Kinetics in the real world: linking molecules, processes, and systems

---

Abstract

Unravelling elementary steps, reaction pathways, and kinetic mechanisms is key to understanding the behaviour of many real-world chemical systems that span from the troposphere or even interstellar media to engines and process reactors. Recent work in chemical kinetics provides detailed information on the reactive changes occurring in chemical systems, often on the atomic or molecular scale. The optimisation of practical processes, for instance in combustion, catalysis, battery technology, polymerisation, and nanoparticle production, can profit from a sound knowledge of the underlying fundamental chemical kinetics. Reaction mechanisms can combine information gained from theory and experiments to enable the predictive simulation and optimisation of the crucial process variables and influences on the system's behaviour that may be exploited for both monitoring and control. Chemical kinetics, as one of the pillars of Physical Chemistry, thus contributes importantly to understanding and describing natural environments and technical processes and is becoming increasingly relevant for interactions in and with the real world.

---

The present issue comprises state-of-the-art contributions with a focus on kinetics in the real world; authors from about 50 institutions in almost 20 countries underline the global interest in this subject. Of the more than 30 articles, many describe the results of international collaborations. Recognising the importance of the subject, the 117th General Assembly of the German Bunsen Society for Physical Chemistry will be held May 10–12, 2018, in Hannover, Germany, under this topical theme, and most of the research communicated in this themed issue will be presented on the occasion of this conference.

Chemistry explains and utilises the transformation of matter, from reactants to desired products. Reaction kinetics provides the quantitative formalism that describes these transformations in terms of the time-dependent changes in the concentrations of all the involved substances – reactants, intermediates, main and side products. It answers the fundamental and highly relevant practical questions: How specifically do molecules react? How much energy is needed, how are energy barriers overcome, and how is the energy distributed along the pathway from reactants to products? What is the probability for a certain pathway to occur under specific boundary conditions? How is it possible to control this process, so that it proceeds in the desired fashion, and how can a specific pathway be promoted? How can reaction rates be determined or calculated and mechanisms analysed and understood at the relevant level of detail?

Since the early developments in kinetics, including, for example, the seminal contributions of Svante Arrhenius on the temperature dependence of reaction rates¹ and Max Bodenstein on pyrolysis² and chain reactions,³ such questions have been the focus of dedicated research. From global kinetic expressions to molecular- and atomic-scale dynamics, tremendous progress has been made both on the fundamental and applied side. Milestones in this development are reflected, among others, by the Nobel prizes awarded to: Sir C. N. Hinshelwood and N. N. Semenov (1956) for research on the mechanisms of chemical reactions and explosions; M. Eigen, R. G. Norrish, and G. Porter (1967) for providing methods to observe extremely fast reactions; D. R. Herschbach, Y. T. Lee, and J. C. Polanyi (1986) for their contributions concerning the reaction dynamics of elementary processes; R. A. Marcus (1992) for the theory of electron transfer reactions; P. J. Crutzen, M. J. Molina, and F. S. Rowland (1995) for unravelling important atmospheric reactions; A. H. Zewail (1999) for the study of transition states by femtosecond spectroscopy; and G. Ertl (2007) for his research to understand the chemical processes on solid surfaces. The Nobel lectures and press releases⁴ convey a good flavour of how intricately fundamental and real-world kinetics aspects are linked. For example, the theoretical developments to describe the electron transfer reactions in chemistry and biology⁵ provide the basis for processes and properties such as light energy fixation by plants, photo- and electrochemical synthesis, conductivity of polymers, and corrosion. Moreover, work sparked off by these laureates and their collaboration partners includes such real-world aspects in a broad number of fields; they deal with preventing safety hazards arising from explosions, understanding the mechanisms of biological interactions, predicting environmental and climate influences of reactive pollutants in the atmosphere, developing efficient catalysts for sustainable production processes, and many other topics. Some related examples include the direct probing of charge-transfer reactions,⁶ the real-time monitoring of enzyme kinetics,⁷ the determination of the details in the mechanisms for ozone destruction⁸ and halogen release at the marine boundary layer.⁹ Prominent industrial processes, such as ammonia synthesis, profit from the possibility of linking macroscopic observables to the microscopic structure of matter and its transformations, which is provided by progress in surface science.^10,11 Decisive knowledge on further important heterogeneously catalysed reactions, such as ammonia oxidation, could build upon the fundamental understanding of the mechanisms describing these changes.¹²

More recent examples for work in real-world kinetics can be found in the context of the transformations towards a lower-carbon energy system and efficient “green” processes, including reaction mechanisms for the production of liquid fuels from biomass¹³ and the conversion of synthesis gas to hydrocarbons,¹⁴ for designing efficient electrocatalysts for automotive fuel cells,¹⁵ for improving the charging and discharging of lithium–air batteries,¹⁶ and for understanding the chemical processes that emit nitrogen oxides from combustion processes.¹⁷ Similarly exciting work describes the real-world kinetic aspects in nature's largest-scale reactors, such as organic aerosol aging¹⁸ and secondary organic aerosol (SOA) formation in the troposphere,^19,20 as well as the reactions of negative ions²¹ and pathways towards polycyclic aromatic hydrocarbons (PAHs) in space.²² While the early developments in kinetics dealt mainly with gas-phase reactions, complexity is a foremost issue and the challenge of current kinetics research that is concerned with boundary layers, phase transitions, solid-state transport, reactions occurring in droplets, emulsions, aerosols, or on the micro- or nano-scale, as well as with biochemical rearrangements in media such as cells. Progress in these increasingly interdisciplinary fields and beyond often involves combinations of experiment, theory, model development and simulation, and numerical predictions with such models increasingly make use of formalisms and routines for mechanism reduction and uncertainty analysis.²³

