Unimolecular Reaction Rate Research Articles

Is Quantum Above-Barrier Reflection Important for Molecular Barrier Crossing Rates?

Understanding quantum tunneling and above-barrier reflection effects on unimolecular and bimolecular reaction rate constants remains challenging to this very day. In many applications, especially when considering moderate-to-high temperatures, the "standard" procedure is to use the parabolic barrier approximation. Recent work has shown though that this may be insufficient, and one cannot ignore anharmonicity. In this work, we study the analytic theory, including anharmonicity obtained when expanding the thermal rate up to order ℏ4. Such theories need high-order derivatives of the potential at the barrier top. We show that such derivatives are computed straightforwardly for six different reactions. We suggest a straightforward methodology for assessing whether the parabolic barrier approximation is valid and show that when the reaction asymmetry is large, this may lead to significant quantum above-barrier reflection and transmission coefficients, which are less than unity.

Thermochemical and kinetic study of the recombination and dissociation reactions R + Br (+M) ⇄ RBr (+M) (R = CH3, CH2Cl, CHCl2, CHClBr, CH2Br and CHBr2)

The kinetics of the recombination and thermal dissociation reactions R + Br (+M) ⇄ RBr (+M) with R = CH3, CH2Cl, CHCl2, CHClBr, CH2Br and CHBr2 were theoretically studied over wide temperature and pressure ranges employing unimolecular reaction rate theory combined with potential energy surfaces derived at the ab initio G4//B3LYP/6-311++G(3df,3pd) level. The resulting rate constants are in good agreement with the available experimental information. Rate constant values for the reactions of CH2Cl, CHCl2, CHClBr, CH2Br and CHBr2 radicals with Br atoms are for a first time reported. Standard enthalpies of formation for the above and related molecules have been calculated.

Direct Kinetic Measurements of a Cyclic Criegee Intermediate; Unimolecular Decomposition of c-(CH2)5COO.

We report the first direct kinetic measurements of a cyclic stabilized Criegee Intermediate. We have measured the unimolecular reaction rate coefficient of cyclohexanone oxide (c-(CH2)5COO) in the temperature 213-296 K and pressure 7-50 Torr ranges using absorption spectrometry. The c-(CH2)5COO was produced by the photolysis of c-(CH2)5CIBr at 213 nm in the presence of O2. We compare the measured fast c-(CH2)5COO unimolecular rate coefficient, 1998 ± 147 s-1 at 296 K, with the literature calculations for the structurally similar E-nopinone oxide formed in β-pinene ozonolysis. The kuni(c-(CH2)5COO)/kuni(E-nopinone oxide) ratio calculated using transition-state theory and density functional theory agrees well with this comparison. We have also measured the bimolecular rate coefficient of the reaction between c-(CH2)5COO and trifluoroacetic acid at 253 K and 10 Torr and obtained the value (8.7 ± 1.0) × 10-10 cm3 molecule-1 s-1. This very large value agrees with previous kinetic measurements for reactions between stabilized Criegee intermediates and halogenated organic acids.

Polyrate 2023: A computer program for the calculation of chemical reaction rates for polyatomics. New version announcement

Polyrate 2023: A computer program for the calculation of chemical reaction rates for polyatomics. New version announcement

Expert Algorithm for Substance Identification Using Mass Spectrometry: Statistical Foundations in Unimolecular Reaction Rate Theory.

This study aims to resolve one of the longest-standing problems in mass spectrometry, which is how to accurately identify an organic substance from its mass spectrum when a spectrum of the suspected substance has not been analyzed contemporaneously on the same instrument. Part one of this two-part report describes how Rice-Ramsperger-Kassel-Marcus (RRKM) theory predicts that many branching ratios in replicate electron-ionization mass spectra will provide approximately linear correlations when analysis conditions change within or between instruments. Here, proof-of-concept general linear modeling is based on the 20 most abundant fragments in a database of 128 training spectra of cocaine collected over 6 months in an operational crime laboratory. The statistical validity of the approach is confirmed through both analysis of variance (ANOVA) of the regression models and assessment of the distributions of the residuals of the models. General linear modeling models typically explain more than 90% of the variance in normalized abundances. When the linear models from the training set are applied to 175 additional known positive cocaine spectra from more than 20 different laboratories, the linear models enabled ion abundances to be predicted with an accuracy of <2% relative to the base peak, even though the measured abundances vary by more than 30%. The same models were also applied to 716 known negative spectra, including the diastereomers of cocaine: allococaine, pseudococaine, and pseudoallococaine, and the residual errors were larger for the known negatives than for known positives. The second part of the manuscript describes how general linear regression modeling can serve as the basis for binary classification and reliable identification of cocaine from its diastereomers and all other known negatives.

