Reaction Rate Coefficients Research Articles

Photodegradation of MC-LR using a novel Au-decorated Ni metal-organic framework (Au/Ni-MOF)

Microcystins (MCs) are toxins produced by cyanobacteria commonly found in harmful algal blooms (HAB) occurring in many surface waters. Conventional methods for removing MC-LR such as membrane filtration and activated carbon are only phase change removal methods and are often expensive in operation and maintenance. It is urgent to develop a rapid, easy-to-use, and cost-effective method for the degradation of MC-LR. In this study, a novel Au-decorated Ni-metal-organic framework (Au/Ni-MOF) was newly developed on a hydrophilic carbon fiber paper (2 cm × 2 cm) using an air spraying method. The Au/Ni-MOF was then applied for the photodegradation of MC-LR in water under UV–Vis. The addition of Au onto the surface of the Ni-MOF resulted in a nearly fivefold enhancement in the reaction rate coefficient (k), reaching a value of 0.0599 min−1 for the photodegradation of MC-LR (initial concentration of 20 ppb). It was found that 94.2% of MC-LR removal was attributed to photodegradation, with the remaining 5.8% from adsorption. The rate coefficient of 20 ppb of MC-LR in the surface water sample (pH 6.0) was 0.06 min−1 likely due to the presence of other contaminates including scavenger agents within the sample which inhibits the degradation reaction of the MC-LR. Overall, this study demonstrated the potential for the novel Au/Ni-MOF to effectively reduce the concentration of the MC-LR toxin in the contaminated water.

Persistent effects of inertia on diffusion-influenced reactions: Theoretical methods and applications.

The Cattaneo-Vernotte model has been widely studied to take momentum relaxation into account in transport equations. Yet, the effect of reactions on the Cattaneo-Vernotte model has not been fully elucidated. At present, it is unclear how the current density associated with reactions can be expressed in the Cattaneo-Vernotte model. Herein, we derive a modified Cattaneo-Vernotte model by applying the projection operator method to the Fokker-Planck-Kramers equation with a reaction sink. The same modified Cattaneo-Vernotte model can be derived by a Grad procedure. We show that the inertial effect influences the reaction rate coefficient differently depending on whether the intrinsic reaction rate constant in the reaction sink term depends on the solute relative velocity or not. The momentum relaxation effect can be expressed by a modified Smoluchowski equation including a memory kernel using the Cattaneo-Vernotte model. When the intrinsic reaction rate constant is independent of the reactant velocity and is localized, the modified Smoluchowski equation should be generalized to include a reaction term without a memory kernel. When the intrinsic reaction rate constant depends on the relative velocity of reactants, an additional reaction term with a memory kernel is required because of competition between the current density associated with the reaction and the diffusive flux during momentum relaxation. The competition effect influences even the long-time reaction rate coefficient.

Combined model for 15N, 13C, and spin-state chemistry in molecular clouds

We present a new gas-grain chemical model for the combined isotopic fractionation of carbon and nitrogen in molecular clouds. To this end, we have developed gas-phase and grain-surface chemical networks where the isotope chemistry of carbon and nitrogen is coupled with a time-dependent description of spin-state chemistry, which is important for nitrogen chemistry at low temperatures. We updated the rate coefficients of some isotopic exchange reactions considered previously in the literature, and here we present a set of new exchange reactions involving molecules substituted in 13C and 15N simultaneously. We applied the model to a series of zero-dimensional simulations representing a set of physical conditions across a prototypical prestellar core, exploring the deviations of the isotopic abundance ratios in the various molecules from the elemental isotopic ratios as a function of physical conditions and time. We find that the 12C/13C ratio can deviate from the elemental ratio to a high factor depending on the molecule, and that there are highly time-dependent variations in the ratios. The HCN/H13CN ratio, for example, can obtain values of less than ten depending on the simulation time. The 14N/15N ratios tend to remain close to the assumed elemental ratio within approximately 10%, with no clearly discernible trends for the various species as a function of the physical conditions. Abundance ratios between 13C-containing molecules and 13C+15N-containing molecules however show somewhat increased levels of fractionation as a result of the newly included exchange reactions, though they still remain within a few tens of percent of the elemental 14N/15N ratio. Our results imply the existence of gradients in isotopic abundance ratios across prestellar cores, suggesting that detailed simulations are required to interpret observations of isotopically substituted molecules correctly, especially given that the various isotopic forms of a given molecule do not necessarily trace the same gas layers.

