MRCI Calculations of the Lowest Potential Energy Surface for CH3OH and Direct ab Initio Dynamics Simulations of the O(1D) + CH4 Reaction

Photochemical pathways in astronomical ices: A computational study of singlet oxygen reactions with hydrocarbons

Complex organic molecules are widespread in different areas of the interstellar medium, including cold areas, such as molecular clouds, where chemical reactions occur in ice. Among the observed molecules are oxygen-bearing organic molecules, which are of high interest given their significant role in astrobiology. Despite the observed rich chemistry, the underlying molecular mechanisms responsible for molecular formation in such cold dilute areas are still not fully understood. In this paper, we study the unique chemistry taking place in astronomically relevant ices, where UV radiation is a central driving force for chemical reactions. Photofragmentation of ice components gives rise to highly reactive species, such as the O(1D) atom. These species provide a pathway for chemical complexity even in cold areas. Using quantum chemistry calculations, we demonstrate that O(1D) reacts barrierlessly with hydrocarbons. Moreover, photoprocessing of the reaction products (and other components of the ice), followed by radical recombination, is found to be an essential part of the overall mechanism. In ice containing O(1D) and hydrocarbons, the formation of formaldehyde in methane ice, acetaldehyde in ethane ice, and carbon monoxide in acetylene ice, and the consumption of alcohol in all systems, was predicted in agreement with experimental results.

Calculations of Dissociation Dynamics of CH3OH on a Global Potential Energy Surface Reveal the Mechanism for the Formation of HCOH; Roaming Plays a Role

The experimental observation of hydroxymethylene, HCOH, following excitation of methanol at 193 nm, was reported recently (Hockey, E. K.; McLane, N.; Martí, C.; Duckett, L.; Osborn, D. L.; Dodson, L. G. Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation. J. Am. Chem. Soc. 2024, 146, 14416-14421, 10.1021/jacs.4c03090). This stimulated us to investigate a dynamic mechanism for its formation using a global potential energy surface for the ground electronic state (S0) of methanol. We show via quasi-classical trajectory calculations that hydroxymethylene is indeed formed under the reasonable assumption that the initially excited state undergoes rapid internal conversion to S0. From the trajectories, fractional yields of the six major product channels are determined as a function of excitation energy, and the rate of production of each is determined from the fractions and rate of methanol disappearance. Roaming is observed in trajectories leading to the OH and CH3 products as well as in those leading to CH2OH + H and non-reactive trajectories. A "frustrated" roaming accounts for roughly 20% of the HCOH production.

The Reaction NCN + H2: Quantum Chemical Calculations, Role of 1NCN Chemistry, and 3NCN Absorption Cross Section

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. Recently, in a combined shock tube and flame modeling study, the so far neglected reaction NCN + H₂ and the related chemistry of the main product HNCN turned out to be significant for NO modeling under fuel-rich conditions. In this study, the reaction has been thoroughly revisited by detailed quantum chemical rate constant calculations both for the singlet ¹NCN and triplet ³NCN pathways. Optimized geometries and vibrational frequencies of reactants, products, and transition states were calculated on B3LYP/aug-cc-pVQZ level with single-point energy calculations carried out against the optimized structures using CASPT2/aug-cc-pVQZ. The determined rate constants for the ¹NCN + H₂ reaction as well as the newly measured high temperature absorption cross section of ³NCN made a reevaluation of the shock tube data of the previous work necessary, finally revealing quantitative agreement between experiment and theory. Moreover, the new directly measured Doppler-limited absorption cross section data, σ(³NCN, λ = 329.1302 nm) = 2.63 × 10⁹ × exp(-1.96 × 10^-3 × T/K) cm²/mol (±23%, p = 0 bar, T = 870-1700 K), are in agreement with previously reported values based on detailed spectroscopic simulations. Hence, a long-standing debate about a reliable high temperature ³NCN absorption cross section has been resolved. Whereas ³NCN + H₂ resembles a simple abstraction type reaction with the exclusive products HNCN + H, the singlet radical reaction is initiated by the insertion into the H-H bond. Up to pressures of 100 bar, the main products of the subsequent decomposition of the H₂NCN intermediate are HNCN + H as well, with minor contributions of CN + NH₂ toward higher temperatures. Although much faster than the triplet reaction, the singlet radical insertion is actually rather slow, due to the necessary reorganization of the HOMO electron density in ¹NCN that is equally distributed over the two N atom sites. In general, the distinct reactivity differences call for a separate treatment of ¹NCN and ³NCN chemistry. However, as the main reaction products in case of the H₂ reaction are the same and as the population of the ¹NCN in thermal equilibrium remains low, a properly weighted effective rate constant k(NCN + H₂ → HNCN + H) = 2.62 × 10⁴ × (T/K)^2.78 × exp(-97.6 kJ/mol/RT) cm³ mol^-1s^-1(±30%, 800 K < T < 3000 K, p < 100 bar) is recommended for inclusion into flame models that, as yet, do not explicitly account for ¹NCN chemistry.

