Product Branching Ratios Research Articles

Dynamics simulations and statistical modeling of thermal decomposition of 1-ethyl-3-methylimidazolium dicyanamide and 1-ethyl-2,3-dimethylimidazolium dicyanamide.

Quasi-classical, direct dynamics trajectories were calculated at the B3LYP/6-31G* level of theory, in an attempt to understand decomposition mechanisms of 1-ethyl-3-methylimidazolium dicyanamide (EMIM(+)DCA(-)) and 1-ethyl-2,3-dimethylimidazolium dicyanamide (EMMIM(+)DCA(-)). The trajectories showed many dissociation paths for these two ionic liquids. Using trajectory results as a guide, structures of transition states and products that might be important for decomposition of these two compounds were determined using density functional theory calculations. Rice-Ramsperger-Kassel-Marcus (RRKM) theory was then utilized to examine properties of energized ionic liquids and to determine unimolecular rates for crossing various transition states. On the basis of RRKM modeling, initial decomposition paths for energized EMIM(+)DCA(-) correspond to formation of an N-heterocyclic carbene and acid pair via transfer of the C2 proton of EMIM(+) to DCA(-), and evolution of methylimidazole and ethylimidazole via SN2 alkyl abstraction by DCA(-). Similar decomposition paths were identified for energized EMMIM(+)DCA(-), except that the reactivity of C2 of the imidazolium cation is significantly reduced upon substitution of a methyl group for a hydrogen atom at this position. The present work demonstrates that dynamics simulations, in conjunction with statistical modeling, are able to provide insight into decomposition mechanisms, kinetics, and dynamics for alkylimidazolium-based ionic liquids and to predict product branching ratios and how they vary with decomposition temperatures.

Toward the Oxidation of the Phenyl Radical and Prevention of PAH Formation in Combustion Systems.

The reaction of the phenyl radical (C6H5) with molecular oxygen (O2) plays a central role in the degradation of poly- and monocyclic aromatic radicals in combustion systems which would otherwise react with fuel components to form polycyclic aromatic hydrocarbons (PAHs) and eventually soot. Despite intense theoretical and experimental scrutiny over half a century, the overall reaction channels have not all been experimentally identified. Tunable vacuum ultraviolet photoionization in conjunction with a combustion simulating chemical reactor uniquely provides the complete isomer specific product spectrum and branching ratios of this prototype reaction. In the reaction of phenyl radicals and molecular oxygen at 873 K and 1003 K, ortho-benzoquinone (o-C6H4O2), the phenoxy radical (C6H5O), and cyclopentadienyl radical (C5H5) were identified as primary products formed through emission of atomic hydrogen, atomic oxygen and carbon dioxide. Furan (C4H4O), acrolein (C3H4O), and ketene (C2H2O) were also identified as primary products formed through ring opening and fragmentation of the 7-membered ring 2-oxepinoxy radical. Secondary reaction products para-benzoquinone (p-C6H4O2), phenol (C6H5OH), cyclopentadiene (C5H6), 2,4-cyclopentadienone (C5H4O), vinylacetylene (C4H4), and acetylene (C2H2) were also identified. The pyranyl radical (C5H5O) was not detected; however, electronic structure calculations show that it is formed and isomerizes to 2,4-cyclopentadienone through atomic hydrogen emission. In combustion systems, barrierless phenyl-type radical oxidation reactions could even degrade more complex aromatic radicals. An understanding of these elementary processes is expected to lead to a better understanding toward the elimination of carcinogenic, mutagenic, and environmentally hazardous byproducts of combustion systems such as PAHs.

The sudden vector projection model for reactivity: mode specificity and bond selectivity made simple.

