H2 Elimination Reactions Research Articles

Understanding Decomposition Mechanism of Acetone under Electric Pulsed Discharge: Generation of C3O+ and C3H4O+ ions

AbstractGas‐phase investigations on acetone, under the influence of electric pulsed discharge has provided distinct insight related to its decomposition. Time‐of‐flight mass spectrometry has been utilized to identify different products generated upon dissociation/ionization of acetone, under the influence of electric discharge. In addition to molecular (C3H6O+) and protonated acetone (C3H7O+) ions, predominant ions signal corresponding to C3H4O+, C3O+ and CH3CO+ were observed in the mass spectrum. Under discharge condition, acetone undergoes soft dissociation/ionization producing C3H4O+ and C3O+ ions, which are distinctive from usual fragment ions reported upon decomposition of acetone, in the gas‐phase. Though metastable in nature, significant ion yield has been obtained for C3O+ ions. Computational studies have been carried out to understand the mechanism of acetone decomposition in gas‐phase. Based on experimental and computational findings, generation of these ions (C3O+ and C3H4O+) has been ascribed to step‐wise H2 elimination reaction originating from acetone molecule, followed by ionization of resultant fragments under the influence of electric pulsed discharge. Present studies are of relevance for plasma pyrolysis of volatile organic compounds and laboratory astrochemical investigations.

Low-Energy Electron Interactions with Resveratrol and Resorcinol: Anion States and Likely Dissociation Pathways.

We report a computational study of the anion states of the resveratrol (RV) and resorcinol (RS) molecules, also investigating dissociative electron attachment (DEA) pathways. RV has well-known beneficial effects in human health, and its antioxidant activity was previously associated with DEA reactions producing H2. Our calculations indicate a valence bound state and four resonances ( to ) for that system. While the computed thermodynamic thresholds are compatible with DEA reactions producing H2 at 0 eV, the well-known mechanism involving vibrational Feshbach resonances built on a dipole bound state should not be present in RV. Our results suggest that the shallow valence bound state is expected to account for H2 elimination, probably involving / couplings along the vibration dynamics. The RS molecule is also an oxidant and a subunit of RV. Because two close-lying hydroxyl groups are found in the RS moiety, the H2-elimination reaction in RV should take place at the RS site. Our calculations point out a correspondence between the anion states of RV and RS and even between the thresholds. Nevertheless, the absence of bound anion states in RS, indicated by our calculations, is expected to suppress the H2-formation channel at 0 eV. One is led to conclude that the ethene and phenol subunits in RV stabilize the state, thus switching on the DEA mechanism producing H2.

Discriminating between the dissociative photoionization and thermal decomposition products of ethylene glycol by synchrotron VUV photoionization mass spectrometry and theoretical calculations.

Understanding the combustion chemistry of biofuel compounds is of great importance in the intelligent selection of next-generation alternative fuels. Ethylene glycol (C6H10O2) is a prototypical representative of potential biofuels. In this work, the thermal decompositions along with the dissociative ionization of ethylene glycol are studied by synchrotron VUV photoionization mass spectrometry. As a part of the dissociative ionization study, the appearance energies of seven fragments are measured. Using the theoretical calculation results, the possible formation channels of these fragments are proposed. In particular, the productions of CH3OH+ and CH3OH2+ are suggested to be from the isomerization/dissociation process, where double proton transfer processes are highlighted. Using a tunable VUV source, the high-temperature pyrolysis products of ethylene glycol are differentiated from the dissociative ionization products. Specifically, two isomeric products vinyl alcohol and acetaldehyde by H2O elimination are obtained. Formaldehyde and methanol from direct C-C bond cleavage are identified. The fragmentations of fragile radicals such as hydroxymethyl, methoxy and ethoxy are used to explain the missing products from the direct C-C and C-O bond dissociation reactions. There is no experimental evidence for the occurrence of the H and H2 elimination reactions which may have not been accessed under the present temperature conditions.

Hierarchy of Commonly Used DFT Methods for Predicting the Thermochemistry of Rh-Mediated Chemical Transformations.

