temperature behavior of the heat capacity at the thermochromic phase transition. Here we develop a tight binding (TB) model for the variation of the energy of the bond as it breaks as a function of temperature. We use a statistical mechanical approach to incorporate this variation in a partition function, calculate a free energy, and obtain the heat capacity of the phase transition. A distribution of such heat capacities is shown to fit the data for the leuco dye. The heat capacity distribution is compared to the experimentally determined heat capacity by differential scanning calorimetry (DSC).

Investigation on the Thermal Dissociation of Vinyl Nitrite with a Saddle Point Involved.

Hybrid and double-hybrid density functionals are employed to explore the O–NO bond dissociation mechanism of vinyl nitrite (CH2=CHONO) into vinoxy (CH2=CHO) and nitric monoxide (NO). In contrast to previous investigations, which point out that the O–NO bond dissociation of vinyl nitrite is barrierless, our computational results clearly reveal that a kinetic barrier (first-order saddle point) in the O–NO bond dissociation is involved. Furthermore, a radical–radical adduct is recommended to be present on the dissociation path. The activation and reaction enthalpies at 298.15 K for the vinyl nitrite dissociation are calculated to be 91 and 75 kJ mol–1 at the M062X/MG3S level, respectively, and the calculated reaction enthalpy compares very well with the experimental result of 76.58 kJ mol–1. The M062X/MG3S reaction energetics, gradient, Hessian, and geometries are used to estimate vinyl nitrite dissociation rates based on the multistructural canonical variational transition-state theory including contributions from hindered rotations and multidimensional small-curvature tunneling at temperatures from 200 to 3000 K, and the rate constant results are fitted to the four-parameter Arrhenius expression of 4.2 × 109 (T/300)4.3 exp[−87.5(T – 32.6)/(T2 + 32.62)] s–1.

A quantum-chemical study of the CO dissociation mechanism on low-index Miller planes of ϴ-Fe3C

Abstract Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe3C in the context of Fischer-Tropsch synthesis. Compared to the most reactive (111) surface of bcc-Fe on which CO binds in a 7-fold coordination, CO prefers to locate in 3-fold or 4-fold sites on the carburized surfaces due to the presence of interstitial C atoms at or below the surface. An important finding is that the lowest activation energies for direct CO bond dissociation are associated with the presence of step-like sites, similar to the case of metallic surfaces. We could identify such sites for 3 out of the 9 investigated surfaces, namely the (111), 1 1 ¯ 1 , and (010) terminations of ϴ-Fe3C. On the other hand, H-assisted CO dissociation is preferred on the 0 1 ¯ 1 , (001), and (100) surfaces. The other (011), (110), and (101) surfaces are inert with CO dissociation barriers close to or exceeding the CO adsorption energy. A kinetic analysis shows that the (111) surface (direct CO dissociation) and the ( 0 1 ¯ 1 ) surface (H-assisted CO dissociation via HCO) display comparable CO bond dissociation rates, much higher than the rates computed for the other surfaces. Together these two surfaces make up ca. 28% of the surface enclosing a Wulff nanoparticle of ϴ-Fe3C. Using an atomic population analysis, we show that the activation barrier for C-O bond dissociation correlates well with the bond order of adsorbed CO. This implies that pre-activation of CO is important for lowering the overall activation barrier. The present work demonstrates that the high-temperature ϴ-Fe3C phase is highly active towards CO bond dissociation, which is the essential first step in the Fischer-Tropsch reaction. Several of the exposed surfaces present lower overall CO dissociation barriers than α-Fe (known to be unstable under Fischer-Tropsch conditions) and the χ-carbide of Fe (usually assumed to be the most stable phase of Fe-carbide under Fischer-Tropsch conditions). Notably, the activity of the (111) surface is higher than that of a stepped cobalt surface.