CONSPECTUS: Mode specificity is defined by the differences in reactivity due to excitations in various reactant modes, while bond selectivity refers to selective bond breaking in a reaction. These phenomena not only shed light on reaction dynamics but also open the door for laser control of reactions. The existence of mode specificity and bond selectivity in a reaction indicates that not all forms of energy are equivalent in promoting the reactivity, thus defying a statistical treatment. They also allow the enhancement of reactivity and control product branching ratio. As a result, they are of central importance in chemistry. This Account discusses recent advances in our understanding of these nonstatistical phenomena. In particular, the newly proposed sudden vector projection (SVP) model and its applications are reviewed. The SVP model is based on the premise that the collision in many direct reactions is much faster than intramolecular vibrational energy redistribution in the reactants. In such a sudden limit, the coupling of a reactant mode with the reaction coordinate at the transition state, which dictates its ability to promote the reaction, is approximately quantified by the projection of the former onto the latter. The SVP model can be considered as a generalization of the venerable Polanyi's rules, which are based on the location of the barrier. The SVP model is instead based on properties of the saddle point and as a result capable of treating the translational, rotational, and multiple vibrational modes in reactions involving polyatomic reactants. In case of surface reactions, the involvement of surface atoms can also be examined. Taking advantage of microscopic reversibility, the SVP model has also been used to predict product energy disposal in reactions. This simple yet powerful rule of thumb has been successfully demonstrated in many reactions including uni- and bimolecular reactions in the gas phase and gas-surface reactions. The success of the SVP model underscores the importance of the transition state in controlling mode-specific and bond-selective chemistry.

Revisiting the photodissociation dynamics of the phenyl radical.

We have reinvestigated the photodissociation dynamics of the phenyl radical at 248 nm and 193 nm via photofragment translational spectroscopy under a variety of experimental conditions aimed at reducing the nascent internal energy of the phenyl radical and eliminating signal from contaminants. Under these optimized conditions, slower translational energy (P(E(T))) distributions for H-atom loss were seen at both wavelengths than in previously reported work. At 193 nm, the branching ratio for C2H2 loss vs. H-atom loss was found to be 0.2 ± 0.1, a significantly lower value than was obtained previously in our laboratory. The new branching ratio agrees with calculated Rice-Ramsperger-Kassel-Marcus rate constants, suggesting that the photodissociation of the phenyl radical at 193 nm can be treated using statistical models. The effects of experimental conditions on the P(E(T)) distributions and product branching ratios are discussed.

Nonadiabatic photofragmentation dynamics of BrCN−

The photofragmentation dynamics of BrCN(-) in the 270-355 nm and the 430-600 nm wavelength regions is explored both experimentally and theoretically. In the case of excitation between 430 nm and 600 nm, it is found that the molecular ion accesses two dissociation channels with a measured 60:40 branching ratio that is nearly constant over this range of photon energies. The dominant product channel corresponds to Br(-) + CN, while the second channel correlates to spin-orbit excited Br(*) with CN(-). A larger wavelength dependence of the branching ratio is observed at shorter wavelengths, where the fraction of Br(-) based products ranges from 80% to 95% at 355 nm and 270 nm, respectively. These branching ratios are reproduced and the mechanisms are explored by quantum dynamics calculations based on ground and excited state potential energy surfaces for BrCN(-), evaluated at the SO-MRCISD level of theory. It is found that the electronic states that correlate to the two observed product channels are coupled through the spin-orbit terms in the electronic Hamiltonian. The strength of this coupling displays a strong dependence on the Br-CN angle. Specifically, after promotion to the excited state that is energetically accessible with 430-600 nm photons, it is found that when the wave packet accesses Br-CN separations of between 4 Å and 6 Å, predominantly the Br(-) + CN products are formed when the Br-CN angle is smaller than 120°. For larger values of the Br-CN angle, the Br(*) + CN(-) channel dominates. At the shorter wavelength excitation, the dynamics is complicated by a pair of states that correlate to electronically excited CN(*) + Br(-) products that borrow oscillator strength from the bright state, leading to an increase in the amount of Br(-) relative to CN(-). The implications of these findings are discussed and compared to the experimentally measured product branching ratios for the photodissociation of BrCN(-).

