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.

Experimental and Theoretical Study of the Kinetics of the CH3 + HBr → CH4 + Br Reaction and the Temperature Dependence of the Activation Energy of CH4 + Br → CH3 + HBr.

The rate coefficient of the reaction of CH3 with HBr was measured and calculated in the temperature range 225-960 K. The results of the measurements performed in a flow apparatus with mass spectrometric detection agree very well with the quasiclassical trajectory calculations performed on a previously developed potential energy surface. The experimental rate coefficients are described well with a double-exponential fit, k1(exp) = [1.44 × 10-12 exp(219/T) + 6.18 × 10-11 exp(-3730/T)] cm3 molecule-1 s-1. The individual rate coefficients below 500 K accord with the available experimental data as does the slightly negative activation energy in this temperature range, -1.82 kJ/mol. At higher temperatures, the activation energy was found to switch sign and it rises up to about an order of magnitude larger positive value than that below 500 K, and the rate coefficient is about 50% larger at 960 K than that around room temperature. The rate coefficients calculated with the quasiclassical trajectory method display the same tendencies and are within about 8% of the experimental data between 960 and 300 K and within 25% below that temperature. The significant variation of the magnitude of the activation energy can be reconciled with the tabulated heats of formation only if the activation energy of the reverse CH4 + Br reaction also significantly increases with the temperature.

Temperature and Pressure Dependence of the Reaction between Ethyl Radical and Molecular Oxygen: Experiments and Master Equation Simulations.

We have used laser-photolysis - photoionization mass-spectrometry to measure the rate coefficient for the reaction between ethyl radical and molecular oxygen as a function of temperature (190-801 K) and pressure (0.2-6 Torr) under pseudo-first-order conditions ([He] ≫ [O2] ≫ [C2H5•]). Multiple ethyl precursor, photolysis wavelength, reactor material, and coating combinations were used. We reinvestigated the temperature dependence of the title reaction's rate coefficient to resolve inconsistencies in existing data. The current results indicate that some literature values for the rate coefficient may indeed be slightly too large. The experimental work was complemented with master equation simulations. We used the current and some previous rate coefficient measurements to optimize the values of key parameters in the master equation model. After optimization, the model was able to reproduce experimental falloff curves and C2H4 + HO2• yields. We then used the model to perform simulations over wide temperature (200-1500 K) and pressure (10-4-102 bar) ranges and provide the results in PLOG format to facilitate their use in atmospheric and combustion models.

An experimental and computational study of the reaction between pent-3-en-2-yl radicals and oxygen molecules: switching from pure stabilisation to pure decomposition with increasing temperature.

We have used laser-photolysis-photoionization mass spectrometry, quantum chemical calculations, and master equation simulations to investigate the kinetics of the reaction between (E/Z)-pent-3-en-2-yl a resonance-stabilised hydrocarbon radical, and molecular oxygen. The time-resolved experiments were performed over a wide temperature range (240-750 K) at relatively low pressures (0.4-7 Torr) under pseudo-first-order conditions (excess [O2]). Helium bath gas was used in most experiments, but nitrogen was employed in a few measurements to investigate the effect of a heavier collider on the kinetics of the studied reaction. The experimental traces were directly used to optimise parameters in the master equation model using the recently implemented trace fitting feature in the MESMER program. At low temperatures (T < 300 K), the reaction proceeds by barrierless recombination reactions to form peroxyl adducts, and the radical traces are single-exponential. Between 326 K and 376 K, equilibration between the reactants and the peroxyl adducts is observed, and the radical traces are multi-exponential. Interestingly, at temperatures above 500 K, single-exponential decays were again observed, although the reaction is much slower than at low temperatures. The master equation simulations revealed that at both low and high temperatures, the radical decay rate is governed by a single eigenvalue. At low temperatures, this eigenvalue corresponds to recombination reactions, and at high temperatures to the phenomenological formation of bimolecular products. Between low and high temperatures (the exact temperature thresholds depend on [O2]), there is a region of avoided crossing in which the rate coefficient "jumps" from one eigencurve to the other. Although chemically significant eigenvalues are not well separated from internal energy relaxation eigenvalues at elevated temperatures (600 K at 0.01 bar, 850 K at 100 bar), we observed that many of the Bartis-Widom rate coefficients produced by the master equation model were valid up to 1500 K. Our simulations predict that the most important reaction channel at high temperatures is the formation of (E/Z)-penta-1,3-diene and hydroperoxyl. The experimentally constrained master equation model was used to simulate the title reaction over a wide range of conditions. To facilitate the use of our results in autoignition and combustion models, modified Arrhenius representations are given for the most important reaction 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.

