Potential Energy Surface For Reaction Research Articles

Exploring the potential energy surface for reaction of SWCNT with NO2+: A model reaction for oxidation of carbon nanotube in acid solution

The interaction of single-walled carbon nanotubes (SWCNT) with NO2+, an active species present in HNO3/H2SO4 acid mixture was studied by quantum mechanical calculations. In addition to the pristine (P) form of an armchair (5,5) SWCNT, two other species containing Stone–Wales (SW) and mono-vacancy (V1) defects were considered in order to model the distinct defective regions on the carbon nanotube surface. For the P and SW regions, the ether (COC) functional group was predicted as the main product, with an epoxide (CCO) found as a reactive intermediate. The Gibbs free energy barriers were predicted to be 31.7 (P) and 37.8kcalmol−1 (SW) in aqueous solution at 298.15K and 1atm. The mechanism involving the V1 region leads to the carbonyl group (CO) as the main product, which is formed spontaneously upon NO2+ adsorption without energy barrier. A higher energy mechanism was also described for V1 region, passing through a transition state characterized as an oxaziridine-like ring. Through this pathway an alkoxy (CO−) is firstly formed and reacts with the neighbor carbon yielding the ether (COC) functional group. The activation Gibbs free energies were 4.5 and 11.2kcalmol−1 for the first (CO− formation) and second (COC formation) steps, respectively. The results reported here suggest that at the beginning of oxidation in acid medium, the vacancy regions (V1) are firstly oxidized leading to the carbonyl (CO) functional groups, followed by reactions at the topological defective parts (P and SW) of the tube surface where the ether (COC) function is the main product.

Antioxidant activities of [60]fullerene derivatives from chalcone, flavone and flavanone: A ONIOM approach via H-atom and electron transfer mechanism

Antioxidant properties of C60 flavonoid conjugates were computationally examined via their O–H bond dissociation enthalpies (BDEs) and ionization energies (IEs) using two-layer ONIOM and PM6 methods, respectively. Eight ONIOM((RO)B3LYP/6-311++G(2df,2p):PM6) models were evaluated by computing BDE(O–H)s of a series of polyphenol. Synthetic mechanism of C60 flavonoid conjugates was also explored via the potential energy surfaces of reaction between C60 and malonate flavonoid derivatives (chalcone, flavone and flavanone) at the B3LYP/6-31G(d)//PM6. Antioxidant activities of C60 flavonoid conjugates were discussed via hydrogen atom transfer, single electron transfer mechanisms and the effect of C60 on the BDE(O–H)s and IEs of these compounds.

Submerged Barriers in the Ni(+) Assisted Decomposition of Propionaldehyde.

The reaction dynamics of the Ni(+) mediated decarbonylation of propionaldehyde was assessed using the single photon initiated decomposition rearrangement reaction (SPIDRR) technique. The exothermic production of Ni(+)CO was temporally monitored and the associated rate constants, k(E), were extracted as a function of activating photon energy. In addition, the reaction potential energy surface was calculated at the UCCSD(T)/def2-TZVP//PBEPBE/cc-pVDZ level of theory to provide an atomistic description of the reaction profile. The decarbonylation of propionaldehyde can be understood as proceeding through parallel competitive reaction pathways that are initiated by Ni(+) insertion into either the C-C or C-H bond of the propionaldehyde carbonyl carbon. Both paths lead to the elimination of neutral ethane and are governed by submerged barriers. The lower energy sequence is a consecutive C-C/C-H addition process with a submerged barrier of 14 350 ± 600 cm(-1). The higher energy sequence is a consecutive C-H/C-C addition process with a submerged barrier of 15 400 ± 600 cm(-1). Both barriers were determined using RRKM calculations fit to the experimentally determined k(E) values. The measured energy difference between the two barriers agrees with the DFT computed difference in rate limiting transition-state energies, 18 413 and 19 495 cm(-1).

