The non-statistical dynamics of the 18O + 32O2 isotope exchange reaction at two energies

Theoretical dynamics studies of the CH3 + HBr → CH4 + Br reaction: effects of isotope substitution and vibrational excitation of CH3

The rate coefficient for two deuterium substituted isotopologues of reaction CH3 + HBr → CH4 + Br has been determined using the quasiclassical trajectory (QCT) method. We used the analytical potential energy surface (PES) fitted to high-level ab initio points in earlier work. The PES exhibits a pre-reaction van der Waals complex and a submerged potential barrier. The rate coefficients of the deuterated isotopologue reactions, similarly to the pure-protium isotopologue, show significant deviation from the Arrhenius law, namely, the activation energy is negative below about 600 K and positive above it: k[CH3 + DBr] = 1.35 × 10-11 exp(- 2472/T) + 5.85 × 10-13 exp(335/T) and k[CD3 + HBr] = 2.73 × 10-11 exp(- 2739/T) + 1.46 × 10-12 exp(363/T). The CH3 + DBr reaction is slower by a factor of 1.8, whereas CD3 + HBr isotopologue is faster by a factor of 1.4 compared to the HBr + CH3 system across a wide temperature range. The isotope effects are interpreted in terms of the properties of various regions of the PES. Quantum state-resolved simulations revealed that the reaction of CH3 with HBr becomes slower when any of the vibrational modes of the methyl radical is excited. This contradicts the assumption that vibrational excitation of methyl radicals enhances its reactivity, which is of historical importance: this assumption was used as an argument against the existence of negative activation energy in a decade-long controversy in the 1980s and 1990s.

Lifetimes and decay mechanisms of isotopically substituted ozone above the dissociation threshold: matching quantum and classical dynamics

Energies and lifetimes of vibrational resonances were computed for 18O-enriched isotopologue 50O3 = {16O16O18O and 16O18O16O} of the ozone molecule using hyperspherical coordinates and the method of complex absorbing potential. Various types of scattering resonances were identified, including roaming OO-O rotational states, the series corresponding to continuation of bound vibrational resonances of highly excited bending or symmetric stretching vibrational modes. Such a series become metastable above the dissociation limit. The coupling between the vibrationally excited O2 fragment and rotational roaming gives rise to Feshbach type resonances in ozone. Different paths for the formation and decay of symmetric 16O18O16O and asymmetric species 16O16O18O were also identified. The symmetry properties of the total rovibronic wave functions of the 18O-enriched isotopologues are discussed in the context of allowed dissociation channels.

Theoretical dynamics studies of the CH3 + HBr → CH4 + Br reaction: integral cross sections, rate constants and microscopic mechanism

Quantum and quasi-classical dynamics calculations have been performed for the reaction of HBr with CH₃. The accurate ab initio-based potential energy surface function developed earlier for this reaction displays a potential well corresponding to a reactant complex and a submerged potential barrier. The integral cross sections were calculated on this potential energy surface using both a six-degree-of-freedom reduced dimensional quantum dynamics and the quasi-classical trajectory method and very good agreement was found between the two approaches. The cross sections were found to diverge when the collision energy decreases, indicating that the reactant attraction is responsible for the dynamics at low collision energy. The quantum mechanical and the quasi-classical rate constants also agree very well and almost exactly reproduce the experimental results at low temperatures up to 540 K. The negative activation energy observed experimentally is confirmed by the calculations and is a consequence of the long-range attraction between the reactants. From the classical trajectories mechanistic details have been extracted. It is found that at very low collision energy, the reacting system crosses the potential barrier because the forces within the complex guide them, although some 30% is reflected from the product side of the barrier. When the collision energy increases, the system does not follow the most favorable path and the reactants are, with increasing probability, reflected from the repulsive walls of the nonreactive parts of the reactants, providing a picture beyond the decreasing excitation function.