An experimental and computational study of the reaction between 2-methylallyl radicals and oxygen molecules: optimizing master equation parameters with trace fitting.

We have investigated the reaction between 2-methylallyl radicals and oxygen molecules with experimental and computational methods. Kinetic experiments were conducted in a tubular laminar flow reactor using laser photolysis for radical production and photoionization mass spectrometry for detection. The reaction was investigated as a function of temperature (203-730 K) and pressure (0.2-9 torr) in helium and nitrogen bath gases. At low temperatures (T < 410 K), the reaction proceeds by a barrierless reaction to form 2-methylallylperoxyl. Equilibration of the peroxyl adduct and the reactants was observed between 350-410 K. Measurements were extended to even higher temperatures, up to 730 K, but no reaction could be observed. Master equation simulations of the reaction system were performed with the MESMER program. Kinetic parameters in the master equation model were optimized by direct fitting to time-resolved experimental 2-methylallyl traces. Trace fitting is a recently implemented novel feature in MESMER. The trace approach was compared with the more traditional approach where one uses experimental rate coefficients for parameter optimization. The optimized parameters yielded by the two approaches are very similar and do an excellent job at reproducing the experimental data. The optimized master equation model was then used to simulate the reaction under study over a wide temperature and pressure range, from 200 K and 0.01 bar to 1500 K and 100 bar. The simulations predict a small phenomenological rate coefficient under autoignition conditions; about 1 × 10-18 cm3 s-1 at 400 K and 5 × 10-16 cm3 s-1 at 1000 K. We provide modified Arrhenius expressions in PLOG format for the most important product channels to facilitate the use of our results in combustion models.

The Role of Zero-Point Vibration and Reactant Attraction in Exothermic Bimolecular Reactions with Submerged Potential Barriers: Theoretical Studies of the R + HBr → RH + Br (R = CH3, HO) Systems.

The dynamics of the reactions CH3 + HBr → CH4 + Br and HO + HBr → H2O + Br have been studied using the quasiclassical trajectory method to explore the interplay of the vibrational excitation of the breaking bond and the potential energy surface characterized by a prereaction van der Waals well and a submerged barrier to reaction. The attraction between the reactants is favorable for the reaction, because it brings together the reactants without any energy investment. The reaction can be thought to be controlled by capture. The trajectory calculations indeed provide excitation functions typical to capture: the reaction cross sections diverge when the collision energy is reduced toward zero. Excitation of reactant vibration accelerates both reactions. The barrier on the potential surface is so early that the coupling between the degrees of freedom at the saddle point geometry is negligible. However, the trajectory calculations show that when the breaking bond is stretched at the time of the encounter, an attractive force arises, as if the radical approached a HBr molecule whose bond is partially broken. As a result, the dynamics of the reaction are controlled more by the temporary "dynamical", vibrationally induced than by the "static" van der Waals attraction even when the reactants are in vibrational ground state. The cross sections are shown to drop to very small values when the amplitude of the breaking bond's vibration is artificially reduced, which provides an estimate of the reactivity due to the "static" attraction. Without zero-point vibration these reactions would be very slow, which is a manifestation of a unique quantum effect. Reactions where the reactivity is determined by dynamical factors such as the vibrationally enhanced attraction are found to be beyond the range of applicability of Polanyi's rules.

Solid-Phase "Self-Hydrolysis" of [Zn(NH3)4MoO4@2H2O] Involving Enclathrated Water-An Easy Route to a Layered Basic Ammonium Zinc Molybdate Coordination Polymer.

