Dissociation Pathways Research Articles

N-H and N-C Bond Dissociation Pathways in Ultraviolet Photodissociation of Dimethylamine.

We investigated the interlinked N-H and N-C photochemistry of primary and secondary amines via the state-resolved detection of vibrationally excited CH3 product and H atom product by 200-235 nm dimethylamine photodissociation using resonance-enhanced multiphoton ionization (REMPI) and velocity map imaging (VMI) techniques. The out-of-plane bending (ν2) vibrationally excited CH3 showed a bimodal translational energy distribution that became unimodal with a near-zero product yield at longer photolysis wavelengths (λphotolysis). In contrast, a unimodal distribution was observed for the C-H stretching (νCH) vibrationally excited CH3 products with an almost constant product yield in the examined λphotolysis region. We ascribed the state-specific energy releases of the CH3 products to two reaction pathways based on calculations of the potential energy surface (PES): the direct N-CH3 dissociation pathway and the indirect N-CH3 dissociation pathway via the N-H bond conical intersection. Meanwhile, the H atom product showed a bimodal energy distribution similar to the ammonia photodissociation model, with an excited-state counterproduct channel that became accessible at a shorter λphotolysis. These results suggest that the N-H and N-C bond dissociations are connected, and these dissociations cause different photochemistry between primary/secondary amines and tertiary amines.

Water molecule elimination from the protonated methanol dimer ion-An example of a size-selective intracluster reaction.

The abundance of extraterrestrial methanol makes the reaction between methanol molecules in a molecular cluster a possible key step in the search for mechanisms for the formation of more complex molecules under the conditions of the interstellar medium as well as circumstellar and planetary atmospheres. The reaction leading to the formation of the dimethyl ether ion from a methanol molecule interacting with a protonated methanol ion via the elimination of a water molecule is a basic mechanism for the formation of complex organic molecules. Here, we experimentally examine such reactions in the gas phase, analyzing the production and reactivity of protonated cluster ions formed by the ionization of a supersonic jet of methanol. Focusing especially on the post-collisional relaxation of the protonated methanol dimer and trimer ions after high-energy single collisions, the results indicate a strong size selectivity favoring the occurrence of this reaction only in the dimer ion. To elucidate this behavior, the velocity distribution of the eliminated water molecule was measured using an event-by-event coincidence analysis. These results are interpreted using quantum chemical calculations of the dissociation pathways. It turns out that in the dimer case, two transition states are able to contribute to this intracluster reaction. In the trimer case, methanol evaporation appears as the most energetically favorable relaxation pathway.

The synergistic effect in the heteronuclear double-atom catalysts on CO activation: Insights from DFT calculations

In order to identify the synergistic effect in the heteronuclear double-atom catalysts (DACs) on CO activation, the homonuclear and heteronuclear DACs have been designed based on graphene supporter. And the three CO dissociation pathways have been considered using the density functional theory (DFT) calculation. It reveals that the heteronuclear DACs (MoX@NC) possesses higher catalytic ability than the homonuclear DACs (2X@NC) for CO activation. And the unpaired d-orbital electron plays a significant role on CO activation, which can change the magnetic moment of the DACs. The coordination atoms offer similar local coordination environment for the homonuclear and heteronuclear DACs. However, the Bader charge displays that there is a strong electronic interaction between Mo atom and other transition metals of dimer for the heteronuclear DACs. The synergistic effect of the heteronuclear DACs originates from the enhancement of the intermetallic interaction between the Mo atom and other transition metals. The work offers rational guideline for the synthesis of DACs with excellent catalytic performances, and has great potential for application in various catalytic fields.

Electrochemical Oxidation of Aliphatic Carboxylates: Kinetics, Thermodynamics, Mechanism, and the Role of Hydrogen Bonding

AbstractThe oxidation of tetra‐n‐butylammonium acetate, propionate, and pivalate was studied in rigorously anhydrous acetonitrile by conventional linear sweep and convolution voltammetry (LSV and ConV, respectively). The results suggest oxidation occurs via a concerted dissociative electron transfer pathway (RCO2−→R⋅+CO2+e−). The addition of water lowers the intrinsic barrier, signaling a possible change in mechanism to stepwise dissociative electron transfer. In rigorously dry acetonitrile, the oxidation potentials of CH3CO2− (0.60±0.09), CH3CH2CO2−, (0.47±0.05) and (CH3)3CCO2− (0.40±0.04 V vs. Ag/AgNO3 (CH3CN, 0.1 M)) are reported. These values parallel the stabilities of the resulting free radicals, consistent with a possible concerted pathway, although differential solvation of the carboxylate anions cannot be completely excluded as a contributor to this trend.

