Spectroscopy and dynamics of radicals, clusters and ions

Transient species, including radicals, ions and molecular as well as metal clusters, play a central role in the chemistry of complex systems. Radicals and ions appear as reactive intermediates in combustion processes, the atmosphere and interstellar space. Molecular clusters, held together by noncovalent bonds, permit to study the weak interactions that govern the structure and function of biomolecules, in particular hydrogen bonds. Metal clusters, on the other hand, bridge the gap between isolated molecules and solids and provide insight into properties like catalytic activity. From the experimentalists’ point of view all these species have one thing in common: since they are unstable at room temperature and/or atmospheric pressure, they can only be studied under isolated conditions.

Given the chemical importance of transient species it is no surprise that their spectroscopic investigation has a long history. The methods to generate them in the early days of molecular spectroscopy were not much different from today: electric discharges, pyrolysis, and later photolysis were used. However, there were significant challenges to overcome. The species were studied in low pressure gas cells, and collisions were often unavoidable, destroying kinetically labile radicals and ions. In addition, species that were only stable at low temperature eluded characterisation. Consequently, there was a continuing search for new experimental strategies. One particular milestone was the advent of matrix isolation spectroscopy in 1954.¹ Molecules were stored in frozen rare gases, providing a cold and chemically inert environment especially for, but not limited to, vibrational and electronic spectroscopy. Much of the development was driven by the group of George Pimentel in Berkeley. After it was shown that even ions can be stored and studied in rare gas matrices, one of his former students, Vladimir Bondybey, started to investigate vibronic interactions in open shell systems, like Jahn–Teller and Renner–Teller interactions.²

Matrices had still some shortcomings: The rotational structure was obscured, mass selection was not possible for a long time, and the stored molecules were subject to small perturbations by the matrix, shifting spectroscopic transitions relative to those of the isolated molecules. Free jet spectroscopy based on the adiabatic expansion of molecules into the vacuum permitted studying reactive species at low temperatures under collision-free conditions, beginning in the mid 1970’s. Multiphoton laser excitation schemes and mass spectrometric detection later allowed a species- and state-selective investigation of molecules.

The key technical innovation in the area of metal clusters was the introduction of laser vaporisation as a generation method. Two groups developed the approach in parallel: in the same volume of the Journal of Chemical Physics, V. E. Bondybey and J. H. English³ reported the laser induced fluorescence spectrum of Pb₂ in a free jet, while R. Smalley and coworkers published a mass spectrum of aluminium clusters.⁴ Suddenly, a whole new research field opened up: metal cluster science started to flourish and developed into what is now called “nanoscience”.

Investigations on molecular ions were also driven by new technology. With the advent of ion guides, Paul and Penning (ICR) traps, it became possible to store isolated ions and study their reactivity. One now examines the chemistry of ions surrounded by a large number of solvent molecules, closely mimicking the behaviour of liquids. Bondybey and coworkers showed that even precipitation reactions have their equivalent in the cluster.⁵ Finally, molecular clusters have become nanodroplets, and the dream of studying solution phase chemistry under well defined conditions in the gas phase is coming true! The experimentalist will happily admit that much of the insight gained over the last couple of years was achieved because of the intimate relation between experiment and theory. The availability of ever faster, more powerful and affordable processors in combination with sophisticated codes for electronic structure calculations allows to tackle questions almost routinely that were untractable only a few years ago.

We are now at the point where investigations of the structure and dynamics of radicals, clusters and ions are driven by the chemical relevance of the species of interest rather than the technological development. This special issue, containing a selection of original work by leading groups, provides an overview of the field as it stands today:

• Many reactive species have now been studied in free jets, using wavelength from IR to XUV. Investigations focus not only on their structure, but also on the kinetics and dynamics of their reactions.

• Since radicals are important intermediates in combustion processes there has been a lot of effort dedicated to their in situ detection. One contribution in this issue shows⁶ how hydrocarbon radicals present in a flame can be detected in an isomer-selective way using photoionization detection.

• Photodetachment of mass-selected anions allows to access parts of the neutral potential energy surface not accessible by other means, for example collision complexes of chemical reactions. In combination with coincidence detection techniques more complicated reactions can be studied and in turn used to calibrate calculations.⁷

• Matrix isolation, especially in combination with electronic structure calculations, continues to be a powerful tool. Its major advantage is the opportunity to store molecules over hours to study them at leisure. Numerous contributions in this issue highlight the various applications of the technique that can now even be combined with mass selection. The spectroscopy of matrix isolated molecules also permits to investigate the interactions of molecules with a solid. An invited review⁸ highlights the historical development and its ties to modern applications like holography and molecular data storage.

