Lawrence B. Harding

Roaming radical kinetics in the decomposition of acetaldehyde.

A novel theoretical framework for predicting the branching between roaming and bond fission channels in molecular dissociations is described and applied to the decomposition of acetaldehyde. This reduced dimensional trajectory (RDT) approach, which is motivated by the long-range nature of the roaming, bond fission, and abstraction dynamical bottlenecks, involves the propagation of rigid-body trajectories on an analytic potential energy surface. The analytic potential is obtained from fits to large-scale multireference ab initio electronic structure calculations. The final potential includes one-dimensional corrections from higher-level electronic structure calculations and for the effect of conserved mode variations along both the addition and abstraction paths. The corrections along the abstraction path play a significant role in the predicted branching. Master equation simulations are used to transform the microcanonical branching ratios obtained from the RDT simulations to the temperature- and pressure-dependent branching ratios observed in thermal decomposition experiments. For completeness, a transition-state theory treatment of the contributions of the tight transition states for the molecular channels is included in the theoretical analyses. The theoretically predicted branching between molecules and radicals in the thermal decomposition of acetaldehyde is in reasonable agreement with the corresponding shock tube measurement described in the companion paper. The prediction for the ratio of the tight to roaming contributions to the molecular channel also agrees well with results extracted from recent experimental and experimental/theoretical photodissociation studies.

Methylene: ab initio vibronic analysis and reinterpretation of the spectroscopic and negative ion photoelectron experiments☆

Abstract The potential curves from extensive ab initio configuration interaction calculations on the three lowest states of methylene ( 3 B 1 , 1 A 1 , and 1 B 1 ) and one state ( 2 B 1 ) of CH − 2 are reported. The Franck-Condon factors from the vibrational wave-functions obtained using these potential curves lead to excellent agreement with the observed 1 B 1 - 1 A 1 spectrum (Herzberg) and also lead to an excellent fit with the photoelectron spectra of CH − 2 (Lineberger), showing that the lowest three bands in the observed negative ion spectrum are hot bands (from vibrationally excited CH − 2 ). Reassignment of the observed spectrum based on these calculations leads to the prediction of a 1 A 1 - 3 B 1 splitting of 0.39 ± 0.05 eV (9 kcal); theoretical value: 0.45 eV.

Kinetics of CH + N2 Revisited with Multireference Methods

The potential energy surface for the CH + N2 reaction was reexamined with multireference ab initio electronic structure methods employing basis sets up to aug-cc-pvqz. Comparisons with related CCSD(T) calculations were also made. The multireference ab initio calculations indicate significant shortcomings in single reference based methods for two key rate-limiting transition states. Transition state theory calculations incorporating the revised best estimates for the transition state properties provide order of magnitude changes in the predicted rate coefficient in the temperature range of importance to the mechanism for prompt NO formation. At higher temperatures, two distinct pathways make a significant contribution to the kinetics. A key part of the transition state analysis involves a variable reaction coordinate transition state theory treatment for the formation of H + NCN from HNCN. The present predictions for the rate coefficients resolve the discrepancy between prior theory and very recent experimental measurements.

Statistical Theory for the Kinetics and Dynamics of Roaming Reactions

We present a statistical theory for the effect of roaming pathways on product branching fractions in both unimolecular and bimolecular reactions. The analysis employs a separation into three distinct steps: (i) the formation of weakly interacting fragments in the long-range/van der Waals region of the potential via either partial decomposition (for unimolecular reactants) or partial association (for bimolecular reactants), (ii) the roaming step, which involves the reorientation of the fragments from one region of the long-range potential to another, and (iii) the abstraction, addition, and/or decomposition from the long-range region to yield final products. The branching between the roaming induced channel(s) and other channels is obtained from a steady-state kinetic analysis for the two (or more) intermediates in the long-range region of the potential. This statistical theory for the roaming-induced product branching is illustrated through explicit comparisons with reduced dimension trajectory simulations for the decompositions of H(2)CO, CH(3)CHO, CH(3)OOH, and CH(3)CCH. These calculations employ high-accuracy analytic potentials obtained from fits to wide-ranging CASPT2 ab initio electronic structure calculations. The transition-state fluxes for the statistical theory calculations are obtained from generalizations of the variable reaction coordinate transition state theory approach. In each instance, at low energy the statistical analysis accurately reproduces the branching obtained from the trajectory simulations. At higher energies, e.g., above 1 kcal/mol, increasingly large discrepancies arise, apparently due to a dynamical biasing toward continued decomposition of the incipient molecular fragments (for unimolecular reactions). Overall, the statistical theory based kinetic analysis is found to provide a useful framework for interpreting the factors that determine the significance of roaming pathways in varying chemical environments.