Diffusion Monte Carlo Calculations of Zero-Point Energies of Methanol and Deuterated Methanol

Diffusion Monte Carlo (DMC) simulations have been used to obtain quantum zero-point energies of methanol and all its isotopologs and isotopomers, using a new, accurate semi-global potential energy surface. This potential energy surface is a precise, permutationally invariant fit to 6676 ab initio energies, obtained at the CCSD(T)-F12b/aug-cc-pVDZ level of theory. Quantum zero-point energies of deuterated methanol isotopomers are very close to each other and so a simple statistical argument can be used to estimate the populations of each isotopomer at very low-temperatures. The DMC simulations also indicate that there is virtually zero probability for H/D exchange in the zero-point state. © 2019 Wiley Periodicals, Inc.

A global full-dimensional potential energy surface and quasiclassical trajectory study of the O(1D) + CH4 multichannel reaction

The QCT calculations based on an accurate global full-dimensional PES are capable of reproducing the experimental dynamic features for O(¹D) + CH₄.

Trapped Abstraction in the O((1)D) + CHD3 → OH + CD3 Reaction

Despite significant progress made in past decades, it is still challenging to elucidate dynamics mechanisms for polyatomic reactions, in particular, involving complex formation. The reaction of O((1)D) with methane has long been regarded as a prototypical polyatomic system of direct insertion reaction in which the O((1)D) atom can insert into the C-H bond of methane to form a "hot" methanol intermediate before decomposition. Here, we report a combined theoretical and experimental study on the O((1)D) + CHD3 reaction, on which good agreement between theory and experiment is achieved. Our study revealed that this complex-forming reaction actually proceeds via a trapped abstraction mechanism, rather than an insertion mechanism as has long been thought. We anticipate that this reaction mechanism should also be responsible for the reaction of O((1)D) with ethane and propane, as well as many other chemical reactions with deep wells in the interaction region.

Imaging the O((1)D) + CD4 → OD + CD3 Reaction Dynamics: The Threshold of Abstraction Pathway

The O((1)D) + CD4 → OD + CD3 reaction was investigated using the crossed molecular beam technique with sliced velocity map imaging at four different collision energies: 1.6, 2.8, 4.6, and 6.8 kcal/mol. The vibrational ground state product CD3 was detected using a (2 + 1) resonance-enhanced multiphoton ionization (REMPI). Remarkably different features were found in the forward and backward scatterings, and gradually changed with the collision energy. These features were attributed to two distinctive reaction mechanisms-insertion and abstraction-that occur on the ground and excited state surfaces, respectively. Contributions from the two mechanisms were extracted from the experiment results, and a positive correlation was found between the abstraction proportion and the collision energy. The threshold for the abstraction pathway was determined and compared with results from calculations.

Spherical electron cloud hopping molecular dynamics simulation on dissociative recombination of protonated water

Dissociative recombination (DR) of H(3)O(+) with electrons at zero collision energy has been studied by a direct ab initio molecular dynamics method on four low-lying electronic states of the system. Initial conditions for trajectories are determined by a spherical electron cloud hopping (SECH) model, while nonadiabatic effects are considered through a surface hopping scheme. The energies, forces, and nonadiabatic coupling strengths (NACS) used in trajectory propagations are calculated on-the-fly via state-average complete active self-consistent field (CASSCF) theory with full valence electrons. Dynamics results show that the H(3)O(+) DR is ultrafast and yields diversity of products. Product branching fractions are predicted to be 0.660 for (OH + 2H), 0.230 for (H(2)O + H), 0.108 for (OH + H(2)), and 0.002 for (O + H + H(2)), which are in excellent agreement with the heavy-ion storage ring experimental results. Kinetic energies of the eliminated hydrogen atoms are large and show a bimodal distribution.