The accuracy and reliability of 17 commonly used density functionals in conjunction with Poisson–Boltzmann finite solvation model were gauged for predicting the free energy of Rh(I)- and Rh(III)-mediated chemical transformations such as ligand exchange, hydride elimination, dihydrogen elimination, chloride affinity, and silyl hydride bond activation reactions. In total, six Rh-mediated reactions were examined, and the computed density functional theory results were then subjected to comparison with the experimentally reported values. For reaction A, involving replacement of N2 with η2-H2 over Rh(I), MPWB1K-D3, B3PW91, B3LYP, and BHandHYLP emerged to be the best functionals of all the tested methods in terms of their deviations ≤2 kcal mol–1 from experimental data. For reaction B, in which exchange of η2-C2H4 with N2 over Rh(I) takes place, MPWB1K-D3 and M06-2X-D3 functionals performed the best, while as for reaction C (hydride elimination reaction in Rh(III) complex), it is PBE functional that showed impressive performance. Similarly, for reaction D (H2 elimination reaction in Rh(III) complex), PBE0-D3 and PBE-D3 showed exceptional results compared to other functionals. For reaction E (H2O/Cl– exchange), the PBE0 again shows impressive performance as compared to other functionals. For reaction F (Si–H activation), M06-2X-D3, PBE0-D3, and MPWB1K-D3 functionals are undoubtedly the best functionals. Overall, PBE0-D3 and MPWB1K-D3 functionals were impressive in all cases with lowest mean unsigned errors (3.2 and 3.4 kcal mol–1, respectively) with respect to experimental Gibbs free energies. Thus, these two functionals are recommended for studying Rh-mediated chemical transformations.

Splitting of Hydrogen Sulfide by Group 14 Elements (Si, Ge, Sn, Pb) in Excess Argon at Cryogenic Temperatures.

The water gas reaction C + H2O → CO + H2 has been employed for centuries; however little is known for analogous reaction M + H2S → MS + H2 (M = Si, Ge, Sn, Pb). In addition, this latter reaction is intriguing in its function of converting pollutant H2S to clean energy source H2. We report herein the reactions of laser-ablated Group 14 atom M (M = Si, Ge, Sn, Pb) with H2S using matrix isolation infrared spectroscopy as well as the state-of-the-art quantum chemical calculations. Important reactive intermediates HMSH with high active H (protonic Hδ+ and hydridic Hδ-) are identified. In addition, the reaction mechanisms are established for insertion reaction M + H2S → HMSH, and photoinduced H2 elimination reactions of HMSH → MS+ H2 and H2SiS → SiS+ H2 in low-temperature matrices.

The insertion and H2 elimination reactions of H2GeFMgF germylenoid with RH (R = Cl, SH, PH2)

In present paper, the insertion and H2 elimination reactions of H2GeFMgF germylenoid with RH (R = Cl, SH, PH2) were investigated by means of B3LYP and QCISD calculation methods. One transition state and one intermediate were found along the potential energy surface in each insertion reaction, while for the H2 elimination reactions, only one transition state was found between the reactants and products in each reaction process. Both for the insertion and H2 elimination reactions, RH reactivity increases in the following order: H–Cl > H–SH > H–PH2. The insertion and H2 elimination reactions were compared, and the results demonstrated that the H2 elimination should be more favorable than the corresponding insertion. The solvent effects on these two types of reactions were considered. The calculated results indicated that the solvents could accelerate the reactions by reducing their barrier heights.

