Chemical processes of ions—transport and reactivity

85th International Discussion Meeting of the Deutsche Bunsen-Gesellschaft für Physikalische Chemie, Marburg, 15–17 September, 2004

Elementary chemical processes of electrically charged molecular entities play a pivotal role in many areas of chemistry, as well as of physics and biology. Examples are chemical processes in natural or technical discharges (lightning, plasma processes), chemical reactions in the ionosphere and in interstellar clouds (ion–molecule reactions), surface analysis and modification by ion beams (ion–surface reactions), metal ion catalysis, and signal transfer in ion channels of biological relevance. In most of the areas mentioned above we have seen in recent years significant progress both in terms of techniques and understanding. The scientific relevance of work in this field has been recognized by the award of the Nobel Prize for chemistry to Georg A. Olah (1994) for his work on carbocation chemistry and to Roderick MacKinnon (2003) for structural and mechanistic studies of ion channels, just to mention a few examples. The technical relevance is reflected e.g. in the application of reactive ion etching and ion catalysis.

The 85th International Bunsen Discussion Meeting ‘Chemical Processes of Ions—Transport and Reactivity’ was held at the University of Marburg during 15–17 September, 2004, with the specific aim to bring together chemists, physicists and biologists in an interdisciplinary effort to shed new light on the common underlying laws governing the transport and the reactivity of charged molecules. The current special issue puts together a collection of papers presented at this meeting. It contains contributions covering the most relevant topics of the meeting.

One important prerequisite of the investigation of reactivity and transport of ions is the selective formation of molecular ions. Two of the most widely used techniques are the resonance enhanced multiphoton ionization (REMPI) and the pulsed field ionization (PFI). The selective formation of BCl₃ ions by the PFI technique is contributed by Ng et al. employing both VUV laser and synchrotron radiation.¹ This work also accurately determines the bond dissociation energy of the B–Cl bond. The formation of state-selected ions of several ammonia isotopomers by the REMPI technique is described by Weitzel et al.² These studies provide the basis for future state-selected ion–molecule studies. Structural properties of ions can be derived from optical spectroscopy but also from chemical probing, e.g. in a cold ion trap, as discussed by Gerlich for the CH₅⁺ ion.³ Currently, it is difficult to distinguish whether a ‘protonated methane’ structure or a CH₃–H₂^+⌉ adduct is more relevant. An invaluable piece of information is coming from ab initio calculations, which today often predict which region it would be interesting to perform experiments in. An example is presented by Hochlaf for several structural isomers of the C₃S⁺ ion and its formation from C₃⁺.⁴ An overview over the current state of the art in unimolecular decay of energy selected ions is given by Baer et al.⁵ Both the energetics and the kinetics of fragmentation of CH₂BrCl⁺ and P(C₂H₅)³⁺ ions are included as examples.

A central part of the meeting dealt with ion–molecule reactions. The importance of complex formation in low temperature ion–molecule reactions is pointed out by Nikitin and Troe.⁶ In particular, rotational quantum effects are predicted for very low collision energy. While relevant conditions for ion–molecule reactions in the outer atmospheres and in interstellar space are typically low pressure and low temperature, relevant conditions for plasma processes may include high pressure and high temperature. The contribution from Viggiano et al. surveys a representative number of ion–molecule studies ranging from 15–700 Torr obtained at variable temperature.⁷ Complex forming association reactions and thermal dissociation can be described within the same framework of unimolecular reaction theory. This is demonstrated by Troe, in particular for the N₂⁺ + N₂ (+ M) ⇔ N₄⁺ (+ M) reaction.⁸ While the previous studies looked at reactions under thermal conditions, an example of photo-initiated bimolecular reactions is contributed by Schlemmer et al.⁹ The authors show that exciting the C–H stretching vibration in C₂H₂⁺ is an order of magnitude more effective than the bending vibration in promoting hydrogen abstraction.