Selective C[sbnd]H bond activation of ethane by free gold clusters

The activation and potential dissociation of ethane mediated by small cationic gold clusters Aux+ (x = 2–4) has been explored by infrared multiple photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations. The calculations show that the interaction between the gold clusters and ethane is mainly governed by the mixing of the ethane CH3 bond-forming orbitals π−(CH3) with gold d-orbitals. While the CC single bond appears to be unaffected, this mixing leads to the selective activation of up to two ethane C–H bonds and a reduction of the activation barrier for C–H bond dissociation to up to 0.82 eV, making the reaction kinetically feasible at room temperature. In agreement with this, experimental IR-MPD spectra of the complexes Au2(C2H6)2+ and Au2(C2D6)2+ show strong indications for the co-existence of two isomeric structures, one representing the encounter complex and one where a single ethane C–H bond is dissociated resulting in a complex which contains an ethyl group and a bridge-bonded H atom along with a second, adsorbed C2H6 molecule. A similar C–H bond dissociation mechanism is theoretically predicted for the Au3(C2H6)y+ (y = 2,3) and Au4(C2H6)y+ (y = 2,3) complexes, albeit thermodynamically less favorable. IR-MPD spectra of Au3(C2H6)y+ (y = 2,3) and Au4(C2H6)y+ (y = 2,3) confirm the encounter product to be the dominant one, although the co-existence of isomers containing ethyl groups cannot be excluded. Various pathways for C–H bond activation are theoretically explored and the ethane activation mechanism is compared to the gold mediated activation of methane and ethylene.

Laser photolysis studies of ω-bond dissociation in aromatic carbonyls of five-membered rings induced by triplet sensitization

Photoinduced ω-bond cleavage was investigated in seven newly synthesized aromatic ketones having five-membered rings (furan and thiophene) and/or phenyl group connected to the phenylthiyl moiety with CS bond. Following direct laser excitation of these ketones in acetonitrile, the transient absorption spectra that appeared were shown to be due to the corresponding triplet states. No chemical reactions occurred upon direct excitation. However, triplet sensitization of these aromatic ketones using xanthone revealed no formation of their triplet states. Instead absorption spectra of the phenylthiyl radical (PTR) appeared as a result of CS bond cleavage. These observations indicate that higher triplet states were reactive leading to CS bond dissociation. The CS bond dissociation mechanism upon triplet sensitization for the studied carbonyl compounds was discussed considering energy diagrams of the higher excited states.

Single Molecule Force Spectroscopy Reveals the pH-Dependent Mechanical Strength of the Alkynyl-Gold Bond.

Alkynyl groups are widely used as the gold-binding motifs for many applications, including the stabilization of gold nanoparticles, surface protection and surface chemical conjugation. However, at the fundamental level, the strength and binding mechanism of the alkynyl-gold bonds were yet to be revealed. Here, we use atomic force microscopy (AFM) based single molecule force spectroscopy to directly quantify the mechanical strength of the alkynyl-gold bond one bond at a time. We find that alkynyl-gold bonds are relatively strong with rupture forces in a broad range from 200 pN to 400 pN, which are stronger than many coordination bonds studied so far but weaker than a thiol-gold bond under similar experimental conditions. More interestingly, we find that the dissociation forces of alkynyl-gold bonds depend on the pH of the solution, suggesting a heterolytic rupture mechanism. The average rupture forces decrease from ∼270 pN at pH 7.4 to ∼100 pN at pH 3.4. Moreover, we find that the solvent conditions can also significantly affect the dissociation of alkynyl-gold bonds, indicating the existence of different binding modes and different dissociation pathways. Together, our results provide direct quantification of the mechanical strength of alkynyl-gold bonds and reveal the complex bond dissociation mechanism. This study may shed new light on the modulation of the stabilities of alkynyl-gold bonds at the bulk level for various biomedical and nanotechnological applications.

Using the Lewis Acid Me3 Si-F-Al(ORF )3 To Prepare Phosphino-Phosphonium Cations with the Least-Coordinating Anion [(RF O)3 Al-F-Al(ORF )3

By reaction of two equivalents of Me3 Si-F-Al(ORF )3 1 with an equimolar amount of PPh2 Cl, the salt [Ph2 P-PPh2 Cl]+ [(RF O)3 Al-F-Al(ORF )3 ]- 2 is prepared smoothly in 91 % yield (NMR, XRD). The synthesis of [Ph2 P-PPh3 ]+ [(RF O)3 Al-F-Al(ORF )3 ]- 3 is best achieved by a two-step reaction: first, two equivalents of 1 react with one PPh3 to give [Me3 Si-PPh3 ]+ [(RF O)3 Al-F-Al(ORF )3 ]- 4 (NMR, XRD), which, upon reaction with PPh2 Cl, yields pure 3 and Me3 SiCl (NMR, XRD). Typically, a stoichiometry of two equivalents of 1 with respect to one equivalent of the chloride donor should be used. Otherwise, the residual strong Lewis acidity of the [(RF O)3 Al-F-Al(ORF )3 ]- anion in the presence of the [F-Al(ORF )3 ]- anion-that forms with less than two equivalents of 1-leads to further chloride exchange reactions that complicate work-up. This route presents the easiest way to introduce the least-coordinating [(RF O)3 Al-F-Al(ORF )3 ]- anion into a system. We expect a wide use of this route in all areas, in which chloride-bond heterolysis in combination with very weakly coordinating anions is desirable. Additionally, we performed calculations on the bond dissociation mechanisms of [R2 P-PMe3 ]+ and the isoelectronic Me2 P-SiMe3 and Me2 Si-PMe3 in dependence of the solvent permittivity. These calculations show, especially for the neutral reference compounds, a heavy influence of the solvent on the dissociation mechanism, which is why we suggest investigating these properties in solution instead of gas phase.