• Studies on metal clusters have always been motivated by the urge to understand catalytic activity. This field is now well on its way from studying simple model systems to investigating the dynamics of real catalytic chemical reactions in unprecedented detail. In an intriguing paper a study on the trimerization of acetylene on size-selected clusters deposited on a MgO film is presented.⁹

• Investigations of molecular clusters with a small number of solvent molecules has developed into studies of nanodroplets, permitting new insight into solute/solvent interactions.

• The interpretation of fragmentation pathways in mass spectra relies on the decay of excited molecular and cluster ions,¹⁰ often involving unpaired electrons.

• Several contributions demonstrate the capability of computational chemistry. Even the notoriously difficult anionic species are now tackled as shown for the DNA-bases and photodetachment processes in anionic salt/water clusters.¹¹

One of the most influential and productive scientists in the whole area is Vladimir E. Bondybey. He was one of the driving forces behind the research on matrix isolated molecules and one of the first experimentalists to investigate the Jahn–Teller and Renner–Teller effect in open shell ions. He pioneered research on metal clusters, solving two long-standing riddles in molecular orbital theory, namely bonding in Be₂ and in Cr₂, which became benchmarks for electronic structure calculations.¹² Already in the mid-1980s he studied cluster chemistry in an ICR trap,¹³ an approach now widely popular in the whole ion–molecule community. And the last decade witnessed new contributions from his group to low temperature physics, nanodroplet chemistry and still more previously unidentified radicals and ions. As past and present coworkers we have the privilege to experience his exceptional creativity and dedication first-hand, and enjoy being part of an innovative, international team on the leading edge of science.

On the occasion of his 65th birthday, we dedicate this special issue of Physical Chemistry Chemical Physics to Vladimir, and wish him many more productive years in science.

Ingo Fischer, Universität Würzburg

Martin Beyer, Technische Universität München

Markku Räsänen, University of Helsinki

Acknowledgements

We cordially thank all authors for their contributions to this special issue, many of whom are long-term colleagues, collaborators and friends of Vladimir’s. We are equally deeply indebted to the PCCP editorial office, and thank Susan Appleyard, Hilary Crichton and Philip Earis for their tremendous effort to bring this special issue to life.

References

1. E. Whittle, D. A. Dows and G. C. Pimentel, J. Chem. Phys., 1954, 22, 1943.
2. T. J. Sears, T. A. Miller and V. E. Bondybey, J. Chem. Phys., 1980, 72, 6070.
3. V. E. Bondybey and J. H. English, J. Chem. Phys., 1981, 74, 6978.
4. T. G. Dietz, M. A. Duncan, D. E. Powers and R. E. Smalley, J. Chem. Phys., 1981, 74, 6511.
5. B. S. Fox, M. K. Beyer, U. Achatz, S. Joos, G. Niedner-Schatteburg and V. E. Bondybey, J. Phys. Chem. A, 2000, 104, 1147.
6. C. A. Taatjes, S. J. Klippenstein, N. Hansen, J. A. Miller, T. A. Cool, J.-K. Wang, M. E. Law and P. R. Westmoreland, Phys. Chem. Chem. Phys., 2004, 7 10.1039/b417160h.
7. H.-J. Deyerl and R. E. Continetti, Phys. Chem. Chem. Phys., 2005, 7 10.1039/b414604b.
8. K. K. Rebane, Phys. Chem. Chem. Phys., 2005, 7 10.1039/b417730b.
9. K. Judai, A. S. Wörz, S. Abbet, J.-M. Antonietti, U. Heiz, A. Del Vitto, L. Giordano and G. Pacchioni, Phys. Chem. Chem. Phys., 2005, 7 10.1039/b414399j.
10. D. Schröder, H. Schwarz, S. Polarz and M. Driess, Phys. Chem. Chem. Phys., 7 10.1039/b415078c.
11. A. L. Sobolewski and W. Domcke, Phys. Chem. Chem. Phys., 2005, 7 10.1039/b416353b.
12. V. E. Bondybey, Science, 1985, 227, 125.
13. M. L. Mandich, W. D. Reents and V. E. Bondybey, J. Phys. Chem., 1986, 90, 2315.

---