The conventional wisdom of ion chemistry is to prepare an ion first and then look at its reactivity. Wrede et al. demonstrate that the chemical reaction of a neutral molecule in (ion-like) high Rydberg states closely resembles that of the free ion.¹⁰ While the majority of the contributions in this issue deal with cations, for an important example of an anionic S_N2 reaction Schmatz et al. have calculated cross sections and rate constants.¹¹ Again, the relevance of vibrational excitation in the precursor is investigated in detail.

The broad spectrum of ion surface interactions from low impact energy where soft-landing dominates to high energy where reactive scattering wins, is covered by a contribution from Cooks et al.¹² The low energy regime of a few eV allows for the analysis of e.g. functionalized self-assembled monolayers, while the high energy regime (several 10 eV) may lead to significant surface modification.

The transport of alkali ions through membranes plays a pivotal role for the transfer of signal, e.g. in sensory perception. Important progress in the understanding at the molecular level comes from the synthesis of artificial ion channels. In his contribution Koert discusses the synthesis of gramicidin ion channels together with single channel current measurements.¹³ Conformational switching is demonstrated in the presence of Cs⁺ ions. Charge transport along a DNA chain is crucial for understanding damage and repair processes in DNA. Baranovski et al. describe model calculation for electron and hole hopping between two guanine bases in model DNA compounds.¹⁴

The 85th International Bunsen Discussion Meeting was supported by the German Science Foundation (DFG) the German Fund of the Chemical Industry (FCI) and the Fund of Marburg University (Marburger Universitäts-Stiftung). Further details of the meeting are available via the URL http://www.chemie.uni-marburg.de/∼weitzel/ion2004/ or directly from the author of this editorial.

Karl-Michael Weitzel

Philipps-Universität Marburg

References

1. Jie Yang, Yuxiang Mo, K. C. Lau, Y. Song, X. M. Qian and C. Y. Ng, Phys. Chem. Chem. Phys., 2005, 7, 1518–1526.
2. Moana Nolde, Karl-Michael Weitzel and Colin M. Western, Phys. Chem. Chem. Phys., 2005, 7, 1527–1532.
3. D. Gerlich, Phys. Chem. Chem. Phys., 2005, 7, 1583–1591.
4. H. Ndome and M. Hochlaf, Phys. Chem. Chem. Phys., 2005, 7, 1568–1576.
5. Tomas Baer, Bálint Sztáray, James P. Kercher, A. F. Lago, Andras Bödi, Christopher Skull and Don Palathinkal, Phys. Chem. Chem. Phys., 2005, 7, 1507–1513.
6. E. E. Nikitin and J. Troe, Phys. Chem. Chem. Phys., 2005, 7, 1540–1551.
7. A. A. Viggiano, Abel I. Fernandez and J. Troe, Phys. Chem. Chem. Phys., 2005, 7, 1533–1539.
8. Jürgen Troe, Phys. Chem. Chem. Phys., 2005, 7, 1560–1567.
9. Stephan Schlemmer, Oskar Asvany and Thomas Giesen, Phys. Chem. Chem. Phys., 2005, 7, 1592–1600.
10. Eckart Wrede, Ludger Schnieder, Karen Seekamp-Schnieder, Britta Niederjohann and Karl H. Welge, Phys. Chem. Chem. Phys., 2005, 7, 1577–1582.
11. Carsten Hennig and Stefan Schmatz, Phys. Chem. Chem. Phys., 2005, 7, 1552–1559.
12. Bogdan Gologan, Jason R. Green, Jormarie Alvarez, Julia Laskin and R. Graham Cooks, Phys. Chem. Chem. Phys., 2005, 7, 1490–1500.
13. Ulrich Koert, Phys. Chem. Chem. Phys., 2005, 7, 1501–1506.
14. J. Matulewski, S. D. Baranovskii and P. Thomas, Phys. Chem. Chem. Phys., 2005, 7, 1514–1517.

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