Tuning Cysteine Reactivity and Sulfenic Acid Stability by Protein Microenvironment in Glyceraldehyde-3-Phosphate Dehydrogenases of Arabidopsis thaliana.

Cysteines and H2O2 are fundamental players in redox signaling. Cysteine thiol deprotonation favors the reaction with H2O2 that generates sulfenic acids with dual electrophilic/nucleophilic nature. The protein microenvironment surrounding the target cysteine is believed to control whether sulfenic acid can be reversibly regulated by disulfide formation or irreversibly oxidized to sulfinates/sulfonates. In this study, we present experimental oxidation kinetics and a quantum mechanical/molecular mechanical (QM/MM) investigation to elucidate the reaction of H2O2 with glycolytic and photosynthetic glyceraldehyde-3-phosphate dehydrogenase from Arabidopsis thaliana (cytoplasmic AtGAPC1 and chloroplastic AtGAPA, respectively). Although AtGAPC1 and AtGAPA have almost identical 3D structure and similar acidity of their catalytic Cys149, AtGAPC1 is more sensitive to H2O2 and prone to irreversible oxidation than AtGAPA. As a result, sulfenic acid is more stable in AtGAPA. Based on crystallographic structures of AtGAPC1 and AtGAPA, the reaction potential energy surface for Cys149 oxidation by H2O2 was calculated by QM. In both enzymes, sulfenic acid formation was characterized by a lower energy barrier than sulfinate formation, and sulfonate formation was prevented by very high energy barriers. Activation energies for both oxidation steps were lower in AtGAPC1 than AtGAPA, supporting the higher propensity of AtGAPC1 toward irreversible oxidation. QM/MM calculations coupled to fingerprinting analyses revealed that two Arg of AtGAPA (substituted by Gly and Val in AtGAPC1), located at 8-15 Å distance from Cys149, are the major factors responsible for sulfenic acid stability, underpinning the importance of long-distance polar interactions in tuning sulfenic acid stability in native protein microenvironments.

Expanding the applicability of electrostatic potentials to the realm of transition states.

Central to any reaction mechanism study, and sometimes a challenging job, is tracing a transition state in a reaction path. For the first time, electrostatic potentials (ESP) of the reactants were used as guiding tactics to predict whether there is a possibility of any transition state in a reaction surface. The main motive behind this strategy is to see whether the directionality nature of the transition state has something to do with the anisotropic natures of the ESP with their embedded directionalities. Strategically, some atmospherically important, but simple, reactions have been chosen for this study, which heretofore were believed to be barrierless. By carefully analysing the ESP maps of the reactants, regions of possible interactions were located. Using the bilinear interpolation of the 2D grids of the ESP surfaces, search co-ordinates were fine-tuned for a local gradient based approach for the search of a transition state. Out of the three reactions studied in this work, we were able to successfully locate transition states, for the first time, in two cases and the third one still proved to be barrierless. This gives a clear indication that though ESP maps can qualitatively predict the possibility of a transition state; it is not always true that there should definitely be a transition state, as some of the reaction surfaces may genuinely be barrierless. But, nevertheless this strategy definitely has credential to be tested for many more reactions, either new or already established, and may be applied to create the initial search co-ordinates for any well-established transition state search method. Moreover, we have observed that the analysis of the ESP maps of the reactants were very much useful in explaining the nature of interactions existing in those observed transition states and we hope the same can also be extended to any transition state in an electrostatically driven reaction potential energy surface.

Construction of molecular reactive potential energy surfaces based on neural networks

In this article, recent developments in construction of potential energy surfaces using neural networks are reviewed. Based on the previous studies, a systematical method is proposed for iteratively selecting new molecular conformations. The problem for data sampling is now fully resolved under strict standards. A series of potential energy surfaces have been constructed based on highly accurate ab initio energies, using this data selecting scheme and combined with several fitting methods as well as extensive dynamics calculations. Highly reliable dynamics results can be obtained from these surfaces.

