Tight Transition State Research Articles

Dual-level strategy for quantitative kinetics for the reaction between ethylene and hydroxyl radical.

The atmospheric reactions are mainly initiated by hydroxyl radical (OH). Here, we choose the C2H4 + OH reaction as a model reaction for other reactions of OH with alkenes. We use the GMM(P).L//CCSD(T)-F12a/cc-pVTZ-F12 theoretical method as the benchmark results close to the approximation of CCSDTQ(P)/CBS accuracy to investigate the C2H4 + OH reaction. The rate constants for the C2H4 + OH reaction at high-pressure limit were calculated by using the dual-level strategy. It integrates the transition state theory rate constant calculated by GMM(P).L//CCSD(T)-F12a/cc-pVTZ-F12 with the canonical variational transition state theory containing small-curvature tunneling (CVT/SCT) calculated by using the M11-L functional method with the MG3S basis set. The rate constants of C2H4 + OH at different pressures were obtained by using both the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory and master equation method. The calculated results uncover that both the calculated rate constants at different pressures and temperatures are quantitatively consistent with the values obtained by the experimental measurements in the C2H4 + OH reaction. We find that the post-CCSD(T) contributions to the barrier height for the C2H4 + OH reaction are significant with the calculated value of -0.38 kcal/mol. We also find that the rate determining step is only dominated by the tight transition state under atmospheric conditions, whereas previous investigations indicated that the rate constants were controlled by both the loose and tight transition states in the C2H4 + OH reaction. The present findings unravel that it is an important factor for the effect of torsional anharmonicity on quantitative kinetics.

Experimental and modeling study of the N, N-dimethylformamide pyrolysis at atmospheric pressure

As a polar solvent, N, N-Dimethylformamide (DMF) is a potential source of volatile organic compounds during industrial processing. The potential energy surface and pressure-dependent rate coefficients of DMF primary and secondary thermal decomposition were theoretically investigated by automatic ab initio calculations. A singlet aminohydroxycarbene route generating CO is found to be the most energetically favored channel among the unimolecular initiation reactions with a tight transition state. A kinetic model was proposed for DMF pyrolysis prediction and validated against the speciation data newly obtained from synchrotron vacuum ultraviolet photoionization mass spectrometry in a flow reactor within 773 K–1048 K and literature data in a jet stirred reactor within 450 K–900 K at atmospheric pressure. Satisfactory agreements were obtained for the major intermediates and products, but the model failed to capture the formation of some oxygenated compounds, possibly due to the missing reaction pathway for bimolecular addition. During pyrolysis, DMF is mainly consumed by the hydrogen abstraction reaction as the relatively high CN bond energy hinders the barrierless homolytic cleavage. Unlike aldehydes where the carbonyl site is favored, H-abstraction from DMF methyl sites is prevalent, although the acylamino group readily undergoes decarbonylation and β-scission forming oxygenated and nitrogenous parts. Consequently, CO and HCN are the major pyrolysis products. Besides, by incorporating essential high temperature kinetics, extended validation was conducted against the available intermediate temperature autoignition delay times (1000–1350 K, 2 bar) and laminar burning velocity data, and the reason for the discrepancy between the model prediction and experiments was inferred according to the sensitivity analyses.

Kinetic isotope effect study of N-6 methyladenosine chemical demethylation in bicarbonate-activated peroxide system.

N-6 methyladenosine is the most abundant nucleic acid modification in eukaryotes and plays a crucial role in gene regulation. The AlkB family of alpha-ketoglutarate-dependent dioxygenases is responsible for nucleic acid demethylation. Recent studies have discovered that a chemical demethylation system using hydrogen peroxide and ammonium bicarbonate can effectively demethylate nucleic acids. The addition of ferrous ammonium sulfate boosts the oxidation rate by forming a Fenton reagent with hydrogen peroxide. However, the specific mechanism and key steps of this process remain unclear. In this study, we investigate the influence of ferrous ammonium sulfate concentration on the kinetic isotope effect (KIE) of the chemical demethylation system using LC-MS. As the concentration of ferrous ions increases, the observed KIE decreases from 1.377 ± 0.020 to 1.120 ± 0.016, indicating a combination of the primary isotope effect and inverse α-secondary isotope effect with the ion pairing effect. We propose that the initial hydrogen extraction is the rate-limiting step and observe a tight transition state structure in the formation of the hm6A process through the analysis of KIE trends. The concentration-dependent KIE provides a novel perspective on the mechanism of chemical demethylation and offers a chemical model for enzyme-catalyzed demethylation.