Gas-phase reaction kinetics of 1,3-disilacyclobutane in a hot-wire chemical vapor deposition reactor

The reaction kinetics of the decomposition of 1,3-disilacyclobutane (DSCB) was investigated using a hot-wire chemical vapor deposition reactor. The reaction products were monitored using a vacuum ultraviolet laser single-photon ionization source coupled with a time-of-flight mass spectrometer. Steady-state approximation was used to determine the rate constants for three main decomposition pathways of DSCB: the exocyclic H2 elimination (k1), the cycloreversion (k2), and the ring-opening via 1,2-H shift (k3). Separate k2 and k3 were not obtained, but 2k2+k3 and k1 were determined. The activation energy (Ea) for the exocyclic H2 elimination reaction was determined to be 63.5kJ·mol−1. Compared to the Ea value of 43.6kJ·mol−1 previously obtained under collision-free conditions at much lower pressures, the value from this work is higher. This is attributed to the filament aging caused by the formation of silicon carbide (SiC) and tungsten sub-carbide (W2C) on the wire surface. The heterogeneous reactions on the metal surface also led to a much faster decay constant, koverall, for DSCB than those for each individual decomposition pathway, k1 and 2k2+k3.

Theoretical prediction on H2 elimination reactions of H2GeLiF with RH (R = Cl, SH, and PH2)

The H2 elimination reactions of H2GeLiF with RH (R = Cl, SH, PH2) have been studied using the DFT B3LYP and QCISD methods. The calculated results indicate that all the mechanisms of the three reactions are identical to each other and under the same condition the H2 elimination reactions should occur easily in the order of H-Cl > H-SH > H-PH2. The solvent effects on the H2 elimination reactions were also calculated and it was found that the larger the dielectric constant, the easier the H2 elimination reactions. Compared with the insertion reactions of H2GeLiF with RH (R = Cl, SH, PH2), the H2 elimination reactions have the lower activation barriers and should be more favorable.

Hydrogen Release from Ammonia Alane-Based Materials: Formation of Cyclotrialazane and Alazine

In previous papers (Nguyen et al. J. Phys. Chem. C 2008, 112, 5662–5667 and J. Phys. Chem. C 2009, 113, 18914–18926), formation of H2 molecules from ammonia alane monomer (AAl) and dimers (AAl)2 was shown to be facilitated by the addition of one or more alane or ammonia molecules that can play the role of efficient bifunctional catalyst. Ammonia alane emerges as a good starting compound for building up materials for chemical hydrogen storage (CHS). Further exploration of the products based on the H2 release from ammonia alane were carried out using coupled-cluster theory computations together with the aug-cc-pVTZ basis set (based on MP2/aug-cc-pVDZ optimized geometries). Our ab initio MO calculations for the first time led to the identification of cyclotrialazane [(H2AlNH2)3], alazine [(HAlNH)3], and its oligomer [H2Al(HNAlH)2NH2] that are produced along the multistep dehydrogenation processes from the reactions of ammonia alane and AlH3NH2AlH2NH3. The formation of alazine (homologue of borazine) as the final product in our H2 elimination reactions is an important feature because of its long-debated existence. The present reaction path analyses show that the formation of this compound is an important phenomenon for explaining the entire dehydrogenation process.

Addition and elimination reactions of H2 in ruthenaborane clusters: A computational study

Ruthenaborane clusters have been modelled by performing density functional theory calculations using the B3LYP functional. The calculations gain insights into hydrogen storage and the H–H bond activation by ruthenaboranes. To study the nature of the chemical bond of H2 molecules attached to ruthenaboranes, we carried out structural optimizations for different ruthenaborane clusters and determined transition state structures for their hydrogenation addition/elimination reactions. Calculations of the reaction pathways yielded different transition-state structures involving molecular hydrogen bonded to the cluster or formation of metal hydrides. The H–H bond of H2 seems to be activated by the ruthenaborane clusters as activation energies of 24–42 kcal/mol were calculated for the H2 addition reaction. The calculated Gibbs free energy for the H2 addition reaction is 14–27 kcal/mol. The calculated activation energies and the molecular structures of the [(C5Me5)Ru2B10H16], [(C5Me5)Ru2B8H14] and [(C5Me5)Ru2B8H12] clusters with different degree of hydrogenation are compared. The mechanisms of the H2 addition and elimination reactions of the studied clusters suggest that they might be useful as hydrogen storage materials due to their ability to activate the H–H bond. They also serve as an example of the ability of hypoelectronic metallaboranes to reversibly or irreversibly bind hydrogen.