Numerous examples in the present article collection address problems in combustion and atmospheric chemistry. In combustion, the aspects of chemical and physical complexity concern the enormous number of thousands of elementary reactions²⁴ and the harsh reaction conditions in engines, gas turbines, and combustors at high temperatures and/or elevated pressures.²⁵ The combined efforts of experiment and theory have played a substantial role in providing a solid basis for combustion mechanisms.^26,27 New low-temperature combustion approaches aim at significant decreases of engine emissions, but they involve additional reaction classes leading to fuel-specific peroxy species formation.^28,29 Experimental evidence for such labile compounds has only been obtained quite recently,³⁰ using sophisticated in situ mass spectrometric techniques that employ photoionisation with synchrotron-generated tuneable vacuum ultraviolet radiation.³¹ Highly oxygenated intermediates from low-temperature combustion that resemble those found in the atmospheric oxidation processes of hydrocarbons have been identified very recently,³² with structural assignment aided by isotope exchange reactions. Combustion reaction mechanisms also depend highly on the fuel's molecular structure, leading to additional reaction pathways for alternative transportation fuels that are becoming available from biomass, including esters, ethers, alcohols, carbonyl compounds, and furan derivatives.^33–35 A more complete picture from sustainable biofuel production to these fuels' combustion reactions under realistic engine conditions, as well as to the associated emissions from the tailpipe, is only just emerging.³⁶ To enable the mechanisms to represent the important details of the overall kinetics, studies of the elementary reactions for current and emerging fuels and the respective intermediates are of great value.^37,38 A topic of high fundamental interest and eminent practical importance is the formation of soot particles from the gas phase, a process of enormous complexity that is still insufficiently understood in spite of more than two decades of research,^39,40 in part because of the experimental challenges involved in the detection of just-nucleating particles and their precursors.^41,42

Kinetic studies of these and other combustion-related systems employ a plethora of experimental devices with shock tubes, rapid compression machines, flow and jet-stirred reactors, flames, and combustors. Spectroscopic, mass spectrometric, and/or gas chromatographic analysis permit the reaction progress of the system in question to be followed, not only monitoring the reactants and products, but also the key intermediates, including reactive radicals. Enormous progress in theory has been made in calculating reaction pathways and rate coefficients by advanced quantum chemical methods and statistical rate theory. Such results favourably complement experimental investigations and support model development; theoretical methods can also provide access to reactions that are not easy to study experimentally, for example radical–radical reactions and the temperature dependences of individual reaction channels.

The articles in this issue span a broad range from individual reactions to complete mechanisms, as well as to methods for their systematic analysis. The reaction mechanisms for the combustion of transportation fuels serve the important role of connecting the relevant chemical aspects with the performance of the real engine. Real-world fuels, such as gasoline and diesel fuel, are mixtures of many chemical components, including additives and improvers, and not much attention had been devoted to providing generally applicable composite mechanisms that can predict their behaviour reliably. The article by Westbrook et al. (DOI: 10.1039/c7cp07901j) reports a systematic approach for the development of combustion mechanisms that can be used for fuel mixtures under a large variety of practical conditions. The rate coefficients for thousands of elementary reactions are typically included in such mechanisms for practical fuels, and reliable experiments or calculations for these expressions are a prerequisite for predictive simulation quality. Many gas-phase reactions in combustion chemistry, astrochemistry, atmospheric chemistry, and heterogeneous catalysis involve reaction energy barriers, the magnitude of which is crucial to obtain accurate reaction rates, as discussed by Chan and Simmie (DOI: 10.1039/c7cp08045j). Their investigations are dedicated to establishing several reference databases for combustion and atmospheric chemistry using a consistent high-level quantum chemical approach. This is especially needed for larger species for which computationally more efficient density functional theory approaches must be employed, the reliability of which can be assessed against such reference databases. Automatic reaction mechanism generator routines are utilised in the contribution of Zhang et al. (DOI: 10.1039/c7cp07058f), which reports an evaluation of anti-knock agents for gasoline combustion. Several substituted phenols were added to n-butane, a smaller representative of the alkanes in gasoline, and a chemical reaction mechanism for the combustion reactions of the fuel blends with 1465 species and 27 428 reactions was generated to capture the effects of substituted phenol anti-knock additives in an engine-near closed adiabatic homogeneous batch reactor. The authors consider this fundamental kinetic approach to be advantageous compared to engineering procedures for rating fuels and fuel mixtures with single parameters, such as octane numbers, since fuels with identical octane numbers could show distinctly different ignition kinetics in modern engines.

Among the most important classes of combustion reactions are the abstraction reactions of hydrogen from the fuel molecule or from its decomposition products, since they can initiate or sustain the radical chain reactions that lead to fuel consumption and oxidation. On the CBS-QB3 level of theory, Van de Vijver et al. (DOI: 10.1039/c7cp07771h) have calculated a large set of rate coefficients for the intramolecular hydrogen abstraction reactions of alkanes, alkenes, and alkynes for a large temperature range and derived a new group additivity model, including correlations for tunnelling corrections. The results favourably compare both with experimental data and higher-level theoretical calculations. H-atom abstraction reactions are also important to understand the auto-ignition of fuels. For the potential next-generation biofuel isopentanol, Parab et al. (DOI: 10.1039/c7cp08077h) have performed quantum chemical calculations of the key initiation reactions, namely H-abstraction reactions from isopentanol by H atom and HO₂ radical. Based on a thorough sensitivity analysis of the toluene reaction system, Pelucchi et al. (DOI: 10.1039/c7cp07779c) have calculated H-abstraction reactions by OH, HO₂, O, and O₂. Their work is concerned with the improvement of the combustion mechanism for toluene, a reference fuel representative of alkylated aromatic fuels and a key precursor for the PAHs that are known to precede soot formation. In this regard, the presented theoretical study of the reaction of the resulting benzyl radical with O₂, which proceeds on a multi-well potential energy surface, is another key step in the toluene combustion process.