Rate coefficients and product branching ratios for the oxidation of phenyl and naphthyl radicals: A theoretical RRKM-ME study

Theoretical VRC-TST/RRKM-ME calculations were performed to evaluate total rate coefficients and product branching ratios for the oxidation of phenyl and 1- and 2-naphthyl radicals with O2 at temperatures relevant to combustion (1500, 2000, and 2500K) and pressures of 0.01, 0.1, 1.0, and 10atm. The results give the rate coefficients in the range of 3.0–5.5×10−11cm3 molecule−1s−1 with slightly positive temperature dependence, activation energies varying within 2.3–3.3kcal/mol, and pre-exponential factors of 7–10×10−11cm3 molecule−1s−1. The dominant reaction channel in all three cases is elimination of the oxygen atom from peroxy complexes formed at the initial O2 addition step and leading to the phenoxy and naphthoxy radical products. The contribution of this channel increases with temperature. Chemically-activated phenoxy and naphthoxy radicals either decompose to the cyclopentadienyl+CO and indenyl+CO products, respectively, or undergo thermal equilibration. The relative yields of the decomposition/equilibration products strongly depend on temperature and pressure in the way that a temperature growth favors decomposition, whereas an increase in pressure favors equilibration. At the lowest temperature considered, 1500K, the reactions also yield significant amounts of pyranyl+CO (phenyl+O2) or 1-benzopyranyl+CO (1-naphthyl+O2). A comparison of the phenyl+O2 and naphthyl+O2 reactions reveals that although the general trends in the oxidation kinetics of phenyl and naphthyl radicals are similar, the size and especially the position of the radical site in the aromatic moiety may affect the details of the mechanism and relative product yields.

Reaction mechanism and product branching ratios of the CH + C₃H₈ reaction: a theoretical study.

The C4H9 potential energy surface accessed by the reaction of methylidyne radical, CH (X(2)Π), with propane, C3H8, including possible intermediates, transition states and dissociation products, has been studied by ab initio and density functional calculations at the CCSD(T)/CBS//B3LYP/6-311G(d,p) level of theory. The computed relative energies and molecular parameters were utilized to calculate collision-energy-dependent unimolecular rate constants at the zero-pressure limit for isomerization and dissociation channels of the C4H9 adducts formed in the entrance reaction channels. The rate constants were used to evaluate the product branching ratios in the CH + C3H8 reaction under single-collision conditions. The results show that the reaction can produce mostly ethene (C2H4) + ethyl radical (C2H5) and propene (C3H6) + methyl radical (CH3), and up to 14% of various butene isomers (C4H8) + H. The product branching ratios are sensitive to the initial reaction adduct (a butyl radical, C4H9) formed in the entrance channels via barrierless insertion of the CH radical into the terminal and middle C-H bonds of propane or, possibly, into the single C-C bonds. A more definite answer on relative contributions of various available CH insertion channels can be obtained through ab initio quasiclassical trajectory calculations, which are proposed for the future. The results allowed us to conclude that the CH + C3H8 reaction does not result in major amounts in the direct growth of the carbon-skeleton to four-carbon C4H8 products via the CH-for-H exchange because C-C bond cleavages in C4H9 radicals are generally more preferable than C-H bond cleavages.

A selected ion flow tube study of the ion molecule association reactions of protonated (MH+), nitrosonated (MNO+) and dehydroxidated (M−OH)+ carboxylic acids (M) with H2O

A selected ion flow tube, SIFT, study has been carried out of the reactions of H3O+, NO+ and O2+ with four carboxylic acids: formic acid, acetic acid, propionic acid and butyric acid. Ethanol was also included in order to provide data that could facilitate SIFT-MS analyses of formic acid in its presence. The product ion branching ratios of the primary reactions were obtained and compared with the already known values, thus improving the certainty of kinetic data for quantification of these compounds. The primary products of the H3O+ reactions, the protonated molecules MH+, and the primary products of the NO+ reactions, nitrosonated (MNO+) and dehydroxidated (M−OH)+ molecules, are observed to react with the molecules of water by three body association. Original data were obtained for the kinetics of these cluster-forming reactions. The three body rate constant for the associations of MH+ with H2O in He have been determined in the units of 10−27cm3s−1 for formic, acetic, propionic and butyric acids as 1.4, 2.4, 3.8 and 4.9, respectively. The three body rate constant for the associations of MNO+ with H2O in He were obtained in the same units as 2.1, 1.1, 0.15 and 0.7. Results will improve the certainty of identification and accuracy of quantification of vapors of carboxylic acids by SIFT-MS in medical and food science research.

