Synergies between experimental and theoretical studies of gas phase reactions

The field of gas phase reaction kinetics has been a central discipline in the physical chemistry/chemical physics arena for over well over a century. In the 1950s and 60s developments in flash photolysis and flow tubes allowed studies of radical species, but until relatively recently, experimental studies focused on the removal of radical species, with little information on products. Additionally, experimental and theoretical treatments of reactions were generally presented in separate papers often with a publication gap of several years. In the last two decades, advances in experimental techniques have transformed the field with the result that there is an increasing quantity of a high quality data available for analysis over much wider ranges of conditions and with more information of reaction products (P. W. Seakins, Ann. Rep. Progr. Chem., Sect. C: Phys. Chem., 2007, 103). In the same period theoretical tools have advanced to the point where they can be used on a routine basis by all workers in the field to analyse experimental data. Consequently, there is an increasing trend in the presentation of kinetic rate data to also report results from modelling studies in order to extend and interpret the raw data.

The objectives of this special issue of PCCP are to highlight developments in theoretical and experimental techniques, to provide a showcase for the presentation of state of the art joint experimental and theoretical work, and to consider challenges for the subject in the future.

In the introduction to their paper, Carl et al. (S. A. Carl et al., DOI: 10.1039/b705505f) present a vivid picture of the gradual and step changes that have occurred in experimental techniques. Some of the major developments in experimental methods have been in expanding the conditions under which elementary kinetics can be studied. Prominent amongst these has been the coupling of Laval expansion systems with laser flash photolysis and laser induced fluorescence, pioneered by Smith, Sims and Rowe (I. W. M. Smith et al., Chem.–Eur. J., 1997, 3, 1925) for studying reactions down to less than 10 K. At the other extreme of temperature, several groups have been applying time resolved techniques to shock tube measurements and several examples are presented in this issue (N. K. Srinivasan et al., DOI: 10.1039/b702267k; Hui Xu et al., DOI: 10.1039/b703124f; J. Dammeier et al., DOI: 10.1039/b704197g).

New developments in the monitoring of radical reactants and products have also come to prominence in the last decade or so. Possibly the most significant are the developments in mass spectroscopy. The pioneering work of Gutman and Slagle in coupling flash photolysis to mass spectrometry has been taken forward by groups at Brookhaven and Leeds and an example of work from the definitive experimental apparatus coupled to the Berkeley advanced light source is featured in this issue (Fabien Goulay et al., DOI: 10.1039/b614502g). Chemical ionization mass spectrometry is also generating exciting results and studies from Percival and co-workers are presented here (Max R. McGillen et al., DOI: 10.1039/b703035e; M. Teresa Raventós-Duran et al., DOI: 10.1039/b703038j).

Optical spectroscopy still remains an important tool. Taatjes (Adam M. Knepp et al., DOI: 10.1039/b705934e) and McDonald (R. Glen Macdonald, DOI: 10.1039/b701900a) have been instrumental in developing techniques in the IR region of the spectrum and sensitivity of absorption methods in general have been enhanced by modulation techniques (J. Dammeier et al., DOI: 10.1039/b704197g). However, we should not ignore the important role that instrumented chambers play in generating high quality kinetic data, especially for complex systems and on product branching ratios. As theory becomes able to tackle larger and more complex systems, opportunities for interpretation of experimental results from chambers and validation of theory by chambers will increase. Good examples of the high quality data that can be obtained from chamber work are presented by Orlando (J. Orlando, DOI: 10.1039/b706819k), Wallington and co-workers (T. Yamanaka et al., DOI: 10.1039/b702933k) and Melluki and co-workers (G. Solignac et al., DOI: 10.1039/b703741b).

The theory and modelling of gas phase reaction systems has advanced because of developments on two fronts: the first of these is electronic structure theory. Central to the understanding of any reaction system is, of course, the potential energy surface (PES) on which it takes place. The explosion in computer power and the availability of electronic structure codes make the calculation of the PES an almost routine exercise. The synergy here is clear, gas phase systems are dominated by radical reactions and such systems present something of a challenge for electronic structure codes and so experimental data are vital in validating these codes. At the same time, as the systems that are examined become ever more complex, it becomes increasingly difficult to ascribe observed data of any one elementary reaction and the use of electronics structure calculations to map out reaction pathways, with some adjustments based on data, has had a tremendous impact on a number of studies. This special issue includes an invited article by Harding, Klippenstein and Jasper (Lawrence B. Harding et al., DOI: 10.1039/b705390h) that reviews and presents material on electronic structure methods and an application of these calculations to mapping out reaction pathways is demonstrated in the work of Hermans et al. (Ive Hermans et al., DOI: 10.1039/b704351a).