Optically Triggered Stepwise Double-Proton Transfer in an Intramolecular Proton Relay: A Case Study of 1,8-Dihydroxy-2-naphthaldehyde

1,8-Dihydroxy-2-naphthaldehyde (DHNA), having doubly intramolecular hydrogen bonds, was strategically designed and synthesized in an aim to probe a long-standing fundamental issue regarding synchronous versus asynchronous double-proton transfer in the excited state. In cyclohexane, DHNA shows the lowest lying S0 →S1 (π-π*) absorption at ∼400 nm. Upon excitation, two large Stokes shifted emission bands maximized at 520 and 650 nm are resolved, which are ascribed to the tautomer emission resulting from the first and second proton-transfer products, denoted by TA* and TB*, respectively. The first proton transfer (DHNA* → TA*) is ultrafast (< system response of 150 fs), whereas the second proton transfer is reversible, for which the rates of forward (TA* → TB*) and backward (TA* ← TB*) proton transfer were determined to be (1.7 ps)(-1) and (3.6 ps)(-1), respectively. The fast equilibrium leads to identical population lifetimes of ∼54 ps for both TA* and TB* tautomers. Similar excited-state double-proton transfer takes place for DHNA in a single crystal, resulting in TA* (560 nm) and TB* (650 nm) dual-tautomer emission. A comprehensive 2D plot of reaction potential energy surface further proves that the sequential two-step proton motion is along the minimum energetic pathway firmly supporting the experimental results. Using DHNA as a paradigm, we thus demonstrate unambiguously a stepwise, proton-relay type of intramolecular double-proton transfer reaction in the excited state, which should gain fundamental understanding of the multiple proton transfer reactions.

Kinetic mechanisms of hydrogen abstraction reactions from methanol by methyl, triplet methylene and formyl radicals

Although H-abstraction reactions from methanol by methyl radical (CH3), triplet methylene radical (3CH2) and formyl radical (HCO) are important for methanol pyrolysis and combustion, the kinetic mechanisms of those reactions are indistinct and need to be better understood. Ab initio calculations were performed to optimize geometries of transition states using Gaussian 09W package with the B3LYP density functional at the 6-311++G (3df, 3pd) basis set. Energy barrier and minimum energy path (MEP) on the reaction potential energy surface were calculated with MP2 method at the same basis set. The canonical variational transition state theory (CVT) with Eckart tunneling correction were employed to compute rate constants over the temperature range 298–2000K. Results show that, for reaction CH3OH+CH3, barrier height for hydroxymethyl channel (R1a) is 12.59kcal/mol and for methoxyl channel (R1b) is 13.82kcal/mol. Rate constants for channels R1a and R1b are k1a=3.635×10−31T5.58exp(−1962/T)cm3molecule−1s−1, k1b=4.342×10−28T4.61exp(−4284/T)cm3molecule−1s−1, respectively. For reaction CH3OH+3CH2, barrier heights for hydroxymethyl channel (R2a) and methoxyl channel (R2b) are 7.68kcal/mol and 9.19kcal/mol, respectively. Rate constant for channel R2a is k2a=5.885×10−28T4.19exp(−1813/T)cm3molecule−1s−1 and for channel R2b is k2b=2.266×10−28T4.26exp(−2833/T)cm3molecule−1s−1. For reaction CH3OH+HCO, barrier heights for hydroxymethyl channel (R3a) and methoxyl channel (R3b) are 21.88kcal/mol and 28.83kcal/mol, respectively. Rate constant for channel R3a is k3a=8.156×10−38T7.37exp(−4900/T)cm3molecule−1s−1 and for channel R3b is k3b=2.007×10−27T4.34exp(−11,605/T)cm3molecule−1s−1. This study is beneficial for improving the kinetic mechanism of methanol pyrolysis and combustion.

Reactions of Th(+) + H2, D2, and HD Studied by Guided Ion Beam Tandem Mass Spectrometry and Quantum Chemical Calculations.

