|Title||Infrared Photodissociation Spectroscopy of Cluster Anions in the Gas Phase|
|Year of Publication||2008|
Infrared photodissociation spectroscopy has been applied to mass-selected anion molecule complexes in the gas phase. In combination with quantum chemical calculations, this technique has proven to be very successful for gaining insight into the structures and interaction behavior of such species. We have used the “Ar nanomatrix” approach (which means tagging of the target clusters with a small number of Ar atoms) in order to produce cold complexes close to their ground state equilibrium structures and to facilitate dissociation upon absorption of one infrared
The first part of this work deals with the investigation of the hydration of anions. While the hydration behavior of atomic anions such as halides is well understood, not much is known about the interaction between metal anions and water. Infrared spectra of M-·H2O (M = Au, Ag, Cu) have been measured in this study and it has been shown that they introduce a new motif for the solvation of small atomic anions, intermediate between the clear-cut hydration motifs known so far due to the shallowness of their potential energy curves. A second focus of the work on anion hydration has been on complexes of water molecules and anions with extended negative charge distribution such as the C6FnH6-n-·(H2O)m (n = 4 - 6, m = 1,2) and SF6-·(H2O)m (m = 1 - 3) clusters. While the binding motifs of water ligands to the fluorobenzenes have been found to correspond mostly to the structures displayed by other anions where the charge is not localized in a small part of the molecule (such as anions with triatomic domains), the SF6-·(H2O)m (m = 1 - 3) complexes show another binding motif, reminiscent of the heavier halide-water complexes. Moreover, the hydration shell of the sulfur hexafluoride anion was found to exhibit delayed onset of water-water network formation, leading to water-water interaction only upon binding of a third water ligand.
An intramolecular, infrared triggered reaction is described in the example of the SF6-·HCOOH complex. It was found that the reaction could be influenced by the degree of Ar solvation, effectively shutting down upon attachment of two or more Ar atoms with the Ar acting as a coolant. The structure of the complex and three different reaction channels identified could be determined. Aided by high-level quantum calculations, a possible reaction pathway is proposed.
Lastly, a study on A-·C6FnH6-n (n = 0 - 5, A = Cl, I, SF6) is presented. This system is of considerable interest in the context of anion recognition via interactions with electron-deficient aromatic systems. Varying the number of fluorine atoms around the carbon ring one at a time offers the possibility of tuning the electronic properties of the aromatic molecule. Arenes with a high degree of fluorination offer two competing binding motifs to an anion, namely binding to the top of the ring (displaying a positive electrostatic potential) and binding to the periphery of the ring via hydrogen bonding to one of the CH groups, which become increasingly acidic upon increasing the number of fluorine atoms. It has been shown that the latter prevails up to pentafluorobenzene, so that full fluorination of the ring is needed in the case of fluorinated benzenes to make the binding motif switch to the top of the ring.