Triplet states of the amide group. Trapped electron spectra of formamide and related molecules

Abstract Trapped electron (TE) spectra are obtained using ion cyclotron resonance detection of scavenged electrons. The lowest singlet-triplet transitions, 3 (n→π * ), in formamide (HCONH 2 ) and N,N-dimethyl formamide (HCONMe 2 ) are found at vertical energies of 5.30 and 5.00 eV, respectively. An unresolved band containing the 3 (π→π * ) and 3 (n→3s) states appears at higher energies, centered at 6.60 and 6.00 eV, respectively. The TE spectra of formaldehyde (HCHO), acetaldehyde (MeCHO) and acetone (Me 2 CO) are obtained for comparison and are used along with results from ab initio theoretical calculations in establishing assignments. Singlet-triplet transitions dominate the spectra of all of these carbonyl containing molecules, to the exclusion of low lying singlet-singlet transitions. This is in agreement with other TE spectra and the expectation that (dσ/d E ) will be higher near threshold for singlet-triplet as compared to singlet-singlet transitions.

Separability of Tight and Roaming Pathways to Molecular Decomposition.

Recent studies have questioned the separability of the tight and roaming mechanisms to molecular decomposition. We explore this issue for a variety of reactions including MgH(2) → Mg + H(2), NCN → CNN, H(2)CO → H(2) + CO, CH(3)CHO → CH(4) + CO, and HNNOH → N(2) + H(2)O. Our analysis focuses on the role of second-order saddle points in defining global dividing surfaces that encompass both tight and roaming first-order saddle points. The second-order saddle points define an energetic criterion for separability of the two mechanisms. Furthermore, plots of the differential contribution to the reactive flux along paths connecting the first- and second-order saddle points provide a dynamic criterion for separability. The minimum in the differential reactive flux in the neighborhood of the second-order saddle point plays the role of a mechanism divider, with the presence of a strong minimum indicating that the roaming and tight mechanisms are dynamically distinct. We show that the mechanism divider is often, but not always, associated with a second-order saddle point. For the formaldehyde and acetaldehyde reactions, we find that the minimum energy geometry on a conical intersection is associated with the mechanism divider for the tight and roaming processes. For HNNOH, we again find that the roaming and tight processes are dynamically separable but we find no intrinsic feature of the potential energy surface associated with the mechanism divider. Overall, our calculations suggest that roaming and tight mechanisms are generally separable over broad ranges of energy covering most kinetically relevant regimes.

Temperature and Pressure-Dependent Rate Coefficients for the Reaction of Vinyl Radical with Molecular Oxygen

State-of-the-art calculations of the C2H3O2 potential energy surface are presented. A new method is described for computing the interaction potential for R + O2 reactions. The method, which combines accurate determination of the quartet potential along the doublet minimum energy path with multireference calculations of the doublet/quartet splitting, decreases the uncertainty in the doublet potential and thence the rate constants by more than a factor of 2. The temperature- and pressure-dependent rate coefficients are computed using variable reaction coordinate transition-state theory, variational transition-state theory, and conventional transition-state theory, as implemented in a new RRKM/ME code. The main bimolecular product channels are CH2O + HCO at lower temperatures and CH2CHO + O at higher temperatures. Above 10 atm, the collisional stabilization of CH2CHOO directly competes with these two product channels. CH2CHOO decomposes primarily to CH2O + HCO. The next two most significant bimolecular products are OCHCHO + H and (3)CHCHO + OH, and not C2H2 + HO2. C2H3 + O2 will be predominantly chain branching above 1700 K. Uncertainty analysis is presented for the two most important transition states. The uncertainties in these two barrier heights result in a significant uncertainty in the temperature at which CH2CHO + O overtakes all other product channels.