Diagonal Born-Oppenheimer corrections to the ground electronic state potential energy surfaces of ozone: improvement of ab initio vibrational band centers for the 16O3, 17O3 and 18O3 isotopologues

Mass-dependent diagonal Born-Oppenheimer corrections (DBOCs) to the ab initio electronic ground state potential energy surface for the main ¹⁶O₃ isotopologue and for homogeneous isotopic substitutions ¹⁷O₃ and ¹⁸O₃ of the ozone molecule are reported for the first time. The system being of strongly multiconfigurational character, multireference configuration interaction wave function ansatz with different complete active spaces was used. The reliable DBOC calculations with the targeted accuracy were possible to carry out up to about half of the dissociation threshold D₀. The comparison with the experimental band centers shows a significant improvement of the accuracy with respect to the best Born-Oppenheimer (BO) ab initio calculations reducing the total root-mean-squares (calculated-observed) deviations by about a factor of two. For the set of ¹⁶O₃ vibrations up to five bending and four stretching quanta, the mean (calculated-observed) deviations drop down from 0.7 cm^-1 (BO) to about 0.1 cm^-1, with the most pronounced improvement seen for bending states and for mixed bending-stretching polyads. In the case of bending band centers directly observed under high spectral resolutions, the errors are reduced by more than an order of magnitude down to 0.02 cm^-1 from the observed levels, approaching nearly experimental accuracy. A similar improvement for heavy isotopologues shows that the reported DBOC corrections almost remove the systematic BO errors in vibrational levels below D₀/2, though the scatter increases towards higher energies. The possible reasons for this finding, as well as remaining issues are discussed in detail. The reported results provide an encouraging accuracy validation for the multireference methods of the ab initio theory. New sets of ab initio vibrational states can be used for improving effective spectroscopic models for analyses of the observed high-resolution spectra, particularly in the cases of accidental resonances with "dark" states requiring accurate theoretical predictions.

Non-adiabatic Excited-State Molecular Dynamics: Theory and Applications for Modeling Photophysics in Extended Molecular Materials

Optically active molecular materials, such as organic conjugated polymers and biological systems, are characterized by strong coupling between electronic and vibrational degrees of freedom. Typically, simulations must go beyond the Born-Oppenheimer approximation to account for non-adiabatic coupling between excited states. Indeed, non-adiabatic dynamics is commonly associated with exciton dynamics and photophysics involving charge and energy transfer, as well as exciton dissociation and charge recombination. Understanding the photoinduced dynamics in such materials is vital to providing an accurate description of exciton formation, evolution, and decay. This interdisciplinary field has matured significantly over the past decades. Formulation of new theoretical frameworks, development of more efficient and accurate computational algorithms, and evolution of high-performance computer hardware has extended these simulations to very large molecular systems with hundreds of atoms, including numerous studies of organic semiconductors and biomolecules. In this Review, we will describe recent theoretical advances including treatment of electronic decoherence in surface-hopping methods, the role of solvent effects, trivial unavoided crossings, analysis of data based on transition densities, and efficient computational implementations of these numerical methods. We also emphasize newly developed semiclassical approaches, based on the Gaussian approximation, which retain phase and width information to account for significant decoherence and interference effects while maintaining the high efficiency of surface-hopping approaches. The above developments have been employed to successfully describe photophysics in a variety of molecular materials.

Mechanism Change in the Dynamics of the O' + O2 → O'O + O Atom Exchange Reaction at High Collision Energies

