Multiply charged ions in the gas phase

When the elder of this editorial team began to study the properties of small molecular dications, in the 1980s, any conversation on his research topic had to be prefaced by a lengthy explanation of why investigating the chemical physics of small di-positive ions might be in any way interesting or relevant. The contents of this Themed Issue show how dramatically this field has widened and developed over the intervening 30 years, and also that those tentative early ideas, of the relevance of multiply-charged ion chemistry in a variety of fields, have been realized. The papers in this volume clearly show that various aspects of the physics and chemistry of multiply-charged ions now represent vibrant research fields in their own right, involving topics ranging from the fundamental properties of small multiply charged ions, which are still of interest and now studied at a level of detail un-imaginable in the 1980s, via studies of the reactivity of larger dications, to investigations of multiply-charged biomolecules.

From the days of pioneering investigations, scientists have been intrigued by the nature of the bonding, and the associated kinetic and thermodynamic stability, of small highly-charged species. In this issue we can read about such fundamental properties of diatomic dications, trications and even tetracations, in the work of Corral et al. (DOI: 10.1039/C1CP21534E), Alves et al. (DOI: 10.1039/C1CP20735K), Wu et al. (DOI: 10.1039/C1CP21345H) and Brites et al. (DOI: 10.1039/C1CP21567A; DOI: 10.1039/C1CP21566C). Multiply-charged anions are usually much harder to generate than their positively-charged counterparts, largely because of the possibility of electron autodetachment leading to a decrease in the ion's charge. However, an exciting prediction of small metastable trianions, which are stable with respect to such autodetachment, can be found in the article of Sommerfeld and Bhattarai (DOI: 10.1039/C1CP21570A). Such predictions lay down a gauntlet to experimentalists to generate and detect these species.

As in other areas of chemistry, the accurate measurement of critical quantities which define the properties (energetics, lifetimes…) of multiply-charged ions relies on careful benchmark experiments. For example, such measurements provide reliable experimental values quantifying the electronic structure of the target species. Common benchmark values for multiply charged ions are ionization energies, which are of particular value as their calculation allows the ready evaluation of the effectiveness of various theoretical methodologies for computational investigations of multiply-charged ions. More details on this fundamental theme can be found in the articles of Lablanquie et al. (DOI: 10.1039/C1CP21546A), Eland et al. (DOI: 10.1039/C1CP21654F) and Püttner et al. (DOI: 10.1039/C1CP21584A).

Another topical issue to emerge in recent years is the extensive bimolecular reactivity of multiply-charged ions. Their significant charge density can make small multiply charged ions very reactive, and their high internal energy allows energetic access to transformations which are not possible for neutral or singly charged reactants. In this issue new insights into dicationic reactivity are described in articles of Marçalo et al. (DOI: 10.1039/C1CP21399G), Ascenzi et al. (DOI: 10.1039/C1CP21634A) and Parkes et al. (DOI: 10.1039/C1CP21612K).

Many general principles governing their properties and reactivity are common to all multiply charged ions. However, in contrast, the environmental venues in which multiply-charged ions play (or may play!) an important role, are rather diverse. The role of multiply-charged ions in the interstellar environment, a region where the potential role of these species has long been speculated upon, is discussed in the Perspective article of Böhme (DOI: 10.1039/C1CP21814J). The chemistry of dications in the rather denser media of planetary atmospheres is addressed in the work of Thissen et al. (DOI: 10.1039/C1CP21957J). Moving to even higher densities multiply-charged ions in solution can be stabilized via their interaction with solvent molecules. The intriguing properties of such microsolvated multiply-charged ions are described in the articles of Milner et al. (DOI: 10.1039/C1CP21538H) and Perera et al. (DOI: 10.1039/C1CP21586H).

Over the past few years, multiply-charged ions have also become very important in analytic mass spectrometry, as their formation allows tractable mass spectrometric analysis of large, and hence heavy, biomolecules. As mass spectrometry plays an increasingly important role in biomolecular structure determination, it is of immediate interest to understand the details and effects of multiple-charging. In this issue such fundamental studies related to peptide or protein analysis are presented in the articles of Sterling et al. (DOI: 10.1039/C1CP20277D), Saminathan et al. (DOI: 10.1039/C1CP21522A), Jensen et al. (DOI: 10.1039/C1CP21549C), Eizaguirre et al. (DOI: 10.1039/C1CP21578G) and Hassell et al. (DOI: 10.1039/C1CP21581G). Understanding the role and fate of multiply-charged ions in analytical mass spectrometry requires both a detailed explanation of their fragmentation pathways and also the ability to model such species reliably. This volume contain a significant step towards these goals, presented in the article of Stace and Bichoutskaia (DOI: 10.1039/C1CP21573F).

We hope that this special issue presents both a topical snap-shot of the state-of-the-art, and an informative overview of current topics of interest, in the field of multiply-charged ions. We have tried to assemble a series of papers which give a representative picture of the ever widening field in which multiply-charged species play a role. If we have succeeded, this success is principally due to all the contributing authors who made the effort to write for this special issue; we would both like to express our sincere thanks to our contributors for this support. Finally, special thanks are also due to the editorial team of PCCP for their professional work and assistance, which made our editorial duties very straightforward.

We hope this issue acts both as a resource to those who work in the field and also serves to intrigue those who currently only deal with singly-charged species to broaden their horizons and enter the multiply-charged world.

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