Designing metal hydride complexes for water splitting reactions: a molecular electrostatic potential approach.

The hydridic character of octahedral metal hydride complexes of groups VI, VII and VIII has been systematically studied using molecular electrostatic potential (MESP) topography. The absolute minimum of MESP at the hydride ligand (Vmin) and the MESP value at the hydride nucleus (VH) are found to be very good measures of the hydridic character of the hydride ligand. The increasing/decreasing electron donating feature of the ligand environment is clearly reflected in the increasing/decreasing negative character of Vmin and VH. The formation of an outer sphere metal hydride-water complex showing the HH dihydrogen interaction is supported by the location and the value of Vmin near the hydride ligand. A higher negative MESP suggested lower activation energy for H2 elimination. Thus, MESP features provided a way to fine-tune the ligand environment of a metal-hydride complex to achieve high hydridicity for the hydride ligand. The applicability of an MESP based hydridic descriptor in designing water splitting reactions is tested for group VI metal hydride model complexes of tungsten.

Theoretical Study on the Ring-Opening of 1,3-Disilacyclobutane and H2 Elimination

The kinetics and thermochemistry of the decomposition pathways for 1,3-disilacyclobutane (1,3-DSCB) in the gas phase were studied using the second-order Møller-Plesset (MP2) perturbation theory and coupled cluster methods with single, double, and perturbative triple excitations (CCSD(T)). The reactions examined include 2 + 2 cycloreversion to form two silenes by either a concerted or a stepwise mechanism, 1,1-, 1,2-, and 1,3-H(2) elimination, and the ring-opening initiated by 1,2-H shift to form an open-chain 1,3-disilabut-1-ylidene, which undergoes further decomposition to produce two pairs of silene/silylene species. The structures of the transition states for the concerted and the stepwise 2 + 2 cycloreversion pathways are found to resemble closely those reported for the head-to-tail and head-to-head dimerization, respectively. Comparison of the activation barriers demonstrates unambiguously that the stepwise cycloreversion (ΔH(0)(‡) = 66.1 kcal/mol) is favored over the concerted one (ΔH(0)(‡) = 77.3 kcal/mol). A new pathway was established from the 1,4-diradical intermediate in the stepwise cycloreversion to form 1-silylmethylsilene via 1,3-H shift. The concerted 1,1-H(2) elimination is shown to have the lowest activation barrier of all H2 elimination reactions. Overall, the 1,2-H shift in 1,3-DSCB with concerted ring-opening to form 1,3-disilabut-1-ylidene is the most kinetically and thermodynamically favorable decomposition pathway, both at 0 and 298 K.

Density functional theory studies on the mechanism of activation of methane by homonuclear bimetallic Ni–Ni

The reaction mechanisms of methane catalyzed by homonuclear bimetallic Ni–Ni have been investigated theoretically in this paper. Activation of methane by homonuclear bimetallic Ni–Ni is proposed to proceed in two effective catalytic reaction routes, Route I and Route II. The homonuclear bimetallic Ni–Ni inserts in the C–H bond to form the ring-hydride and chain-hydride in Route I and Route II, respectively. In this study, we examine the isomerization reaction pathway for ring-hydride (Route I). Undergoing the isomerization reaction, the chain-product CH3Ni–Ni–H is formed. From the chain-hydride CH3Ni2–H, two kinds of distinguishable reaction paths have been considered including isomerization reaction and H2 elimination reactions in Route II. The H2 elimination reactions are proposed to proceed along three parallel reaction pathways with the activation of the second C–H bond on the potential energy surfaces (PESs). The pathway of rearranging to isomer in Route II is preferred to the others. In comparison with heteronuclear bimetallic NiM+ (M=Cu, Ag) and homonuclear bimetallic Pt2+, the dehydrogenation of methane induced by homonuclear bimetallic Ni–Ni should be more favored thermodynamically and should give more branching.

