Structure and Reactions of Aliphatic Bridged Bifunctional Radical Ions: Exploring Fine‐Tuning in Radiation Chemistry

Abstract

Bridged bifunctional molecules of general structure X(CH2)nY are useful models for investigations into the effects of intra‐ and intermolecular interactions in primary radiation‐induced processes, with a number of implications for radiation chemistry, radiobiology, and molecular electronics. This short Review presents an outline of recent studies on the structure and reactivity of aliphatic bifunctional radical ions in low‐temperature matrices using EPR spectroscopy and quantum‐chemical calculations. Both EPR data and DFT calculations for radical cations show that the delocalization of charge and spin density occurs if XY or X and Y have close electronic properties. If the difference in the ionization potentials between X and Y is large, localized radical cations are observed with ground‐state properties close to those of the corresponding monofunctional species. Nevertheless, the remote second functional group may affect the photochemistry of such radical cations at a specific bridge length, probably due to intramolecular interactions in a certain conformation. The stabilization of bridged bifunctional radical anions containing two carbonyl groups was found to be very sensitive to the local environment, which may be described in terms of a microsolvation model at the MP2 level of theory. Two independent reactions pathways with excess electrons, yielding different types of localized radical anions, were demonstrated for asymmetrical bifunctional molecules. The obtained results and their implications are discussed in terms of fine‐tuning effects in radiation‐induced chemical processes in condensed phases.

Astrochemically Relevant Radicals and Radical–Molecule Complexes: A New Insight from Matrix Isolation

“…Organic radical cations remained elusive for several decades because of their high reactivity, even at low temperatures. The breakthrough was achieved in the 1980s, and since that time, a large number of radical cations produced from complex organic molecules was studied by EPR in detail due to the invention of a convenient and simple laboratory technique based on their stabilization in halocarbon (Freon) matrices at moderately low temperatures (typically, 77 K) (several reviews are available [14,[126][127][128]). An overview of EPR studies on moderate-size organic radical cations stabilized in noble gas matrices made in our laboratory was given elsewhere [14,17].…”