To better understand the fuel-structure-specific reactions toward PAH and soot precursor formation, Ruwe et al. (DOI: 10.1039/c7cp07743b) have investigated non-premixed flames of three C₅ fuels with different structures, namely n-pentane, 1-pentene, and 2-methyl-2-butene. They have analysed the intermediate species and their mole fractions obtained with a combination of electron impact ionisation and photoionisation mass spectrometry and attribute the striking differences in the aromatic species concentrations to the differences in the fuel-structure-specific intermediate species pool. The mechanism of soot nucleation is analysed in the article by Kholghy et al. (DOI: 10.1039/c7cp07803j) who have investigated the importance of reactive PAH dimerisation by comparing their reaction model to recent experiments with laser-induced incandescence in a so-called nucleation flame. This is a flame in which particle nucleation takes place but particle growth is reduced due to the particular flame conditions. Their study shows that chemical bond formation after the dimerisation of small PAHs can stabilise such dimers and enhance the soot concentration by orders of magnitude, in good agreement with experiments.

Since methylated furans are recognised as one family of promising biofuels, Tranter et al. (DOI: 10.1039/c7cp07775k) have investigated the high-temperature pyrolysis of 2-methyl furan. They have studied the important multi-channel unimolecular decomposition at different temperatures and pressures in a diaphragm-less shock tube equipped with laser Schlieren densitometry and complemented their experiments with theoretical calculations. For n-butanol, another biofuel and fuel additive that can be renewably produced, Li et al. (DOI: 10.1039/c7cp08518d) have studied the reaction mechanism in flames of toluene doped with n-butanol to understand the effect of the additive on the formation of soot precursors. Their analysis has relied on synchrotron vacuum ultraviolet photoionisation mass spectrometry to determine the concentrations of multiple intermediates. A kinetic mechanism was developed to represent the combustion of these fuel mixtures, relying on the obtained experimental information for its critical examination. The contribution by Chen et al. (DOI: 10.1039/c7cp08164b) is devoted to the combustion reactions of tetrahydropyran, a next-generation fuel accessible from cellulosic biomass. Their study compares the kinetics related to the ring-opening pathways for this heterocyclic molecule with those for cyclohexane, reporting striking differences in the reaction mechanisms for these structurally quite similar fuel molecules. Specifically, they have directly measured the temporal behaviour of important chain carriers resulting from the fuel radical oxidation reaction with molecular oxygen, namely the OH and HO₂ radicals, by time-resolved near- and mid-infrared absorption in a pulsed photolysis experiment at temperatures of 500–750 K.

Auto-ignition reactions, in this case for the hydrogen–air system, are the focus of the article by Yu et al. (DOI: 10.1039/c7cp07213a). Safety considerations for this system and for hydrocarbon auto-ignition demand reliable predictions with accurate models that can, however, be numerically handled for practical purposes. To reduce the computational load, the authors evaluate numerical approaches for automatic mechanism reduction. Along a similar line, the article by Miyoshi (DOI: 10.1039/c7cp07736j) is concerned with the hundreds of organic compounds and thousands of reactions that must be considered in the auto-ignition mechanisms for practical transportation fuels. Since computational fluid dynamic codes for practical combustion systems and industrial applications can only include significantly lower numbers of reactions, the author analyses an approximate numerical solution of the underlying differential equations governing the reaction system, starting from the H₂–O₂ system and evaluating it for a primary reference fuel. Essentially based on a pseudo-first-order treatment of the chain carrier kinetics, a mechanism-based rationalisation for a long-established empirical method to estimate the occurrence of knock in spark-ignition engines could be given.

Thermal conversion processes of practical relevance include the gasification of biomass that may be considered as an energy source with a reduced or neutral carbon dioxide balance. The article by Singla et al. (DOI: 10.1039/c7cp07552a) addresses the challenges that are introduced by chlorinated species for end uses of the gasification products, such as in gas turbines, engines, and fuel cells. A highly important compound in this context is methyl chloride, the pyrolysis and oxidation kinetics of which has been inspected by ab initio calculations and kinetic modelling. The results indicate that problems associated with this species may mainly be expected at low-temperature gasification conditions in fluidised bed reactors whereas it may be unproblematic in other gasifier designs.