Competitive activation of C–H and C–F bonds in gas phase reaction of Ir+ with CH3F: A DFT study

The spin-forbidden reaction mechanism of Ir+ with CH3F to yield H2 or HF has been investigated using density functional theory (DFT) calculations. The overall H2 elimination reaction is calculated to be exothermic, by 36.3 kcal/mol. One pathway is found on three potential energy surfaces (PESs), whereas the HF-elimination reaction is exothermic by 32.8 kcal/mol, two reaction pathways are identified on the quintet and triplet surfaces, only one reaction pathway is identified on the singlet surface. The reaction mechanism can be explained by the simple donor–acceptor model. According to the revealed mechanism and the calculated potential energy surfaces, H2-elimination is energetically much more favorable than HF-elimination. These conclusions are consistent with the experimental observations.

Why is Pt4+ the Least Efficient Cationic Cluster in Activating the C−H Bond in Methane? Two-State Reaction Computational Investigation

The two-state reaction mechanism of Pt4+ with CH4 on the quartet and doublet potential energy surfaces has been investigated at the B3LYP level. Crossing points between the potential energy surfaces are located using different methods, and possible spin inversion processes are discussed by means of spin−orbit coupling (SOC) calculations. As a result, after the C−H insertion intermediates, two distinct H2 elimination reaction paths have been found, and the second C−H bond broken is regarded as the rate-determining step for two reaction paths. For the first pathway, there is a minimal energy crossing point (MECP) between the two surfaces, where the reacting system will change its spin multiplicities from the quartet state to the doublet state near this crossing region because the magnitude of the spin multiplicity mixing increases in a small energy gap between high- and low-spin states and will greatly enhance the probability of the intersystem crossing. The single P1ISC and double P2ISC passes estimated at MECP (SOC = 201.25 cm−1) are approximately 0.51 and 0.76, respectively. Not only will the reaction overcome a spin-change-induced barrier (ca. 6 kcal/mol) but it also will overcome an adiabatic barrier (ca. 24.7−20.8 kcal/mol), which will greatly reduced the efficiency of the reaction system. As for the second reaction path, similarly, there is a crossing seam between CP-2 and CP-3, the spin-change-induced barrier is very high, about 15.86−18.86 kcal/mol, which also indicates a less favorable process kinetically. Therefore, the lack of a thermodynamic driving force is an important factor contributing to the low efficiency of the reaction system. These conclusions are consistent with the experimental observations.

Experimental and theoretical studies on decomposition of pyrrolidine

A laminar premixed pyrrolidine/oxygen/argon flame with volume ratio C/O/N of 1.0/2.4/0.25 at 40mbar was investigated using molecular-beam mass spectrometry (MBMS) with tunable synchrotron vacuum ultraviolet (VUV) photoionization. About 50 species were identified from photoionization efficiency (PIE) spectra, including various air pollutants such as nitric oxide, ammonia, hydrogen cyanide, acetonitrile, formaldehyde, acetaldehyde, and propanal. Some radicals were detected including C4H8N, C4H7, C3H5, C3H3, C2H5, CHO, and CH3. Detected molecular intermediates include the isomers of C2H4O (ethenol and acetaldehyde), C3H4 (propyne and allene), C4H6 (1,3-butadiene and 1-butyne) and C4H8 (1-butene and 2-butene). Based on the experimental observations, the decomposition and ring-opening pathways of pyrrolidine were calculated at G3B3 level. Also, likely reaction channels from the products of H- and H2-elimination reactions were proposed. This combined experimental and theoretical investigation intends to shed light on the decomposition channels of pyrrolidine which may assist the development of a kinetic model for pyrrolidine pyrolysis and combustion.