Survey of the multi-stage mass spectrometry of common ionic liquid cations and some polyfluorinated ions

Ionic liquids are useful for many applications and the structural diversity as a class is ever increasing. Knowledge of gas-phase dissociation pathways can inform analytical method development for the characterization of novel syntheses, degradation products, or environmental contaminants, and can inform our knowledge of the fate of ionic liquid species in the gas phase (e.g., after ejection by electrospray spacecraft thrusters). In this work, the N-heterocyclic (NHC) ions:1-butyl-1-methylpiperidinium (BMPi), 1-methyl-1-propylpiperidinium (MPPi), 1-butyl-1-methylpyrrolidinium (BMPyr), 1-ethyl-1-methylpyrrolidinium (EMPyr), 1-butylpyridinium (BPy), and 1-Butyl-1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl) imidazolium (BFIm) were studied. An additional fluorinated anion, nonafluorobutanesulfonate (NFBS), was also studied. Collision-induced dissociation mass spectrometry (CID-MS), as MS2 and MS3 using an ion trap mass spectrometer and energy-resolved breakdown curves using a quadrupole time-of-flight mass spectrometer, were used to determine dissociation patterns. Assignment of proposed dissociation pathways were made based on the experimental results and complementary computational chemistry. It was observed that for aromatic NHCs, cleavage at the C–N bond was preferred, to generate alkene-based neutral losses. Non-aromatic NHCs typically generated both alkane- and alkene-based neutral losses. Breakdown curves, supported by theoretical thermodynamic calculations, suggest that the loss of the alkane is the favorable dissociation pathway. Additionally, some ring-opening and ring contraction reactions via C–C bond cleavage were observed in non-aromatic species, whereas the intact aromatic NHC is typically a stable terminal ion. The polyfluorinated imidazolium fragmented via the breaking of C–N, C–C, and C–F bonds. The fluorinated anion, NFBS, fragmented by C–C and C–S bond cleavage to generate fluoroalkene, sulfur dioxide, and sulfur trioxide neutral losses. Beyond these broad stroke findings, the dissociation pathways of the seven ionic liquid ions were mapped using evidence from MS3, breakdown curves, and computational chemistry together assigning product ions as either arising from competitive or sequential pathways and proposing tentative structures. Some of the ionic liquid ions showed class-based product ions which will be useful in analytical development for their characterization. Finally, these results were contextualized within previous literature reports on other ionic liquid species.

Probing Conical Intersection in the Multipathway Isomerization of CH3Cl Using Coulomb Explosion.

Ubiquitous ultrafast isomerization is paramount in photoexcited molecules, in which non-adiabatic coupling among multiple electronic states can occur. We use the pump-probe Coulomb explosion imaging method to study the isomerization of CH3Cl molecules. We find that the isomerization under our strong field pump-probe scheme proceeds along multiple pathways, which are encoded in several distinct branches of the time-resolved kinetic energy release spectra for the CH2++HCl+ Coulomb explosion channel. Apart from the isomerized dissociative pathway in neutral and cationic excited states, the pump laser can also induce coherent vibrational dynamics in two coupled intermediate states and set up the initial conditions for the two concurrently proceeding isomerization pathways. The isomerization of CH3Cl provides an intriguing example of a chemical reaction consisting of multiple pathways and non-adiabatic dynamics.

Roaming in highly excited states: The central atom elimination of triatomic molecule decomposition.