Dynamics of reactions O((1)D)+C(6)H(6) and C(6)D(6)

The reaction between O((1)D) and C(6)H(6) (or C(6)D(6)) was investigated with crossed-molecular-beam reactive scattering and time-resolved Fourier-transform infrared spectroscopy. From the crossed-molecular-beam experiments, four product channels were identified. The major channel is the formation of three fragments CO+C(5)H(5)+H; the channels for formation of C(5)H(6)+CO and C(6)H(5)O+H from O((1)D)+C(6)H(6) and OD+C(6)D(5) from O((1)D)+C(6)D(6) are minor. The angular distributions for the formation of CO and H indicate a mechanism involving a long-lived collision complex. Rotationally resolved infrared emission spectra of CO (1<or=upsilon<or=6) and OH (1<or=upsilon<or=3) were recorded with a step-scan Fourier-transform spectrometer. At the earliest applicable period (0-5 mus), CO shows a rotational distribution corresponding to a temperature of approximately 1480 K for upsilon=1 and 920-700 K for upsilon=2-6, indicating possible involvement of two reaction channels; the vibrational distribution of CO corresponds to a temperature of approximately 5800 K. OH shows a rotational distribution corresponding to a temperature of approximately 650 K for upsilon=1-3 and a vibrational temperature of approximately 4830 K. The branching ratio of [CO]/[OH]=2.1+/-0.4 for O((1)D)+C(6)H(6) and [CO]/[OD]>2.9 for O((1)D)+C(6)D(6) is consistent with the expectation for an abstraction reaction. The mechanism of the reaction may be understood from considering the energetics of the intermediate species and transition states calculated at the G2M(CC5) level of theory for the O((1)D)+C(6)H(6) reaction. The experimentally observed branching ratios and deuterium isotope effect are consistent with those predicted from calculations.

Quantum force molecular dynamics study of the reaction of O atoms with HOCO

The reaction of HOCO with O atoms has been studied using a direct ab initio dynamics approach based on the scaling all correlation UCCD/D95(d,p) method. Ab initio calculations point to two possible reaction mechanisms for the O+HOCO-->OH+CO2 reaction. They are a direct hydrogen abstraction and an oxygen addition reaction through a short-lived HOC(O)O intermediate. The dynamics results show that only the addition mechanism is important under the conditions considered here. The lifetime of the HOC(O)O complex is predicted to be 172+/-15 fs. This is typical of a direct and fast radical-radical reaction. At room temperature, the calculated thermal rate coefficient is 1.44 x 10(-11) cm(3) mol(-1) s(-1) and its temperature dependence is rather weak. The two kinds of reactive trajectories are illustrated in detail.

Kinetics of the Reaction of Methyl Radical with Hydroxyl Radical and Methanol Decomposition

The CH3 + OH bimolecular reaction and the dissociation of methanol are studied theoretically at conditions relevant to combustion chemistry. Kinetics for the CH3 + OH barrierless association reaction and for the H + CH2OH and H + CH3O product channels are determined in the high-pressure limit using variable reaction coordinate transition state theory and multireference electronic structure calculations to evaluate the fragment interaction energies. The CH3 + OH --> 3CH2 + H2O abstraction reaction and the H2 + HCOH and H2 + H2CO product channels feature localized dynamical bottlenecks and are treated using variational transition state theory and QCISD(T) energies extrapolated to the complete basis set limit. The 1CH2 + H2O product channel has two dynamical regimes, featuring both an inner saddle point and an outer barrierless region, and it is shown that a microcanonical two-state model is necessary to properly describe the association rate for this reaction over a broad temperature range. Experimental channel energies for the methanol system are reevaluated using the Active Thermochemical Tables (ATcT) approach. Pressure dependent, phenomenological rate coefficients for the CH3 + OH bimolecular reaction and for methanol decomposition are determined via master equation simulations. The predicted results agree well with experimental results, including those from a companion high-temperature shock tube determination for the decomposition of methanol.

