Methanediol from cloud-processed formaldehyde is only a minor source of atmospheric formic acid.

Atmospheric formic acid is severely underpredicted by models. A recent study proposed that this discrepancy can be resolved by abundant formic acid production from the reaction (1) between hydroxyl radical and methanediol derived from in-cloud formaldehyde processing and provided a chamber-experiment-derived rate constant, k1 = 7.5 × 10-12 cm3 s-1. High-level accuracy coupled cluster calculations in combination with E,J-resolved two-dimensional master equation analyses yield k1 = (2.4 ± 0.5) × 10-12 cm3 s-1 for relevant atmospheric conditions (T = 260-310 K and P = 0-1 atm). We attribute this significant discrepancy to HCOOH formation from other molecules in the chamber experiments. More importantly, we show that reversible aqueous processes result indirectly in the equilibration on a 10 min. time scale of the gas-phase reaction [Formula: see text] (2) with a HOCH2OH to HCHO ratio of only ca. 2%. Although HOCH2OH outgassing upon cloud evaporation typically increases this ratio by a factor of 1.5-5, as determined by numerical simulations, its in-cloud reprocessing is shown using a global model to strongly limit the gas-phase sink and the resulting production of formic acid. Based on the combined findings in this work, we derive a range of 1.2-8.5 Tg/y for the global HCOOH production from cloud-derived HOCH2OH reacting with OH. The best estimate, 3.3 Tg/y, is about 30 times less than recently reported. The theoretical equilibrium constant Keq (2) determined in this work also allows us to estimate the Henry's law constant of methanediol (8.1 × 105 M atm-1 at 280 K).

Mechanism, thermochemistry, and kinetics of the reversible reactions: C2H3 + H2 ⇌ C2H4 + H ⇌ C2H5.

High-level coupled cluster theory, in conjunction with Active Thermochemical Tables (ATcT) and E,J-resolved master equation calculations, was used in a study of the title reactions, which play an important role in the combustion of hydrocarbons. In the set of radical/radical reactions leading to soot formation in flames, the addition of H-atoms to alkenes is likely a common reaction, triggering the isomerization of complex hydrocarbons to aromatics. The heats of formation of C2H3, C2H4, and C2H5 are established to be 301.26 ± 0.30 at 0 K (297.22 ± 0.30 at 298 K), 60.89 ± 0.11 (52.38 ± 0.11), and 131.38 ± 0.22 (120.63 ± 0.22) kJ mol-1, respectively. The calculated rate constants from first principles agree well with experiments where they are available. Under conditions typical of high temperature combustion - where experimental work is very challenging with a consequent dearth of accurate data - we provide high-level theoretical results for kinetic modeling.

High-accuracy first-principles-based rate coefficients for the reaction of OH and CH3OOH.

The ˙OH-initiated oxidation of methyl hydroperoxide, which plays an important role in the atmospheric chemistry of methane, was theoretically characterized using high-accuracy composite amHEAT-345(Q) coupled-cluster calculations followed by a two-dimensional E,J resolved master equation analysis. The reaction is found to proceed through two distinct hydrogen-bonded pre-reactive complexes leading to two product channels, in accord with the experimental observations: (i) ˙OH + CH3OOH → CH3OO˙ + H2O with a yield of 0.8 ± 0.1, and (ii) ˙OH + CH3OOH → HCHO + ˙OH + H2O with a yield of 0.2 ± 0.1. The calculated reaction enthalpies are within 0.2 kcal mol-1 of the benchmark ATcT values. Overall thermal rate coefficients obtained from first principles are found to be in the low-pressure limit at atmospheric pressure; the total rate coefficient can be expressed over the T = 200-450 K range as k(T) = 5.0 × 10-12 × T-0.152 × exp(287/T) cm3 s-1, strongly supporting the experimental results of Vaghjiani and Ravishankara (J. Phys. Chem. 1989, 93, 1948), with which this expression agrees within ca. 15%. The current results show that (i) is the principal reaction channel and support the view that, due to its inherently fast transformations, CH3OOH is an important redistribution species for HOx˙ radicals in the Earth's atmosphere.

Reaction of Methylidyne with Ethane: The C-C Insertion Is Unimportant.