Reactions of atomic and molecular ions with acetone, 1,1,1-trifluoroacetone, and hexafluoroacetone: An investigation of the effects of molecular structure on the dynamics and kinetics of ion–molecule reactions

A selected ion flow tube study of the reactions of a series of gas-phase atomic cations (S+, Xe+, O+, Kr+, N+, Ar+ and Ne+) and molecular ions (SFn+ (n=1–5), CFn+ (n=1–3), CF2Cl+, H3O+, NO+, N2O+, CO2+, CO+, and N2+) spanning a large range of recombination energies (6.3–21.6eV), with acetone, 1,1,1-trifluoroacetone, and hexafluoroacetone has been undertaken with the objective of exploring the nature of the reaction ion chemistry as the methyl groups in acetone are substituted for CF3. The reaction rate coefficients and product ion branching ratios for all 66 reactions, measured at 298K, are reported. The experimental reaction rate coefficients are compared to theoretically calculated collisional values. Several distinct reaction processes were observed among the large number of reactions studied, including charge transfer (non-dissociative and dissociative), abstraction, ion–molecule associations and, in the case of the reactions involving the reagent ion H3O+, proton transfer.

Effects of CF3 and CH3 Groups on the Threshold Energy for the Unimolecular Interchange Reaction of Cl- and F-Atoms in CF3CHFCH2Cl and CH3CHFCH2Cl

The recombination reactions of CH2Cl radicals with CF3CHF and with CH3CHF radicals were used to generate CF3CHFCH2Cl and CH3CHFCH2Cl molecules with 90-92 kcal mol(-1) of vibrational energy. The experimental rate constants for elimination of HCl and HF and the interchange of Cl and F atoms were measured and compared to RRKM calculated rate constants to assign the threshold energy for each unimolecular reaction channel. The Cl/F interchange reaction is approximately 18% of the total unimolecular reaction for both molecules. The product branching ratios and some rate constants also could be measured for the unimolecular reactions of the rearranged molecules, CF3CHClCH2F and CH3CHClCH2F. The most important result is that the CH3 group lowers the threshold for Cl/F interchange relative to CH2FCD2Cl, as expected for an electron-density donating group, and the CF3 group, an electron-density withdrawing group, increases the threshold energy relative to CH2FCD2Cl. The CH3 and CF3 groups alter the threshold energies of the HCl and HF elimination reactions in such a way so as to maintain the same branching fraction for the interchange reaction. The results from density functional theory using the B3PW91 method with the 6311+G(2d,p) and G-31G(d',p') basis sets are used to discuss the trends in threshold energies for the Cl/F interchange and the HF and HCl elimination reactions.

Activation of Methane by FeO+: Determining Reaction Pathways through Temperature-Dependent Kinetics and Statistical Modeling

The temperature dependences of the rate constants and product branching ratios for the reactions of FeO(+) with CH4 and CD4 have been measured from 123 to 700 K. The 300 K rate constants are 9.5 × 10(-11) and 5.1 × 10(-11) cm(3) s(-1) for the CH4 and CD4 reactions, respectively. At low temperatures, the Fe(+) + CH3OH/CD3OD product channel dominates, while at higher temperatures, FeOH(+)/FeOD(+) + CH3/CD3 becomes the majority channel. The data were found to connect well with previous experiments at higher translational energies. The kinetics were simulated using a statistical adiabatic channel model (vibrations are adiabatic during approach of the reactants), which reproduced the experimental data of both reactions well over the extended temperature and energy ranges. Stationary point energies along the reaction pathway determined by ab initio calculations seemed to be only approximate and were allowed to vary in the statistical model. The model shows a crossing from the ground-state sextet surface to the excited quartet surface with large efficiency, indicating that both states are involved. The reaction bottleneck for the reaction is found to be the quartet barrier, for CH4 modeled as -22 kJ mol(-1) relative to the sextet reactants. Contrary to previous rationalizations, neither less favorable spin-crossing at increased energies nor the opening of additional reaction channels is needed to explain the temperature dependence of the product branching fractions. It is found that a proper treatment of state-specific rotations is crucial. The modeled energy for the FeOH(+) + CH3 channel (-1 kJ mol(-1)) agrees with the experimental thermochemical value, while the modeled energy of the Fe(+) + CH3OH channel (-10 kJ mol(-1)) corresponds to the quartet iron product, provided that spin-switching near the products is inefficient. Alternative possibilities for spin switching during the reaction are considered. The modeling provides unique insight into the reaction mechanisms as well as energetic benchmarks for the reaction surface.