Kinetic energy dependent reactions of Th(+) with H2, D2, and HD were studied using a guided ion beam tandem mass spectrometer. Formation of ThH(+) and ThD(+) is endothermic in all cases with similar thresholds. Branching ratio results for the reaction with HD indicate that Th(+) reacts via a statistical mechanism, similar to Hf(+). The kinetic energy dependent cross sections for formation of ThH(+) and ThD(+) were evaluated to determine a 0 K bond dissociation energy (BDE) of D0(Th(+)-H) = 2.45 ± 0.07 eV. This value is in good agreement with a previous result obtained from analysis of the Th(+) + CH4 reaction. D0(Th(+)-H) is observed to be larger than its transition metal congeners, TiH(+), ZrH(+), and HfH(+), believed to be a result of lanthanide contraction. The reactions with H2 were also explored using quantum chemical calculations that include a semiempirical estimation and explicit calculation of spin-orbit contributions. These calculations agree nicely and indicate that ThH(+) most likely has a (3)Δ1 ground level with a low-lying (1)Σ(+) excited state. Theory also provides the reaction potential energy surfaces and BDEs that are in reasonable agreement with experiment.

Low- and intermediate-temperature oxidation of ethylcyclohexane: A theoretical study

To get a better understanding of low temperature oxidation chemistry of long-side-chain cycloalkanes, the low- and intermediate-temperature oxidation mechanisms and kinetics of ethylcyclohexane (ECH) have been investigated by using a combination of electronic-structure theory, transition state theory (TST) and Rice-Ramsberger-Kassel-Marcus/Master-Equation (RRKM/ME) theory in this work. The high pressure limit rate constants for reactions with tight transition states are obtained by the TST, while the high pressure limit rate constants for barrierless reactions are obtained by the variational transition state theory (VTST). The rate constants in the fall-off range for pressure-dependent reactions are obtained by RRKM/ME theory. Depending on the position of the extracted H atom on the ring or on the side chain, the oxidation of ECH is mainly initiated by H-abstraction with OH radical to form cyclic C8H15 radicals having six isomers (R1–R6) at low (500−900 K) and intermediate (900−1100 K) temperature and in this study we focus on the kinetics of the reactions starting from these isomers. At low temperature, it is shown that, for addition reactions of O2 to cyclic C8H15 radicals, the calculated rate constants when the unpaired electron of the radicals is in the side-chain are smaller than when in the ring. The intramolecular isomerization reactions of six cycloalkylperoxy radicals (R1OO–R6OO) by H-shift can lead to a large number of hydroperoxycycloalkyl radicals (QOOH) and it is shown that the 1,5 H-shift is more competitive than the 1,6 H-shift in the R1OO–R6OO. Furthermore, unimolecular elimination reactions of QOOH can form OH, HO2, cyclic ethers, ketones, aldehydes and conjugated olefins. The calculated results indicate that the formation cyclic ethers of ECH oxidation are inclined to 1-oxaspiro[3.5]octane, 8-methyl-7-oxa-bicyclo[4.2.0]octane, 7-methyl-6-oxa-bicyclo[3.2.1]octane and 2-ethyl-6-oxa-bicyclo[3.1.1]heptane. At intermediate temperature, the most important reaction type of R1–R6 consumption is the C–C bond fission to form smaller products. The reaction pathways and potential energy surfaces to form the main products (ethylene, butadiene, 1-butene, and so on) of R1–R6 decomposition are also performed and the rate constants of the corresponding reactions are calculated by using the TST. Rate constants of these reactions are fitted by a nonlinear least-squares method to the form of a modified Arrhenius rate expression, which can be directly used for the modeling study of low temperature oxidation of cycloalkanes.

Mechanistic and Kinetic Study of Singlet O2 Oxidation of Methionine by On-Line Electrospray Ionization Mass Spectrometry.