Polyrate 2023: A computer program for the calculation of chemical reaction rates for polyatomics. New version announcement

Polyrate 2023: A computer program for the calculation of chemical reaction rates for polyatomics. New version announcement

Kinetics of Sulfur Trioxide Reaction with Water Vapor to Form Atmospheric Sulfuric Acid.

Although experimental methods can be used to obtain the quantitative kinetics of atmospheric reactions, experimental data are often limited to a narrow temperature range. The reaction of SO3 with water vapor is important for elucidating the formation of sulfuric acid in the atmosphere; however, the kinetics is uncertain at low temperatures. Here, we calculate rate constants for reactions of sulfur trioxide with two water molecules. We consider two mechanisms: the SO3···H2O + H2O reaction and the SO3 + (H2O)2 reaction. We find that beyond-CCSD(T) contributions to the barrier heights are very large, and multidimensional tunneling, unusually large anharmonicity of high-frequency modes, and torsional anharmonicity are important for obtaining quantitative kinetics. We find that at lower temperatures, the formation of the termolecular precursor complexes, which is often neglected, is rate-limiting compared to passage through the tight transition states. Our calculations show that the SO3···H2O + H2O mechanism is more important than the SO3 + (H2O)2 mechanism at 5-50 km altitudes. We find that the rate ratio between SO3···H2O + H2O and SO3 + (H2O)2 is greater than 20 at altitudes between 10 and 35 km, where the concentration of SO3 is very high.

Theoretical Investigation of Rate Rules for H-Intermigration Reactions for Cyclic Alkylperoxy Radicals

As a starting channel, the H-intermigration reaction of alkylperoxy radicals (ROO radicals) that yields hydroperoxyl alkyl radicals (QOOH radicals) determines the low-temperature chemistry of alkanes. In this work, this type of reaction was investigated for typical cyclic alkanes, which are important fuel components and soot precursors, using theoretical ab initio methods. First, all the molecular geometries and vibrational frequencies were computed using the density functional theory method and the single point energies were refined using the post-Hartree fork method (M062X/6-311G(d,p)//DLPNO-CCSD(T)/CBS). Then, high-pressure limit rate constants were evaluated with tight transition state theory, with which tunneling effects were considered using the Eckart model and low-frequency torsion modes were modeled as hindered rotors. Pressure-dependent rate constants were also calculated for typical reaction channels. Rate expressions in the Arrhenius form for 91 reactions are proposed. All reactions were categorized into seven reaction types and the rate rule for each reaction type was estimated with uncertainty factors of three to six. These rules can be potentially used in the development of low-temperature kinetic mechanisms for cycloalkanes. A comparison between different reaction types was also performed and the favorable channels are discussed.

Asymmetric Synthesis of Vicinal Tetrasubstituted Diamines via Reductive Coupling of Ketimines Templated by Chiral Diborons.

We herein describe the chiral diboron-templated asymmetric homocoupling of aryl alkyl ketimines, providing for the first time a series of chiral vicinal tetrasubstituted diamines with excellent ee values and good to high yields. The powerful and efficient diboron-participated [3,3]-sigmatropic rearrangement is successfully demonstrated by the homocoupling of a variety of ketimines thanks to the rational design and engineering of chiral diborons. Systematic DFT studies suggest that two chiral diborons adopt different conformational assembling strategies to couple the diboron template with ketimine substrates in their tight concerted transition states to ensure the excellent enantioselectivities. The synthetic value of chiral vicinal tetrasubstituted diamines is demonstrated by the asymmetric α-bromination of aliphatic aldehydes by employing a chiral vicinal tetrasubstituted diamine-based organocatalyst.

Asymmetric Synthesis of Vicinal Tetrasubstituted Diamines via Reductive Coupling of Ketimines Templated by Chiral Diborons

AbstractWe herein describe the chiral diboron‐templated asymmetric homocoupling of aryl alkyl ketimines, providing for the first time a series of chiral vicinal tetrasubstituted diamines with excellent ee values and good to high yields. The powerful and efficient diboron‐participated [3,3]‐sigmatropic rearrangement is successfully demonstrated by the homocoupling of a variety of ketimines thanks to the rational design and engineering of chiral diborons. Systematic DFT studies suggest that two chiral diborons adopt different conformational assembling strategies to couple the diboron template with ketimine substrates in their tight concerted transition states to ensure the excellent enantioselectivities. The synthetic value of chiral vicinal tetrasubstituted diamines is demonstrated by the asymmetric α‐bromination of aliphatic aldehydes by employing a chiral vicinal tetrasubstituted diamine‐based organocatalyst.