Many of the oxidation reactions that matter in the combustion of organic compounds can also be relevant in atmospheric chemistry, albeit at somewhat different conditions of temperature, pressure, and chemical composition. It is thus not surprising that the chemical kinetics of partly oxidised intermediates are being investigated for either purpose.^38,43 Similar to combustion, atmospheric chemistry is a real-world example for the interplay of chemical and physical complexity. For many years it was believed that ozone in the troposphere essentially originated from the downward transport of ozone from the stratosphere, where it is produced from the photolysis of molecular oxygen. However, several key reactions involving nitrogen oxides are involved in producing ozone in the troposphere.⁴⁴ In the key gas-phase reaction sequence, nitric oxide is oxidised with hydroperoxyl or organic peroxy radicals to nitrogen dioxide. NO₂ can then be photolysed by radiation in the near ultraviolet region to form atomic oxygen, which combines with molecular oxygen to produce ozone. These chemical processes, together with the input and removal of ozone by physical processes, such as transfer from the stratosphere and removal at land and water surfaces, determine the ozone abundance in the troposphere. Next to ozone reactions related to the hydroxy radical, which acts as the main “detergent” in the atmosphere, ozone is central to the overall rate of oxidation of most trace pollutants in the troposphere, a process which prevents the build-up in air of toxic pollutants. Consequently, the scientific community has dedicated enormous effort to understanding the kinetics and related photochemistry of the mechanisms of tropospheric ozone production, involving hydroxyl radicals, organic peroxy radicals, and nitrogen oxides, as well as the ozone-induced atmospheric degradation of organic and inorganic compounds.

From the large body of experimental data on atmospheric oxidation processes and the role of the hydroxyl radical, recommendations have been made available as prerequisites for models of tropospheric ozone production and loss.⁴⁵ These evaluations consider the general importance of complex-forming bimolecular reactions at ambient temperatures and pressures encountered in the troposphere and provide a fundamentally-based treatment of the pressure dependence of rate coefficients to assist the parameterisation of atmospheric reaction rates.⁴⁶ Because of the huge amount and variety of volatile organic compounds present in the atmosphere that can potentially influence ozone production through the specific peroxy radicals formed in their atmospheric oxidation and degradation, this work is far from being completed. Aerosol particles produced during the oxidation of volatile organic and nitrogen- and sulphur-containing compounds have a fundamental influence on the atmospheric radiation balance, due to the backscatter of incoming sunlight. Global climate models must therefore adequately represent the chemically and photochemically induced aerosol production pathways. Additionally, tropospheric ozone acts as a greenhouse gas by itself. Climate models with detailed and explicit chemical schemes are increasingly used to predict trends in climate change and explore atmospheric composition. The reactions of ozone with alkenes proceed via highly reactive diradical species, known as Criegee intermediates, which are long thought to be responsible for the oxidation of atmospheric SO₂.⁴⁷ However, quantitative measurements of the reaction kinetics of these elusive species could be obtained only recently.⁴⁸

In the context of radiative forcing and climate change, understanding the reaction mechanisms of soot formation from combustion in transportation, power generation, and rural and residential burning is highly relevant; black carbon is estimated, albeit with considerable uncertainties, to be potentially the second-most important human emission in terms of anthropogenic climate forcing, after carbon dioxide, and of about similar influence as methane.^49,50 Uncertainties in such estimates relate, for example, to the interactions of black carbon formation and oxidation reactions with liquid substrates present in the atmosphere, such as clouds and aerosol particles. Kinetic and photochemical data for atmospheric chemistry including heterogeneous reactions involving liquid particles are of high importance in the troposphere and at the marine boundary layer. Data collections and evaluations for such reactions are becoming available⁵¹ and must be considered as work in continuous progress, similar to those for gas-phase reactions mentioned above.⁴⁵ Detailed studies of the pertinent reactions at liquid surfaces shed light on such important interactions at natural air–water interfaces,⁵² as for example in the recent study by Kleber et al.⁵³ on the oleic acid–ozone oxidation model system, where the authors employed quantitative time-resolved vibrational sum frequency generation spectroscopy to probe the reaction at the respective oleic acid monolayer. The influence of water vapour on the kinetics of the highly important Criegee intermediates has been analysed in the recent work of Berndt et al.;⁵⁴ they used a production scheme of the simplest representative formaldehyde oxide, CH₂OO, by the reaction of ethene with ozone and measured the kinetics under highly diluted water conditions in a free-jet flow system so that the reaction with the water monomer could be separated from that with the dimer. A lot of details as those from such examples highlighted here contribute to an extensive picture of the overall processes.

The oxidation and combination reactions of organic species are a target in both combustion and atmospheric chemistry; similarly, molecular growth reactions are of interest in both combustion and astrochemistry. Thomas et al. (DOI: 10.1039/c8cp00357b) have employed crossed molecular beam experiments accompanied by numerical calculations to assess the role of resonantly stabilised radicals in molecular growth processes that lead to PAHs. Specifically, they have investigated the formation of three different C₅H₃ isomers via the bimolecular reactions of singlet/triplet C₂ with methylacetylene, CH₃CCH, d₃-methylacetylene, CD₃CCH, and 1-butyne, C₂H₅CCH, using a newly designed source of C₂ molecules. They could identify several previously unknown pathways including methyl loss in the reaction with 1-butyne. Schleier et al. (DOI: 10.1039/c7cp07893e) have addressed the reactions of the resonantly stabilised allyl radical C₃H₅ with O₂ that matters in combustion as well as in tropospheric environments. Their study employed a new kinetic flow reactor setup coupled to a synchrotron-based photoelectron–photoion coincidence spectroscopy instrument for background-free and isomer-selective detection of reactive species. The oxidation reactions of the isomers CH₃O and CH₂OH with O₂ are the focus of the contribution of Assaf et al. (DOI: 10.1039/c7cp05770a). They proceed at considerably different rates, as analysed by the direct measurement of the time-resolved profiles of the hydroperoxy radical HO₂. By adapting the oxygen concentrations in this system it was even possible to determine rate expressions for the radical–radical cross reaction CH₃O + HO₂, which was also supported by theoretical calculations.