We report a reaction apparatus developed to monitor singlet oxygen ((1)O2) reactions in solution using on-line ESI mass spectrometry and spectroscopy measurements. (1)O2 was generated in the gas phase by the reaction of H2O2 with Cl2, detected by its emission at 1270 nm, and bubbled into aqueous solution continuously. (1)O2 concentrations in solution were linearly related to the emission intensities of airborne (1)O2, and their absolute scales were established based on a calibration using 9,10-anthracene dipropionate dianion as an (1)O2 trapping agent. Products from (1)O2 oxidation were monitored by UV-Vis absorption and positive/negative ESI mass spectra, and product structures were elucidated using collision-induced dissociation-tandem mass spectrometry. To suppress electrical discharge in negative ESI of aqueous solution, methanol was added to electrospray via in-spray solution mixing using theta-glass ESI emitters. Capitalizing on this apparatus, the reaction of (1)O2 with methionine was investigated. We have identified methionine oxidation intermediates and products at different pH, and measured reaction rate constants. (1)O2 oxidation of methionine is mediated by persulfoxide in both acidic and basic solutions. Persulfoxide continues to react with another methionine, yielding methionine sulfoxide as end-product albeit with a much lower reaction rate in basic solution. Density functional theory was used to explore reaction potential energy surfaces and establish kinetic models, with solvation effects simulated using the polarized continuum model. Combined with our previous study of gas-phase methionine ions with (1)O2, evolution of methionine oxidation pathways at different ionization states and in different media is described.

Non-equilibrium reaction and relaxation dynamics in a strongly interacting explicit solvent: F + CD3CN treated with a parallel multi-state EVB model.

We describe a parallelized linear-scaling computational framework developed to implement arbitrarily large multi-state empirical valence bond (MS-EVB) calculations within CHARMM and TINKER. Forces are obtained using the Hellmann-Feynman relationship, giving continuous gradients, and good energy conservation. Utilizing multi-dimensional Gaussian coupling elements fit to explicitly correlated coupled cluster theory, we built a 64-state MS-EVB model designed to study the F + CD3CN → DF + CD2CN reaction in CD3CN solvent (recently reported in Dunning et al. [Science 347(6221), 530 (2015)]). This approach allows us to build a reactive potential energy surface whose balanced accuracy and efficiency considerably surpass what we could achieve otherwise. We ran molecular dynamics simulations to examine a range of observables which follow in the wake of the reactive event: energy deposition in the nascent reaction products, vibrational relaxation rates of excited DF in CD3CN solvent, equilibrium power spectra of DF in CD3CN, and time dependent spectral shifts associated with relaxation of the nascent DF. Many of our results are in good agreement with time-resolved experimental observations, providing evidence for the accuracy of our MS-EVB framework in treating both the solute and solute/solvent interactions. The simulations provide additional insight into the dynamics at sub-picosecond time scales that are difficult to resolve experimentally. In particular, the simulations show that (immediately following deuterium abstraction) the nascent DF finds itself in a non-equilibrium regime in two different respects: (1) it is highly vibrationally excited, with ∼23 kcal mol(-1) localized in the stretch and (2) its post-reaction solvation environment, in which it is not yet hydrogen-bonded to CD3CN solvent molecules, is intermediate between the non-interacting gas-phase limit and the solution-phase equilibrium limit. Vibrational relaxation of the nascent DF results in a spectral blue shift, while relaxation of the post-reaction solvation environment results in a red shift. These two competing effects mean that the post-reaction relaxation profile is distinct from what is observed when Franck-Condon vibrational excitation of DF occurs within a microsolvation environment initially at equilibrium. Our conclusions, along with the theoretical and parallel software framework presented in this paper, should be more broadly applicable to a range of complex reactive systems.

Oxidation Dynamics of Methionine with Singlet Oxygen: Effects of Methionine Ionization and Microsolvation.