An aerial humidity-induced solid-phase hydrolytic transformation of the [Zn(NH3)4]MoO4@2H2O (compound 1@2H2O) with the formation of [(NH4)xH(1-x)Zn(OH)(MoO4)]n (x = 0.92-0.94) coordination polymer (formally NH4Zn(OH)MoO4, compound 2) is described. Based on the isostructural relationship, the powder XRD indicates that the crystal lattice of compound 1@2H2O contains a hydrogen-bonded network of tetraamminezinc (2+) and molybdate (2-) ions, and there are cavities (O4N4(µ-H12) cube) occupied by the two water molecules, which stabilize the crystal structure. Several observations indicate that the water molecules have no fixed positions in the lattice voids; instead, the cavity provides a neighborhood similar to those in clathrates. The @ symbol in the notation is intended to emphasize that the H2O in this compound is enclathrated rather than being water of crystallization. Yet, signs of temperature-dependent dynamic interactions with the wall of the cages can be detected, and 1@2H2O easily releases its water content even on standing and yields compound 2. Surprisingly, hydrolysis products of 1 were observed even in the absence of aerial humidity, which suggests a unique solid-phase quasi-intramolecular hydrolysis. A mechanism involving successive substitution of the ammonia ligands by water molecules and ammonia release is proposed. An ESR study of the Cu-doped compound 2 (2#dotCu) showed that this complex consists of two different Cu2+(Zn2+) environments in the polymeric structure. Thermal decomposition of compounds 1 and 2 results in ZnMoO4 with similar specific surface area and morphology. The ZnMoO4 samples prepared from compounds 1 and 2 and compound 2 in itself are active photocatalysts in the degradation of Congo Red dye. IR, Raman, and UV studies on compounds 1@2H2O and 2 are discussed in detail.

(Me2NH2)10[H2-Dodecatungstate] polymorphs: dodecatungstate cages embedded in a variable dimethylammonium cation + water of crystallization matrix.

Two polymorphs and a solvatomorph of a new dimethylammonium polytungstate-decakis(dimethylammonium) dihydrogendodecatungstate, (Me2NH2)10(W12O42)·nH2O (n = 10 or 11)-have been synthesized. Their structures were characterized by single-crystal X-ray diffraction and solid-phase NMR methods. The shape of the dodecatungstate anions is essentially the same in all three structures, their interaction with the cations and water of crystallization, however, is remarkably variable, because the latter forms different hydrogen-bonded networks, and provides a highly versatile matrix. Accordingly, the N-H⋯O and C-H⋯O hydrogen bonds are positioned in each crystal lattice in a variety of environments, characteristic to the structure, which can be distinguished by solid-state 1H-CRAMPS, 13C, 15N CP MAS and 1H-13C heteronuclear correlation NMR. Thermogravimetry of the solvatomorphs also reflect the difference and multiformity of the environment of the water molecules in the different crystal lattices. The major factors behind the variability of the matrix are the ability of ammonium cations to form two hydrogen bonds and the rigidity of the polyoxometalate anion cage. The positions of the oxygen atoms in the latter are favourable for the formation of bifurcated and trifurcated cation-anion hydrogen bonds, some which are so durable that they persist after the crystals are dissolved in water, forming ion associates even in dilute solutions. The H atom involved in furcated hydrogen bonds cannot be exchanged by deuterium when the compound is dissolved in D2O. An obvious consequence of the versatility of the matrix is the propensity of these compounds to form multiple polymorphs.

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.

Kinetics of the Methyl-Vinyl Radical + O2 Reactions Associated with Propene Oxidation.