High-accuracy coupled-cluster calculations in combination with the E,J-resolved master-equation analysis are used to study the reaction mechanism and kinetics of methylidyne with ethane. This reaction plays an important role in the combustion of hydrocarbon fuels and in interstellar chemistry. Two distinct mechanisms, the C-C and the C-H insertions of CH in C2H6, are characterized. The C-C insertion pathway is identified to have a large barrier of 34.5 kcal mol-1 and hence plays no significant role in kinetics. The C-H insertion pathway is found to have no barrier, leading to a highly vibrationally excited n-C3H7 radical, which rapidly dissociates (within 50 ps) to yield CH3 + C2H4 and H + C3H6 in a roughly 7:3 ratio. These findings are in good agreement with an experimental result that indicates that about 20% of the reaction goes to H + C3H6. The reaction of the electronically excited quartet state of the CH radical with C2H6 is examined for the first time and found to proceed as a direct H-abstraction via a small barrier of 0.4 kcal mol-1 to yield triplet CH2 and C2H5. The reaction on the quartet state surface is negligibly slow at low temperatures characteristic of interstellar environments but becomes important at high combustion temperatures.

The CH(X2Π) + H2O reaction: two transition state kinetics.

The reaction of ground state methylidyne (CH) with water vapor (H2O) is theoretically re-investigated using high-level coupled cluster computations in combination with semi-classical transition state theory (SCTST) and two-dimensional master equation simulations. Insertion of CH into a H-O bond of H2O over a submerged barrier via a well-skipping mechanism yielding solely H and CH2O is characterized. The reaction kinetics is effectively determined by the formation of a pre-reaction van der Waals complex (PRC, HC-OH2) and its subsequent isomerization to activated CH2OH in competition with PRC re-dissociation. The tunneling effects are found to be minor, while variational effects in the PRC → CH2OH step are negligible. The calculated rate coefficient k(T) is nearly pressure-independent, but strongly depends on temperature with pronounced down-up behavior: a high value of 2 × 10-10 cm3 s-1 at 50 K, followed by a fairly steep decrease down to 8 × 10-12 cm3 s-1 at 900 K, but increasing again to 5 × 10-11 cm3 s-1 at 3500 K. Over the T-range of this work, k(T) can be expressed as: k(T, P = 0) = 2.31 × 10-11 (T/300 K)-1.615 exp(-38.45/T) cm3 s-1 for T = 50-400 K k(T, P = 0) = 1.15 × 10-12 (T/300 K)0.8637 exp(892.6/T) cm3 s-1 for T = 400-1000 K k(T, P = 0) = 4.57 × 10-15 (T/300 K)3.375 exp(3477.4/T) cm3 s-1 for T = 1000-3500 K.

Thermal Decomposition of CH3O: A Curious Case of Pressure-Dependent Tunneling Effects.

The thermal unimolecular decomposition of a methoxy radical (CH3O), a key intermediate in the combustion of methane, methanol, and other hydrocarbons, was studied using high-level coupled-cluster calculations, followed by E,J-resolved master equation analyses. The experimental results available for a wide range of temperature and pressure are in striking agreement with the calculations. In line with a previous theoretical study that used a one-dimensional master equation, the tunneling correction is found to exhibit a marked pressure dependence, being the largest at low pressure. This curious effect on the tunneling enhancement also affects the calculated kinetic isotope effect, which falls initially with pressure but is predicted to rise again at high pressures. These findings serve to reconcile a set of conflicting results regarding the importance of tunneling in this prototype unimolecular reaction and also motivate further experimental investigation. This study also exemplifies how changes in the energy redistribution due to collisions manifest in the tunneling rates.

Pragmatic Solution for a Fully E,J-Resolved Master Equation.

Including the effects of total angular momentum is essential to determine highly accurate pressure-dependent phenomenological rate coefficients when there are significant changes of rotational constants along the reaction coordinate. In this work, a deterministic (matrix) method has been used to solve a completely E,J-resolved two-dimensional master equation (2DME) for reaction systems that have many intermediates and many products. The practicality of the method is due to the need to obtain just a few eigenvalues and corresponding eigenvectors. Three examples are provided in order to test the performance of the implementation. It is found that the impact of rotational energy transfer via collisions on a loose transition state (TS) is more noticeable than a tight TS. The calculated results are then compared with those obtained from a recent fixed-J 2DME model.

A master equation simulation for the •OH + CH3OH reaction.

A combined (fixed-J) two-dimensional master-equation/semi-classical transition state theory/variational Rice-Ramsperger-Kassel-Marcus approach has been used to compute reaction rate coefficients of •OH with CH3OH over a wide range of temperatures (10-2500 K) and pressures (10-1-104 Torr) based on a potential energy surface that has been constructed using a modification of the high accuracy extrapolated ab initio thermochemistry (HEAT) protocol. The calculated results show that the title reaction is nearly pressure-independent when T > 250 K but depends strongly on pressure at lower temperatures. In addition, the preferred mechanism and rate constants are found to be very sensitive to temperature. The reaction pathway CH3OH + •OH → CH3O• + H2O proceeds exclusively through tunneling at exceedingly low temperatures (T ≤ 50 K), typical of those established in interstellar environments. In this regime, the rate constant is found to increase with decreasing temperature, which agrees with low-temperature experimental results. The thermodynamically favored reaction pathway CH3OH + •OH → •CH2OH + H2O becomes dominant at higher temperatures (T ≥ 200 K), such as those found in Earth's atmosphere as well as combustion environments. By adjusting the ab initio barrier heights slightly, experimental rate constants from 200 to 1250 K can be satisfactorily reproduced.

