Kinetics of the Gas-Phase Reaction of OH with HCl

OH + HCl Reaction on the Surface of Ice: An Ab Initio Molecular Dynamics Study

We have investigated the OH + HCl reaction on the surface of ice using Born-Oppenheimer molecular dynamics (BOMD) simulation. The present work revealed that the OH + HCl reaction becomes ∼1 order of magnitude faster on the ice surface compared to the bare reaction. The BOMD simulation also indicates that the Cl radical formed on the ice surface through the title reaction can form two hydrogen bonds at a time with the water molecules present on the ice surface; hence, the Cl radical cannot escape from the ice surface easily.

Strong non-Arrhenius behavior at low temperatures in the OH + HCl → H2O + Cl reaction due to resonance induced quantum tunneling

The OH + HCl reaction possesses many Feshbach resonances trapped in the hydrogen bond well in the entrance channel, which substantially enhance the reaction rates at low temperatures.

Effect of microsolvation on the mode specificity of the OH˙(H2O) + HCl reaction

The present study investigates the mode specificity in the microsolvated OH˙(H₂O) + HCl reaction using on-the-fly direct dynamics simulation. To the best of our knowledge, this is the first study which aims to gain insights into the effect of microsolvation on the mode selectivity. Our investigation reveals that, similar to the gas phase OH˙ + HCl reaction, the microsolvated reaction is also predominantly affected by the vibrational excitation of the HCl mode, whereas the OH vibrational mode behaves as a spectator. Interestingly, in contrast to the behavior of the bare reaction, the integral cross section at the ground state of the microsolvated reaction decreases with an increase in translational energy. However, for the vibrational excited states, the reactivity of the microsolvated reaction is found to be higher than that of the bare reaction within the selected range of translational energies.

Kinetics of the OH+HCl→H2 O+Cl reaction: Rate determining roles of stereodynamics and roaming and of quantum tunneling

The OH + HCl → H₂ O + Cl reaction is one of the most studied four-body systems, extensively investigated by both experimental and theoretical approaches. Here, as a continuation of our previous work on the OH + HBr and OH + HI reactions, which manifest an anti-Arrhenius behavior that was explained by stereodynamic and roaming effects, we extend the strategy to understand the transition to the sub-Arrhenius behavior occurring for the HCl case. As previously, we perform first-principles on-the-fly Born-Oppenheimer molecular dynamics calculations, thermalized at four temperatures (50, 200, 350, and 500 K), but this time we also apply a high-level transition-state-theory, modified to account for tunneling conditions. We find that the theoretical rate constants calculated with Bell tunneling corrections are in good agreement with extensive experimental data available for this reaction in the ample temperature range: (i) simulations show that the roles of molecular orientation in promoting this reaction and of roaming in finding the favorable path are minor than in the HBr and HI cases, and (ii) dominating is the effect of quantum mechanical penetration through the energy barrier along the reaction path on the potential energy surface. The discussion of these results provides clarification of the origin on different non-Arrhenius mechanisms observed along this series of reactions. © 2018 Wiley Periodicals, Inc.

Accurate Determination of Tunneling-Affected Rate Coefficients: Theory Assessing Experiment

The thermal rate coefficients of a prototypical bimolecular reaction are determined on an accurate ab initio potential energy surface (PES) using ring polymer molecular dynamics (RPMD). It is shown that quantum effects such as tunneling and zero-point energy (ZPE) are of critical importance for the HCl + OH reaction at low temperatures, while the heavier deuterium substitution renders tunneling less facile in the DCl + OH reaction. The calculated RPMD rate coefficients are in excellent agreement with experimental data for the HCl + OH reaction in the entire temperature range of 200-1000 K, confirming the accuracy of the PES. On the other hand, the RPMD rate coefficients for the DCl + OH reaction agree with some, but not all, experimental values. The self-consistency of the theoretical results thus allows a quality assessment of the experimental data.

A global coupled cluster potential energy surface for HCl + OH ↔ Cl + H2O

A new and more accurate full-dimensional global potential energy surface (PES) for the ground electronic state of the ClH₂O system is developed by using the permutation invariant polynomial-neural network (PIP-NN) method to fit 15 777 points obtained using an explicitly correlated unrestricted coupled-cluster method with single, double, and perturbative triple excitations (UCCSD(T)-F12b).

