Combined crossed beam and theoretical studies of the C(1D) + CH4 reaction

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

OH(2Π) + C2H4 Reaction: A Combined Crossed Molecular Beam and Theoretical Study

The reaction between the ground-state hydroxyl radical, OH(²Π), and ethylene, C₂H₄, has been investigated under single-collision conditions by the crossed molecular beam scattering technique with mass-spectrometric detection and time-of-flight analysis at the collision energy of 50.4 kJ/mol. Electronic structure calculations of the underlying potential energy surface (PES) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of product branching fractions on the derived PES for the addition pathway have been performed. The theoretical results indicate a temperature-dependent competition between the anti-/syn-CH₂CHOH (vinyl alcohol) + H, CH₃CHO (acetaldehyde) + H, and H₂CO (formaldehyde) + CH₃ product channels. The yield of the H-abstraction channel could not be quantified with the employed methods. The RRKM results predict that under our experimental conditions, the anti- and syn-CH₂CHOH + H product channels account for 38% (in similar amounts) of the addition mechanism yield, the H₂CO + CH₃ channel for ∼58%, while the CH₃CHO + H channel is formed in negligible amount (<4%). The implications for combustion and astrochemical environments are discussed.

Direct Observation of Ethylidene (CH3CH), the Elusive High-Energy Isomer of Ethylene

Despite experimental efforts spanning more than 80 years, there has been no direct observation of free ethylidene (CH₃CH), the simplest alkyl-substituted carbene. Here, we report that ethylidene is indefinitely stable in the absence of collisions if produced in the triplet ground state at energies below the threshold for intersystem crossing. Near-UV photolysis of gaseous methylketene, or propenal (followed by isomerization to methylketene), leads to CO loss producing triplet ethylidene, which is detected by photoionization mass spectrometry. Electronically excited singlet ethylidene is also produced, rapidly undergoing isomerization by a 1,2-hydrogen atom shift, producing highly vibrationally excited ethylene. The measured product translational energy distributions verify the theoretically calculated enthalpy of formation of triplet ethylidene and are consistent with a singlet-triplet energy gap of approximately 12.5 kJ/mol.

Universal crossed beam imaging studies of polyatomic reaction dynamics

Crossed-beam imaging studies of polyatomic reactions show surprising dynamics not anticipated by extrapolation from smaller model systems.

Important features of the potential energy surface of the methylamine plus O(1D) reaction

This research presents an ab initio characterization of the potential energy surface for the methylamine plus ¹D oxygen atom reaction, which may be relevant to interstellar chemistry. Geometries and harmonic vibrational frequencies were determined for all stationary points at the CCSD(T)/aug-cc-pVTZ level of theory. The focal point method along with several additive corrections was used to obtain reliable CCSDT(Q)/CBS potential energy surface features. Extensive conformational analysis and intrinsic reaction coordinate computations were performed to ensure accurate chemical connectivity of the stationary points. Five minima were determined to be possible products of this reaction and three novel transition states were found that were previously unreported or mislabeled in the literature. The pathways we present can be used to guide further searches for NH₂ containing species in the interstellar medium.

Low-Temperature Rate Constants and Product-Branching Ratios for the C(1D) + H2O Reaction

The gas-phase reaction between atomic carbon in its first electronically excited ¹D state and water has been studied over the 50-296 K temperature range using a supersonic flow apparatus. C(¹D) atoms were produced by pulsed ultraviolet multiphoton dissociation of carbon tetrabromide; a process that also generates ground-state atomic carbon C(³P). The reaction was followed by detecting product H-atoms by pulsed vacuum ultraviolet laser-induced fluorescence. Two types of experiment were performed. First, temperature-dependent rate constants were derived by recording H-atom formation curves at various gas-phase water concentrations at each temperature. Secondly, temperature-dependent H-atom yields were extracted by comparing the H-atom fluorescence intensities generated by the target C(¹D) + H₂O reaction with those of a reference reaction. The second-order rate constants are large and increase to low temperature, whereas the measured H-atom yields are close to the theoretical maximum value of 2 above 100 K. At 50 K, neither rate constants nor H-atom yields could be derived because of H-atom formation by quantum tunneling in the activated C(³P) + H₂O reaction. The present results are discussed in the context of earlier work on the C(¹D)/C(³P) + H₂O reactions.

An Experimental and Theoretical Investigation of 1-Butanol Pyrolysis

Bioalcohols are a promising family of biofuels. Among them, 1-butanol has a strong potential as a substitute for petrol. In this manuscript, we report on a theoretical and experimental characterization of 1-butanol thermal decomposition, a very important process in the 1-butanol combustion at high temperatures. Advantage has been taken of a flash pyrolysis experimental set-up with mass spectrometric detection, in which the brief residence time of the pyrolyzing mixture inside a short, resistively heated SiC tube allows the identification of the primary products of the decomposing species, limiting secondary processes. Dedicated electronic structure calculations of the relevant potential energy surface have also been performed and RRKM estimates of the rate coefficients and product branching ratios up to 2,000 K are provided. Both electronic structure and RRKM calculations are in line with previous determinations. According to the present study, the H₂O elimination channel leading to 1-butene is more important than previously believed. In addition to that, we provide experimental evidence that butanal formation by H₂ elimination is not a primary decomposition route. Finally, we have experimental evidence of a small yield of the CH₃ elimination channel.