High-accuracy extrapolated ab initio thermochemistry. IV. A modified recipe for computational efficiency.

A number of economical modifications to the high-accuracy extrapolated ab initio thermochemistry (HEAT) model chemistry are evaluated. The two resulting schemes, designated as mHEAT and mHEAT+, are designed for efficient and pragmatic evaluation of molecular energies in systems somewhat larger than can be practically studied by the unapproximated HEAT scheme. It is found that mHEAT+ produces heats of formation with nearly subchemical (±1 kJ/mol) accuracy at a substantially reduced cost relative to the full scheme. Total atomization energies calculated using the new thermochemical recipes are compared to the results of the HEAT-345(Q) model chemistry, and enthalpies of formation for the three protocols are also compared to Active Thermochemical Tables. Finally, a small selection of transition states is studied using mHEAT and mHEAT+, which illuminates some interesting features of reaction barriers and serves as an initial benchmark of the performance of these model chemistries for chemical kinetics applications.

Three-Dimensional Master Equation (3DME) Approach.

The master equation technique is a standard tool to interpret gas-phase experimental kinetic results as well as to provide phenomenological rate coefficients for modeling. When there are significant changes of rotational constants along the reaction coordinate from a reactant through a transition state (TS) to product(s), including effects of angular momentum explicitly in a master equation model becomes vitally important. In this work, assuming that the K quantum number is adiabatic for both the TS and reactant, we developed an algorithm for pragmatic solutions of a three-dimensional master equation (3DME) that involves internal energy, total angular momentum ( J), and its projection K. Two examples (one is for a thermally activated isomerization of CH3NC to CH3CN via a tight TS, and the other is for a thermally activated dissociation of NH3 to H + NH2 via a loose, variational TS) are given. In addition, comparison of 3DME results with experiment as well as with those of 1DME and 2DME are documented.

Thermal Decomposition of Potential Ester Biofuels. Part I: Methyl Acetate and Methyl Butanoate.

Two methyl esters were examined as models for the pyrolysis of biofuels. Dilute samples (0.06-0.13%) of methyl acetate (CH3COOCH3) and methyl butanoate (CH3CH2CH2COOCH3) were entrained in (He, Ar) carrier gas and decomposed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from the methyl esters were detected and identified by vacuum ultraviolet photoionization mass spectrometry. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures in the pulsed microreactor were about 20 Torr and residence times through the reactors were roughly 25-150 µs. Reactor temperatures of 300-1600 K were explored. Decomposition of CH3COOCH3 commences at 1000 K, and the initial products are (CH2═C═O and CH3OH). As the microreactor is heated to 1300 K, a mixture of CH2═C═O and CH3OH, CH3, CH2═O, H, CO, and CO2 appears. The thermal cracking of CH3CH2CH2COOCH3 begins at 800 K with the formation of CH3CH2CH═C═O and CH3OH. By 1300 K, the pyrolysis of methyl butanoate yields a complex mixture of CH3CH2CH═C═O, CH3OH, CH3, CH2═O, CO, CO2, CH3CH═CH2, CH2CHCH2, CH2═C═CH2, HCCCH2, CH2═C═C═O, CH2═CH2, HC≡CH, and CH2═C═O. On the basis of the results from the thermal cracking of methyl acetate and methyl butanoate, we predict several important decomposition channels for the pyrolysis of fatty acid methyl esters, R-CH2-COOCH3. The lowest-energy fragmentation will be a 4-center elimination of methanol to form the ketene RCH═C═O. At higher temperatures, concerted fragmentation to radicals will ensue to produce a mixture of species: (RCH2 + CO2 + CH3) and (RCH2 + CO + CH2═O + H). Thermal cracking of the ß C-C bond of the methyl ester will generate the radicals (R and H) as well as CH2═C═O + CH2═O. The thermochemistry of methyl acetate and its fragmentation products were obtained via the Active Thermochemical Tables (ATcT) approach, resulting in ΔfH298(CH3COOCH3) = -98.7 ± 0.2 kcal mol-1, ΔfH298(CH3CO2) = -45.7 ± 0.3 kcal mol-1, and ΔfH298(COOCH3) = -38.3 ± 0.4 kcal mol-1.

High-level theoretical study of the reaction between hydroxyl and ammonia: Accurate rate constants from 200 to 2500 K.