Cavity-Modified Unimolecular Dissociation Reactions via Intramolecular Vibrational Energy Redistribution.

While the emerging field of vibrational polariton chemistry has the potential to overcome traditional limitations of synthetic chemistry, the underlying mechanism is not yet well understood. Here, we explore how the dynamics of unimolecular dissociation reactions that are rate-limited by intramolecular vibrational energy redistribution (IVR) can be modified inside an infrared optical cavity. We study a classical model of a bent triatomic molecule, where the two outer atoms are bound by anharmonic Morse potentials to the center atom coupled to a harmonic bending mode. We show that an optical cavity resonantly coupled to particular anharmonic vibrational modes can interfere with IVR and alter unimolecular dissociation reaction rates when the cavity mode acts as a reservoir for vibrational energy. These results lay the foundation for further theoretical work toward understanding the intriguing experimental results of vibrational polaritonic chemistry within the context of IVR.

Quantum-Chemical Kinetic Study of the Unimolecular Decomposition Reactions of 1-Bromo-3-Chloropropane

The pressure and temperature dependence of the thermal decomposition of 1-bromo-3-chloropropane has been theoretically investigated. The reaction takes place majorly through the elimination of HBr. Molecular properties of 1-bromo-3-chloropropane and transition states were derived from MN15/6-311++G(3df,3pd) and G4 quantum-chemical calculations. The resulting rate constants obtained from the unimolecular reaction rate theory for the high- and low-pressure limits of reaction BrCH2CH2CH2Cl → CH2CHCH2Cl + HBr at 400-1000 K were k∞ = 6.1 × 1013 exp(-57.2 kcal mol-1/RT) s-1 and k0 = [BrCH2CH2CH2Cl] 1.45 × 10-1 (T/1000 K)-7.9 exp(-55.9 kcal mol-1/RT) cm3 molecule-1 s-1. A value of -26.3 ± 1.0 kcal mol-1 for the standard enthalpy of formation of 1-bromo-3-chloropropane at 298 K was derived.

Large Pressure Effects Caused by Internal Rotation in the s-cis-syn-Acrolein Stabilized Criegee Intermediate at Tropospheric Temperature and Pressure.

Criegee intermediates are important atmospheric oxidants, and quantitative kinetics for stabilized Criegee intermediates are key parameters for atmospheric modeling but are still limited. Here we report barriers and rate constants for unimolecular reactions of s-cis-syn-acrolein oxide (scsAO), in which the vinyl group makes it a prototype for Criegee intermediates produced in the ozonolysis of isoprene. We find that the MN15-L and M06-2X density functionals have CCSD(T)/CBS accuracy for the unimolecular cyclization and stereoisomerization of scsAO. We calculated high-pressure-limit rate constants by the dual-level strategy that combines (a) high-level wave function-based conventional transition-state theory (which includes coupled-cluster calculations with quasiperturbative inclusion of quadruple excitations because of the strongly multiconfigurational character of the electronic wave function) and (b) canonical variational transition-state theory with small-curvature tunneling based on a validated density functional. We calculated pressure-dependent rate constants both by system-specific quantum Rice-Ramsperger-Kassel theory and by solving the master equation. We report rate constants for unimolecular reactions of scsAO over the full range of atmospheric temperature and pressure. We found that the unimolecular reaction rates of this larger-than-previously studied Criegee intermediate depend significantly on pressure. Particularly, we found that falloff effects decrease the effective unimolecular cyclization rate constant of scsAO by about a factor of 3, but the unimolecular reaction is still the dominant atmospheric sink for scsAO at low altitudes. The large falloff caused by the inclusion of the stereoisomerization channel in the master equation calculations has broad implications for mechanistic analysis of reactions with competitive internal rotations that can produce stable rotamers.