The extreme velocity and the large available energy of atoms with hyperthermal kinetic energies can give rise to novel mechanisms and behavior of chemical reactions unseen at thermal conditions. Crossed-molecular-beams experiments combined with isotope labeling on the reaction of hyperthermal O atoms with O₂ molecules have provided an example of the arising complexity of such systems. Quasiclassical trajectory (QCT) calculations proved to be instructive in the exploration of the microscopic mechanism of the reactive and inelastic scattering observed, and a new mechanism has been identified: there are reactive collisions in which the potential energy remains repulsive during the entire encounter ("direct" reactions in which, in a sense, no complex is formed). In this work, the effect of the magnitude of the collision energy on this mechanism is explored. At hyperthermal collision energies, the reaction is characterized by a unique impact parameter window favorable for reaction through complex formation, while the direct collisions take place exclusively at small impact parameters. In direct reactive collisions, contributing as much as 12% to the reaction cross section, first the existing bond is broken, and the new bond is formed afterward. This kind of collision is unique to extremely high collision energies. Analysis of various correlations was used to find out the details of the reaction dynamics. The observed phenomena indicate that when the collision energy is extremely high, one can expect deviation from what an extrapolation from the more familiar energy ranges would predict.

The Role of Ozone Vibrational Resonances in the Isotope Exchange Reaction 16O16O + 18O → 18O16O + 16O: The Time-Dependent Picture

We consider the time-dependent dynamics of the isotope exchange reaction in collisions between an oxygen molecule and an oxygen atom: ¹⁶O¹⁶O + ¹⁸O → ¹⁶O¹⁸O + ¹⁶O. A theoretical approach using the multiconfiguration time-dependent Hartree method was employed to model the time evolution of the reaction. Two potential surfaces available in the literature were used in the calculations, and the results obtained with the two surfaces are compared with each other as well as with results of a previous theoretical time-independent approach. A good agreement for the reaction probabilities with the previous theoretical results is found. Comparing the results obtained using two potential energy surfaces allows us to understand the role of the reef/shoulder-like feature in the minimum energy path of the reaction in the isotope exchange process. Also, it was found that the distribution of final products of the reaction is highly anisotropic, which agrees with experimental observations and, at the same time, suggests that the family of approximated statistical approaches, assuming a randomized distribution over final exit channels, is not applicable to this case.

First-Principles Computed Rate Constant for the O + O2 Isotopic Exchange Reaction Now Matches Experiment

We show, by performing exact time-independent quantum molecular scattering calculations, that the quality of the ground electronic state global potential energy surface appears to be of utmost importance in accurately obtaining even as strongly averaged quantities as kinetic rate constants. The oxygen isotope exchange reaction, ¹⁸O + ³²O₂, motivated by the understanding of a complex long-standing problem of isotopic ozone anomalies in the stratosphere and laboratory experiments, is explored in this context. The thermal rate constant for this key reaction is now in quantitative agreement with all experimental data available to date. A significant recent progress at the frontier of three research domains, advanced electronic structure calculations, ultrasensitive spectroscopy, and quantum scattering calculations, has therefore permitted a breakthrough in the theoretical modeling of this crucial collision process from first principles.

Electronic quenching of O((1)D) by Xe: Oscillations in the product angular distribution and their dependence on collision energy

The dynamics of the O((1)D) + Xe electronic quenching reaction was investigated in a crossed beam experiment at four collision energies. Marked large-scale oscillations in the differential cross sections were observed for the inelastic scattering products, O((3)P) and Xe. The shape and relative phases of the oscillatory structure depend strongly on collision energy. Comparison of the experimental results with time-independent scattering calculations shows qualitatively that this behavior is caused by Stueckelberg interferences, for which the quantum phases of the multiple reaction pathways accessible during electronic quenching constructively and destructively interfere.

Huge Quantum Symmetry Effect in the O + O2 Exchange Reaction

We report extensive, full quantum-mechanical calculations for the (16)O + (16)O(16)O → (16)O(16)O + (16)O collisions, for both inelastic and atom exchange processes, using a time-independent method based on hyperspherical coordinates. The rates obtained in the present study are much larger than the previously reported ones for this system. The discrepancy is attributed to a huge symmetry effect that was missing in the studies so far. This effect differs from the well-known isotope effect. Importance of this quantum effect is further confirmed by comparison with results for the (16)O + (18)O(18)O → (16)O(18)O + (18)O, exchange reaction.