The bimolecular rate coefficients of reactions CH3CCH2 + O2 (1) and cis/ trans-CH3CHCH + O2 (2a/3a) have been measured using a tubular laminar flow reactor coupled with a photoionization mass spectrometer (PIMS). These reactions are relevant in the combustion of propene. Pulsed excimer laser photolysis of a ketone or a bromide precursor molecule at 193 or 248 nm wavelength was used to produce radicals of interest homogeneously along the reactor. Time-resolved experiments were performed under pseudo-first-order conditions at low pressure (0.3-1.5 Torr) over the temperature range 220-660 K. The measured bimolecular rate coefficients were found to be independent of bath gas concentration. The bimolecular rate coefficients possess negative temperature dependence at low temperatures ( T < 420 K) and appear to be independent of temperature at high temperatures ( T > 420 K). Observed products of the reaction CH3CCH2 + O2 were CH3 and H2CO, while for the reaction cis/trans-CH3CHCH + O2, observed products were CH3CHO and HCO. Current results indicate that the reaction mechanism of both reactions is analogous to that of C2H3 + O2. Methyl substitution of the vinyl radical changes its reactivity toward O2 upward by ca. 50% if it involves the α-position and downward by ca. 30% if the methyl group takes either of the ß-positions, respectively.

Unexpected Sequential NH3/H2O Solid/Gas Phase Ligand Exchange and Quasi-Intramolecular Self-Protonation Yield [NH4Cu(OH)MoO4], a Photocatalyst Misidentified before as (NH4)2Cu(MoO4)2.

[NH4Cu(OH)MoO4] as active photocatalyst in the decomposition of Congo Red when irradiated by UV or visible light has been prepared in an unusual ammonia/water ligand exchange reaction of [tetraamminecopper(II)] molybdate, [Cu(NH3)4]MoO4. [Cu(NH3)4]MoO4 was subjected to moisture of open air at room temperature. Light blue orthorhombic [Cu(NH3)(H2O)3]MoO4 was formed in 2 days as a result of an unexpected solid/gas phase ammonia-water ligand exchange reaction. This complex does not lose its last ammonia ligand on further standing in open air; however, a slow quasi-intramolecular (self)-protonation reaction takes place in 2-4 weeks, producing a yellowish-green microcrystalline material, which has been identified as a new compound, [NH4Cu(OH)MoO4], ( a = 10,5306 Å, b = 6.0871 Å, c = 8.0148 Å, ß = 64,153°, C2, Z = 4). Mechanisms are proposed for both the sequential ligand exchange and the self-protonation reactions supported by ab initio quantum-chemical calculations and deuteration experiments as well. The [Cu(NH3)(H2O)3]MoO4 intermediate transforms into NH4Cu(OH)(H2O)2MoO4, which loses two waters and yields [NH4Cu(OH)MoO4]. Upon heating, both [Cu(NH3)4]MoO4 and [Cu(NH3)(H2O)3]MoO4 decompose, losing three NH3 and three H2O ligands, respectively, and stable [Cu(NH3)MoO4] is formed from both. The latter can partially be hydrated in boiling water into [NH4Cu(OH)MoO4. This compound can also be prepared in pure form by boiling the saturated aqueous solution of [Cu(NH3)4]MoO4. All properties of [NH4Cu(OH)MoO4] match those of the active photocatalyst described earlier in the literature under the formulas (NH4)2[Cu(MoO4)2] and (NH4)2Cu4(NH3)3Mo5O20.

Substituent effect on the dynamics of the inclusion complex formation between protoberberine alkaloids and cucurbit[7]uril.

The kinetics of entry into and exit from the cavity of cucurbit[7]uril (CB7) was studied by the stopped-flow method in water at various temperatures using pharmaceutically important natural isoquinoline alkaloids as guest compounds. The rate constant of the alkaloid-CB7 complex dissociation was separately determined exploiting the very strong competitive binding of the 1-adamantylammonium cation to CB7. The enthalpy and entropy of activation for the release of berberine from CB7 were significantly lower than those found in the case of the other alkaloids, suggesting different dissociation dynamics. CB7 initially moved from the energetically most stable position encompassing the isoquinoline moiety of berberine to the smaller benzodioxole part, which could leave the macrocycle with less structural distortion. Despite the same thermodynamic parameters of berberine and palmatine inclusion, the latter compound was encapsulated in and set free from CB7 much slower due to the more substantial steric hindrance. The most rapid entry into CB7 and the most exothermic binding were found for epiberberine and coptisine, the alkaloids substituted with the less spacious dioxole ring on their isoquinoline moiety.