Chemical reactions are generally assumed to proceed from reactants to products along the minimum energy path (MEP). However, straying from the MEP-roaming-has been recognized as an unconventional reaction mechanism and found to occur in both the ground and first excited states. Its existence in highly excited states is however not yet established. We report a dissociation channel to produce electronically excited fragments, S(1D)+O2(a1Δg), from SO2 photodissociation in highly excited states. The results revealed two dissociation pathways: One proceeds through the MEP to produce vibrationally colder O2(a1Δg) and the other yields vibrationally hotter O2(a1Δg) by means of a roaming pathway involving an intramolecular O abstraction during reorientation motion. Such roaming dynamics may well be the rule rather than the exception for molecular photodissociation through highly excited states.

Photodissociation of the CH2Br radical: A theoretical study.

Bromine atom (Br) reactions lead to ozone depletion in the troposphere and stratosphere. Photodegradation of bromocarbons is one of the main sources of bromine atoms in the atmosphere. Here, we use high-level abinitio methods, including spin-orbit effects, to study the photodissociation of the CH2Br radical. All possible fragmentation pathways, namely CH2Br + hν → CH2 + Br, HCBr + H, and CBr + H2, have been analyzed. Potential-energy curves of the ground and several excited electronic states along the corresponding dissociating bond distance of each pathway have been calculated. Considering the actinic fluxes of solar irradiation in the troposphere and in the stratosphere in the relevant range of frequencies, it is found that the first five excited states of CH2Br can be accessed from the ground state. Analysis of the potential curves shows that the pathways producing CH2 + Br and HCBr + H can proceed through a fast direct dissociation mechanism, while the pathway leading to CBr + H2 involves much slower dissociation mechanisms like internal conversion between electronic states, predissociation, or tunneling through exit barriers. The main implications are that the two faster channels are predicted to be dominant, and the slower pathway is expected to be less relevant. The tropospheric and stratospheric solar actinic fluxes also allow for further dissociation of the HCBr and CBr fragments, generating additional Br atoms, provided that they survive possible collisions with other atmospheric reagents. Finally, we discuss the possible effect of each of the three CH2Br dissociation pathways on the depletion of atmospheric ozone.

The structural and electronic split: Boron vs aluminum hydrides

We systematically investigated the structural evolution of boron (B) and aluminum (Al) hydrides using various DFT and ab initio methods, aiming to reveal the similarities and differences in their geometric and electronic structures. While B hydrides have been extensively studied both experimentally and theoretically, less is known about its group 13 heavier congener, Al. Extensive global minimum searches of the B2Hx (Al2Hx) and B3Hy (Al3Hy) hydrides (x = [0–6], y = [0–9]) were performed to identify the most stable geometric structures for each stoichiometry. In most of the series, B and Al hydrides exhibit qualitatively different structures, except for the most saturated X2H5 and X2H6 stoichiometries. Chemical bonding analyses employing adaptive natural density partitioning and electron localization function methods identified notable differences between B and Al hydrides in most of the compositions. B hydrides predominantly possess two-center (2c) and three-center (3c) bonding elements, suggesting a relatively balanced electron distribution. On the contrary, Al hydrides tend to retain unpaired electrons or lone pairs on Al atoms, forming a large number of closely lying isomers with various combinations of 1c, 2c, 3c, and 4c bonding elements. Thermodynamic stability analyses revealed that all studied clusters demonstrated stability toward various H/H2 dissociation pathways, with Al hydrides being less stable than B counterparts.

Absolute Photoionization Cross Section and Dissociative Ionization Pathways of Alpha-Pinene.

The absolute photoionization cross section of the monoterpenoid, alpha-pinene (AP), is presented together with the relative photoionization cross sections of its dissociative fragments for the first time. Experiments are performed via multiplexed vacuum ultraviolet (VUV) synchrotron photoionization (PI) mass spectrometry in the 8.0-11.0 eV energy range. Experimental work is conducted at the Advanced Light Source of the Lawrence Berkeley National Laboratory. Dissociative fragments were identified at m/z 121, 94, 93, 92, and 80. The photoionization cross section for the parent mass at 11.0 eV was determined to be 17±4 Mb with a total ionization cross section of 92±23 Mb at the same photon energy. Experimental appearance energies of dissociative ionization fragments and potential dissociative ionization pathways calculated at the G4 level of theory are presented as well.