Radiation Processing of Formamide and Formamide:Water Ices on Silicate Grain Analogue

Lyman-α (121.6 nm) photon and 1 keV electron-beam irradiation of pure HCONH2 (FA) ice and H2O:HCONH2 ice mixtures on high-surface-area SiO2 nanoparticles have been investigated with FT-IR spectroscopy and temperature programmed desorption (TPD). Lyman-α photolysis of pure amorphous FA ice grown at 70 K and crystalline FA ice produced by annealing to 165 K gives spectral signatures between 2120 and 2195 cm(-1) that we assign primarily to OCN(-) and CO. The OCN(-) and CO yields are ∼25% less abundant for crystalline FA ice. Photon and electron processing also produces H2 that is released from the ice between ∼90 and 140 K. A decrease in the H2 TPD peak is seen for irradiated crystalline HCONH2 ice. Lyman-α photolysis of H2O:HCONH2 mixed ices increases OCN(-) and CO production, suggesting a catalytic role of H2O. Also, for pure FA, 1 keV electron irradiation slightly increases the yield of OCN(-), while CO decarboxylation is selectively prevented. CO is also not produced in H2O:HCONH2 ices upon electron irradiation. Dissociative ionization, direct dissociative excitation, and dissociative electron attachment (DEA) channels are accessible in the Lyman-α (121.6 nm) photon and 1 keV electron-beam energy range. DEA energetically favors OCN(-) and H(-) formation, with the latter leading to H2 formation. The FA fragment product identities, yields, and branching ratios are considerably different relative to the gas phase and depend upon the radiation type, ice structure, and the presence of SiO2 nanoparticles. The latter may increase ion-electron recombination and radical recombination rates. The main products observed suggest very different condensed-phase dissociation channels from those reported for gas-phase dissociation. Formation of ions/products from FA is not negligible upon Lyman-α photolysis or electron irradiation, both of which could process ices in interstellar regions as well as in Titan's atmosphere.

Conical intersection seam and bound resonances embedded in continuum observed in the photodissociation of thioanisole-d3

Herein, the multi-dimensional nature of the conical intersection seam has been experimentally revealed in the photodissociation reaction of thioanisole-d3 (C6H5SCD3) excited on S1, giving C6H5S·(Ã or X̃]) +·CD3 products. The translational energy distribution of the nascent·CD3 fragment, reflecting the relative yields of the C6H5S·(Ã) and C6H5S·(X̃) products, was measured at each S1 vibronic band using the velocity map ion imaging technique. Direct access of the reactant flux to the conical intersection seam leads to the increase of the nonadiabatic transition probability resulting in sharp resonances in the X̃/ÃC6H5S·product branching ratio at several distinct S1 vibronic bands. The nature of the S1 vibronic bands associated with such dynamic resonances was clarified by the mass-analyzed threshold ionization spectroscopy. The bound state embedded in continuum generated by the conical intersection is observed as a distinct dynamic resonance, revealing the nature of the nuclear motion responsible for the nonadiabatic coupling of two potential energy surfaces at the conical intersection. The multi-dimensional facets of the conical intersection seam in terms of its detailed structure and dynamic role are discussed with the aid of theoretical calculations.

Trajectory surface hopping study of the O((3)P) + C2H2 reaction dynamics: effect of collision energy on the extent of intersystem crossing.

Intersystem crossing (ISC) dynamics plays an important role in determining the product branching in the O((3)P) + C2H2 reaction despite the necessarily small spin-orbit coupling constant values. In this study we investigate the effect of collision energy on the extent of the contribution of a spin non-conserving route through ISC dynamics to the product distributions at the initial collision energies 8.2, 9.5, and 13.1 kcal/mol. A direct dynamics trajectory surface hopping method is employed with potential energy surfaces generated at the unrestricted B3LYP/6-31G(d,p) level of theory to perform nonadiabatic dynamics. To make our calculation simpler, nonadibatic transitions were only considered at the triplet-singlet intersections. At the crossing points, Landau-Zener transition probabilities were calculated using spin-orbit coupling constant values computed at the same geometry. The Landau-Zener model for the title reaction is validated against a more rigorous Tully's fewest switches method and found to be working reasonably well as expected because of weak spin-orbit coupling. We have compared our results with the recent crossed molecular beam experiments and observed a very good agreement with respect to the primary product branching ratios. Our calculation revealed that there is no noticeable effect of the initial collision energy on the overall product distributions that corroborates the recent experimental findings. Our calculation indicates, however, that the extent of intersystem crossing contributions varies significantly with collision energy, needed to be verified, experimentally.

