Mixed quantum-classical theory for the collisional energy transfer and the rovibrational energy flow: application to ozone stabilization

Collisional alignment and molecular rotation control the chemi-ionization of individual conformers of hydroquinone with metastable neon

The relationship between the shape of a molecule and its chemical reactivity is a central tenet in chemistry. However, the influence of the molecular geometry on reactivity can be subtle and result from several opposing effects. Here, using a crossed-molecular-beam experiment in which individual rotational quantum states of specific conformers of a molecule are separated, we study the chemi-ionization reaction of hydroquinone with metastable neon atoms. We show that collision-induced alignment of the reaction partners caused by geometry-dependent long-range forces influences reaction pathways, which is, however, countered by molecular rotation. The present work provides insights into the conformation-specific stereodynamics of complex polyatomic systems and illustrates the capability of advanced molecule-control techniques to unravel these effects.

Development of a potential energy surface for the O3–Ar system: rovibrational states of the complex

Collisional stabilization is an important step in the process of atmospheric formation of ozone.

On stabilization of scattering resonances in recombination reaction that forms ozone

Calculations of energy transfer in the recombination reaction that forms ozone are carried out within the framework of the mixed quantum/classical theory and using the dimensionally reduced 2D-model of ozone molecule, with bending motion neglected. Recombination rate coefficients are obtained at room temperature for symmetric and asymmetric isotopomers of singly and doubly substituted isotopologues. The processes of resonance formation, spontaneous decay, collisional dissociation, and stabilization by bath gas (Ar) are all characterized and taken into account within the steady-state approximation for kinetics. The focus is on stabilization step, where the mysterious isotopic η-effect was thought to originate from. Our results indicate no difference in cross sections for stabilization of scatteringresonances in symmetric and asymmetric isotopomers. As practical results, the general and simple analytic models for stabilization and dissociation cross sections are presented, which can be applied to resonances in any ozone molecule, symmetric or asymmetric, singly or doubly substituted. Present calculations show some isotope effect that looks similar to the experimentally observed η-effect, and the origin of this phenomenon is in the rates of formation/decay of scatteringresonances, determined by their widths, that are somewhat larger in asymmetric isotopomers than in their symmetric analogues. However, the approximate two-dimensional model used here is insufficient for consistent and reliable description of all features of the isotopic effect in ozone. Calculations using an accurate 3D model are still needed.

Inelastic Scattering of Identical Molecules within Framework of the Mixed Quantum/Classical Theory: Application to Rotational Excitations in H2 + H2

Theoretical foundation is laid out for description of permutation symmetry in the inelastic scattering processes that involve collisions of two identical molecules, within the framework of the mixed quantum/classical theory (MQCT). In this approach, the rotational (and vibrational) states of two molecules are treated quantum-mechanically, whereas their translational motion (responsible for scattering) is treated classically. This theory is applied to H2 + H2 system, and the state-to-state transition cross sections are compared versus those obtained from the full-quantum calculations and experimental results from the literature. Good agreement is found in all cases. It is also found that results of MQCT, where the Coriolis coupling is included classically, are somewhat closer to exact full-quantum results than results of the other approximate quantum methods, where those coupling terms are neglected. These new developments allow applications of MQCT to a broad variety of molecular systems and processes.

A full-dimensional model of ozone forming reaction: the absolute value of the recombination rate coefficient, its pressure and temperature dependencies

Rigorous calculations of scattering resonances in ozone are carried out for a broad range of rotational excitations with a detailed analysis of their properties and contribution into recombination process.

Rotational quenching of H2O by He: mixed quantum/classical theory and comparison with quantum results

The mixed quantum/classical theory (MQCT) formulated in the space-fixed reference frame is used to compute quenching cross sections of several rotationally excited states of water molecule by impact of He atom in a broad range of collision energies, and is tested against the full-quantum calculations on the same potential energy surface. In current implementation of MQCT method, there are two major sources of errors: one affects results at energies below 10 cm(-1), while the other shows up at energies above 500 cm(-1). Namely, when the collision energy E is below the state-to-state transition energy ΔE the MQCT method becomes less accurate due to its intrinsic classical approximation, although employment of the average-velocity principle (scaling of collision energy in order to satisfy microscopic reversibility) helps dramatically. At higher energies, MQCT is expected to be accurate but in current implementation, in order to make calculations computationally affordable, we had to cut off the basis set size. This can be avoided by using a more efficient body-fixed formulation of MQCT. Overall, the errors of MQCT method are within 20% of the full-quantum results almost everywhere through four-orders-of-magnitude range of collision energies, except near resonances, where the errors are somewhat larger.

Introduction to chemistry and applications in nature of mass independent isotope effects special feature

Stable isotope ratio variations are regulated by physical and chemical laws. These rules depend on a relation with mass differences between isotopes. New classes of isotope variation effects that deviate from mass dependent laws, termed mass independent isotope effects, were discovered in 1983 and have a wide range of applications in basic chemistry and nature. In this special edition, new applications of these effects to physical chemistry, solar system origin models, terrestrial atmospheric and biogenic evolution, polar paleo climatology, snowball earth geology, and present day atmospheric sciences are presented.

On molecular origin of mass-independent fractionation of oxygen isotopes in the ozone forming recombination reaction

Theoretical treatment of ozone forming reaction is developed within the framework of mixed quantum/classical dynamics. Formation and stabilization steps of the energy transfer mechanism are both studied, which allows simultaneous capture of the delta zero-point energy effect and η-effect and identification of the molecular level origin of mass-independent isotope fractionation. The central role belongs to scattering resonances; dependence of their lifetimes on rotational excitation, asymmetry; and connection of their vibrational wave functions to two different reaction channels. Calculations, performed within the dimensionally reduced model of ozone, are in semiquantitative agreement with experiment.

The Physical Chemistry of Mass-Independent Isotope Effects and Their Observation in Nature

Historically, the physical chemistry of isotope effects and precise measurements in samples from nature have provided information on processes that could not have been obtained otherwise. With the discovery of a mass-independent isotopic fractionation during the formation of ozone, a new physical chemical basis for isotope effects required development. Combined theoretical and experimental developments have broadened this understanding and extended the range of chemical systems where these unique effects occur. Simultaneously, the application of mass-independent isotopic measurements to an extensive range of both terrestrial and extraterrestrial systems has furthered the understanding of events such as solar system origin and evolution and planetary atmospheric chemistry, present and past.

Collisional stabilization of van der Waals states of ozone

The mixed quantum-classical theory developed earlier [M. Ivanov and D. Babikov, J. Chem. Phys. 134, 144107 (2011)] is employed to treat the collisional energy transfer and the ro-vibrational energy flow in a recombination reaction that forms ozone. Assumption is that the van der Waals states of ozone are formed in the O + O(2) collisions, and then stabilized into the states of covalent well by collisions with bath gas. Cross sections for collision induced dissociation of van der Waals states of ozone, for their stabilization into the covalent well, and for their survival in the van der Waals well are computed. The role these states may play in the kinetics of ozone formation is discussed.