Electronic structures and reaction dynamics of open-shell species

Free radicals are broadly defined as short-lived transient (chemically unstable) species (atoms, molecules, or ions). They typically have open-shell configurations with one or more unpaired electrons, and they are highly reactive. Free radicals are common in a wide range of reactive chemical environments, such as combustion, plasmas, atmosphere, and the interstellar environment, and they play important roles in these chemistries. For example, complex atmospheric and combustion chemistries are composed of and governed by many elementary processes involving free radicals. Consequently, characterization of the open-shell species and understanding their chemical reactivity are pivotal to assessing the reaction mechanisms in the complex reactive environments.

Since the first spectroscopic identification of the organic free radical, triphenylmethyl radical, by Gomberg in 1900, there have been a great number of studies in many aspects of the free radicals, such as their electronic structures, spectroscopy, chemical kinetics, and reaction dynamics, as well as their roles in various reactive environments. This is, for example, attested by the seminal volume by Herzberg, The Spectra and Structures of Simple Free Radicals, among many others. The interests in open-shell species have continued to grow strongly, and free radicals have been the topics of many scientific meetings, including the biannual International Symposium on Free Radicals, since 1956.

Compared with closed-shell molecules, the studies of free radicals, both experiments and theories, are more challenging, especially for their reactivity at the molecular level. Free radicals are open-shell systems and they typically possess multiple low-lying electronic states; these states, along with their non-adiabatic processes, complicate their spectroscopy and reaction dynamics. The structure, energetics, and chemical reactivity are fundamental and inseparable. Sophisticated theories are required. Experimentally, it is difficult to characterize the free radicals and their reactions due to the difficulty of their preparation and their highly reactive nature, and sensitive and selective techniques are needed to detect radicals and their reaction products.

This Themed Issue of PCCP highlights recent advances in experimental and theoretical studies of the fundamental aspects of the open-shell species. This issue concentrates on the isolated open-shell species; the topics include spectroscopy and potential energy surfaces, photochemistry, and reaction dynamics of free radicals in the gas phase, as well as in atmospheric chemistry, combustion, and other chemical processes. This Themed Issue is loosely connected to the Symposium on Electronic Structures and Reaction Dynamics of Open-shell Species at the 235th American Chemical Society National Meeting in April 2008, New Orleans, Louisiana, USA.

The 11 papers collected in this Themed Issue of PCCP can be organized into three groups: (1) electronic structures and spectroscopy of open-shell species (Evangelista et al., p. 4728; Stanton and Okumura, p. 4742; and Wren et al., p. 4745), (2) photodissociation of free radicals and photodissociation that involves open-shell products (Radenovic et al., p. 4754; Zheng et al., p. 4761; Dooley et al., p. 4770; and Silva et al., p. 4777), and (3) reaction dynamics of elementary bimolecular reactions of free radicals (Leonori et al., p. 4701; Zhang et al., p. 4707; Lin et al., p. 4715; and Chen et al., p. 4722). Evangelista et al., p. 4728, have described a companion perturbation theory for the state-specific Mukherjee multireference coupled cluster method and its application to the lowest-lying electronic states of meta-benzyne (a diradical). Stanton and Okumura, p. 4742, have provided new insights, from both experiment and theory, into the complex spectroscopy and vibronic level structures of NO₃ (an important radical in atmospheric chemistry). Wren et al., p. 4745, have resolved the previously debated issue on the singlet–triplet splittings of the dihalocarbenes using high-resolution photoelectron spectroscopy. Photodissociation dynamics of three diatomic open-shell species are investigated with high-resolution techniques in three papers. The vibrational state resolved predissociation of OH (A²Σ⁺) and ClO (A²Π_3/2) are studied using velocity-map imaging by Radenovic et al., p. 4754, and Dooley et al., p. 4770, respectively; the fine-structure branching ratios, angular distributions, and alignment parameters of the atomic photofragments are characterized, revealing detailed non-adiabatic dynamics. The ultraviolet photodissociation of the SD radical in vibrationally ground and excited states is examined using the Rydberg tagging technique by Zheng et al., p. 4761; the fine-structure branching ratios of S(³P_J) and angular distributions of D atom have also revealed detailed non-adiabatic photodissociation dynamics in the diatomic open-shell species. Silva et al., p. 4777, have characterized the various alkyl radical products in the photodissociation of heptane isomers using the ion imaging technique. Two papers describe the experimental studies of elementary bimolecular reactions of free radicals. Leonori et al., p. 4701, have characterized the rate coefficients, product branching ratios, and reaction mechanism of the S(¹D) + C₂H₄ reaction in the crossed-beam and low-temperature kinetic experiments. Zhang et al., p. 4707, have investigated the dynamics of the reaction of ethynyl (C₂H) radical with allene using the crossed-beam technique, indicating possibilities of neutral–neutral reaction pathways that involve ethynyl in planetary atmospheres and interstellar chemistry. Finally, two theoretical papers are presented and describe the theoretical studies of elementary bimolecular reactions of open-shell species. Lin et al., p. 4715, have examined the impacts of reactant rotational excitation in the tri-atomic system H + O₂→ OH + O (a key reaction in combustion) using quantum wave packet, quasi-classical trajectory, and phase space theory calculations. Chen et al., p. 4722, have developed a new full dimensional potential energy surface for the HO₂ + NO reaction (important in atmospheric chemistry) and investigated the reaction and isomerization mechanisms using quasi-classical trajectory calculations.

This Themed Issue of PCCP has witnessed significant progresses in both the experimental and theoretical studies of free radicals. Various experimental methods (flash pyrolysis, photolysis, electric discharge, etc.) are utilized to generate intense and clean sources of radicals. High-intensity lasers are used for diagnostics of radicals and for detections of reaction products of radicals. Experimental techniques such as the universal crossed-beam machine, velocity map imaging, and Rydberg tagging have been used, with increasing sophistication and resolution. One highlight in this issue is the high-resolution studies of photodissociation of open-shell species; the studies of the open-shell excited states expand from spectroscopy to photodissociation, going beyond the Franck–Condon region to a larger region of the excited surface/curve that ultimately connects to the photoproducts. The theories have also become more sophisticated, with increasing capabilities of treating both the ground and electronic excited states of open-shell species with chemical accuracy, as well as for larger and complex systems. It is important to point out that close interactions between the sophisticated experiments and the high-level theories are essential for fundamental understanding of the electronic structures and reaction dynamics of the open-shell species. Indeed, several papers in this Themed Issue have demonstrated the successes of these fruitful interactions.

We would like to thank all the participants in the Symposium on Electronic Structures and Reaction Dynamics of Open-shell Species at the 235th American Chemical Society National Meeting, and in particular all the authors who have submitted materials to this Themed Issue of PCCP. We also thank the staff of the Royal Society of Chemistry, especially Philip Earis and Nicola Nugent, for their persistent efforts to bring this issue to the finish line.

Jingsong Zhang, University of California, Riverside

Martin Head-Gordon, University of California, Berkeley

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