Atmospheric Chemical Reactions of Monoethanolamine Initiated by OH Radical: Mechanistic and Kinetic Study

Monoethanolamine (MEA) is a benchmark and widely utilized solvent in amine-based postcombustion CO2 capture (PCCC), a leading technology for reducing CO2 emission from fossil fuel power plants. The large-scale implementation of PCCC would lead to inevitable discharges of amines to the atmosphere. Therefore, understanding the kinetics and mechanisms of the transformation of representative amine MEA in the atmosphere is of great significance for risk assessment of the amine-based PCCC. In this study, the H-abstraction reaction of MEA with ·OH, and ensuing reactions of produced MEA-radicals, including isomerization, dissociation, and bimolecular reaction MEA-radicals+O2, were investigated by quantum chemical calculation [M06-2X/aug-cc-pVTZ//M06-2X/6-311++G(d,p)] and kinetic modeling. The calculated overall rate constant [(7.27 × 10(-11)) cm(3) molecule(-1) s(-1)] for H-abstraction is in excellent agreement with the experimental value [(7.02 ± 0.46) × 10(-11) cm(3) molecule(-1) s(-1)]. The results show that the product branching ratio of NH2CH2 · CHOH (MEA-β) (43%) is higher than that of NH2 · CHCH2OH (MEA-α) (39%), clarifying that MEA-α is not an exclusive product. On the basis of the unveiled reaction mechanisms of MEA-radicals + O2, the proton transfer reaction mass spectrometry signal (m/z 60.044), not recognized in the experiment, was identified.

Unraveling Organocuprate Complexity: Fundamental Insights into Intrinsic Group Transfer Selectivity in Alkylation Reactions

The near thermal conditions of an ion-trap mass spectrometer were used to examine the intrinsic gas-phase reactivity and selectivity of nucleophilic substitution reactions. The well-defined organocuprate anions [CH3CuR](-) (R = CH3CH2, CH3CH2CH2, (CH3)2CH, PhCH2CH2, PhCH2, Ph, C3H5, and H) were reacted with CH3I. The rates (reaction efficiencies, ϕ) and selectivities (the product ion branching ratios) were compared with those of [CH3CuCH3](-) reacting with CH3I. Alkyl R groups yielded similar efficiencies, with selectivity for C-C bond formation at the coordinated R group. Inclusion of unsaturated R groups curbed the overall reactivity (ϕ = 1 to 2 orders of magnitude lower). With the exception of R = PhCH2CH2, these switched their selectivity to C-C bond formation at the CH3 group. Replacing an organyl ligand with R = H significantly enhanced the reactivity (8-fold), resulting in the selective formation of methane. Unique decomposition and side-reactions observed include: (1) spontaneous β-hydride elimination from [RCuI](-) byproducts; and (2) homocoupling of the pre-existing organocuprate ligands in [CH3CuC3H5](-), as shown by deuterium labeling. DFT (B3LYP-D/Def2-QZVP//B3LYP/SDD:6-31+G(d)) predicts that the alkylation mechanism for all species is via oxidative addition/reductive elimination (OA/RE). OA is the rate-limiting step, while RE determines selectivity: the effects of R on each were examined.

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

WO3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO3 surface can improve the kinetics for water oxidation and increase the branching ratio for O2 production. Ir-based OECs were attached to WO3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO3 photoanodes in 1 M H2SO4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO3/electrolyte interface. Sputtering of IrO2 layers on WO3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O2 yield at 1.2 V vs. RHE from ~0% for bare WO3 to 50-70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO2/WO3 junction, which provided a shunt pathway for electrocatalytic IrO2 behavior with the WO3 photoanode under reverse bias. Although other OECs were tested, only IrO2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.

Direct measurements of the total rate constant of the reaction NCN + H and implications for the product branching ratio and the enthalpy of formation of NCN.