A general formulation of the quasiclassical trajectory method for reduced-dimensionality reaction dynamics calculations.

Dimension reduction by freezing the unimportant coordinates is widely used in intramolecular and reaction dynamics calculations when the solution of the accurate full-dimensional nuclear Schrödinger equation is not feasible. In this paper we report on a novel form of the exact classical internal-coordinate Hamiltonian for full and reduced-dimensional vibrational motion of polyatomic molecules with the purpose of using it in quasiclassical trajectory (QCT) calculations. The derivation is based on the internal to body-fixed frame transformation, as in the t-vector formalism, however it does not require the introduction of rotational variables to allow cancellation of non-physical rotations within the body-fixed frame. The formulas needed for QCT calculations: normal mode analysis and state sampling as well as for following the dynamics and normal-mode quantum number assignment at instantaneous states are presented. The procedure is demonstrated on the CH4, CD4, CH3D and CHD3 isotopologs of methane using three reduced-dimensional models, which were previously used in quantum reactive scattering studies of the CH4 + X → CH3 + HX type reactions. The reduced-dimensional QCT methodology formulated this way combined with full-dimensional QCT calculations makes possible the classical validation of reduced-dimensional models that are used in the quantum mechanical description of the nuclear dynamics in reactive systems [A. Vikár et al., J. Phys. Chem. A, 2016, 120, 5083-5093].

Adiabatic Switching Extended To Prepare Semiclassically Quantized Rotational-Vibrational Initial States for Quasiclassical Trajectory Calculations.

An approximation-free adiabatic switching method to generate semiclassically quantized ensembles of rovibrational states of polyatomic molecules for use as initial conditions in quasiclassical trajectory calculations is presented. Vibrational states are prepared, starting from an ensemble of classical states corresponding to the desired quantum state of the normal-mode Hamiltonian by slowly switching on the anharmonicity in internal coordinates, thereby avoiding rotational contamination. To generate rovibrational states, an extension is proposed: The vibrationally quantized molecules are slowly spun up to the desired quantized angular momentum. The ensembles obtained with adiabatic switching for CH4 are insensitive to the choice of internal coordinates and stationary; furthermore, their mean energies agree remarkably well with the quantum mechanical values: The zero-point energy and 15 vibrational levels of the first three polyads are within 20 cm-1, the rotational levels are between J = 1 and 50 within 1%, and the standard deviation is always <1%. Adiabatic switching produces classical state ensembles with significantly better properties than normal-mode sampling, making them more appropriate in quasiclassical trajectory calculations.

Dimol Emission of Oxygen Made Possible by Repulsive Interaction.

For the energy emitted in a textbook example of chemiluminescence, the peculiar red light produced by singlet molecular oxygen is about twice that of the spin-forbidden O2(a1Δg) → O2(X3∑g-) transition. Theoretical studies suggest that the O2(a1Δg)-O2(a1Δg) van der Waals interaction is weak, and at room temperature no long-lived complex is formed. Our high-level ab initio calculations show that in the bound domain of the dimer, the oscillator strength is very small, but increases at smaller intermolecular separations, where, however, the interaction is repulsive. We propose that the emission is induced by collisions: it takes place "on-the-fly", when the collision energy allows the system to access the repulsive part of the potential energy surface where the oscillator strength is relatively large. The contribution of different orientations of the two O2 molecules to the emission has been evaluated with a simple semiclassical model. The position of the emission peak is in accord with the experiment, and the estimated rate coefficient of collision-induced emission averaged over orientation is in reasonable agreement with the measurements.

Testing the Palma-Clary Reduced Dimensionality Model Using Classical Mechanics on the CH4 + H → CH3 + H2 Reaction.