A Theoretical Approach to the Complex Chemical Evolution of Phosphorus in the Interstellar Medium

The study of phosphorus chemistry in the interstellar medium has become a topic of growing interest in astrobiology because it is plausible that a wide range of P-bearing molecules were introduced in the early Earth by the impact of asteroids and comets on its surface, enriching prebiotic chemistry. Thanks to extensive searches in recent years, it has become clear that P mainly appears in the form of PO and PN in molecular clouds and star-forming regions. Interestingly, PO is systematically more abundant than PN by factors typically of ∼1.4–3, independently of the physical properties of the observed source. In order to unveil the formation routes of PO and PN, in this work we introduce a mathematical model for the time evolution of the chemistry of P in an interstellar molecular cloud and analyze its associated chemical network as a complex dynamical system. By making reasonable assumptions, we reduce the network to obtain explicit mathematical expressions that describe the abundance evolution of P-bearing species and study the dependences of the abundance of PO and PN on the system’s kinetic parameters with much faster computation times than available numerical methods. As a result, our model reveals that the formation of PO and PN is governed by just a few critical reactions, and fully explains the relationship between PO and PN abundances throughout the evolution of molecular clouds. Finally, the application of Bayesian methods constrains the real values of the most influential reaction rate coefficients making use of available observational data.

CdS quantum dots/nano-TiO2 incorporated wood as a long-term stable and easily separable photocatalytic adsorbent for efficient Cr(VI) removal

To address chromium pollution in water resources, a novel photocatalytic adsorbent, CdS quantum dots/nano-TiO2 incorporated wood (QNIW), was synthesized for efficient removal of Cr(VI), which displayed both long-term stability and easy separability. We extensively analyzed QNIW using a range of analytical techniques to investigate its morphology, structure, optical characteristics, as well as adsorption and photocatalytic properties. The adsorption experiments revealed QNIW's remarkable capacity for Cr(VI) adsorption in the absence of light, reaching a maximum value of 482.1 mg/g calculated using the Langmuir model. This value was 8.4 times greater than that of natural wood. The outcomes of adsorption and photocatalytic experiments showcased QNIW's superior performance. Specifically, the photocatalytic reaction rate coefficient of QNIW was measured to be 0.0661 ± 0.0078 min−1 under specific conditions: UV light intensity of 500 mW/cm2, adsorbent concentration of 0.25 g/L, pH of 2, initial Cr(VI) concentration of 20 mg/L, and temperature of 20 ℃. Remarkably, this coefficient exceeded that of pure nano-TiO2 by an impressive factor of 6.2. Importantly, QNIW exhibited exceptional stability, preserving 92.2 % of its initial removal efficiency after twelve weeks storage and maintaining 83.4 % efficiency even after undergoing five consecutive adsorption–desorption cycles. Thus, this study offers valuable insights into the progress of photocatalytic adsorbents characterized by high efficiency, enduring stability, and facile separability from treated wastewater.

Influence of temperature on the heterogeneous reaction of toluene to N-containing organic compounds using in situ DRIFTS

Heterogeneous reactions of toluene/NO2 with and without O3 on α-Fe2O3 particles was studied as a function of temperature by using in situ diffuse reflectance infrared Fourier transform spectrometry (DRIFTS). The results show that the formation of R-ONO2 group is identified on the α-Fe2O3 particles surface, indicating the formation of benzyl nitrate in the presence of toluene/NO2 with or without O3 at different temperatures from 253 K to 298 K. From the analysis of the spectral changes, the reaction kinetics, such as reaction rate and uptake coefficient are sensitive to temperature. During the heterogeneous reaction of toluene/NO2, the formation rate of organic nitrate increases from (2.38 ± 0.07) × 1017 ions g−1 s−1 to (5.95 ± 0.09) × 1017 ions g−1 s−1 when the temperature decreases from 278 K to 253 K. When the trace gases were toluene/NO2/O3, the reaction rate increases about 3 folds with the increasing of temperature from 253 K to 298 K. In the absence of ozone, nitrification from heterogeneous reaction is exothermic process. However, the addition of ozone dominates the whole reaction process, resulting the different change trends of formation rate at similar range of temperatures. Furthermore, reactive uptake coefficient increases from (6.73 ± 0.02) × 10−7 to (1.64 ± 0.05) × 10−6 with decreasing temperature from 278 K to 253 K during the heterogeneous reaction of toluene/NO2 on α-Fe2O3 particles. The values of activation energy for the heterogeneous reaction of toluene/NO2 and toluene/NO2/O3 are determined to be −8.60 ± 0.91 kJ mol−1 and 15.14 ± 2.51 kJ mol−1, respectively. It implies that the heat change is an important factor in affecting the heterogeneous reaction under the seasonal temperature changes.

