Catalysis in Hydrogen-Bridged Radical Cations

Abstract

Hydrogen-bridged radical cations (HBRCs) are an intriguing subclass of ion-molecule complexes. They may act as key intermediates of remarkable stability in both association and dissociation reactions of heteroatom-containing molecular ions. The H-bridge of such an HBRC can promote isomerization of its ionic component by H-transfer. Proton-transport catalysis (PTC) is a prime example. Here, a neutral molecule promotes the smooth transformation of an ion into its H-shift isomer by consecutive proton-transfer reactions. A celebrated case is the water-catalyzed isomerization of CH 3 OH•+ into its more stable distonic isomer • CH 2 OH 2 + . Other early studies of PTC also deal with catalyzing 1,2-H shifts in association reactions. This short review focuses on more recent combined experimental and computational studies of catalysis in HBRCs. Mechanisms involving both proton and H atom transfers have been proposed for a variety of systems of H-shift isomers. It has also become clear that PTC is not confined to bimolecular reactions. It also features in the unimolecular chemistry of heteroatom-containing ions, which have a tendency to isomerize to HBRCs en route to their dissociation.

Dissociative Photoionization of Glycerol and its Dimer Occurs Predominantly via a Ternary Hydrogen-Bridged Ion–Molecule Complex

“…[31][32][33] In the context of ionic rearrangements in low-energy radical species, hydrogen-bridged radical cations, in particular those containing a O...H...O or N...H...O moiety, have been proposed to form stable intermediates. [34][35][36][37] Moreover, such observations have not been limited to the gas phase. For example, a hydrogen-bridged ion has been proposed to play a key role in the B 12 -catalyzed dehydration reaction of ethylene glycol.…”

“…Furthermore, several product fragments are found to feature hydrogen-bridged ion–molecule interactions, which allow for charge and spin delocalization between fragments. Hydrogen-bridged ion–molecule interactions are found to typically lie between 5 and 35 kcal/mol and are proposed to play a key role in biology and chemistry, such as protein folding, enzyme activity, , biomolecular recognition and sensors, surface adsorption, self-assembly in supramolecular chemistry and molecular crystals, − electrolytes, ion solvation, and ionic clusters. − In the context of ionic rearrangements in low-energy radical species, hydrogen-bridged radical cations, in particular those containing a O···H···O or N···H···O moiety, have been proposed to form stable intermediates. − Moreover, such observations have not been limited to the gas phase. For example, a hydrogen-bridged ion has been proposed to play a key role in the B 12 -catalyzed dehydration reaction of ethylene glycol .…”

Water-Assisted Proton Transfer in the Sequential Hydration of Benzonitrile Radical Cation C6H5CN•+(H2O)n: Transition to Hydrated Distonic Cation •C6H4CNH+(H2O)n with n ≥ 4

“…The benzonitrile radical cation (C 6 H 5 CN •+ ) is a prototype for an ionized aromatic molecule containing a cyano group that plays an important role in the functionalization of many bimolecular systems. − The hydration of the benzonitrile radical cation is of particular interest in view of the comparison with the stepwise hydration of the benzene and pyridine radical cations. The interest in the interactions of the benzonitrile radical cations with water and other polar molecules is also motivated by the recent discovery of benzonitrile, one of the simplest nitrogen-bearing aromatic molecules, in the interstellar medium. − The ion in its classic form could be hydrated by water molecules, forming relatively weak CH δ+ ···O hydrogen bonds similar to the benzene radical cation or a water-facilitated proton shift could form a distonic ion ( • C 6 H 4 CNH + ), where a water cluster is hydrogen-bonded to the −CNH + group [ • C 6 H 4 CNH + (H 2 O) n ]. − The driving force for the water-facilitated proton shift is the formation of strong IHBs in the hydrated cluster ions.…”