Li et al. (DOI: 10.1039/c7cp05008a) have studied, at the CCSD(T)/aug-cc-pVTZ//B3LYP-D3/aug-cc-pVDZ level of theory, the influence of a single water molecule on the reaction of ClO with HO₂. With ClO as one of the abundant chlorine reservoir species in the troposphere, their research addresses the frequently raised issue of the potentially important effect of ubiquitously present water on atmospheric reactions. Interestingly, it is concluded in this case that a single water molecule has no important effect on the overall reaction rate and hence the tropospheric ClO cycle. Monge-Palacios et al. (DOI: 10.1039/c7cp08538a) have reported a theoretical investigation of the isomerisation of a six carbon atom Criegee intermediate, regarded as a surrogate for larger such species derived from monoterpenes or other larger important volatile organic compounds. The authors observe that formic acid exerts a catalytic effect on the studied reaction, which they have analysed regarding the different possible channels available to the conformers of this Criegee intermediate, concluding that the promotion effect of carboxylic acids on the formation of the resulting vinylhydroperoxides may contribute to secondary organic aerosol formation.

The consideration of the effects of a single additional complex-forming molecule on the outcome of the elementary reactions – water or formic acid in the two preceding examples – is only the first step in bridging the gap between gas-phase and condensed-phase chemistry taking place in the atmosphere, often accompanied by the presence or involvement of charged species. For example, negative ion reactions in the troposphere are involved in charged aerosol particle formation. The CO₃⁻ radical anion reactions with nitric acid are the focus of the article by van der Linde et al. (DOI: 10.1039/c7cp07773d); they have studied the reactions of the non-, mono- and dihydrated anion under high-vacuum conditions and show that, similar to the corresponding HCl reaction, the rate is clearly dependent on the number of water molecules solvating the CO₃⁻ ion. Michenfelder et al. (DOI: 10.1039/c7cp07774b) have investigated the reaction dynamics of nitrophenolates by femtosecond broadband absorption spectroscopy, since nitroaromatic compounds are discussed as a potential photolytic source for nitrous acid in the atmosphere. While the photochemistry of nitrophenols has been studied in detail before, not much attention was devoted to the corresponding anions, the nitrophenolates. The authors have detected surprising stimulated emission in their assessment of the photo-induced reaction chemistry of these compounds. Organic and aqueous solutions were compared and seen to exhibit quite different temporal behaviour, associated with the magnitude of the dipole moment of the solvent.

Tropospheric aqueous-phase chemistry is also the focus of the articles by Schaefer and Herrmann (DOI: 10.1039/c7cp08571k) and Hoffmann et al. (DOI: 10.1039/c7cp08576a). Since oxygenated organic compounds are widely present in the troposphere, both from primary emissions and from the gas-phase oxidation reactions of precursor volatile organic compounds, it is important to study their degradation and conversion reactions. These reactions may occur both in the gas and the liquid phase, recognizing the important presence of cloud droplets, haze, fog, or hygroscopic particles in the atmosphere. Schaefer and Herrmann (DOI: 10.1039/c7cp08571k) have thus investigated the temperature-dependent oxidation of carbonyls and diols by the OH radical in aqueous solution using laser-induced flash photolysis and a kinetic competition method and determined numerous rate coefficients for such reactions. Hoffmann et al. (DOI: 10.1039/c7cp08576a) have constructed a kinetic model for the aqueous-phase oxidation of substituted monocyclic aromatic hydrocarbons, typically present in the troposphere as a result of automobile emission or from wood combustion.

It is evident from these studies of atmospheric and environmental chemistry that a high level of detail is addressed regarding specific reaction pathways and mechanisms. Many pieces in this large reactive chemistry puzzle can contribute together to resolve fundamental questions and provide information relevant for grand challenges, such as climate change.

Reaction kinetics also matters in numerous industrial and high-tech processes, for which only a few examples can be given here. Ongoing efforts are undertaken to link polymerisation kinetics on the level of elementary reactions with large-scale process applications, where today's macromolecular simulation capabilities often target molecular weight distributions of all types of polymerisation processes.⁵⁵ Kinetic effects are shown to have decisive influence on the shapes of colloidal metal nanocrystals that are of use in multiple processes from catalysis to electronics, photonics, information storage, energy conversion and storage, environmental protection, and medicine.⁵⁶ Mechanism development for multiscale modelling of metal-catalysed reactions⁵⁷ has been reported similarly to the importance of kinetic control in processes to produce carbon nanotubes⁵⁸ and graphene^59,60 with intriguing properties for such diverse applications as sensing, hydrogen storage, electronic and optoelectronic devices, solar cells, energy storage, super capacitors, lithium batteries, and fuel cells. From more global kinetic analysis regarding rate-controlling steps to more details regarding the influences of reactive intermediates, such studies can also involve the treatment of individual pathways, such as for fluorine compounds that may be used in microelectronics fabrication.⁶¹

The present article collection includes a number of interesting contributions towards such real-world processes. Sela et al. (DOI: 10.1039/c7cp06827a) have studied the decomposition of tetramethylsilane in a shock tube by means of high-repetition-rate mass spectrometry to establish a mechanism for the synthesis of silicon carbide nanoparticles or thin films. Such mechanisms can be used in process optimisation and upscaling. Scholz et al. (DOI: 10.1039/c7cp07729g) present an investigation of the exciton photon dynamics in BI₃ from experiments using transient absorption spectroscopy. Such materials as this trivalent bismuth compound with octahedral halide coordination exhibiting a double-perovskite structure are promising candidates for solar light harvesting and optoelectronics, where they could replace toxic lead-based compounds, and knowledge of the photo-induced carrier dynamics is instrumental to overcome performance bottlenecks in such semiconductor materials.

