Spectroscopically Probing Solvent-Mediated Folding in Dicarboxylate Dianions**

Dicarboxylate salts play an important role in many areas of science including atmospheric, bioand synthetic chemistry. For example, they are used as antitumor drugs, building blocks for metalorganic framework materials and are found in aerosol particles comprising photochemical smog. Isolated dicarboxylate dianions are stable in the gas phase and serve as model systems for multiply charged anions. 5] The presence of two charge centers separated by a hydrophobic, aliphatic chain also makes them ideal for studying charge-screening and solvent-mediated effects. In the present study, we use gas phase infrared (IR) spectroscopy of microhydrated suberate (SA) dianion-water clusters, SA(H2O)0-28, together with quantum chemical calculations to establish relations between conformational changes and spectroscopic features. We then analyze how hydration can drive a conformational transition in a dianion and what role the hydrogen-bonded network plays. Gas-phase action spectroscopy is a powerful tool to study the effects of microhydration on the configuration of dicarboxylate dianions, adding one water molecule at a time. Anion photoelectron spectra in combination with quantum chemical calculations 8, 9] found a delicate dependence of the conformation of the dicarboxylate dianion on the degree of hydration, the aliphatic chain length, as well as on temperature. A more detailed insight into the folding mechanism requires structural information, which is challenging to extract from the photoelectron data. IR photodissociation (IRPD) spectroscopy combined with high-level quantum chemical calculations on microhydrated anions is able to supply this information and thus leads to a considerably more detailed understanding of this hydration-mediated folding process at the molecular level. Recent IRPD spectra of SA and its monohydrate SA·H2O revealed, how the addition of a water molecule effects the spectroscopic signature of the dianion. The water molecule binds to one of the carboxylate groups of the quasi-linear dianion, causing characteristic shifts of the intense IR-active carboxylate stretching bands. In more detail, the two symmetric (νS) and two antisymmetric (νA) carboxylate stretching modes in SA are quasidegenerate, because each pair of modes is weakly coupled as a result of the large distance in-between the carboxylate groups. Addition of a single water molecule lifts this degeneracy and leads to a characteristic splitting of both bands (compare n=0 and n=1 spectra in Figure 1).