We report an in-depth study on the gas-phase reactions of singlet O2[a(1)Δg] with methionine (Met) at different ionization and hydration states (including deprotonated [Met - H](-), hydrated deprotonated [Met - H](-)(H2O)1,2, and hydrated protonated MetH(+)(H2O)1,2), using guided-ion-beam scattering mass spectrometry. The measurements include the effects of collision energy (Ecol) on reaction cross sections over a center-of-mass Ecol range from 0.05 to 1.0 eV. The aim of this study is to probe the influences of Met ionization and hydration on its oxidation mechanism and dynamics. Density functional theory calculations, Rice-Ramsperger-Kassel-Marcus modeling, and quasi-classical, direct dynamics trajectory simulations were performed to examine the properties of various complexes and transition states that might be important along reaction coordinates, probe reaction potential energy surfaces, and to establish the atomic-level mechanism for the Met oxidation process. No oxidation products were observed for the reaction of [Met - H](-) with (1)O2 due to the high-energy barriers located in the product channels for this system. However, this nonreactive property was altered by the microsolvation of [Met - H](-); as a result, hydroperoxides were captured as the oxidation products for [Met - H](-)(H2O)1,2 + (1)O2. For the reaction of MetH(+)(H2O)1,2 + (1)O2, besides formation of hydroperoxides, an H2O2 elimination channel was observed. The latter channel is similar to what was found in the reaction of dehydrated MetH(+) with (1)O2 (J. Phys. Chem. B 2011, 115, 2671). The reactions of hydrated protonated and deprotonated Met are all inhibited by Ecol, becoming negligible at Ecol ≥ 0.5 eV. The kinetic and dynamical consequences of microsolvation on Met oxidation and their biological implications are discussed.

An implementation of the Levenberg–Marquardt algorithm for simultaneous-energy-gradient fitting using two-layer feed-forward neural networks

We present in this study a new and robust algorithm for feed-forward neural network (NN) fitting. This method is developed for the application in potential energy surface (PES) construction, in which simultaneous energy-gradient fitting is implemented using the well-established Levenberg–Marquardt (LM) algorithm. Three fitting examples are demonstrated, which include the vibrational PES of H2O, reactive PESs of O3 and ClOOCl. In the three testing cases, our new LM implementation has been shown to work very efficiently. Not only increasing fitting accuracy, it also offers two other advantages: less training iterations are utilized and less data points are required for fitting.

The catalytic activity of Pt6M (M = Pt, Ru, Sn) cluster for methanol partial oxidation

Using the density functional theory (B3PW91), we have investigated the reaction mechanism of the initial adsorption and dehydrogenation steps of methanol on Pt6M (M=Pt, Ru, Sn) clusters. Pt6M (M=Pt, Ru, Sn) clusters are used to simulate the catalysts. Two adsorption and dehydrogenation mechanisms of methanol were studied: one used the hydroxyl-hydrogen atom and methyl-hydrogen atom approach the Pt and M atoms of Pt6M, and the other used hydroxyl-oxygen atom and hydroxyl-hydrogen atom attack the M and Pt atoms of Pt6M, respectively. The reaction potential energy surfaces (PES) show that on pure Pt7 cluster the favorable dehydrogenation channel is the decomposition of CH3 adsorption complex. On the Pt6Sn cluster the decomposition of hydroxyl adsorption complex is more favorable. On Pt6Ru cluster, both pathways are dominant. The adsorption energy, energy barrier, dissociation energy, NBO analysis and frontier molecular orbital analysis are discussed at B3PW91/LANL2DZ level of theory. The addition of Ru and Sn enhance the catalytic activity of pure Pt for methanol partial oxidation reaction. We may conclude that Pt6Ru bimetallic catalyst is most effective among Pt6M (M=Pt, Ru, Sn) catalysts for initial adsorption and dehydrogenation reaction of methanol in DMFCs.

A QUANTUM STUDY OF THE CHEMICAL FORMATION OF CYANO ANIONS IN INNER CORES AND DIFFUSE REGIONS OF INTERSTELLAR MOLECULAR CLOUDS

The barrierless, exothermic reactions of H – with HCnN cyanopolyynes, with n = 1 and 3, are analyzed using ab initio calculations of the interaction forces. The shape of the reactive potential energy surface suggests the most efficient approach of H – to be on a nearly collinear arrangement on the H-side of HCnN. Using simple transition state formulation of the reaction rates, which are obtained via calculation of the partition functions of each transition state configuration, provides a new non-Langevin behavior of the reaction which can help explain the unexpectedly large density of CN – formation found in observations. A similar procedure is also employed for the reaction of H – with HC 3 N and the differences in the results, indicating a lower efficiency of the latter reactivity compared with that for CN –, are discussed in this paper.