The article by Limbach et al. (DOI: 10.1039/c7cp07770j) presents ¹H NMR spectroscopy studies of the H₂, HD, and D₂ equilibration reactions between the gas phase and surface hydrides of Ru nanoparticles stabilised with polyvinylpyrrolidone or hexadecylamine. Solid metal nanoparticles are widely employed in catalysis. The NMR tubes were used as batch reactors in these experiments and two different mechanisms were evaluated, including dissociative and associative exchange. The results reveal that under the typical conditions where these transition metal particles are employed as catalysts, namely at room temperature and normal pressures, the associative exchange mechanism dominates.

The study of Moore et al. (DOI: 10.1039/c7cp07754h) is concerned with the kinetics of the phase transitions of spodumene, a lithium aluminium silicate that is regarded as a prime candidate for increased lithium demand for batteries, since it occurs in sufficient natural abundance and offers high lithium content. The authors have employed time-resolved in situ synchrotron-based X-ray diffraction to analyse the conversion kinetics between the α- and β-spodumene modifications, a key processing step for lithium extraction from this ore.

Drache et al. (DOI: 10.1039/c7cp07768h) introduce a new holistic approach for modelling the emulsion polymerisation of styrene using ab initio Monte Carlo simulations. Whereas emulsion polymerisation has multiple advantages including environmentally friendly processing, excellent heat dissipation, and low viscosity of the emulsion, the kinetics of such processes is rather complex. A model was developed that describes the entire process including the elemental reactions in the aqueous phase, the radical transfer into individual particles, and the radical polymerisation inside the particles taking into account chain-length-dependent rate coefficients. The simulation results are in good agreement with the experiments.

The interactions of cationic polymers with cell surfaces, biosensors, or bioelectrodes provide the background for the article of McGeachy et al. (DOI: 10.1039/c7cp07353d). As a prerequisite for developing models for the interaction kinetics of polycations with such surfaces, they have estimated the equilibrium charge densities at the surface from interface-induced signal changes in second harmonic generation spectra and also performed time-dependent studies to address the reversibility of the polycation adsorption process.

Aitken and Coote (DOI: 10.1039/c7cp07562f) have contributed theoretical work to investigate the question of whether pH-switchable charged functional groups can be used in the electrocatalysis of Diels–Alder reactions, which are well known in synthetic organic chemistry for the regio- and stereochemically controlled formation of six-membered carbon ring structures. The concept of electrocatalysed Diels–Alder reactions is based on local electric fields generated in the molecules in favourable orientation that could thus influence the regioselectivity. Experimental proof of such general concepts has been demonstrated in scanning tunnelling microscopy experiments, but the process is more challenging in solution. The presented work demonstrates pH switches for polar and nonpolar systems including 2-pyrone and cyclopentene.

Many of the above-mentioned kinetic studies required reliable simulation tools to set up and solve the underlying differential equations describing the time-dependent observables. For real-world applications and the simulation of complex experimental setups, it is often necessary to couple chemical, transport, and heat transfer models. Owing to this complexity, the potential and transferability of an individual mechanism that has been designed to perform well for a given experimental situation is often difficult to assess. To overcome this practical problem, a new open software tool named CaRMeN (C̲a̲talytic R̲eaction M̲e̲chanism N̲etwork) for analysing and developing kinetics for real-world reacting systems is presented by Gossler et al. (DOI: 10.1039/c7cp07777g). The advantages of the approach for improving the current manual workflow is demonstrated in a case study for the conversion of methane over rhodium catalysts for a large range of conditions. The software tool is intended to replace tedious and error-prone manual procedures by a systematic numerical approach, and it is reported to be user-friendly and easily adaptable. In optimising and upscaling real-world processes, such approaches promise high value.

The present themed issue impressively shows that real-world kinetics spans bridges from the most fundamental aspects, as in the work of Kushnarenko et al. (DOI: 10.1039/c7cp08561c), to the eminent challenges in climate change, as in the article by Anderson and Clapp (DOI: 10.1039/c7cp08331a).

Since the time scales of intramolecular vibrational energy redistribution (IVR) and chemical reaction are of fundamental importance in physical chemistry, Kushnarenko et al. (DOI: 10.1039/c7cp08561c) have used time-resolved femtosecond pump–probe experiments in a hollow waveguide to probe the IVR process by means of direct, real-time observation of the redistribution dynamics. They have chosen the system of propargyl halides, HCCCH₂X (X = Cl, Br, I) to analyse the IVR behaviour of these bichromophoric molecules after the excitation of the first overtone of the CH stretching vibrations of the acetylenic (sp²) CCH and alkyl (sp³) CH₂X groups. Upon excitation with 150 fs pulses, monitoring of the energy loss and arrival with IR and UV absorption spectroscopy is achieved. Their investigation shows quite different relaxation times for the involved processes at the two different chromophore sites, depending also on the nature of the halide. Such direct analysis of the time-resolved energy flow in a molecule highly assists our present understanding of reaction dynamics and kinetics.