Influence analysis of uncertainty of chemical reaction rate under different reentry heights on the plasma sheath and terahertz transmission characteristics

Plasma sheath prediction of hypersonic vehicle is critical to radio frequency (RF) blackout solutions as well as vehicle design and development. In this paper, an optimized chemical reaction (OCR) model by considering the difference in reaction rate coefficient under different re-entry heights is proposed to obtain more accurate plasma sheath distribution. In the OCR model, the correction coefficient of chemical reaction rate at different heights is obtained by deriving the relationship between vehicle surface temperature and reaction rate. Further, through the coupling with hypersonic plasma flow model and co-simulation with terahertz transmission model, the influence of reaction rate uncertainty on the plasma sheath and terahertz transmission characteristics under different heights is studied. The reaction rate uncertainty has little effect on the collision frequency and sheath thickness, but has a significant effect on the electron density. Numerical simulations and comparisons with flight data validate the electron density prediction accuracy of OCR model was improved by more than 13.1% compared with the traditional chemical reaction (TCR) model, which causes prediction accuracy of signal attenuation at 0.05 THz improves about 25%, and it gradually decreases with the increasing terahertz frequency. The above conclusions are of great significance for the communication attenuation prediction and RF blackout schemes.

The reactive Graetz problem for a first order homogeneous decomposition reaction, and the limit of moderately large reaction constants

The long term goal of the present study is to facilitate the inversion of experimental data on cluster evaporation in laminar flow inside tubes. This requires the determination of reactant and product fluxes as functions of three dimensionless governing parameters: the axial position x along the tube, the reaction rate coefficient K, and the ratio γ of product and reactant diffusivities. The reactant flux is first calculated by separation of variables, including the accurate determination of the first 35 eigenvalues for 13 discrete values of K in the range 0<K<500. This approach is far less efficient for the product flux because the eigenfunctions are not standard functions. A numerical solution is also implemented for the reactant and its product, though it can hardly cover the full range of the three relevant parameters. The most common experimental situation involves relatively large values of K, for which a three term asymptotic solution is developed in negative powers of K1/2, with errors of order K−2. These approximate results represent well the numerically calculated reactant fluxes for K > 10 in terms of two functions, H0(s) and H1(s), reported in tables. The product fluxes require two functions, I0(s,γ), I1(s,γ). These we compute for six values of γ, and report as simple analytical fits covering continuously the full physically relevant range 1<γ<1.5. The desired ability to analyze experimental data to yield kinetic information is therefore achieved without the need for additional numerical solutions to partial differential equations.

Stochastic Simulation of Continuous Time Random Walks: Minimizing Error in Time-Dependent Rate Coefficients for Diffusion-Limited Reactions

Stochastic Simulation of Continuous Time Random Walks: Minimizing Error in Time-Dependent Rate Coefficients for Diffusion-Limited Reactions

Predicting chlorine demand by peracetic acid in drinking water treatment.

Peracetic acid (PAA) may be used in drinking water treatment for pre-oxidation and mussel control at the intake. PAA may exert a downstream chlorine demand, but full details of this reaction have not been reported. There are three possible mechanisms of this demand: (1) PAA may react directly with chlorine; (2) PAA exists in equilibrium with hydrogen peroxide, which is known to react with chlorine; and (3) as H2O2 reacts with chlorine, PAA will hydrolyze to form more H2O2 to re-establish PAA/H2O2 equilibrium, thereby serving as an indirect reservoir of chlorine demand. While the H2O2 reaction with chlorine is well known, the other mechanisms of possible PAA-induced chlorine demand have not previously been investigated. The observed molar stoichiometric ratio of PAA to free chlorine (n) for the presumed direct PAA+free chlorine reaction was determined to be approximately 2, and the corresponding observed reaction rate coefficients at pH 6, 7, 8, and 9 were 2.76, 3.14, 1.61, 10.1 M-n·s-1, respectively (at 25°C). With these estimated values, a kinetic model was built to predict the chlorine demand by PAA. The results suggest that chlorine demand from PAA is likely to be negligible over the course of several days (e.g., < 20% chlorine loss) for most conditions except for high pH (e.g., >8) and high PAA:Cl2 molar ratios (e.g., >2:1).