Comment on "Microhydration of Biomolecules: Revealing the Native Structures by Cold Ion IR Spectroscopy".

This Viewpoint presents a re-examination of the conclusions of a study reported in The Journal of Physical Chemistry Letters (Saparbaev, et al. 2021, 12, 907) that compared the structure of microsolvated ions formed by electrospray ionization to those formed in the gas-phase via a previously published cryogenic ion trap approach. We conducted additional experiments that clearly show that most of the observed differences in the IR spectra can be accounted for by considering the different spectroscopic action schemes used to obtain them. In particular, the presence of the D2-tag induces shifts in some of the N-H and O-H peaks which need to be carefully considered before comparing spectra. Once these spectral effects are taken into account, we show that both clustering approaches yield similar cluster structures for the small GlyH+(H2O)n species. Using unimolecular reaction rate theory, we also show that for the small complexes considered here, only the gas-phase equilibrium distribution of conformers should be expected in both experimental approaches. In addition, the barrier heights necessary to kinetically trap high-energy conformers at 298 K is explored using a series of model polyalanine chains.

Atmospheric oxidation of gaseous anthracene and phenanthrene initiated by OH radicals

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants. In the atmosphere, PAHs are oxidized to oxygenated PAHs (OPAHs) and nitrated PAHs (NPAHs). However, the formation mechanism of OPAHs and NPAHs are unclear. Here we investigated the oxidation mechanism of anthracene (ANT) and phenanthrene (PHE) initiated by OH radical using quantum chemistry and chemical kinetic calculations. The oxidation starts by forming PAH-OH adducts which then react with O2 and NO2 in the atmosphere. Reactions of PAH-OH and O2 start mainly by reversible OO addition, forming PAH-OH-OO radicals, which then undergo backward decomposition to PAH-OH + OO, forward unimolecular isomerizations to OPAH products, and bimolecular reaction with NO. Effective rate of each route depends on the equilibrium constant of O2 addition and the rates of forward unimolecular and bimolecular reactions of PAH-OH-OO. We predicted that ANT-9-OH, PHE-4-OH, and PHE-1-OH would react slowly with O2 with effective rates at 298 K of ~18, ~50, and ~18 s−1, forming OPAH (2,10-AQ for ANT); therefore, they would also react with atmospheric NO2, forming significant amounts of 9-NANT, 3-NPHE, and 2-NPHE, respectively. ANT-1-OH and PHE-10-OH would react rapidly with O2, forming OPAHs only. Predicted formation of 9-NANT and 3-NPHE agrees with field measurements in urban atmosphere where NPAHs are more likely formed from reactions of PAHs with OH and/or NO3. On the other hand, observation of 9-NPHE might suggest a primary source of NPAHs such as biomass burning because 9-NPHE is highly unlikely formed in OH- and/or NO3-initiated reaction of PHE.

Solvent softness effects on unimolecular chemical reaction rate constants

The stochastic hard collision (SHC) model for a coarse-grained solvent has been used to solvate a class of model chemical reactions. Although there is precedent from our prior work that the use of nondeterministic interaction potentials can lead to accurate dynamics at long-enough time scales, this work provides evidence that this model has dynamical consistency specifically in the response to the reactant motion. That is, the agreement between the theoretical rate constants and those obtained from the stochastic simulations provides support for the SHC solvent without sacrificing the correlations in the dynamics of the reacting solutes.

Unimolecular decomposition rates of a methyl-substituted Criegee intermediate syn-CH3CHOO.

Criegee intermediates play important roles in atmospheric chemistry. Methyl Criegee intermediate, CH3CHOO, has two conformers, syn- and anti-conformers. Syn-CH3CHOO would undergo fast unimolecular decomposition to form OH radical via 1,4 H-atom transfer. In this work, unimolecular decomposition of syn-CH3CHOO was probed in real time with UV absorption spectroscopy at 278–318 K and 100–700 torr. We used water vapor as the scavenger of anti-CH3CHOO to distinguish the absorption signals of the two conformers. After removing the contributions from reactions with radical byproducts, reaction with water vapor and wall loss, we obtained the unimolecular reaction rate coefficient of syn-CH3CHOO (at 300 torr), which increases from (67 ± 15) s−1 at 278 K, (146 ± 31) s−1 at 298 K, to (288 ± 81) s−1 at 318 K with an Arrhenius activation energy of ca. 6.4 kcal mol−1 and a weak pressure dependence for 100–700 torr. Compared to previous studies, this work provides temperature dependent unimolecular rates of syn-CH3CHOO at higher pressures, which are more relevant to atmospheric conditions.