Reaction processes that matter on the largest scales are addressed in the article by Anderson and Clapp (DOI: 10.1039/c7cp08331a) who discuss the mechanism of free radical reactions involved in stratospheric ozone loss and of increased climate forcing by carbon dioxide and methane emitted from fossil fuel use in light of the state of the climate and climate change as well as in regard of influences on human health. The highly interesting analysis, based upon decades of research and given from the perspective of one of the pioneers in this field, examines potential misconceptions and pivotal turning points regarding the climate change debate. Rather than focusing on “global warming”, a term discussed as imprecise in characterising the current climate state, influences that lead to irreversible changes in the climate structure and to feedback are shown to merit more detailed inspections. As one exemplary phenomenon, the injection of water vapour into the stratosphere over the land mass of the USA in summer and its influence on stratospheric ozone is addressed. The impressive article, which is highly suited as reading material for students, researchers, and policy makers, demonstrates multiple links between individual reaction pathways and global atmospheric behaviour, including the delicate balance of ozone that is, by the way of skin cancer incidence, of direct influence on human health, but also on other environmental conditions.

Of course, the specific topics addressed within the limits of this issue can only serve as an incomplete excerpt of the many real-world reactive systems that demand a detailed understanding of practically relevant reaction kinetics. Dealing with a level of complexity that could only be handled since very recently, this collection of studies indicates the rise of an era of research in the field of fundamental and applied reaction kinetics, i.e. kinetics in the real world.