Experimental and Theoretical Investigation of the Reaction of NH2 with NO at Very Low Temperatures.

The first experimental study of the low-temperature kinetics of the gas-phase reaction between NH2 and NO has been performed. A pulsed laser photolysis-laser-induced fluorescence technique was used to create and monitor the temporal decay of NH2 in the presence of NO. Measurements were carried out over the temperature range of 24-106 K, with the low temperatures achieved using a pulsed Laval nozzle expansion. The negative temperature dependence of the reaction rate coefficient observed at higher temperatures in the literature continues at these lower temperatures, with the rate coefficient reaching 3.5 × 10-10 cm3 molecule-1 s-1 at T = 26 K. Ab initio calculations of the potential energy surface were combined with rate theory calculations using the MESMER software package in order to calculate and predict rate coefficients and branching ratios over a wide range of temperatures, which are largely consistent with experimentally determined literature values. These theoretical calculations indicate that at the low temperatures investigated for this reaction, only one product channel producing N2 + H2O is important. The rate coefficients determined in this study were used in a gas-phase astrochemical model. Models were run over a range of physical conditions appropriate for cold to warm molecular clouds (10 to 30 K; 104 to 106 cm-3), resulting in only minor changes (<1%) to the abundances of NH2 and NO at steady state. Hence, despite the observed increase in the rate at low temperatures, this mechanism is not a dominant loss mechanism for either NH2 or NO under dark cloud conditions.

Uncertainty in phosphine photochemistry in the Venus atmosphere prevents a firm biosignature attribution

Context. The possible detection of phosphine (PH3) in the clouds of Venus has raised the question as to which processes could produce such large abundances of PH3. Previous studies suggested that abiotic processes including photochemical production cannot explain the claimed PH3 concentrations. However, the photochemistry of phosphorus-bearing species in the atmosphere of Venus is not well known. Aims. We aim to assess the abiotic production of PH3 considering the effect of uncertainties in the chemical rate coefficients of phosphorus-containing reactions. Methods. Using a photochemical column model, we simulated Venus-like conditions and varied the chemical rate coefficients with a Monte Carlo (MC) approach in order to estimate the associated error in the PH3 abundances throughout the atmosphere. Results. Current uncertainties and missing data in photochemical rate coefficients lead to a variation of about six orders of magnitude in the modelled PH3 abundance on Venus, assuming photochemical production of PH3 from tetraphosphorus hexoxide (P4O6) pathways. Our results suggest an abiotically produced upper limit of 2 ppb PH3 between 50 and 60 km. These concentrations are in the range of a recent reanalysis of Atacama Large Millimeter Array (ALMA) data, suggesting planet-averaged abundances in PH3 of 1–4 ppb above 55 km. Future observations of phosphorus monoxide (PO) on Venus would be beneficial for increasing our confidence in assessing PH3 as a biosignature. Conclusions. We conclude that due to the large uncertainties in phosphorus chemistry, even a firm detection of several ppb PH3 in the Venus atmosphere would not necessarily mean a biological origin.

Cl atoms-initiated degradation of 1-Chlorobutane and 2-Chlorobutane: Kinetics, product analysis and atmospheric implications

The relative rate method was employed to investigate the kinetics of the Cl-initiated reactions of 1-chlorobutane (1-CB) and 2-chlorobutane (2-CB) over 263–363 K, and the measured rate coefficients at room temperature are (1.04 ± 0.24) × 10−10 and (5.84 ± 0.27) × 10−11 cm3 molecule−1 s−1, respectively. The Arrhenius equations for the title reactions were derived to be k1-CB + Cl (T = 263–363 K) = (2.77 ± 0.72) × 10−11 exp [(422 ± 79)/T] and k2-CB + Cl (T = 263–363 K) = (1.40 ± 0.32) × 10−11 exp [(415 ± 70)/T] cm3 molecule−1 s−1, respectively. The products were analysed qualitatively using gas chromatography-mass spectrometry (GC-MS), and the reaction mechanism was proposed for the reactions. The rate coefficients for the title reactions were calculated computationally over the temperature range of 200–400 K using canonical variational transition state theory with appropriate tunnelling corrections at CCSD(T)/6–311++G(2d,2p)//BHandHLYP/6–311++G(2d,2p) level of theory to complement our experimentally measured kinetic parameters. The experimental and theoretical data obtained were used to evaluate the impact of the studied molecules in the troposphere.