Description of unimolecular reaction rates of Lennard-Jones clusters

The unimolecular rate constant of Lennard-Jones clusters with N = 54 − 56 atoms has been investigated in constant energy molecular dynamics simulations and compared with the detailed balance predictions. Decays were found to be statistical and absolute rate constants agree quantitatively with detailed balance predictions. The level densities required for the comparison of simulated and calculated rate constants were obtained from separate simulations using the integrated microcanonical temperature. The quantitative agreement between simulations and theory requires that the classical degeneracy of the ground state is included into the level densities.

Low-Temperature Kinetic Isotope Effects in CH3OH + H → CH2OH + H2 Shed Light on the Deuteration of Methanol in Space.

We calculated reaction rate constants including atom tunneling for the hydrogen abstraction reaction CH3OH + H → CH2OH + H2 with the instanton method. The potential energy was fitted by a neural network that was trained to UCCSD(T)-F12/VTZ-F12 data. Bimolecular gas-phase rate constants were calculated using microcanonic instanton theory. All H/D isotope patterns on the CH3 group and the incoming H atom are studied. Unimolecular reaction rate constants, representing the reaction on a surface, down to 30 K, are presented for all isotope patterns. At 30 K, they range from 4100 for the replacement of the abstracted H by D to ∼8 for the replacement of the abstracting H to ∼2 to 6 for secondary KIEs. The 12C/13C kinetic isotope effect is 1.08 at 30 K, while the 16O/18O kinetic isotope effect is extremely small. A simple kinetic surface model using these data predicts high abundances of the deuterated forms of methanol.

Unimolecular Reaction Rate Measurement of syn-CH3CHOO.

The unimolecular reactions of Criegee intermediates (CIs) are thought to be one of the significant sources of atmospheric OH radicals. However, stark discrepancies exist in the unimolecular reaction rate of the methyl-substituted CI CH3CHOO, typically from ozonolysis of alkenes such as trans-2-butene, between the results of ozonolysis of alkene experiments and the up-to-date theoretical calculations. That no further progress has been made since the method that directly produces CIs in the laboratory was developed is mostly attributed to the existence of two conformers, syn- and anti-CH3CHOO, and the methodological limitations of sensitive conformer-specific detection. We report a conformer-specific measurement of the unimolecular reaction rate of syn-CH3CHOO by using a high-repetition-rate laser-induced fluorescence method. At 298 K, the observed value of 182 ± 66 s-1 is in good agreement with recent theoretical calculations.

Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of α-Pinene and β-Pinene by Hydroxyl Radicals.

Atmospheric oxidation of monoterpenes (emitted primarily by evergreen trees) is known to contribute to the formation and growth of aerosol particles. While recent research has tied the formation of organic aerosol to unimolecular chemistry of the organic peroxy radicals (RO2) formed in the oxidation of monoterpenes, the fundamental physical chemistry of these RO2 remains obscure. Here we use isomer-specific measurements and ab initio calculations to determine the unimolecular reaction rates and products of RO2 derived from the hydroxyl radical (OH) oxidation of α-pinene and β-pinene. Among all of the structural isomers of the first-generation RO2 from both monoterpenes, we find that the first-generation RO2 produced following opening of the four-membered ring undergo fast unimolecular reactions (4 ± 2 and 16 ± 5 s-1 for α-pinene and β-pinene, respectively) at 296 K, in agreement with high-level ab initio calculations. The presence of the hydroxy group and carbon-carbon double bond in the ring-opened RO2 enhances the rates of these unimolecular reactions, including endo-cyclization and H-shift via transition states involving six- and seven-membered rings. These reaction rate coefficients are sufficiently large that unimolecular chemistry is the dominant fate of these monoterpene-derived RO2 in the atmosphere. In addition, the overall yields of first-generation α-pinene and β-pinene hydroxy nitrates, C10H17NO4, at 296 K and 745 Torr are measured to be 3.3 ± 1.5% and 6.4 ± 2.1%, respectively, for conditions where all RO2 are expected to react with NO ([NO] > 1000 ppbv). These yields are lower than anticipated.