The effect of spin-orbit splitting on the association kinetics of barrierless halogen atom-hydrocarbon radical reactions.

The effect of the geometry dependence of spin-orbit splitting on transition state theory (TST) predictions for radical-radical recombination rate coefficients is examined. The effects are illustrated with direct ab initio variable-reaction-coordinate (VRC)-TST calculations for the reactions of two types of hydrocarbon radicals (R = CH(3) and CH(2)CHCH(2)) with three halogen atoms (X = F, Cl, and Br). These halogen atoms exhibit a range of spin-orbit interaction strengths, while their interactions with the two hydrocarbon radicals exhibit a range of attractiveness. The transition state dividing surfaces for these barrierless reactions occur over a range of R-X fragment separations ( approximately 3-7 A) where the magnitude of the spin-orbit splitting is strongly geometry dependent. Perturbative models for incorporating the energetic effect of spin-orbit splitting into barrierless kinetics are presented and tested. Simply neglecting the variation in the spin-orbit splitting is demonstrated to contribute an error of less than 15% to the predicted rate coefficients for all but the CH(2)CHCH(2) + Br reaction, where its neglect increases the rate by up to a factor of 2. For the CH(2)CHCH(2) + Br reaction, the effect of spin-orbit splitting is not perturbative and instead qualitatively changes the long-range interaction potential and association dynamics. The present theoretical predictions are compared with available experimental measurements and previous theoretical work. For the CH(3) + F association reaction, the errors associated with limitations in the basis set and in the active space are studied, and a detailed comparison is made between VRC-TST and rigid rotor-harmonic oscillator variational TST.

Unconventional Peroxy Chemistry in Alcohol Oxidation: The Water Elimination Pathway.

Predictive simulation for designing efficient engines requires detailed modeling of combustion chemistry, for which the possibility of unknown pathways is a continual concern. Here, we characterize a low-lying water elimination pathway from key hydroperoxyalkyl (QOOH) radicals derived from alcohols. The corresponding saddle-point structure involves the interaction of radical and zwitterionic electronic states. This interaction presents extreme difficulties for electronic structure characterizations, but we demonstrate that these properties of this saddle point can be well captured by M06-2X and CCSD(T) methods. Experimental evidence for the existence and relevance of this pathway is shown in recently reported data on the low-temperature oxidation of isopentanol and isobutanol. In these systems, water elimination is a major pathway, and is likely ubiquitous in low-temperature alcohol oxidation. These findings will substantially alter current alcohol oxidation mechanisms. Moreover, the methods described will be useful for the more general phenomenon of interacting radical and zwitterionic states.

Ab Initio Computations and Active Thermochemical Tables Hand in Hand: Heats of Formation of Core Combustion Species

The fidelity of combustion simulations is strongly dependent on the accuracy of the underlying thermochemical properties for the core combustion species that arise as intermediates and products in the chemical conversion of most fuels. High level theoretical evaluations are coupled with a wide-ranging implementation of the Active Thermochemical Tables (ATcT) approach to obtain well-validated high fidelity predictions for the 0 K heat of formation for a large set of core combustion species. In particular, high level ab initio electronic structure based predictions are obtained for a set of 348 C, N, O, and H containing species, which corresponds to essentially all core combustion species with 34 or fewer electrons. The theoretical analyses incorporate various high level corrections to base CCSD(T)/cc-pVnZ analyses (n = T or Q) using H2, CH4, H2O, and NH3 as references. Corrections for the complete-basis-set limit, higher-order excitations, anharmonic zero-point energy, core-valence, relativistic, and diagonal Born-Oppenheimer effects are ordered in decreasing importance. Independent ATcT values are presented for a subset of 150 species. The accuracy of the theoretical predictions is explored through (i) examination of the magnitude of the various corrections, (ii) comparisons with other high level calculations, and (iii) through comparison with the ATcT values. The estimated 2σ uncertainties of the three methods devised here, ANL0, ANL0-F12, and ANL1, are in the range of ±1.0-1.5 kJ/mol for single-reference and moderately multireference species, for which the calculated higher order excitations are 5 kJ/mol or less. In addition to providing valuable references for combustion simulations, the subsequent inclusion of the current theoretical results into the ATcT thermochemical network is expected to significantly improve the thermochemical knowledge base for less-well studied species.