Revisiting the Gas-phase Chemical Rate Coefficients at High Temperatures in CLOUDY

A two-body gas-phase reaction rate coefficient can be given by the usual Arrhenius-type formula which depends on temperature. The UMIST Database for Astrochemistry is a widely used database for reaction rate coefficients. They provide fittings for coefficients valid over a particular range of temperatures. The permissible upper-temperature limits vary over a wide range: from 100 to 41,000 K. A wide range of temperatures occurs in nature; thus, it requires evaluating the rate coefficients at temperatures outside the range of validity. As a result, a simple extrapolation of the rate coefficients can lead to unphysically large values at high temperatures. These result in unrealistic predictions. Here we present a solution to prevent the gas-phase reaction coefficients from diverging at a very high temperature. We implement this into the spectral synthesis code CLOUDY which operates over a wide range of temperatures from CMB to 1010 K subject to different astrophysical environments.

New insight into the oxidation of propene at elevated pressure

Comprehensive experimental and kinetic studies of propene (C3H6) oxidation were conducted by using a jet-stirred reactor at 12.0 atm within 725–1020 K under both fuel-lean (Φ = 0.5) and fuel-rich (Φ = 3.0) conditions. Molecular species concentration profiles were obtained by using online gas chromatography and gas chromatography-mass spectrometry, including CO, CO2, CH4, C2H4, C2H6, A-C3H4, C3H8, 1,3-C4H6, 1-C4H8, nC4H10, CH3CHO, C2H3CHO, CH3COCH3, C3H6O1-2 CH3CH2CHO, and C6H6. Sixteen products and intermediates including light hydrocarbons, oxygenated and aromatic species were identified and quantified. Based on Aramco Mech 3.0 and previous works, the rate coefficients of some key elementary reactions were updated, and a detailed kinetic reaction model was developed with reasonable predictions involving 587 and 3037 reactions. Rate-of-production analysis indicates the four main pathways for high-pressure oxidation of C3H6, namely hydrogen atom abstraction reactions, OH addition reactions, H atom addition reactions, and the formation of C3H6O1-2. Sensitivity analysis reveals that H2O2(+M) <=> 2OH(+M) and the H-abstraction of C3H6 by OH radicals are the most promoting and inhibiting reactions for C3H6 consumption, respectively. The benzene formation during C3H6 oxidation was analyzed, mainly generated by the C1 + C5 channel and consumed by OH addition. It was found that increasing pressure can decrease the initial reaction temperature and increase the conversion of C3H6. The importance of addition and hydrogen abstraction reactions in the oxidation of C3H6 was identified by analyzing the pressure effect. To confirm its validity, the proposed kinetic model could be used to the previous literature data, including JSR oxidation at around 1 atm and ignition delay times for C3H6. These experimental and modeling efforts provide a basis for gaining a more comprehensive insight into the oxidation of propene.

Theoretical and experimental study of the OH radical with 3‐bromopropene gas phase reaction rate coefficients temperature dependence

AbstractIn this work, the rate‐determining steps of the OH radical + 3‐bromopropene gas phase reaction were studied, which could explain for the possible negative activation energy observed in experiments. To obtain new kinetic parameters and data for critical revisions, a reinvestigation of the rate coefficient (k) and its temperature dependence was carried out using the PLP‐LIF technique, in the 254‐ to 371‐K range. Moreover, quantum‐mechanical and canonical variational transition state theory calculations were performed, taking into consideration four OH addition and two β‐hydrogen atom abstraction reaction channels. The proposed kinetic model fits to the observed experimental Arrhenius behavior, and three not negligible reaction pathways are described for the first time.

Modelling the effect of bromide on chlorine decay in raw and coagulated surface waters