Probing the dissociation and reassembly pathway of an amyloidogenic homotetramer

Probing the dissociation and reassembly pathway of an amyloidogenic homotetramer

Computational study of ligand dissociation pathways and free-energy barriers using the site-identification by ligand competitive saturation (SILCS) method

Computational study of ligand dissociation pathways and free-energy barriers using the site-identification by ligand competitive saturation (SILCS) method

Pt Nanoparticle-Mn Single-Atom Pairs for Enhanced Oxygen Reduction.

The intrinsic roadblocks for designing promising Pt-based oxygen reduction reaction (ORR) catalysts emanate from the strong scaling relationship and activity-stability-cost trade-offs. Here, a carbon-supported Pt nanoparticle and a Mn single atom (PtNP-MnSA/C) as in situ constructed PtNP-MnSA pairs are demonstrated to be an efficient catalyst to circumvent the above seesaws with only ∼4 wt % Pt loadings. Experimental and theoretical investigations suggest that MnSA functions not only as the "assist" for Pt sites to cooperatively facilitate the dissociation of O2 due to the strong electronic polarization, affording the dissociative pathway with reduced H2O2 production, but also as an electronic structure "modulator" to downshift the d-band center of Pt sites, alleviating the overbinding of oxygen-containing intermediates. More importantly, MnSA also serves as a "stabilizer" to endow PtNP-MnSA/C with excellent structural stability and low Fenton-like reactivity, resisting the fast demetalation of metal sites. As a result, PtNPs-MnSA/C shows promising ORR performance with a half-wave potential of 0.93 V vs reversible hydrogen electrode and a high mass activity of 1.77 A/mgPt at 0.9 V in acid media, which is 19 times higher than that of commercial Pt/C and only declines by 5% after 80,000 potential cycles. Specifically, PtNPs-MnSA/C reaches a power density of 1214 mW/cm2 at 2.87 A/cm2 in an H2-O2 fuel cell.

Separation and identification of the synthetic cathinone isomers dipentylone and N-ethylpentylone using chromatographic and mass spectral characteristics

Synthetic cathinone isomers exhibit very similar structure and physicochemical properties, which makes them unable to be effectively separated and identified, therefore increasing the likelihood of misidentification. In this study, the chromatographic and mass spectral characteristics of a sample by gas chromatography-mass spectrometry (GC–MS) were highly like those of N-ethylpentylone. The powder, however, was finally identified as dipentylone (also known as N, N-dimethylpentylone) by high performance liquid chromatography-linear ion trap-orbitrap mass spectrometry (HPLC-LTQ-Orbitrap-MS) and nuclear magnetic resonance (NMR). Because of the possibility of misidentification, the collision-induced dissociation (CID) and electron ionization (EI) pathways of dipentylone and its isomer N-ethylpentylone were further examined to assist forensic laboratories in identifying the synthetic cathinone isomers in their case work. The separation of the two isomers was successfully achieved through ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS/MS). These results demonstrate that the methods developed in this study can be used to separate and identify the two isomers.

A DFT study for in-situ CO2 utilization realized by calcium-looping dry reforming of methane based on Ni/CaCO3

The calcium-looping dry reforming of methane (CaL-DRM) process couples CO2 capture and dry reforming of methane process using Ni/CaO dual-functional material. This process in-situ converts CO2 captured by CaO through reacting with CH4, catalyzed by Ni, to produce syngas. However, researches on reaction mechanism for CaL-DRM are rare. In this work, the fundamental reaction mechanism of calcium-looping dry reforming of methane is elucidated by density functional theory (DFT) analysis. The energy barriers for elementary reactions involved in various potential pathways were investigated to determine the primary reaction pathway. The DFT analysis shows two possible reaction pathways for the CaL-DRM, with the CH* assisted CaCO3 dissociation pathway being the more favorable one. Along this pathway, CH4 undergoes three dehydrogenation steps to form CH*. Then, CH* reacts with the CaCO3* to produce CHO* and CaCO2*, accompanying the cleavage of one C-O bond in the carbonate. Afterwards, CHO* dissociates into CO* and H*. Finally, another C-O bond in CaCO2* breaks, generating CO* and CaO. Notably, CH4 reduces the energy barrier for CaCO3 dissociation from 3.47 to 2.733 eV. Moreover, the perturbation of electron clouds around O in CaCO3* in the presence of CH or C, as evidenced in electron density differential results, highlights the effective activation of C-O bond by CH or C, thereby promoting CaCO3 decomposition. This work provides further support for the potential mechanism for in-situ CO2 utilization achieved through CaL-DRM.