Application of exact quantum scattering methods in theoretical reaction dynamics of bimolecular reactions is limited by the complexity of the equations of nuclear motion to be solved. Simplification is often achieved by reducing the number of degrees of freedom to be explicitly handled by freezing the less important spectator modes. The reaction cross sections obtained in reduced-dimensionality (RD) quantum scattering methods can be used in the calculation of rate coefficients, but their physical meaning is limited. The accurate test of the performance of a reduced-dimensionality method would be a comparison of the RD cross sections with those obtained in accurate full-dimensional (FD) calculations, which is not feasible because of the lack of complete full-dimensional results. However, classical mechanics allows one to perform reaction dynamics calculations using both the RD and the FD model. In this paper, an RD versus FD comparison is made for the 8-dimensional Palma-Clary model on the example of four isotopologs of the CH4 + H → CH3 + H2 reaction, which has 12 internal dimensions. In the Palma-Clary model, the only restriction is that the methyl group is confined to maintain C3v symmetry. Both RD and FD opacity and excitation functions as well as differential cross sections were calculated using the quasiclassical trajectory method. The initial reactant separation has been handled according to our one-period averaging method [ Nagy et al. J. Chem. Phys. 2016, 144, 014104 ]. The RD and FD excitation functions were found to be close to each other for some isotopologs, but in general, the RD reactivity parameters are lower than the FD reactivity parameters beyond statistical error, and for one of the isotopologs, the deviation is significant. This indicates that the goodness of RD cross sections cannot be taken for granted.

Oscillatory reaction cross sections caused by normal mode sampling in quasiclassical trajectory calculations.

The quasiclassical trajectory (QCT) method is an efficient and important tool for studying the dynamics of bimolecular reactions. In this method, the motion of the atoms is simulated classically, and the only quantum effect considered is that the initial vibrational states of reactant molecules are semiclassically quantized. A sensible expectation is that the initial ensemble of classical molecular states generated this way should be stationary, similarly to the quantum state it is supposed to represent. The most widely used method for sampling the vibrational phase space of polyatomic molecules is based on the normal mode approximation. In the present work, it is demonstrated that normal mode sampling provides a nonstationary ensemble even for a simple molecule like methane, because real potential energy surfaces are anharmonic in the reactant domain. The consequences were investigated for reaction CH4 + H → CH3 + H2 and its various isotopologs and were found to be dramatic. Reaction probabilities and cross sections obtained from QCT calculations oscillate periodically as a function of the initial distance of the colliding partners and the excitation functions are erratic. The reason is that in the nonstationary ensemble of initial states, the mean bond length of the breaking C-H bond oscillates in time with the frequency of the symmetric stretch mode. We propose a simple method, one-period averaging, in which reactivity parameters are calculated by averaging over an entire period of the mean C-H bond length oscillation, which removes the observed artifacts and provides the physically most reasonable reaction probabilities and cross sections when the initial conditions for QCT calculations are generated by normal mode sampling.

Dynamics of Complex-Forming Bimolecular Reactions: A Comparative Theoretical Study of the Reactions of H Atoms with O2((3)Σg(-)) and O2((1)Δg).

The atomic-level mechanism of the reaction of H atoms with triplet and singlet molecular oxygen, H((2)S) + O2((3)Σg(-)) → O((3)P) + OH((2)Πg) ( R1 ) and H((2)S) + O2((1)Δg) → O((3)P) + OH((2)Πg) ( R2 ) is analyzed in terms of the topology of the potential energy surfaces (PES) of the two reactions. Both PES exhibit a deep potential well corresponding to the ground and first excited electronic state of HO2. The ground-state reaction is endothermic with no barrier on either side of the well; the excited-state reaction is exothermic with a barrier in the entrance valley of the PES. The differences of the PES are manifested in properties such as the excitation functions, which show reaction R1 to be much slower and the effect of rotational excitation on reactivity, which speeds up reaction R1 and has little effect on R2 . Numerous common dynamics features arise from the presence of the deep potential well on the PES. Such are the significant role of isomerization (for example, 90% of reactive collisions in R2 involve at least one H atom transfer from one of the O atoms to the other in reaction R2 ), which is shown to give rise to a significant rotational excitation of the product OH radicals. Common is the significant sideways scattering of the products that originates from collisions in propeller-type arrangements induced by the presence of two bands of acceptance around the O2 molecule. The HO2 complex in both reactions proves to behave nonstatistically, with signatures of the dynamics in lifetime distributions, angular distributions, opacity functions, and product quantum-state distributions.