Dependence of the Four-Atom Reaction HBr + OH → Br + H2O on Temperatures between 20 and 2000 K

Using quantum dynamics to study the effect of energy efficiency on the reactivity of the OH + DBr reaction

We report a time-dependent, full dimensional, wave-packet calculation for the reaction of OH + DBr to examine the effect of the energy efficiency on the reactivity. This study shows that the vibrational excitations of the OH and DBr enhance the reaction. However, the rotational excitations of OH and DBr both hinder the reaction. As a result, the vibrational energies of both the OH and DBr reactants are more efficient at promoting the reactivity than the translational energy, while the rotational energies of OH and DBr are less effective than the translational energy. By analyzing the state population of the vibrational and rotational states along the reaction pathway, we also developed an approach in order to explain the enhancement of the vibrational excitation and the hindrance of the rotational excitation of the reaction. We found that the initial-state selected vibrational excited states of OH and DBr are the dominant components, respectively, for surmounting the barrier. However, the initial-state selected rotational excited states of OH and DBr are no longer the dominant states for surmounting the transition state owing to their population changes in the van der Waals well. This quantitative analysis demonstrates the potential well in the entrance valley plays an important role in the energy efficiency with regards to the reactivity.

Rate Constant of the Reaction of OH Radicals with HBr over the Temperature Range 235-960 K

The kinetics of the reaction of hydroxyl radicals with HBr, important in atmospheric and combustion chemistry, has been studied in a discharge flow reactor combined with an electron impact ionization quadrupole mass spectrometer in the temperature range 235-960 K. The rate constant of the reaction OH + HBr → H₂O + Br (1) was determined using both a relative rate method (using the reaction of OH with Br₂ as a reference) and absolute measurements, monitoring the kinetics of OH consumption under pseudo-first-order conditions in excess of HBr. The observed U-shaped temperature dependence of k₁ is well represented by the sum of two exponential functions: k₁ = 2.53 × 10^-11 exp(-364/T) + 2.79 × 10^-13 exp(784/T) cm³ molecule^-1 s^-1 (with an estimated conservative uncertainty of 15% at all temperatures). This expression for k₁, recommended for T = 240-960 K, combined with that from previous low temperature studies, k₁ = 1.06 × 10^-11 (T/298)^-0.9 cm³ molecule^-1 s^-1 at T = 23-240 K, allows to describe the temperature behavior of the rate constant over an extended temperature range 23-960 K. The current direct measurements of k₁ at temperatures above 460 K, the only ones to date, provide an experimental dataset for use in combustion and volcanic plume modeling and an experimental basis to test theoretical calculations.

Vibrational Energy Flow in the Uracil-H2O Complexes

In the uracil-H₂O complex, the vibrational energy initially stored in the OH(v = 1) stretch efficiently transfers to the first overtone-bending mode under a near-resonant condition. The relaxation of the overtone vibration redistributes its energy to uracil and the two hydrogen bonds in the intermolecular zone, which consists of the OH bond and the bonds between nearby C, N, O, and H atoms of uracil. The uracil NH bond and the hydrogen bond it formed with the H₂O molecule, N-H···O, store the major portion of the energy released by the relaxing bending mode, thus forming a localized hot band in the intermolecular zone. Energy transfer to the bonds beyond the zone is found to be not significant. The excited uracil NH is found to transfer its energy to the bending mode, thus indicating that the hydrogen bond of N-H···O is the principal energy pathway in both directions. In the presence of efficient near-resonant energy transfer pathways, the time evolution of the centers of mass distance shows the phenomenon of beats. One global and two different local minima energy structures are considered. The results of energy transfer do not differ significantly, suggesting that the two hydrogen bonds in all three structures have similar contributions to the energy transfer.

Quantum dynamics study of kinetic isotope effects of OD with HBr and DBr

Comparison of kinetic isotope effects between quantum dynamics calculations and experiments shows that they agree well with each other both qualitatively and quantitatively.

Relaxation of the H2O Overtone Bending Vibration in the Water Dimer···Hydroxyl Radical Complex

The relaxation mechanism of the overtone bending vibration in the collision of the water dimer with the vibrationally excited hydroxyl radical is studied by use of trajectory procedures. The transfer of the OH(v = 1) energy to the dimer stretches is followed by a near-resonant first overtone transition to the donor monomer. Nearly a quarter of the trajectories undergo a complex-mode collision, forming the (H₂O)₂···OH complex bound by a hydrogen bond with the lifetime ranging from a subpicosecond scale to >100 ps. The overtone vibration relaxes to the ground state, transferring approximately half of its energy to the dimer hydrogen-bonding (H₂O···H₂O) and the remaining half to the complex hydrogen-bonding (H₂O)₂···OH, via near-resonant pathways, each consisting of a series of intermolecular low-frequency vibrations.