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (C6H5) with propargyl radical (H2CCCH) provides an important prototype for radical-radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300-1000 K and pressure range of 4-10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the C9H8 and C9H7 + H product channels and report experimental isomer-resolved product branching fractions for the C9H8 product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These ab initio transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical-radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products C9H7 + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (C9H7) recombination to indene and H-assisted isomerization that converts less stable C9H8 isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons.

Competing Si2CH4-H2 and SiCH2-SiH4 Channels in the Bimolecular Reaction of Ground-State Atomic Carbon (C(3Pj)) with Disilane (Si2H6, X1A1g) under Single Collision Conditions.

The bimolecular gas-phase reaction of ground-state atomic carbon (C(3Pj)) with disilane (Si2H6, X1A1g) was explored under single-collision conditions in a crossed molecular beam machine at a collision energy of 36.6 ± 4.5 kJ mol-1. Two channels were observed: a molecular hydrogen elimination plus Si2CH4 (reaction 1) pathway and a silane loss channel along with the formation of SiCH2 (reaction 2), with branching ratios of 20 ± 3 and 80 ± 4%, respectively. Both channels involved indirect scattering dynamics via long-lived Si2CH6 reaction intermediate(s); the latter eject molecular hydrogen and silane in "molecular" elimination channels within the rotational plane of the fragmenting intermediate nearly perpendicularly to the total angular momentum vector. These molecular elimination channels are associated with tight exit transition states as reflected in a significant electron rearrangement as visible from the chemical bonding in the light reaction products molecular hydrogen and silane. Once these hydrogenated silicon-carbide clusters are formed within the inner envelope of carbon stars such as of IRC + 10216, the stellar wind can drive both Si2CH4 and SiCH2 to the outside sections of the envelope, where they can be photolyzed. This is of particular importance to unravel potential formation pathways to disilicon monocarbide (Si2C) observed recently in the circumstellar shell of IRC + 10216.

Reaction kinetics and implications of the decomposition and formation of C2H4O isomers

Acetaldehyde, ethylene oxide, and vinyl alcohol are three C2H4O isomers that are ubiquitous in combustion over a wide temperature range. A comprehensive reaction kinetics study has been performed for reactions that occur on the same C2H4O potential energy surface, mainly including the unimolecular reactions of three C2H4O isomers and the recombination reaction of CH3 with HCO. Temperature- and pressure-dependent rate coefficients were updated or newly predicted by performing quantum chemical calculations coupled with RRKM/master equation simulations. The dominance of CC bond fission is reconfirmed for acetaldehyde decomposition at higher temperatures. The negligible contribution of CH4 + CO channel from usual tight intramolecular H-shift transition state provides indirect evidence for previously reported roaming mechanism that was proposed to explain the observation of CH4 + CO in acetaldehyde pyrolysis. For ethylene oxide pyrolysis, current experiment and kinetic prediction have a good consistency in branching ratios between the formation of acetaldehyde and vinyl alcohol products. The radicals of CH3 + HCO can also be directly generated from ethylene oxide through a well-skipping reaction mechanism especially at higher temperatures. For CH3 + HCO recombination, we highlight the importance of CH3CO + H channel that was often overlooked previously. Kinetic simulations show that the incorporation of updated rate coefficients and newly identified channels results in a measurable influence on C2 chemistry, indicating the possibility and necessity of further improvement of core mechanism in combustion kinetic models.

Collaborative control of branching ratio in the O + HO2 → OH + O2 reaction via vibrational and rotational excitation

AbstractTo understand the effect of different vibrational and rotational modes of reactant to enhance the reactivity of the O + HO2 → OH + O2 reaction, we revisited this important atmospheric reaction. We report here a quasi‐classical trajectory (QCT) study of the reaction dynamics on a recently developed full‐dimensional potential energy surface (PES). Our previous work has indicated that this reaction has two pathways, the H abstraction (HA) channel and the O abstraction (OA) channel, which lead to totally different product energy distribution. In this work, we identified that the vibrational excitation of the OH stretching (v1) mode of HO2 is the switch of the HA channel at low collision energy; meanwhile, the rotational excitation can also greatly change the branching ratio of the two pathways. With the excitation of v1 mode, the original negligible HA channel controlled by the tight transition state becomes quite important. This work presents an approach to control the branching ratio via collaboration between vibrational and rotational excitation and will enrich the knowledge of the O + HO2 reaction in atmospheric chemistry and physics.

