Infrared Photodissociation Spectroscopy of Cluster Anions in the Gas Phase

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
photon.

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 C₆F_nH_6-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^-·C₆F_nH_6-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.