Quasiclassical trajectory study of the Cl+CH4 reaction dynamics on a quadratic configuration interaction with single and double excitation interpolated potential energy surface

An ab initio interpolated potential energy surface (PES) for the Cl+CH(4) reactive system has been constructed using the interpolation method of Collins and co-workers [J. Chem. Phys. 102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); Theor. Chem. Acc. 108, 313 (2002)]. The ab initio calculations have been performed using quadratic configuration interaction with single and double excitation theory to build the PES. A simple scaling all correlation technique has been used to obtain a PES which yields a barrier height and reaction energy in good agreement with high level ab initio calculations and experimental measurements. Using these interpolated PESs, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions has been carried out for the Cl+CH(4) and Cl+CD(4) reactions, and the theoretical results have been compared with the available experimental data. It has been shown that the calculated total reaction cross sections versus collision energy for the Cl+CH(4) and Cl+CD(4) reactions is very sensitive to the barrier height. Besides, due to the zero-point energy (ZPE) leakage of the CH(4) molecule to the reaction coordinate in the quasiclassical trajectory (QCT) calculations, the reaction threshold falls below the barrier height of the PES. The ZPE leakage leads to CH(3) and HCl coproducts with internal energy below its corresponding ZPEs. We have shown that a Gaussian binning (GB) analysis of the trajectories yields excitation functions in somehow better agreement with the experimental determinations. The HCl(v'=0) and DCl(v'=0) rotational distributions are as well very sensitive to the ZPE problem. The GB correction narrows and shifts the rotational distributions to lower values of the rotational quantum numbers. However, the present QCT rotational distributions are still hotter than the experimental distributions. In both reactions the angular distributions shift from backward peaked to sideways peaked as collision energy increases, as seen in the experiments and other theoretical calculations.

Quantum Molecular Dynamics Study of the Reaction of O2 with HOCO

The reaction of O2 with HOCO has been studied by using an ab initio direct dynamics method based on the UB3PW91 density functional theory. Results show that the reaction can occur via two mechanisms: direct hydrogen abstraction and an addition reaction through a short-lived HOC(O)O2 intermediate. The lifetime of the intermediate is predicted to be 660 +/- 30 fs. Although it is an activated reaction, the activation energy is only 0.71 kcal/mol. At room temperature, the obtained thermal rate coefficient is 2.1 x 10(-12) cm3 molecule(-1) s(-1), which is in good agreement with the experimental results.

Universal and State-Resolved Imaging of Chemical Dynamics

We showcase the use of high-resolution ion imaging with complementary state-resolved and "universal" vacuum ultraviolet probes to address a broad range of fundamental problems in chemical reaction dynamics. Examples from our recent work include applications in state-correlated unimolecular reactions, ion pair dissociation dynamics and spectroscopy, crossed-beam reactive scattering, and atomic angular momentum polarization in photodissociation. These studies are all directed to achieving a detailed understanding of atomic and molecular interactions, with particular emphasis on reaction mechanisms outside the scope of transition state theory; on spectroscopy and dynamics of highly excited, transient species; on nonadiabatic reaction mechanisms; and on chemical dynamics in polyatomic systems.

Quasiclassical Trajectory Study of the F + CH4 Reaction Dynamics on a Dual-Level Interpolated Potential Energy Surface

An ab initio interpolated potential energy surface (PES) for the F + CH4 reactive system has been constructed using the interpolation method of Collins and co-workers. The ab initio calculations have been performed using second-order Möller-Plesset (MP2) perturbation theory to build the initial PES. Scaling all correlation (SAC) methodology has been employed to improve the ab initio calculations and to construct a dual-level PES. Using this PES, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations and internal energy distributions has been carried out for the F + CH4 and F + CD4 reactions and the theoretical results have been compared with the available experimental data.

A Study of the Unimolecular Dissociation of the 2-Buten-2-yl Radical via the 193 nm Photodissociation of 2-Chloro-2-butene

This work investigates the unimolecular dissociation of the 2-buten-2-yl radical. This radical has three potentially competing reaction pathways: C-C fission to form CH3 + propyne, C-H fission to form H + 1,2-butadiene, and C-H fission to produce H + 2-butyne. The experiments were designed to probe the branching to the three unimolecular dissociation pathways of the radical and to test theoretical predictions of the relevant dissociation barriers. Our crossed laser-molecular beam studies show that 193 nm photolysis of 2-chloro-2-butene produces 2-buten-2-yl in the initial photolytic step. A minor C-Cl bond fission channel forms electronically excited 2-buten-2-yl radicals and the dominant C-Cl bond fission channel produces ground-state 2-buten-2-yl radicals with a range of internal energies that spans the barriers to dissociation of the radical. Detection of the stable 2-buten-2-yl radicals allows a determination of the translational, and therefore internal, energy that marks the onset of dissociation of the radical. The experimental determination of the lowest-energy dissociation barrier gave 31 +/- 2 kcal/mol, in agreement with the 32.8 +/- 2 kcal/mol barrier to C-C fission at the G3//B3LYP level of theory. Our experiments detected products of all three dissociation channels of unstable 2-buten-2-yl as well as a competing HCl elimination channel in the photolysis of 2-chloro-2-butene. The results allow us to benchmark electronic structure calculations on the unimolecular dissociation reactions of the 2-buten-2-yl radical as well as the CH3 + propyne and H + 1,2-butadiene bimolecular reactions. They also allow us to critique prior experimental work on the H + 1,2-butadiene reaction.