The overall rate constant of the reaction (2), NCN + H, which plays a key role in prompt-NO formation in flames, has been directly measured at temperatures 962 K < T < 2425 K behind shock waves. NCN radicals and H atoms were generated by the thermal decomposition of NCN3 and C2H5I, respectively. NCN concentration-time profiles were measured by sensitive narrow-line-width laser absorption at a wavelength of λ = 329.1302 nm. The obtained rate constants are best represented by the combination of two Arrhenius expressions, k2/(cm(3) mol(-1) s(-1)) = 3.49 × 10(14) exp(-33.3 kJ mol(-1)/RT) + 1.07 × 10(13) exp(+10.0 kJ mol(-1)/RT), with a small uncertainty of ±20% at T = 1600 K and ±30% at the upper and lower experimental temperature limits.The two Arrhenius terms basically can be attributed to the contributions of reaction channel (2a) yielding CH + N2 and channel (2b) yielding HCN + N as the products. A more refined analysis taking into account experimental and theoretical literature data provided a consistent rate constant set for k2a, its reverse reaction k1a (CH + N2 → NCN + H), k2b as well as a value for the controversial enthalpy of formation of NCN, ΔfH = 450 kJ mol(-1). The analysis verifies the expected strong temperature dependence of the branching fraction ϕ = k2b/k2 with reaction channel (2b) dominating at the experimental high-temperature limit. In contrast, reaction (2a) dominates at the low-temperature limit with a possible minor contribution of the HNCN forming recombination channel (2d) at T < 1150 K.

Headspace analysis of new psychoactive substances using a Selective Reagent Ionisation-Time of Flight-Mass Spectrometer

The rapid expansion in the number and use of new psychoactive substances presents a significant analytical challenge because highly sensitive instrumentation capable of detecting a broad range of chemical compounds in real-time with a low rate of false positives is required. A Selective Reagent Ionisation-Time of Flight-Mass Spectrometry (SRI-ToF-MS) instrument is capable of meeting all of these requirements. With its high mass resolution (up to m/Δm of 8000), the application of variations in reduced electric field strength (E/N) and use of different reagent ions, the ambiguity of a nominal (monoisotopic) m/z is reduced and hence the identification of chemicals in a complex chemical environment with a high level of confidence is enabled. In this study we report the use of a SRI-ToF-MS instrument to investigate the reactions of H3O+, O2+, NO+ and Kr+ with 10 readily available (at the time of purchase) new psychoactive substances, namely 4-fluoroamphetamine, methiopropamine, ethcathinone, 4-methylethcathinone, N-ethylbuphedrone, ethylphenidate, 5-MeO-DALT, dimethocaine, 5-(2-aminopropyl)benzofuran and nitracaine. In particular, the dependence of product ion branching ratios on the reduced electric field strength for all reagent ions was investigated and is reported here. The results reported represent a significant amount of new data which will be of use for the development of drug detection techniques suitable for real world scenarios.

Role of the Metal, Ligand, and Alkyl/Aryl Group in the Hydrolysis Reactions of Group 10 Organometallic Cations [(L)M(R)]+

The reactions of water with the coordinatively unsaturated group 10 organometallic cations [(L)M(R)]+ (4; where L = 1,10-phenanthroline (phen), neocuproine (neo); M = nickel, palladium, platinum; R = CH3, C6H5, CH2C6H5), formed via decarboxylation of the carboxylate complexes [(L)M(O2CR)]+, were examined in the gas phase using a combination of multistage mass spectrometry experiments and DFT calculations at the M06/SDD6-31+G(d) level of theory. Two main types of primary product ions were observed: the aqua adduct [(L)M(R)(H2O)]+ (5) and the hydroxide [(L)M(OH)]+ (7), formed via a hydrolysis reaction. A secondary product ion, arising from formation of the adduct [(L)M(OH)(H2O)]+, was also observed when L = phen, R = CH3, and M = Pt. The rates of reaction of 4 and the product branching ratios for 5 and 7 were dependent upon the nature of M, L, and R. When L = phen and R = CH3, the hydroxide 7 dominates for Ni, with the adduct 5 as the major product for both Pd and Pt. For R = C6H5 the rate of the reaction i...