Hydrogen abstraction from NH3 by OH to produce H2O and NH2-an important reaction in combustion of NH3 fuel-was studied with a theoretical approach that combines high level quantum chemistry and advanced chemical kinetics methods. Thermal rate constants calculated from first principles agree well (within 5%-20%) with available experimental data over a temperature range that extends from 200 to 2500 K. Quantum mechanical tunneling effects were found to be important; they lead to a decided curvature and non-Arrhenius behavior for the rate constant.

Gas-Phase Formation of the Disilavinylidene (H2 SiSi) Transient.

The hitherto elusive disilavinylidene (H2 SiSi) molecule, which is in equilibrium with the mono-bridged (Si(H)SiH) and di-bridged (Si(H2 )Si) isomers, was initially formed in the gas-phase reaction of ground-state atomic silicon (Si) with silane (SiH4 ) under single-collision conditions in crossed molecular beam experiments. Combined with state-of-the-art electronic structure and statistical calculations, the reaction was found to involve an initial formation of a van der Waals complex in the entrance channel, a submerged barrier to insertion, intersystem crossing (ISC) from the triplet to the singlet manifold, and hydrogen migrations. These studies provide a rare glimpse of silicon chemistry on the molecular level and shed light on the remarkable non-adiabatic reaction dynamics of silicon, which are quite distinct from those of isovalent carbon systems, providing important insight that reveals an exotic silicon chemistry to form disilavinylidene.

Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO-CH2OH), and Glyoxal in a Heated Microreactor.

Both glycolaldehyde and glyoxal were pyrolyzed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from CHO-CH2OH and HCO-CHO were detected and identified by vacuum ultraviolet (VUV) photoionization mass spectrometry. Complementary product identification was provided by argon matrix infrared absorption spectroscopy. Pyrolysis pressures in the microreactor were about 100 Torr, and contact times with the microreactors were roughly 100 µs. At 1200 K, the products of glycolaldehyde pyrolysis are H atoms, CO, CH2═O, CH2═C═O, and HCO-CHO. Thermal decomposition of HCO-CHO was studied with pulsed 118.2 nm photoionization mass spectrometry and matrix infrared absorption. Under these conditions, glyoxal undergoes pyrolysis to H atoms and CO. Tunable VUV photoionization mass spectrometry provides a lower bound for the ionization energy (IE)(CHO-CH2OH) ≥ 9.95 ± 0.05 eV. The gas-phase heat of formation of glycolaldehyde was established by a sequence of calorimetric experiments. The experimental result is ΔfH298(CHO-CH2OH) = -75.8 ± 1.3 kcal mol(-1). Fully ab initio, coupled cluster calculations predict ΔfH0(CHO-CH2OH) of -73.1 ± 0.5 kcal mol(-1) and ΔfH298(CHO-CH2OH) of -76.1 ± 0.5 kcal mol(-1). The coupled-cluster singles doubles and noniterative triples correction calculations also lead to a revision of the geometry of CHO-CH2OH. We find that the O-H bond length differs substantially from earlier experimental estimates, due to unusual zero-point contributions to the moments of inertia.

Communication: Thermal unimolecular decomposition of syn-CH3CHOO: A kinetic study.

The thermal decomposition of syn-ethanal-oxide (syn-CH3CHOO) through vinyl hydrogen peroxide (VHP) leading to hydroxyl radical is characterized using a modification of the HEAT thermochemical protocol. The isomerization step of syn-CH3CHOO to VHP via a 1,4 H-shift, which involves a moderate barrier of 72 kJ/mol, is found to be rate determining. A two-dimensional master equation approach, in combination with semi-classical transition state theory, is employed to calculate the time evolution of various species as well as to obtain phenomenological rate coefficients. This work suggests that, under boundary layer conditions in the atmosphere, thermal unimolecular decomposition is the most important sink of syn-CH3CHOO. Thus, the title reaction should be included into atmospheric modeling. The fate of cold VHP, the intermediate stabilized by collisions with a third body, has also been investigated.

A Steady-State Approximation to the Two-Dimensional Master Equation for Chemical Kinetics Calculations.

In the field of chemical kinetics, the solution of a two-dimensional master equation that depends explicitly on both total internal energy (E) and total angular momentum (J) is a challenging problem. In this work, a weak-E/fixed-J collisional model (i.e., weak-collisional internal energy relaxation/free-collisional angular momentum relaxation) is used along with the steady-state approach to solve the resulting (simplified) two-dimensional (E,J)-grained master equation. The corresponding solutions give thermal rate constants and product branching ratios as functions of both temperature and pressure. We also have developed a program that can be used to predict and analyze experimental chemical kinetics results. This expedient technique, when combined with highly accurate potential energy surfaces, is cable of providing results that may be meaningfully compared to experiments. The reaction of singlet oxygen with methane proceeding through vibrationally excited methanol is used as an illustrative example.