A novel and simple semi-mechanistic model describing the effect of bromide on chlorine decay was developed. A well validated two-reactant parallel second order (2R) chlorine decay model is available for planning and operation. Four parameters - reaction rate coefficients and initial concentrations of fast and slow reacting compounds - define the model. Bromide concentration varies in water bodies but considering the variability requires conducting individual tests and parameter estimation for each bromide concentration. The proposed model was developed and tested using data obtained from six water sources from the US and Australia. The samples included a mix of raw and coagulated waters with different pH (7.5–8.0), DOC (1.43–5.54 mg-C/L), SUVA (1.73–2.90), chlorine dose (2.32–8.29 mg/L), bromide (14–844 μg/L), chlorine to bromide ratios (4.7–230.4), and temperatures (20–23 °C). when it was assumed that the initial concentrations of fast and slow reacting compounds would not be affected by the addition of bromide, a linear relationship was established between fast and slow decay rate coefficients and the initial bromide concentration in each water source. The linearity coefficient (β) ranged from 1.73 ± 7.24E-04 to 3.56 ± 1.79E-03 L/μg-Br, but a common value of 3.00E-03 ± 5.79E-04 described the effect of bromide in all four tested sources (DOC 1.43–5.54 mg/L) from the US at pH 8 units and 20 °C. This model simplifies the consideration of the effect of bromide in chlorine decay and reduces the number of tests and parameters required to simulate water supply systems that incorporate the effect of bromide.

Reaction mechanism for atmospheric pressure plasma treatment of cysteine in solution

Mechanisms for the cold atmospheric plasma (CAP) treatment of cells in solution are needed for more optimum design of plasma devices for wound healing, cancer treatment, and bacterial inactivation. However, the complexity of organic molecules on cell membranes makes understanding mechanisms that result in modifications (i.e. oxidation) of such compounds difficult. As a surrogate to these systems, a reaction mechanism for the oxidation of cysteine in CAP activated water was developed and implemented in a 0-dimensional (plug-flow) global plasma chemistry model with the capability of addressing plasma-liquid interactions. Reaction rate coefficients for organic reactions in water were estimated based on available data in the literature or by analogy to gas-phase reactions. The mechanism was validated by comparison to experimental mass-spectrometry data for COST-jets sustained in He/O2, He/H2O and He/N2/O2 mixtures treating cysteine in water. Results from the model were used to determine the consequences of changing COST-jet operating parameters, such as distance from the substrate and inlet gas composition, on cysteine oxidation product formation. Results indicate that operating parameters can be adjusted to select for desired cysteine oxidation products, including nitrosylated products.

Diffusion combustion of NH3 in a single bubble of fluidized bed

Fluidized bed is a potential equipment for NH3 combustion due to its efficient gas–solid contact and high temperature particles as heat source for ignition. This paper studies the diffusion combustion of NH3 in a single bubble of fluidized bed, using an experimental approach and CFD-DEM simulation incorporated with a skeleton mechanism of 98 elementary reactions. The results show that the diffusion combustion of NH3 intermittently occurs in the bubble injection period and the oxygen is mainly consumed by H + O2 = OH + O. Slow oxidation, instead of combustion, takes place in the bubble rising period and the oxygen is mainly consumed by NNH + O2 = N2 + HO2. With the rupture of the bubble, unreacted NH3 re-burns, generating flames in the freeboard of the bed. The chemical reaction in the bubble is highly correlated to the mass transfer between the emulsion phase and bubble phase. Increasing the fluidization gas velocity promotes the accumulation of oxygen and enhances the reaction rate of slow oxidation of NH3 in the bubble due to the increasing inter-phase mass transfer. The increase of the bed temperature reduces the ignition delay time, and increases the chemical reaction rate and mass transfer coefficient. The changes in oxygen concentration of fluidizing gas not only changes the chemical reaction rate, but also affects the oxygen consumption path. The fraction of NH3 oxidized in the dense bed can be enhanced by increasing the fluidization gas velocity, bed temperature and oxygen concentration of the fluidizing gas, the latter exerting the greatest influence.

Theoretical Insights into the Dynamics of Gas-Phase Bimolecular Reactions with Submerged Barriers.

Much attention has been paid to the dynamics of both activated gas-phase bimolecular reactions, which feature monotonically increasing integral cross sections and Arrhenius kinetics, and their barrierless capture counterparts, which manifest monotonically decreasing integral cross sections and negative temperature dependence of the rate coefficients. In this Perspective, we focus on the dynamics of gas-phase bimolecular reactions with submerged barriers, which often involve radicals or ions and are prevalent in combustion, atmospheric chemistry, astrochemistry, and plasma chemistry. The temperature dependence of the rate coefficients for such reactions is often non-Arrhenius and complex, and the corresponding dynamics may also be quite different from those with significant barriers or those completely dominated by capture. Recent experimental and theoretical studies of such reactions, particularly at relatively low temperatures or collision energies, have revealed interesting dynamical behaviors, which are discussed here. The new knowledge enriches our understanding of the dynamics of these unusual reactions.