Origin of Bath Gas Dependence in Unimolecular Reaction Rates.

The bath gas dependence of thermal unimolecular reaction rates arises from different rates and efficiencies of collisions between reactant and third-body molecules. This study aims to unravel the mechanistic origin of this dependence based on the classical trajectories of methyl isocyanide (CH3NC) colliding with 15 different bath gas molecules (CH3NC, He, Ar, H2, N2, CO, CO2, HCN, NH3, CH4, CH3F, CF4, C2H2, C2H4, and C2H6). The collision frequencies, energy transfer parameters, and relative third-body efficiencies are evaluated from the trajectory calculations. The relative third-body efficiencies of the studied bath gases are found to be in good agreement with available experimental data. The results indicate that differences in collision frequencies are the primary source of the bath gas dependence of the low-pressure rate constants. The nature of the long-range intermolecular interaction, particularly, its anisotropy, is suggested to play a key role in determining the collision frequency.

Theoretical Study of Gas-Phase Unimolecular Decomposition of Simulants of the Nerve Agent VX.

In order to further understand and support approaches for the degradation and destruction of toxic chemicals, the thermal decomposition of the nerve agent VX through possible pericyclic hydrogen transfer reactions is investigated using simulant molecules. A total of four simulant molecules are studied. Three of them have only one possible H-transfer site, while the other has two. They are chosen to bring physical insights into individual steps of the pericyclic reaction mechanism as well as the possible existence of competing mechanisms. The unimolecular reaction rate constants at the high-pressure limit are calculated. Geometries of stationary structures on the potential energy surfaces are calculated with the MP2 method as well as the B3LYP and M06-2X functionals and 6-311++G(d,p), jul-cc-pVTZ, and aug-cc-pVTZ basis sets. The barrier heights are corrected using energy values obtained at the CBS/QB3 level of theory. The contribution of the quantum tunneling effect to the reaction rate constants is included using one-dimensional semiclassical transition state theory. Adiabatic barrier heights, reaction rate constants, and branching ratio of the competing mechanisms are reported.

Unraveling the role of entropy in tuning unimolecular vs. bimolecular reaction rates: The case of olefin polymerization catalyzed by transition metals

Olefin polymerization catalyzed by Group 4 transition metals is studied here as test case to reveal the entropy effects when bimolecular and unimolecular reactions are computed for processes occurring in solution. Catalytic systems characterized by different ligand frameworks, metal, and growing polymeric chain for which experimental data are available have been selected in order to validate the main approaches to entropy calculation. Applying the “standard” protocol results in a strong disagreement with the experimental results and the methods introducing a direct correction of the translational entropy term based on a single experimental parameter emerge as the most reliable. The general and powerful computational tool achieved in this study can represent a further step towards the “catalyst design” to control and predict the molecular mass of the resulting polymers.

Collision Frequency for Energy Transfer in Unimolecular Reactions.

Pressure dependence of unimolecular reaction rates is governed by the energy transfer in collisions of reactants with bath gas molecules. Pressure-dependent rate constants can be theoretically determined by solving master equations for unimolecular reactions. In general, master equation formulations describe energy transfer processes using a collision frequency and a probability distribution model of the energy transferred per collision. The present study proposes a novel method for determining the collision frequency from the results of classical trajectory calculations. Classical trajectories for collisions of several polyatomic molecules (ethane, methane, tetrafluoromethane, and cyclohexane) with monatomic colliders (Ar, Kr, and Xe) were calculated on potential energy surfaces described by the third-order density-functional tight-binding method in combination with simple pairwise interaction potentials. Low-order (including non-integer-order) moments of the energy transferred in deactivating collisions were extracted from the trajectories and compared with those derived using some probability distribution models. The comparison demonstrates the inadequacy of the conventional Lennard-Jones collision model for representing the collision frequency and suggests a robust method for evaluating the collision frequency that is consistent with a given probability distribution model, such as the exponential-down model. The resulting collision frequencies for the exponential-down model are substantially higher than the Lennard-Jones collision frequencies and are close to the (hypothetical) capture rate constants for dispersion interactions. The practical adequacy of the exponential-down model is also briefly discussed.