DFT-based study on the optimal CH3OH decomposition pathways in aqueous-phase: Homolysis versus heterolysis

Our present study indicates that solvent effect can alter CH3OH decomposition pathways and identifies for the first time the presence of a dual reaction mechanism for CH3OH heterolytic decomposition that proceeds through formation of CH2OH and CH3O intermediates, whereas only initial C-H bond dissociation mechanism is identified in homolytic pathways. Furthermore, the homolytic and heterolytic pathways may be able to occur simultaneously on Pt(111) based on our DFT calculations. The present study can provide a new insight into modeling dehydrogenation reactions in aqueous-phase and help to achieve the ultimate goal of the design of Pt-based alloy electrocatalysts for CH3OH decomposition.

Bond dissociation mechanism of ethanol during carbon nanotube synthesis via alcohol catalytic CVD technique: Ab initio molecular dynamics simulation

Dissociation of ethanol on a nickel cluster is investigated by ab initio molecular dynamics simulation to reveal the bond dissociation mechanism of carbon source molecules during carbon nanotube synthesis. C–C bonds in only CHxCO fragments are dissociated on the nickel cluster, whereas there is no preferential structure among the fragments for C–O bond dissociation. The dissociation preference is uncorrelated with the bond dissociation energy of corresponding bonds in freestanding molecules but is correlated with the energy difference between fragment molecules before and after dissociation on the nickel surface. Moreover, carbon-chain formation occurs after C–C bond dissociation in a continuous simulation.

Low-energy electron induced resonant loss of aromaticity: consequences on cross-linking in terphenylthiol SAMs

Aromatic self-assembled monolayers (SAMs) can be used as negative tone electron resists in functional surface lithographic fabrication. A dense and resistant molecular network is obtained under electron irradiation through the formation of a cross-linked network. The elementary processes and possible mechanisms involved were investigated through the response of a model aromatic SAM, p-terphenylthiol SAM, to low-energy electron (0-10 eV) irradiation. Energy loss spectra as well as vibrational excitation functions were measured using High Resolution Electron Energy Loss Spectroscopy (HREELS). A resonant electron attachment process was identified around 6 eV through associated enhanced excitation probability of the CH stretching modes ν(CH)(ph) at 378 meV. Electron irradiation at 6 eV was observed to induce a peak around 367 meV in the energy loss spectra, attributed to the formation of sp(3)-hybridized CHx groups within the SAM. This partial loss of aromaticity is interpreted to be the result of resonance formation, which relaxes by reorganization and/or CH bond dissociation mechanisms followed by radical chain reactions. These processes may also account for cross-linking induced by electron irradiation of aromatic SAMs in general.

Thermal Decomposition Mechanisms of Nitroesters: Ab Initio Modeling of Pentaerythritol Tetranitrate

The lack of fundamental understanding of the intricate interplay between large amounts of chemical energy stored in energetic molecular materials and their sensitivity to detonation initiation represents a stubborn outstanding challenge. This study sheds much needed light on the atomic decomposition mechanisms of nitroesters with an example of pentaerythritol tetranitrate (C5H8N4O12, also known as PETN), used as an explosive and a medication. The performed study is based on density functional theory combined with variational transition-state theory and large-scale massively parallel periodic supercell calculations and presents a detailed analysis of thermal initiation of chemical reactions and their kinetics in the gas-phase PETN molecules, the ideal bulk crystals, and on two low energy surfaces, (101) and (110). The obtained activation barriers, reaction energies, and reaction rates of simulated bond dissociation mechanisms reveal that the overall decomposition of condensed nitroesters is defined by the ...