Ligand effect in racemization and dynamic kinetic resolution of alcohols: Mechanism on cymene ruthenium complexes

A family of ruthenium complexes with different ligands was utilized in racemization of (R)-1-phenylethanol to investigate the potential influence of the ligands coordinated to the ruthenium center. Kinetic experiments showed that 16-electron cymene ruthenium complex with two chloro-bridge bonds and 18-electron ones with easily dissociative ligands are highly active for catalytic racemization of alcohols. Possible racemization mechanism for cymene ruthenium complexes was then proposed. Computational analysis of dissociation energy barrier, NBO analysis and reaction potential energy surface suggest that ligand-dissociation process is the vital step of the racemization catalyzed by cymene ruthenium complexes. Thereafter, these complexes were applied in the DKR of secondary alcohols to verify their efficiency and applicability.

Formation of fulvene in the reaction of C2H with 1,3-butadiene

Products formed in the reaction of C2H radicals with 1,3-butadiene at 4Torr and 298K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C6H6. The photoionization spectrum of the C6H6 product indicates that fulvene is formed with a branching fraction of (57±30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene+H and 3,4-dimethylenecyclobut-1-ene+H products are calculated.

Atmospheric gas phase chemistry of CH2═NH and HNC. A first-principles approach.

Quantum chemical methods were used to investigate the OH initiated atmospheric degradation of methanimine, CH2═NH, the major primary product in the atmospheric photo-oxidation of methylamine, CH3NH2. Energies of stationary points on potential energy surfaces of reaction were calculated using multireference perturbation theory and coupled cluster theory. The results show that hydrogen abstraction dominates over the addition route in the CH2═NH + OH reaction, and that the major primary product is HCN, while HNC and CHONH2 are minor primary products. HNC is found to react with OH exclusively via addition to the carbon atom followed by O-H scission leading to HNCO; N2O is not a product in the atmospheric photo-oxidation of HNC. Additional G4 calculations of the CH2═NH + O3 reaction show that this is too slow to be of importance at atmospheric conditions. Rate coefficients for the CH2═NH + OH and HNC + OH reactions were calculated as a function of temperature and pressure using a master equation model based on the coupled cluster theory results. The rate coefficients for OH reaction with CH2═NH and HNC at 1000 mbar and room temperature are calculated to be 3.0 × 10(-12) and 1.3 × 10(-11) cm(3) molecule(-1) s(-1), respectively. The atmospheric fate of CH2═NH is discussed and a gas phase photo-oxidation mechanism is presented.

Theoretical Study of the Rate Coefficients for CH3NHNH2 + NO2 and Related Reactions

ABSTRACTThe kinetics and mechanisms of H atom abstraction reactions from CH3NHNH2 by NO2 (R1) and related reactions have been investigated theoretically by using ωB97X‐D and CCSD(T)‐F12 quantum chemical calculations and the steady‐state unimolecular master equation analysis based on Rice–Ramsperger–Kassel–Marcus (RRKM) theory. For reaction (R1), both dissociation and isomerization steps between intermediate complexes were found to be important for the distribution of the dissociated bimolecular products. The dominant products of (R1) were found to be cis‐CH3NHNH and HONO at lower temperature. The branching ratios for CH3NNH2 formation paths increased with increasing temperature. On the same reaction potential energy surface, six reactions including isomerization reactions between CH3NNH2 and cis‐/trans‐CH3NHNH catalyzed by HONO were suggested to compete with the reverse reaction of (R1). The temperature‐ and pressure‐dependent rate expressions are proposed for kinetic modeling.