Dynamical Origin of Rebound versus Dissociation Selectivity during Fe-Oxo-Mediated C-H Functionalization Reactions.

The mechanism of catalytic C-H functionalization of alkanes by Fe-oxo complexes is often suggested to involve a hydrogen atom transfer (HAT) step with the formation of a radical-pair intermediate followed by diverging pathways for radical rebound, dissociation, or desaturation. Recently, we showed that in some Fe-oxo reactions, the radical pair is a nonstatistical-type intermediate and dynamic effects control rebound versus dissociation pathway selectivity. However, the effect of the solvent cage on the stability and lifetime of the radical-pair intermediate has never been analyzed. Moreover, because of the extreme complexity of motion that occurs during dynamics trajectories, the underlying physical origin of pathway selectivity has not yet been determined. For the reaction between [(TQA_Cl)FeIVO]+ and cyclohexane, here, we report explicit solvent trajectories and machine learning analysis on transition-state sampled features (e.g., vibrational, velocity, and geometric) that identified the transferring hydrogen atom kinetic energy as the most important factor controlling rebound versus nonrebound dynamics trajectories, which provides an explanation for our previously proposed dynamic matching effect in fast rebound trajectories that bypass the radical-pair intermediate. Manual control of the reaction trajectories confirmed the importance of this feature and provides a mechanism to enhance or diminish selectivity for the rebound pathway. This led to a general catalyst design principle and proof-of-principle catalyst design that showcases how to control rebound versus dissociation reaction pathway selectivity.

Comparison of H2O2 and H2O oxidations on TDMAT absorbed on silicon(100) surface during reaction step of ALD–TiO2 process: A DFT study

Atomic layer deposition (ALD) is a thin film technology with high potential in nano fabrication. There is industrial demand to produce an ALD–TiO2 thin film using a low temperature process for applications of bending electronic devices and biomaterials. TDMAT has gained the most attention as a suitable titanium source to grow an ALD–TiO2 film at extremely low temperatures. In this process, almost all published reports have focused on metal-oxide precursors. The current work investigates the effects of oxidizing agents. For the first time, a theoretical investigation was carried out on the oxidation of H2O2 and H2O on TDMAT absorbed on a silicon (100) surface during the reaction step of the ALD–TiO2 process. We simulated the oxidation process, which continued from the adsorption step of the ALD–TiO2 process, where the TDMAT was used as a titanium precursor. The cluster of the Si9H12 =O2 =Ti[N(CH3)]2 surface model was used to represent the remaining TDMAT molecule absorbed on the Si(100) substrate. The reaction mechanisms of H2O2 oxidizing on this surface were evaluated based on their reaction pathways. The reaction pathway of H2O oxidizing on this surface was calculated at the same level to compare the effect of oxidizers. The DFT at the B3LYP 6–311 G+ (2d,p) and 6–31 G(d,p) theoretical levels were used to calculate the potential energy surface and to optimize the electronic structures in this process. It was found that the reaction mechanism depended on the nature of chemical bond dissociation within the oxidizer molecules during the oxidation process. In the oxidation of H2O2, the H2O2 molecule contained O–O and O–H bonds, which were dissociated during the oxidation process, resulting in different product structures. Only the O–O bond dissociation pathway produced the TiO2 structure. The two species of TDMAT ligands at the surface were completely oxidized using a single H2O2 molecule. The H2O2 oxidation produced more –OH groups (double) compared to H2O oxidation. In H2O oxidation, the H2O molecule contained only the O–H bond within its molecule and required activation energies significantly less than for all pathways of H2O2. The HOMO–LUMO properties and IR frequency characteristics (cm−1) of the plausible product structures were discussed. The results should be useful in guiding the selection of suitable precursors for use in the co–reactant system of the ALD–TiO2 process.

Photodissociation of CO2 via the 1Πg state: Wavelength-dependent imaging studies of O(1D2) photoproducts.