VUV photoprocessing of oxygen-containing polycyclic aromatic hydrocarbons: iPEPICO study of the unimolecular dissociation of ionized benzofuran

Oxygen-containing polycyclic aromatic hydrocarbons (OPAHs) are potential contributors to the 11.3 µm band in interstellar observations. To further explore their role in the interstellar medium, we investigated their fate after photoprocessing by VUV radiation; in particular, we studied the dissociative photoionization of the simplest OPAH, benzofuran, with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. Ionized benzofuran dissociates by loss of CO, followed by a sequential H-atom loss. The parallel HCO-loss channel, leading to the same bicyclic C7H5+ fragment ion, is not competitive at low excess energies above the ionization threshold. However, the collision-induced dissociation tandem mass spectrometry results suggest that CO and HCO may be formed in parallel at higher energies. A Rice–Ramsperger–Kassel–Marcus fragmentation model reproduced the iPEPICO data well, assuming the initial 1,2-H shift transition state to be rate determining to CO loss. The breakdown diagram and the measured dissociation rates agreed well at the CBS-QB3-calculated activation energy of 2.99 eV, which could be relaxed to 3.25 eV, and only a slight adjustment of the ab initio activation entropy. The model barrier to sequential H loss is larger than the computed H-loss threshold and the breakdown diagram rises less steeply than predicted, which indicates suprastatistical kinetic energy release after the tight H-transfer transition state of the first step. HCO cleavage is possible after a ring-opening transition state, which is looser than and isoenergetic with the CO-loss transition state. However, a subsequent ring formation transition state at 3.85 eV is moderately tight, which suppresses HCO loss at low excess energies.

Thermal decomposition of isopentanol: A theoretical calculation and kinetic modeling analysis

Isopentanol (3-methyl-1-butanol, 3M1B) is a prospective bioderived fuel with advantages over ethanol that make it a suitable substitute for gasoline. Although there have been studies on the combustion properties of 3M1B, reaction kinetics for its thermal decomposition is scarce. This study explores the reaction pathways for the thermal decomposition of 3M1B through theoretical calculations. Six reactions with tight transition states, and nine bond dissociation reactions (i.e., barrierless reactions) were considered. For reactions with tight transition states, the CCSD(T)/CBS//M06–2X-D3(0)/def2-TZVP method was used, and the MRCISD(+Q)/CBS//CASPT2/cc-pVDZ method was adopted for the bond dissociation reactions. The conventional transition state theory (CTST), including multi-structural torsional anharmonicity (MS-T) correction, addressed those reactions with tight transition states. The MS-T method thoroughly examined the co-effect of the multiple structures and torsional anharmonicity. Direct bond-breaking reactions were studied using the variational transition state theory (VTST). Subsequently, system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory and Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) were employed to obtain the rate constants for reactions with tight transition states and direct bond-breaking reactions at different temperatures and pressures (T/P) regimes (T = 298–2400 K, P = 0.01–100 atm), respectively. The results showed that water elimination and C-C bond dissociation play a significant role for 3M1B unimolecular decomposition, other molecular elimination and bond dissociation channels can be ignored. Combining the reaction pathways and the corresponding pressure dependent rate constants into the kinetic model resulted in good predictions of the speciation profiles during 3M1B pyrolysis. These data are valuable for the development of the combustion models of 3M1B.

A theoretical study on the mechanism and kinetics of the Dimeric TrimethylAluminum + O2 reaction in the gas phase: A potential chain-initiation in the hypergolic combustion of TMA in air