The second modelling development is the growth methods for simulating the complex pressure and temperature dependence of rate coefficients. Building on the availability of high quality PESs the modelling of rate coefficients has advanced in two ways: the modelling of bimolecular reactions using scattering theory has been an established tool for some time. In this context the term reaction has come to have a very broad meaning in the sense that a reaction can involve the exchange of mass, charge or energy, or any combination of these three. While scattering techniques remain the method of choice for small systems, as system mass increases there is a concomitant increase in the density of states, an increase that quickly exhausts the capabilities of even the most powerful machines. As a consequence, development has also proceeded along statistical lines, possibly the most famous methods being transition state theory and its microcanonical counterpart RRKM theory and their many variants, particularly variational transition state theory. The work by Georgievskii et al. (Yuri Georgievskii et al., DOI: 10.1039/b703261g) gives an indication of the size and complexity of systems that can currently be tackled using RRKM techniques. In conjunction with RRKM theory, the last two decades have also seen considerable advances in the use of master equation methods particularly for systems which have multiple stationary points on the PES (potential energy wells). In this context the master equation provides a link between the electronic structure calculations, energy transfer models and experimental data. Master equation methods are now regularly used to analyse and fit experimental data and allow synergies between experimental and theoretical studies to be exploited to the full. This special issue includes an invited article be Robertson, Pilling, Jitariu and Hillier (Struan H. Robertson et al., DOI: 10.1039/b704736c), that reviews and presents material on master equation methods. Despite their increasingly routine use, much work remains to be done in making master equation methods more robust and one area of weakness, the low temperature region, is discussed by Green and Bhatti (N. Green and Zaheer A. Bhatti, DOI: 10.1039/b704519k).

Several papers in this issue demonstrate the ever increasing trend to combine experimental measurement with high quality modelling. The work of Knepp et al. (Adam M. Knepp et al., DOI: 10.1039/b705934e) on the reaction of OH and HO₂ with cyclohexyl radicals demonstrates this synergy: ab initio calculations were used to build a master equation model which was then used to analyse experimental data. Key potential energy barrier heights could then be adjusted until agreement with experiment was obtained and, in this way, features of the potential energy surface extracted. Baeza-Romero et al. (M. Teresa Baeza-Romero et al., DOI: 10.1039/b702916k) report results of a combined experimental and modelling approach used to reveal the details of a complex reaction: LIF methods are used to monitor OH in the reaction of OH with CH₃COCOH and from the data in conjunction with ab initio/RRKM/master equation calculations these workers were able to infer, amongst other things, that a significant fraction of the CH₃CO radical produced from the decay of CH₃COCO went on to dissociate into CH₃ and CO. Maranzana and co-workers (Andrea Maranzana et al., DOI: 10.1039/b705116f) also demonstrate the use of ab initio/master equation methods in the estimation of PES features from experimental date for the reaction of O₂ with CH₃CO radicals.

The papers in this special issue reflect the breadth of techniques used, but also the wide range of applications; we have contributions looking at the fundamentals of energy transfer (S. Marinakis et al., DOI: 10.1039/b703909c; Sh. Watanabe et al., DOI: 10.1039/b702840g), through studies applicable to atmospheric chemistry (Sarah L. Broadley et al., DOI: 10.1039/b704920j; Paola Cassanelli et al., DOI: 10.1039/b700285h) to very applied topics in combustion (Y. Muharam and J. Warnatz, DOI: 10.1039/b703415f; Katie L. Randolph and Anthony M. Dean, DOI: 10.1039/b702860a). This issue has papers linked to photochemistry (G. Solignac et al., DOI: 10.1039/b703741b; V. Khamaganov et al., DOI: 10.1039/b701382e) and work is not limited to the first row of the periodic table! (Rosa Becerra et al., DOI: 10.1039/b706148j; M. B. Williams et al., DOI: 10.1039/b703957n). This breadth of application is one of the strengths of the field and well stand the subject in good stead for the foreseeable future. Many advances have been driven by combustion chemistry or the need to understand atmospheric issues such as stratospheric ozone depletion or urban air quality. The need to understand and combat climate change has now become the dominant issue. Chemical kinetics plays a central role in unravelling the issues associated with climate change. Combustion chemistry will also be part of the mitigation of climate change either via promoting enhanced efficiency of fossil fuel combustion or understanding the issues behind biofuel production and combustion. Clearly gas phase kinetics has an important future.

A notable champion of the combined experimental/theoretical approach to problems in gas phase kinetics is Prof. Michael J. Pilling, and it is fitting that, in the year that marks his retirement, this special issue collects together papers which span the experimental/theoretical spectrum. Mike has been heavily involved with the Royal Society of Chemistry Faraday Division and has had several European collaborations, so it is particularly appropriate that these papers have been published within PCCP.

It merely remains for us to thank all the authors who have submitted material to this special issue and the staff at PCCP who have done such a good job in putting this issue together.

Paul W. Seakins

Struan H. Robertson

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