Exploring the microscopic solvation of doubly charged anions: symmetric or asymmetric solvation in the CO2–(CH2)4–CO22−·(H2O)2 dicarboxylate dianion cluster?

Optimised geometries and absolute energies of CO2–(CH2)4–CO22−·(H2O)n, n=0,1,2, were determined using density functional methods. The n=1 global minimum was found to contain a bifurcated hydrogen-bond, with the n=2 minimum containing two such bonds. Vertical detachment energies (VDEs) obtained for the n=1,2 global minima at the MP2/6-31+G*//B3LYP/6-31+G* level agree well with experiment, and suggest that the two waters solvate the carboxylate groups separately in CO2–(CH2)4–CO22−·(H2O)2. However, a number of different isomers were identified for both clusters with several isomers producing similar VDEs, indicating that the measured VDEs may contain contributions from multiple isomers. Calculations are presented illustrating that IR predissociation spectra would allow the direct identification of these isomers.