PNNL

Abstract

We present the results of combined experimental photoelectron spectroscopy and theoretical modeling studies of solvated dicarboxylate species (-O2C(CH2)2CO2-, DC22-) in complex with Na+ and K+ metal cations. These ternary clusters can serve as a simple representative of models, in which ion and solute specific effects that play an important role in biological systems are prevalent. The experimental spectra of both Na+ and K+ show the presence of one major broad band that gradually shifts to higher electron binding energy (EBE) with increasing number of waters. For the Na+ case, using ab-initio calculations we have been able to obtain ground state structures that match well experimental EBE values. In the solvation range investigated in this work (up to 6 waters), Na+ retains direct contact with the dicarboxylate species and solvation shell is built in a fairly predictable fashion with the emphasis on the coordination of the negatively charged carboxylate groups accompanied by simultaneous coordination of metal cation. Up to 9 waters, theoretical predictions show that the solvent separated ion pair structure model starts appear albeit with a higher energy than the contact ion pair model. In addition to the main peak, Na+(DC22-)(H2O)n clusters show development of a weak band at lower EBE when the number of water increases to 3. Similar phenomenon is observed for K+ at a higher number of waters (n = 5). The origin of the band cannot be precisely identified. However, ab-initio calculations point out the existence of quaternary complex involved Na+/K+, H2O, OH- and singly protonated dicarboxylate anion (HO2C(CH)2CO2-). Such complex appears to be stabilized at the solvation range corresponding to the appearance of the low EBE band and does match its peak, even though the energy of such complex is fairly high compared to the ternary clusters.