References

1. S. Arrhenius, Z. Phys. Chem., 1889, 4, 226–248.
2. M. Bodenstein, Ber. Dtsch. Chem. Ges., 1893, 13, 2603–2611.
3. M. Bodenstein, Chem. Rev., 1930, 7, 215–223.
4. Available at http://www.nobelprize.org.
5. R. A. Marcus, Angew. Chem., Int. Ed. Engl., 1993, 32, 1111–1122.
6. P. Y. Cheng, D. Zhong and A. H. Zewail, J. Chem. Phys., 1996, 105, 6216–6248.
7. U. Kettling, A. Koltermann, P. Schwille and M. Eigen, Proc. Natl. Acad. Sci. U. S. A., 1998, 95, 1416–1420.
8. L. A. Barrie, J. W. Bottenheim, R. C. Schnell, P. J. Crutzen and R. A. Rasmussen, Nature, 1988, 334, 138–141.
9. R. Vogt, P. J. Crutzen and R. Sander, Nature, 1996, 383, 327–330.
10. G. Ertl, Angew. Chem., Int. Ed., 2008, 47, 3524–3535.
11. T. Rayment, R. Schlögl, J. M. Thomas and G. Ertl, Nature, 1985, 315, 311–313.
12. M. Baerns, R. Imbihl, V. A. Kondratenko, R. Kraehnert, W. K. Offermans, R. A. van Santen and A. Scheibe, J. Catal., 2005, 232, 226–238.
13. Y. Román-Leshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic, Nature, 2007, 447, 982–985.
14. R. A. van Santen, A. J. Markvoort, I. A. W. Filot, M. M. Ghouri and E. J. M. Hensen, Phys. Chem. Chem. Phys., 2013, 15, 17038–17063.
15. M. K. Debe, Nature, 2012, 486, 43–51.
16. B. D. McCloskey, R. Scheffler, A. Speidel, G. Girishkumar and A. C. Luntz, J. Phys. Chem. C, 2012, 116, 23897–23905.
17. P. Glarborg, J. A. Miller, B. Ruscic and S. J. Klippenstein, Prog. Energy Combust. Sci., 2018, 67, 31–68.
18. Y. Rudich, N. M. Donahue and T. F. Mentel, Annu. Rev. Phys. Chem., 2007, 58, 321–352.
19. M. Claeys, B. Graham, G. Vas, W. Wang, R. Vermeylen, V. Pashynska, J. Cafmeyer, P. Guyon, M. O. Andreae, P. Artaxo and W. Maenhaut, Science, 2004, 303, 1173–1176.
20. F. Paulot, J. D. Crounse, H. G. Kjaergaard, A. Kürten, J. M. St. Clair, J. H. Seinfeld and P. O. Wennberg, Science, 2009, 325, 730–733.
21. T. J. Millar, C. Walsh and T. A. Field, Chem. Rev., 2017, 117, 1765–1795.
22. D. S. N. Parker, F. Zhang, Y. S. Kim, R. I. Kaiser, A. Landera, V. V. Kislov, A. M. Mebel and A. G. G. M. Tielens, Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 53–58.
23. T. Turányi and A. S. Tomlin, Analysis of Kinetic Reaction Mechanisms, Springer, Heidelberg, 2014.
24. T. Lu and C. K. Law, Prog. Energy Combust. Sci., 2009, 35, 192–215.
25. D. Healy, D. M. Kalitan, C. J. Aul, E. L. Petersen, G. Bourque and H. J. Curran, Energy Fuels, 2010, 24, 1521–1528.
26. M. J. Pilling, Proc. Combust. Inst., 2009, 32, 27–44.
27. J. A. Miller, M. J. Pilling and J. Troe, Proc. Combust. Inst., 2005, 30, 43–88.
28. E. Ranzi, C. Cavalotti, A. Cuoci, A. Frassoldati, M. Pelucchi and T. Faravelli, Combust. Flame, 2015, 162, 1679–1691.
29. S. S. Merchant, C. F. Goldsmith, A. G. Vandeputte, M. P. Burke, S. J. Klippenstein and W. H. Green, Combust. Flame, 2015, 162, 3658–3673.
30. F. Battin-Leclerc, O. Herbinet, P.-A. Glaude, R. Fournet, Z. Zhou, L. Deng, H. Guo, M. Xie and F. Qi, Angew. Chem., Int. Ed., 2010, 49, 3169–3172.
31. F. Qi, Proc. Combust. Inst., 2013, 34, 33–63.
32. Z. Wang, D. M. Popolan-Vaida, B. Chen, K. Moshammer, S. Y. Mohamed, H. Wang, S. Sioud, M. A. Raji, K. Kohse-Höinghaus, N. Hansen, P. Dagaut, S. R. Leone and S. M. Sarathy, Proc. Natl. Acad. Sci. U. S. A., 2017, 114, 13102–13107.
33. L. S. Tran, B. Sirjean, P.-A. Glaude, R. Fournet and F. Battin-Leclerc, Energy, 2012, 43, 4–18.
34. K. Kohse-Höinghaus, P. Oßwald, T. A. Cool, T. Kasper, N. Hansen, F. Qi, C. K. Westbrook and P. R. Westmoreland, Angew. Chem., Int. Ed., 2010, 49, 3572–3597.
35. S. M. Sarathy, P. Oßwald, N. Hansen and K. Kohse-Höinghaus, Prog. Energy Combust. Sci., 2014, 44, 40–102.
36. W. Leitner, J. Klankermayer, S. Pischinger, H. Pitsch and K. Kohse-Höinghaus, Angew. Chem., Int. Ed., 2017, 56, 5412–5452.
37. G. Friedrichs, J. T. Herbon, D. F. Davidson and R. K. Hanson, Phys. Chem. Chem. Phys., 2002, 4, 5778–5788.
38. N. Faßheber, G. Friedrichs, P. Marshall and P. Glarborg, J. Phys. Chem. A, 2015, 119, 7305–7315.
39. H. Wang, Proc. Combust. Inst., 2011, 33, 41–67.
40. B. S. Haynes and H. G. Wagner, Prog. Energy Combust. Sci., 1981, 7, 229–273.
41. H. A. Michelsen, Proc. Combust. Inst., 2017, 36, 717–735.
42. H. A. Michelsen, C. Schulz, G. J. Smallwood and S. Will, Prog. Energy Combust. Sci., 2015, 51, 2–48.
43. K. Hoyermann, F. Mauß, M. Olzmann, O. Welz and T. Zeuch, Phys. Chem. Chem. Phys., 2017, 19, 18128–18146.
44. P. J. Crutzen, Tellus, 1973, 26, 47–57.
45. R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson, J. A. Kerr and J. Troe, J. Phys. Chem. Ref. Data, 1989, 18, 881–1097.
46. J. Troe, Chem. Rev., 2003, 103, 4565–4576.
47. R. A. Cox and S. A. Penkett, Nature, 1971, 230, 321–322.
48. O. Welz, J. D. Savee, D. L. Osborn, S. S. Vasu, C. J. Percival, D. E. Shallcross and C. A. Taatjes, Science, 2012, 335, 204–207.
49. T. C. Bond, S. J. Doherty, D. W. Fahey, P. M. Forster, T. Berntsen, B. J. DeAngelo, M. G. Flanner, S. Ghan, B. Kärcher, D. Koch, S. Kinne, Y. Kondo, P. K. Quinn, M. C. Sarofim, M. G. Schultz, M. Schulz, C. Venkataraman, H. Zhang, S. Zhang, N. Bellouin, S. K. Guttikunda, P. K. Hopke, M. Z. Jacobson, J. W. Kaiser, Z. Klimont, U. Lohmann, J. P. Schwarz, D. Shindell, T. Storelvmo, S. G. Warren and C. S. Zender, J. Geophys. Res.: Atmos., 2013, 118, 5380–5552.
50. G. Myhre, D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, Anthropogenic and Natural Radiative Forcing, in Climate Change 2013: The Physical Science Basis. Contributions of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P. M. Midgley, Cambridge University Press, Cambridge, UK, and New York, USA, 2013.
51. M. Ammann, R. A. Cox, J. N. Crowley, M. E. Jenkin, A. Mellouki, M. J. Rossi, J. Troe and T. J. Wallington, Atmos. Chem. Phys., 2013, 13, 8045–8228.
52. L. J. Carpenter and P. D. Nightingale, Chem. Rev., 2015, 115, 4015–4034.
53. J. Kleber, K. Laß and G. Friedrichs, J. Phys. Chem. A, 2013, 117, 7863–7875.
54. T. Berndt, R. Kaethner, J. Voigtländer, F. Stratmann, M. Pfeifle, P. Reichle, M. Sipilä, M. Kulmala and M. Olzmann, Phys. Chem. Chem. Phys., 2015, 17, 19862–19873.
55. C. Bauer, K. Becker, T. Herrmann, D. Lilge, M. Roth and M. Busch, Macromol. Chem. Phys., 2010, 211, 510–519.
56. Y. Xia, X. Xia and H.-C. Peng, J. Am. Chem. Soc., 2015, 137, 7947–7966.
57. M. Salciccioli, M. Stamatakis, S. Caratzoulas and D. G. Vlachos, Chem. Eng. Sci., 2011, 66, 4319–4355.
58. R. Brukh and S. Mitra, Chem. Phys. Lett., 2006, 424, 126–132.
59. S. Bhaviripudi, X. Jia, M. S. Dresselhaus and J. Kong, Nano Lett., 2010, 10, 4128–4133.
60. Y. Ito, C. Christodoulou, M. V. Nardi, N. Koch, H. Sachdev and K. Müllen, ACS Nano, 2014, 8, 3337–3346.
61. C. J. Cobos, G. Knight, L. Sölter, E. Tellbach and J. Troe, Phys. Chem. Chem. Phys., 2018, 20, 2627–2636.

---