Role of Double Hydrogen Atom Transfer Reactions in Atmospheric Chemistry

Hydrogen atom transfer (HAT) reactions are ubiquitous and play a crucial role in chemistries occurring in the atmosphere, biology, and industry. In the atmosphere, the most common and traditional HAT reaction is that associated with the OH radical abstracting a hydrogen atom from the plethora of organic molecules in the troposphere via R-H + OH → R + H2O. This reaction motif involves a single hydrogen transfer. More recently, in the literature, there is an emerging framework for a new class of HAT reactions that involves double hydrogen transfers. These reactions are broadly classified into four categories: (i) addition, (ii) elimination, (iii) substitution, and (iv) rearrangement. Hydration and dehydration are classic examples of addition and elimination reactions, respectively whereas tautomerization or isomerization belongs to a class of rearrangement reactions. Atmospheric acids and water typically mediate these reactions. Organic and inorganic acids are present in appreciable levels in the atmosphere and are capable of facilitating two-point hydrogen bonding interactions with oxygenates possessing an hydroxyl and/or carbonyl-type functionality. As a result, acids influence the reactivity of oxygenates and, thus, the energetics and kinetics of their HAT-based chemistries. The steric and electronic effects of acids play an important role in determining the efficacy of acid catalysis. Acids that reduce the steric strain of 1:1 substrate···acid complex are generally better catalysts. Among a family of monocarboxylic acids, the electronic effects become important; barrier to the catalyzed reaction correlates strongly with the pKa of the acid. Under acid catalysis, the hydration of carbonyl compounds leads to the barrierless formation of diols, which can serve as seed particles for atmospheric aerosol growth. The hydration of sulfur trioxide, which is the principle mechanism for atmospheric sulfuric acid formation, also becomes barrierless under acid catalysis. Rate calculations suggest that such acid catalysis play a key role in the formation of sulfuric acid in the Earth's stratosphere, Venusian atmosphere, and on heterogeneous surfaces. Over the past few years, theoretical calculations have shown that these acid-mediated double hydrogen atom transfers are important in the chemistry of Earth's atmosphere as well as that of other planets. This Account reviews and puts into perspective some of these atmospheric HAT reactions and their environmental significance.

Kinetics of the gas-phase reaction of OH with chlorobenzene

The kinetics of the reaction of hydroxyl radicals with chlorobenzene was studied experimentally using a pulsed laser photolysis/pulsed laser induced fluorescence technique over a wide range of temperatures, 298-670 K, and at pressures between 13.33 and 39.92 kPa. The bimolecular rate constants demonstrate different behavior at low and high temperatures. At room temperature, T = 298.8 +/- 1.5 K, the rate constant is equal to (6.02 +/- 0.34) x 10(-13) cm3 molecule(-1) s(-1); at high temperatures (474-670 K), the rate constant values are significantly lower and have a positive temperature dependence that can be described by an Arrhenius expression k1(T) = (1.01 +/- 0.35) x 10(-11) exp[(-2490 +/- 170 K)/T] cm3 molecule(-1) s(-1). This behavior is consistent with the low-temperature reaction being dominated by reversible addition and the high-temperature reaction representing abstraction and addition-elimination channels. The potential energy surface of the reaction was studied using quantum chemical methods, and a transition state theory model was developed for all reaction channels. The temperature dependences of the high-temperature rate constants obtained in calculations using the method of isodesmic reactions for transition states (IRTS) and the CBS-QB3 method are in very good agreement with experiment, with deviations smaller than the estimated experimental uncertainties. The G3//B3LYP-based calculated rate constants are in disagreement with the experimental values. The IRTS-based model was used to provide modified Arrhenius expressions for the temperature dependences of the rate constant for the abstraction and addition-elimination (Cl replacement) channels of the reaction.

Temperature-Dependent Kinetics Study of the Gas-Phase Reactions of OH with n- and i-Propyl Bromide

An experimental, temperature-dependent kinetics study of the gas-phase reactions of hydroxyl radical with n-propyl bromide, OH+n-C3H7Br-->products (reaction 1), and i-propyl bromide, OH+i-C3H7Br-->products (reaction 2), has been performed over wide ranges of temperatures 297-725 and 297-715 K, respectively, and at pressures between 6.67 and 26.76 kPa by a pulsed laser photolysis/pulsed laser-induced fluorescence technique. Data sets of absolute bimolecular rate coefficients obtained in this study for reactions 1 and 2 demonstrate no correlation with pressure and exhibit positive temperature dependencies that can be represented with modified three-parameter Arrhenius expressions within their corresponding experimental temperature ranges: k1(T)=(1.32x10(-17))T1.95 exp(+25/T) cm3 molecule(-1) s(-1) for reaction 1 and k2(T)=(1.56x10(-24))T4.18exp(+922/T) cm3 molecule(-1) s(-1) for reaction 2. The present results, which extend the current kinetics data base of reactions 1 and 2 to high temperatures, are compared with those from previous works. On the basis of the present data and available data from previous studies, the following bimolecular rate coefficient temperature dependencies can be recommended for the purpose of kinetic modeling: k1(T)=(1.89x10(-19))T2.54exp(+301/T) cm3 molecule-1 s-1 for reaction 1 in a temperature range 210-725 K, and k2(T)=(2.83x10(-21))T3.1exp(+521/T) cm3 molecule(-1) s(-1) and k2(T)=(4.54x10(-24))T4.03exp(+860/T) cm3 molecule(-1) s(-1) for reaction 2 in temperature ranges 210-480 and 297-715 K, respectively.