The mechanism and kinetics of the (CH3)6Al2 + O2 reaction and some associated reactions were studied at the CCSD(T)//B3LYP/6–311++(3df,2p) level of theory as well as VTST and RRKM Master Equation calculations. The predicted potential energy surface reveals that the reaction of O2 with TMA dimer has two pathways: one proceeds via a tight transition state with the energy barrier of 27.1 kcal.mol−1 and the other occurs via a loose transition state above the reactants by 9.5 kcal.mol−1, resulting in the formation of the major products, (CH3)2AlOAl(CH3)2OCH3 + CH3. The former, larger radical product was found to be unstable under the ambient condition; it readily fragments to give (CH3)2AlOAl(CH3)OCH3 + CH3 barrierlessly with a small endothermicity, 3.3 kcal.mol−1. (CH3)2AlOAl(CH3)OCH3 was, however, found to be quite stable; its most favorable reaction path taking place by a four-member ring transition state producing CH3-cyc-Al(O)(CH2)Al-CH3 and CH3OH has an energy barrier of 48 kcal.mol−1. The equilibrium constants for the (TMA)2 ⇌ 2TMA reaction at temperatures 100.3, 115, and 155.7 °C were predicted to be 33.3, 92.2, and 1023.7 atm, respectively, agreeing satisfactorily with experimental values, 28.3, 76.7, and 931 atm, respectively, measured by Laubengayer et al. (J. Am. Chem. Soc. 1941, 63, 477). The temperature-dependent rate constants for three investigated reactions have been computed in the range of 300–2000 K and 760 Torr N2. A least-squares analysis of the result for the (TMA)2 + O2 reaction, presented with the modified Arrhenius form, can be written by the expression:k(T) = 2.64 × 10-21T3.08 exp(-9.50 kcal.mol−1/RT) cm3 molecule-1 s−1; T = 300 – 2000 K.

Gas-Phase Study of the Elementary Reaction of the D1-Ethynyl Radical (C2D; X2Σ+) with Propylene (C3H6; X1A′) under Single-Collision Conditions

The bimolecular gas-phase reactions of the D1-ethynyl radical (C2D; X2Σ+) with propylene (C3H6; X1A') and partially substituted D3-3,3,3-propylene (C2H3CD3; X1A') were studied under single collision conditions utilizing the crossed molecular beams technique. Combining our laboratory data with electronic structure and statistical calculations, the D1-ethynyl radical is found to add without barrier to the C1 and C2 carbons of the propylene reactant, resulting in doublet C5H6D intermediate(s) with lifetime(s) longer than their rotational period(s). These intermediates undergo isomerization and unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming predominantly cis/trans-3-penten-1-yne ((HCC)CH═CH(CH3)) and, to a minor amount, 3-methyl-3-buten-1-yne ((HCC)C(CH3)═CH2) via overall exoergic reactions. Although the title reaction does not lead to the cyclopentadiene molecule (c-C5H6, X1A1), high-temperature environments can convert the identified acyclic C5H6 isomers through hydrogen atom assisted isomerization to cyclopentadiene (c-C5H6, X1A1). Since both the ethynyl radical and propylene reactants have been observed in cold interstellar environments such as TMC-1 and the reaction is exoergic and all barriers lie below the energy of the separated reactants, these C5H6 product isomers are predicted to form in those low-temperature regions.

Theoretical Investigations for Kinetics of the Chemical Reactions: H + SiClx (x = 1, 2, 3).

Hydrogen atoms and SiClx (x = 1, 2, 3) radicals coexist during the hydrogenation of silicon tetrachloride (STC, SiCl4), an important process in the fabrication of industrial polysilicon. In this work, the mechanisms and kinetics of the reactions between H and SiClx (x = 1, 2, 3) were studied by theory. The structures and vibrational frequencies of reactants, products, intermediates, and transition states (TSs) were determined at the B2PLYP/may-cc-pVTZ level. The single-point energies of minima and saddle points were refined using the coupled-cluster single-double with triple perturbative (CCSD(T)) with the complete basis set extrapolation method. Some special treatments were designed to obtain reliable wave functions for unimolecular reactions without tight TSs by the density functional theory. Subsequently, Lennard-Jones (L-J) parameters between each intermediate (SiHClx) and bath gas (He) were obtained at the MP2/jul-cc-pVTZ level to derive reliable temperature- and pressure-dependent rate coefficients for unimolecular reactions according to the variational Rice-Ramsperger-Kassel-Marcus theory. For bimolecular reactions, rate coefficients were determined by the variational transition-state theory. The rate coefficients of barrierless reactions were derived based on the loose TSs with the maximum free energy. Finally, the master equation analysis was used to investigate the variation of the rate coefficients with pressure and temperature in the activated paths.

The unimolecular decomposition of dimethoxymethane: channel switching as a function of temperature and pressure.