ChemInform Abstract: Translational Energy Release Experiment and Theory: H2 Elimination Reactions of Small Gas Phase Ions and Correspondence to H-H Bond Activation

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Kinetic correlations for H2 addition and elimination reaction mechanisms during silicon hydride pyrolysis

The mechanism of H(2) addition and elimination reactions in selected silicon hydrides (Si(x)H(y), x = 1-10, y = 4-20) was modeled using quantum chemical calculations, statistical thermodynamics, transition state theory and transition state group additivity. Rate coefficients for 25 H(2) addition reactions were calculated using G3//B3LYP. For nearly every reaction, the overall conversion exhibits two steps. In the addition direction, the reactants first meet to form an adduct which then converts into a saturated silicon hydride via homolytic H-H bond cleavage. Values for the single-event Arrhenius pre-exponential factor, Ã, and the activation energy, E(a), were calculated from the G3//B3LYP rate coefficients, and a group additivity scheme was developed to predict à and E(a). The values predicted by group additivity are more accurate than kinetic correlations currently used in the literature, which rely on representative à values and the Evans-Polanyi correlation. The factors that have the most pronounced effect on à and E(a) were investigated, and stabilization of the divalent silicon atom of the unsaturated silicon hydride with electron-donating substituents was found to influence kinetic parameters considerably. The rate coefficients for H(2) addition reactions were found to correlate reasonably well with the difference in energy between the highest occupied molecular orbital of H(2) (E(HOMO)) and the lowest unoccupied molecular orbital of the reactant silylene (E(LUMO)).

Proton-Transfer and H2-Elimination Reactions of Trimethylamine Alane: Role of Dihydrogen Bonding and Lewis Acid−Base Interactions

Proton-transfer and H(2)-elimination reactions of aluminum hydride AlH(3)(NMe(3)) (TMAA) with XH acids were studied by means of IR and NMR spectroscopy and DFT calculations. The dihydrogen-bonded (DHB) intermediates in the interaction of the TMAA with XH acids (CH(3)OH, (i)PrOH, CF(3)CH(2)OH, adamantyl acetylene, indole, 2,3,4,5,6-pentafluoroaniline, and 2,3,5,6-tetrachloroaniline) were examined experimentally at low temperatures, and the spectroscopic characteristics, dihydrogen bond strength and structures, and the electronic and energetic characteristics of these complexes were determined by combining experimental and theoretical approaches. The possibility of two different types of DHB complexes with polydentate proton donors (typical monodentate and bidentate coordination with the formation of a symmetrical chelate structure) was shown by DFT calculations and was experimentally proven in solution. The DHB complexes are intermediates of proton-transfer and H(2)-elimination reactions. The extent of this reaction is very dependent on the acid strength and temperature. With temperature increases the elimination of H(2) was observed for OH and NH acids, yielding the reaction products with Al-O and Al-N bonds. The reaction mechanism was computationally studied. Besides the DHB pathway for proton transfer, another pathway starting from a Lewis complex was discovered. Preference for one of the pathways is related to the acid strength and the nucleophilicity of the proton donor. As a consequence of the dual Lewis acid-base nature of neutral aluminum hydride, participation of a second ROH molecule acting as a bifunctional catalyst forming a six-member cycle connecting aluminum and hydride sites notably reduces the reaction barrier. This mechanism could operate for proton transfer from weak OH acids to TMAA in the presence of an excess of proton donor.

Infrared Spectra of Metallacyclopropane, Insertion, and Dihydrido Complex Products in Reactions of Laser-Ablated Group 6 Metal Atoms with Ethylene Molecules

Reactions of laser-ablated group 6 metal atoms with ethylene have been investigated. The insertion and dihydrido products (MH-CHCH(2) and MH(2)-C(2)H(2)) are identified from reactions of W and Mo with ethylene isotopomers, whereas products in the Cr spectra are assigned to the insertion and metallacyclopropane (M-C(2)H(4)) complexes. Our experiments with CH(2)CD(2) show that the dihydrido complex is formed by beta-hydrogen transfer in the insertion complex because the MHD-CHCD isotopic product is favored. The present matrix infrared spectra and DFT computational results support the general trend that the higher oxidation-state complexes become more stable on going down the group 6 column. Unlike the cases of group 4 and 5 metals, binary metal hydride (MH(x)) absorptions are not observed in the infrared spectra, suggesting that the H(2)-elimination reactions of ethylene by group 6 metals are relatively slow, consistent with previous gas-phase reaction dynamics studies.