A low temperature investigation of the N(2D) + CH4, C2H6 and C3H8 reactions

Low temperature rate constants for the N(²D) + C₂H₆, C₃H₈ reactions are shown to be much smaller than previously thought.

Low temperature kinetics and theoretical studies of the reaction CN + CH3NH2: a potential source of cyanamide and methyl cyanamide in the interstellar medium

The reaction between cyano radicals (which are ubiquitous in interstellar clouds) and methylamine (a molecule detected in various interstellar sources) has been investigated in a synergistic experimental and theoretical study. The reaction has been found to be very fast in the entire range of temperatures investigated (23-297 K) by using a CRESU apparatus coupled to pulsed laser photolysis - laser induced fluorescence. The global experimental rate coefficient is given by In addition, dedicated electronic structure calculations of the underlying potential energy surface have been performed, together with capture theory and RRKM calculations. The experimental data have been interpreted in the light of the theoretical calculations and the product branching ratio has been established. According to the present study, in the range of temperatures investigated the title reaction is an efficient interstellar route of formation of cyanamide, NH₂CN, another interstellar species. The second most important channel is the one leading to methyl cyanamide, CH₃NHCN (an isomer of aminoacetonitrile), via a CN/H exchange mechanism with a yield of 12% of the global reaction in the entire range of temperatures explored. For a possible inclusion in future astrochemical models we suggest, by referring to the usual expression the following values: α = 3.68 × 10^-12 cm³ molec^-1 s^-1, β = -1.80, γ = 7.79 K for the channel leading to NH₂CN + CH₃; α = 5.05 × 10^-13 cm³ molec^-1 s^-1, β = -1.82, γ = 7.93 K for the channel leading to CH₃NHCN + H.

An experimental and theoretical investigation of the C(1D) + D2 reaction

Rate constants derived from ring polymer molecular dynamics calculations confirm the validity of this method for studying low-temperature complex-forming reactions

Crossed beam polyatomic reaction dynamics: recent advances and new insights

This review summarizes the developments in polyatomic reaction dynamics, focusing on reactions of unsaturated hydrocarbons with O-atoms and methane with atoms/radicals.

State-of-the-Art Thermochemical and Kinetic Computations for Astrochemical Complex Organic Molecules: Formamide Formation in Cold Interstellar Clouds as a Case Study

We describe an integrated computational strategy aimed at providing reliable thermochemical and kinetic information on the formation processes of astrochemical complex organic molecules. The approach involves state-of-the-art quantum-mechanical computations, second-order vibrational perturbation theory, and kinetic models based on capture and transition state theory together with the master equation approach. Notably, tunneling, quantum reflection, and leading anharmonic contributions are accounted for in our model. Formamide has been selected as a case study in view of its interest as a precursor in the abiotic amino acid synthesis. After validation of the level of theory chosen for describing the potential energy surface, we have investigated several pathways of the OH + CH2NH and NH2 + H2CO reaction channels. Our results show that both reaction channels are essentially barrierless (in the sense that all relevant transition states lie below or only marginally above the reactants) and once tunneling is taken into the proper account indicate that the reaction can occur under the low temperature conditions of interstellar environments.

Isomer-Specific Chemistry in the Propyne and Allene Reactions with Oxygen Atoms: CH3CH + CO versus CH2CH2 + CO Products

We report direct experimental and theoretical evidence that, under single-collision conditions, the dominant product channels of the O((3)P) + propyne and O((3)P) + allene isomeric reactions lead in both cases to CO formation, but the coproducts are singlet ethylidene ((1)CH3CH) and singlet ethylene (CH2CH2), respectively. These data, which settle a long-standing issue on whether ethylidene is actually formed in the O((3)P) + propyne reaction, suggest that formation of CO + alkylidene biradicals may be a common mechanism in O((3)P) + alkyne reactions, in contrast to formation of CO + alkene molecular products in the corresponding isomeric O((3)P) + diene reactions, either in combustion or other gaseous environments. These findings are of fundamental relevance and may have implications for improved combustion models. Moreover, we predict that the so far neglected (1)CH3CH + CO channel is among the main reaction routes also when the C3H4O singlet potential energy surface is accessed from the OH + C3H3 (propargyl) entrance channel, which are radical species playing a key role in many combustion systems.

Quasiclassical Trajectory Calculations of the N((2)D) + H2O Reaction Elucidating the Formation Mechanism of HNO and HON Seen in Molecular Beam Experiments

The N((2)D) + H2O is a reaction with competitive product channels, passing through several intermediates. Dynamics of this reaction had been investigated by two of the present authors at two collision energies, Ec, using the crossed molecular beams mass spectrometric method ( Faraday Discuss. 2001 , 119 , 27 - 49 ). The complicated mechanism of this reaction and puzzling results encouraged us to investigate the reaction in a joint experimental/theoretical study. Quasiclassical trajectory (QCT) calculations on an ab initio potential energy surface describing all channels of the title reaction are done with a focus on the N/H exchange channels. Interesting results of QCT calculations, in very good agreement with experimental data, reveal subtle details of the reaction dynamics of the title reaction to HNO/HON + H exit channels by disentangling the different routes to formation of the two possible HNO/HON isomers and therefore assisting in a critical manner the derivation of the reaction mechanism. Results of the present study show that the nonstatistical HNOH intermediate governs exit channels; therefore, the HON channel is as important as that of HNO. The study also confirms that the H2 + NO molecular channel is negligible even though the barrier to its formation is calculated to be well below the reactant asymptote.