Photoelectron spectroscopy of solvated dicarboxylate and alkali metal ion clusters, M+[O2C(CH2)2CO2]2- [H2O]n (M = Na, K; n=1-6)

We present results of combined experimental photoelectron spectroscopy and theoretical modeling studies of solvated dicarboxylate species (-O2C(CH2)(2)CO2-) in complex with Na+ and K+ metal cations. These ternary clusters serve as simple models for the investigation of aqueous ion/solute specific effects that play an important role in biological systems. The experimental characterization of these systems was performed in the presence of up to six solvating waters. In both Na+ and K+ cases, we observe the presence of one major broad band that gradually shifts to higher electron binding energy (EBE) with an increasing number of waters. In the Na+ case further detailed analysis of experimental spectra was performed using ab initio calculations. In particular, we have identified the structures of the lowest energy clusters whose EBE values match well the major band in the experimental spectra. Our results show that evolution of an aqueous solvation shell emphasizes the coordination of the negatively charged carboxylate groups accompanied by simultaneous interaction with metal cations. Calculations also indicate that in the solvation range investigated experimentally (up to 6 waters), Na+ retains direct contact with the dicarboxylate species, i.e. a contact ion-pair (CIP) complex. Preliminary modeling studies show evidence of an alternative solvent separated ion-pair complex once the solvation range approaches 8 waters, however its energy still remains above that of (approximate to 7-8 kcal/ mol(-1)) the CIP complex. At a higher number of waters (n = 3 for Na+ and n = 5 for K+), the experimental spectra also show the development of a weak low energy band. Its origin cannot be precisely identified. Our calculations in the Na+ case point out the existence of a quaternary complex consisting of Na+, H2O, OH- and a singly protonated dicarboxylate anion (HO2C(CH)(2)CO2-). Such a complex appears to be stabilized in the solvation range corresponding to the appearance of the low EBE band and does match its peak, even though the energy of such a complex is fairly high compared to the ternary structure.