Energy Transfer to the Hydrogen Bond in the (H2O)2 + H2O Collision

Trajectory procedures are used to study the collision between the vibrationally excited H₂O and the ground-state (H₂O)₂ with particular reference to energy transfer to the hydrogen bond through the inter- and intramolecular pathways. In nearly 98% of the trajectories, energy transfer processes occur on a subpicosecond scale (≤0.7 ps). The H₂O transfers approximately three-quarters of its excitation energy to the OH stretches of the dimer. The first step of the intramolecular pathway in the dimer involves a near-resonant first overtone transition from the OH stretch to the bending mode. The energy transfer probability in the presence of the 1:2 resonance is 0.61 at 300 K. The bending mode then redistributes its energy to low-frequency intermolecular vibrations in a series of small excitation steps, with the pathway which results in the hydrogen-bonding modes gaining most of the available energy. The hydrogen bonding in ∼50% of the trajectories ruptures on vibrational excitation, leaving one quantum in the bend of the monomer fragment. In a small fraction of trajectories, the duration of collision is longer than 1 ps, during which the dimer and H₂O form a short-lived complex through a secondary hydrogen bond, which undergoes large amplitude oscillations.

Quantum dynamics study of energy requirement on reactivity for the HBr + OH reaction with a negative-energy barrier

A time-dependent, quantum reaction dynamics approach in full dimensional, six degrees of freedom was carried out to study the energy requirement on reactivity for the HBr + OH reaction with an early, negative energy barrier. The calculation shows both the HBr and OH vibrational excitations enhance the reactivity. However, even this reaction has a negative energy barrier, the calculation shows not all forms of energy are equally effective in promoting the reactivity. On the basis of equal amount of total energy, the vibrational energies of both the HBr and OH are more effective in enhancing the reactivity than the translational energy, whereas the rotational excitations of both the HBr and OH hinder the reactivity. The rate constants were also calculated for the temperature range between 5 to 500 K. The quantal rate constants have a better slope agreement with the experimental data than quasi-classical trajectory results.

Branching Ratios and Vibrational Distributions in Water-Forming Reactions of OH and OD Radicals with Methylamines

Reactions of OH and OD radicals with (CH3)3N, (CH3)2NH, and CH3NH2 were studied by Fourier transform infrared emission spectroscopy (FTIR) of the water product molecules from a fast-flow reactor at 298 K. The rate constants (4.4 ± 0.5) × 10(-11), (5.2 ± 0.8) × 10(-11), and (2.0 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) were determined for OD + (CH3)3N, (CH3)2NH, and CH3NH2, respectively, by comparing the HOD emission intensities to the HOD intensity from the OD reaction with H2S. Abstraction from the nitrogen site competes with abstraction from the methyl group, as obtained from an analysis of the HOD and D2O emission intensities from the OD reactions with the deuterated reactants, (CD3)2NH and CD3NH2. After adjustment for the hydrogen-deuterium kinetic isotope effect, the product branching fractions of the hydrogen abstraction from the nitrogen for di- and monomethylamine were found to be 0.34 ± 0.04 and 0.26 ± 0.05, respectively. Vibrational distributions of the H2O, HOD, and D2O molecules are typical for direct hydrogen atom abstraction from polar molecules, even though activation energies are negative because of the formation of pre-transition-state complexes. Comparison is made to the reactions of hydroxyl radicals with ammonia and with other compounds with primary C-H bonds to discuss specific features of disposal of energy to water product.

Stereodirectional Origin of anti-Arrhenius Kinetics for a Tetraatomic Hydrogen Exchange Reaction: Born-Oppenheimer Molecular Dynamics for OH + HBr

Among four-atom processes, the reaction OH + HBr → H2O + Br is one of the most studied experimentally: its kinetics has manifested an unusual anti-Arrhenius behavior, namely, a marked decrease of the rate constant as the temperature increases, which has intrigued theoreticians for a long time. Recently, salient features of the potential energy surface have been characterized and most kinetic aspects can be considered as satisfactorily reproduced by classical trajectory simulations. Motivation of the work reported in this paper is the investigation of the stereodirectional dynamics of this reaction as the prominent reason for the peculiar kinetics: we started in a previous Letter ( J. Phys. Chem. Lett. 2015 , 6 , 1553 - 1558 ) a first-principles Born-Oppenheimer "canonical" molecular dynamics approach. Trajectories are step-by-step generated on a potential energy surface quantum mechanically calculated on-the-fly and are thermostatically equilibrated to correspond to a specific temperature. Here, refinements of the method permitted a major increase of the number of trajectories and the consideration of four temperatures -50, +200, +350, and +500 K, for which the sampling of initial conditions allowed us to characterize the stereodynamical effect. The role is documented of the adjustment of the reactants' mutual orientation to encounter the entrance into the "cone of acceptance" for reactivity. The aperture angle of this cone is dictated by a range of directions of approach compatible with the formation of the specific HOH angle of the product water molecule; and consistently the adjustment is progressively less effective the higher the kinetic energy. Qualitatively, this emerging picture corroborates experiments on this reaction, involving collisions of aligned and oriented molecular beams, and covering a range of energies higher than the thermal ones. The extraction of thermal rate constants from this molecular dynamics approach is discussed and the systematic sampling of the canonical ensemble is indicated as needed for quantitative comparison with the kinetic experiments.

Rate constant calculations of the GeH4 + OH/OD → GeH3 + H2O/HOD reactions using an ab initio based full-dimensional potential energy surface

2D representation of the analytical potential energy surface. The saddle point and the complexes in the entry and exit channels are included.