Photodissociation of CO2 via the 1Πg state is investigated using a time-sliced velocity-mapped ion imaging apparatus combined with a tunable vacuum ultraviolet photolysis source. The main O(1D2) + CO(X1Σ+) channel is directly observed from the measured images of O(1D2) photoproducts at 129.08-134.76nm. The total kinetic energy release spectra determined based on these images show that the energetic thresholds for the O(1D2) + CO(X1Σ+) photoproducts correspond to the thermochemical thresholds for the photodissociation of CO2(v2 = 0) and CO2(v2 = 1). One significant difference among the CO(X1Σ+, v) vibrational distributions for the predominant CO2(v2 = 0) dissociation is that the population of CO(v = 0) becomes favorable at 130.23-133.45nm compared to the Boltzmann-like component (v > 0) that always exists at 129.08-134.76nm. The wavelength dependences of the overall β are found to follow the variation trend of the CO(v = 0) abnormal intensity. The vibrational state-specific β values present a roughly decreasing trend with an increase in v, whereas β(v = 0) appears to be significantly larger than β(v = 1) at 130.23-133.45nm compared to 134.76 and 129.08nm. The non-statistical CO(v = 0) with larger β values at 130.23-133.45nm implies that an additional pathway may open through the conical intersection coupling to the dissociative 21A' state, except for the ever-existing pathway that yields the Boltzmann-like component. In contrast, at 129.08nm, the restoration of the statistical equilibrium in the CO(X1Σ+, v) vibrational distribution may be caused by the emergence of novel dissociation pathways arising from the participation of the 31A″ state.

Exploring the ultrafast and isomer-dependent photodissociation of iodothiophenes via site-selective ionization.

C-I bond extension and fission following ultraviolet (UV, 262 nm) photoexcitation of 2- and 3-iodothiophene is studied using ultrafast time-resolved extreme ultraviolet (XUV) ionization in conjunction with velocity map ion imaging. The photoexcited molecules and eventual I atom products are probed by site-selective ionization at the I 4d edge using intense XUV pulses, which induce multiple charges initially localized to the iodine atom. At C-I separations below the critical distance for charge transfer (CT), charge can redistribute around the molecule leading to Coulomb explosion and charged fragments with high kinetic energy. At greater C-I separations, beyond the critical distance, CT is no longer possible and the measured kinetic energies of the charged iodine atoms report on the neutral dissociation process. The time and momentum resolved measurements allow determination of the timescales and the respective product momentum and kinetic energy distributions for both isomers, which are interpreted in terms of rival 'direct' and 'indirect' dissociation pathways. The measurements are compared with a classical over the barrier model, which reveals that the onset of the indirect dissociation process is delayed by ∼1 ps relative to the direct process. The kinetics of the two processes show no discernible difference between the two parent isomers, but the branching between the direct and indirect dissociation channels and the respective product momentum distributions show isomer dependencies. The greater relative yield of indirect dissociation products from 262 nm photolysis of 3-iodothiophene (cf. 2-iodothiophene) is attributed to the different partial cross-sections for (ring-centred) π∗ ← π and (C-I bond localized) σ∗ ← (n/π) excitation in the respective parent isomers.

Product branching in the photodissociation of oxazole detected by broadband rotational spectroscopy.

The photodissociation of oxazole (c-C3H3NO) following excitation at 193 nm is studied using mm-Wave rotational spectroscopy in a uniform supersonic flow. Molecules entrained in the flow are excited to a ππ* state after which it is believed most relax back to the ground state via ring opening at the O-C[N] bond with subsequent fragmentation. From the line intensities of the probed products, we obtained the branching fractions for seven different products which are the result of five different dissociation pathways. The detected photoproducts and respective branching fractions (%) are the following: HCN (70.4), HCO (22.8), CH2CN (4.2), CH2CO (1.0), CH3CN (1.0), HNC (0.9), HNCO (0.08). We suspect much of the HCO may be formed in conjunction with the isocyanomethyl radical, CH2NC, which we did not probe. We discuss our results in relation to previous work, in particular our own study on the related isomer isoxazole, as well as direct dynamics theoretical simulations from the literature. We also studied the relaxation of a number of vibrationally excited levels of HCN produced at 20 K.