Branching ratios of competing unimolecular reactions often exhibit a complicated temperature and pressure dependence that makes modelling of complex reaction systems in the gas phase difficult. In particular, the competition between steps proceeding via tight and loose transition states is known to present a problem. A recent example from the field of combustion chemistry is the unimolecular decomposition of CH3OCH2OCH3 (DMM), which is discussed as an alternative fuel accessible from sustainable sources. It is shown by a detailed master equation analysis with energy- and angular-momentum-resolved specific rate coefficients from RRKM theory and from the simplified statistical adiabatic channel model, how channel switching of DMM depends on temperature and pressure, and under which experimental conditions which channels prevail. The necessary molecular and energy data were obtained from quantum-chemical calculations at the CCSD(F12*)(T*)/cc-pVQZ-F12//B2PLYP-D3/def2-TZVPP level of theory. A parameterization describing the channel branching over extended ranges of temperature and pressure is derived, and the model is used to simulate shock tube experiments with detection by atomic resonance absorption spectroscopy and time-of-flight mass spectrometry. The agreement between the simulated and experimental concentration-time profiles is very good. The temperature and pressure dependence of the channel branching is rationalized, and the data are presented in a form that can be readily implemented into DMM combustion models.

A crossed molecular beams and computational study on the unusual reactivity of banana bonds of cyclopropane (c-C3H6; ) through insertion by ground state carbon atoms (C(3Pj)).

The mechanism and chemical dynamics of the reaction of ground electronic state atomic carbon C(3Pj) with cyclopropane c-C3H6 have been explored by combining crossed molecular beams experiments with electronic structure calculations of the pertinent triplet C4H6 potential energy surface and statistical computations of product branching ratios under single-collision conditions. The experimental findings suggest that the reaction proceeds via indirect scattering dynamics through triplet C4H6 reaction intermediate(s) leading to C4H5 product(s) plus atomic hydrogen via a tight exit transition state, with the overall reaction exoergicity evaluated as 231 ± 52 kJ mol-1. The calculations indicate that C(3Pj) can easily insert into one of the three equivalent C-C 'banana' bonds of cyclopropane overcoming a low barrier of only 2 kJ mol-1 following the formation of a van der Waals reactant complex stabilized by 15 kJ mol-1. The carbon atom insertion into one of the six C-H bonds is also feasible via a slightly higher barrier of 5 kJ mol-1. These results highlight an unusual reactivity of cyclopropane's banana C-C bonds, which behave more like unsaturated C-C bonds with a π-character than saturated σ C-C bonds, which are known to be generally unreactive toward the ground electronic state atomic carbon such as in ethane (C2H6). The statistical theory predicts the overall product branching ratios at the experimental collision energy as 50% for 1-butyn-4-yl, 33% for 1,3-butadien-2-yl, i-C4H5, and 11% for 1,3-butadien-1-yl, n-C4H5, with i-C4H5 (230 kJ mol-1 below the reactants) favored by the C-C insertion providing the best match with the experimentally observed reaction exoergicity. The C(3Pj) + c-C3H6 reaction is predicted to be a source of C4H5 radicals under the conditions where its low entrance barriers can be overcome, such as in planetary atmospheres or in circumstellar envelopes but not in cold molecular clouds. Both i- and n-C4H5 can further react with acetylene eventually producing the first aromatic ring and hence, the reaction of the atomic carbon with c-C3H6 can be considered as an initial step toward the formation of benzene.

Quantum resonances and roaming dynamics in formaldehyde photodissociation.

The unimolecular dissociation of formaldehyde is studied via excitation to the à band at several excitation energies from just below the ground state radical dissociation threshold to 5000 cm-1 above it. CO product rotational distributions, photofragment excitation spectroscopy and state-correlated slice imaging results are combined with quasi-classical trajectory calculations to reveal manifestations of quantum effects in this complex dissociation process involving interactions among radical, molecular, and roaming pathways. Evidence of nodal structure at the tight transition state to molecular products is investigated and correlations between the CO rotational and H2 vibrational distributions are used to suggest the transition state modes that are responsible. A large modulation of the roaming yield previously identified and associated with roaming resonances at the onset of the H + HCO(v1,v2,v3 = 0,0,0) product channel suggests a similar origin for enhanced roaming and a roaming yield that is strongly dependent on parent rotation on the 2641 band just 15 cm-1 above the H + HCO(0,2,1) threshold. Similar resonances are predicted on other bands that share near coincident energies with HCO product vibrational thresholds.