Direct ab Initio Dynamics Study of the OH + HOCO Reaction

The reaction between OH and HOCO has been examined using the coupled-cluster method to locate and optimize the critical points on the ground-state potential energy surface. The energetics are refined using the coupled-cluster method with basis set extrapolation to the complete basis set (CBS) limit. Results show that the OH + HOCO reaction produces H2O + CO2 as final products and the reaction passes through an HOC(O)OH intermediate. In addition, the OH + HOCO reaction has been studied using a direct dynamics method with a dual-level ab initio theory. Dynamics calculations show that hydrogen bonding plays an important role during the initial stages of the reaction. The thermal rate constant is estimated over the temperature range 250-800 K. The OH + HOCO reaction is found to be nearly temperature-independent at lower temperatures, and at 300 K, the thermal rate constant is predicted to be 1.03 x 10(-11) cm3 molecule(-1) s(-1). In addition, there may be an indication of a small peak in the rate constant at a temperature between 300 and 400 K.

Ab Initio and Direct Dynamics Studies of the Reaction of Singlet Methylene with Acetylene and the Lifetime of the Cyclopropene Complex

The energetics of the (1)CH(2) + C(2)H(2) --> H + C(3)H(3) reaction are accurately calculated using an extrapolated coupled-cluster/complete basis set (CBS) method based on the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets. The reaction enthalpy (0 K) is predicted to be -20.33 kcal/mol. This reaction has no classical barrier in either the entrance or exit channel. However, there are several stable intermediates-cyclopropene (c-C(3)H(4)), allene (CH(2)CCH(2)), and propyne (CH(3)CCH)-along the minimum energy path. These intermediates with zero-point energy corrections lie below the reactants by 87.11 (c-C(3)H(4)), 109.69 (CH(2)CCH(2)), and 110.78 kcal/mol (CH(3)CCH). The vibrationally adiabatic ground-state (VAG) barrier height for c-C(3)H(4) isomerization to allene is obtained as 45.2 kcal/mol, and to propyne as 37.2 kcal/mol. In addition, the (1)CH(2) + C(2)H(2) reaction is investigated utilizing the dual-level "scaling all correlation" (SAC) ab initio method of Truhlar et al., i.e., the UCCSD(SAC)/cc-pVDZ theory. Results show that the reaction occurs via long-lived complexes. The lifetime of the cyclopropene intermediate is obtained as 3.2 +/- 0.4 ps. It is found that the intermediate propyne can be formed directly from reactants through the insertion of (1)CH(2) into a C-H bond of C(2)H(2). However, compared to the major mechanism in which the propyne is produced through a ring-opening of the cyclopropene complex, this reaction pathway is much less favorable. Finally, the theoretical thermal rate constant exhibits a negative temperature dependence, which is in excellent agreement with the previous results. The temperature dependence is consistent with the earlier RRKM results but weaker than the experimental observations at high temperatures.

Product Study of the Reaction of CH3 with OH Radicals at Low Pressures and Temperatures of 300 and 612 K†

The product distribution for the title reaction was studied using our time-of-flight mass spectrometer (TOFMS) connected to a tubular flow reactor. The methyl and hydroxyl radicals were produced by an excimer laser pulse (lambda = 193 nm) photolyzing acetone and nitrous oxide in the presence of excess water or hydrogen. Helium was used as the bath gas; the total density was held constant at 1.2 x 10(17) cm(-3). At 300 K the observations were consistent with singlet methylene ((1)CH(2)) and water as the main product channel with a small contribution of methanol. In contrast, at about 610 K three channels-formaldehyde isomers and methanol in addition to (1)CH(2) + H(2)O-are formed with similar yields. When acetone-d(6) was used, the production of both CHDO and CD(2)O was observed, indicating that two different formaldehyde-producing channels are operating simultaneously. These experimental results are compared with RRKM and master equation calculations on the basis of the properties of the methanol potential energy surface from a recent ab initio study.