When isomerisation is electron transfer: the intriguing story of the iso-halocarbons

Abstract

This review examines recent studies of the spectroscopy, dynamics, reactivity and electronic structure of iso-halocarbons, which are isomers of organic halogens that are critically important reactive intermediates in the chemistry of these species. We approach this review from the viewpoint of isomerisation as electron transfer, in as much as the dominant resonance structure of the isomer is an ion pair of halocarbenium ion and halide anion. We show that this perspective is important in understanding the spectroscopy, dynamics and chemical reactivity of the iso-halocarbons. We examine experimental methods for study of the isomer, focusing on the complementary nature of steady state matrix isolation experiments and time-resolved studies, typically carried out in solution. We review experimental data concerning the solvent dependence of the isomer lifetime in solution, examine S_N1 style nucleophilic reactions of the isomer with water and related solvents, discuss the role of the isomer as methylene transfer agent in cyclopropanation reactions and probe the possible role of the isomer in photoinduced halogen exchange. We then discuss the role of the isomer in the gas-phase chemistry and photochemistry of halocarbons, where isomerisation is demonstrated as a path to molecular elimination. Moving beyond the polyhalomethanes, we describe the existence of proton-coupled electron-transfer pathways involving the isomer in the gem-dihaloethanes, examine the potential importance of spin-orbit coupling and the role of triplet states and briefly discuss the potential importance of similar isomeric structures in other organic molecules. We conclude with future perspectives and research on this intriguing class of intermediates.

Keywords:

• iso-halocarbons
• isomerisation
• electron transfer
• molecular elimination
• photoinduced halogen exchange
• proton-coupled electron transfer

Acknowledgements

The author acknowledges the many students, post-doctoral fellows and collaborators who have contributed to this work, including Aimable Kalume, Lisa George, Nicole Cunningham, T. J. Preston, M. Dutta, Fleming Crim, Patrick El-Khoury, Alexander Tarnovsky, Bruce Ault, Brian Esselman, James Wagner, Robert McMahon and Richard Dawes. Financial support from the National Science Foundation (CHE-1057951) is gratefully acknowledged, as is receipt of a John J. Eisch fellowship to Aimable Kalume. Finally, the author is grateful for many fruitful discussions with Prof. Rajendra Rathore.