Intramolecular Coulomb repulsion and anisotropies of the repulsive Coulomb barrier in multiply charged anions

Photoelectron spectra of the three isomers of the benzene dicarboxylate dianion (o-, m-, and p-BDC2−) were measured in the gas phase at five photon energies. Detachment features from the carboxylate groups and the π electrons of the ring were clearly observed and distinguished. The electron binding energies were found to increase from the very small value of ∼0.2 eV in o-BDC2− to about 1.0 eV in p-BDC2−, due to the reduced Coulomb repulsion as the two excess charges become farther apart. We found that the repulsive Coulomb barrier (RCB) for detaching electrons from the carboxylates decreases from o-BDC2− to p-BDC2−. However, the RCB for detaching the ring π electrons was found to be significantly higher and remain constant for the three isomers. This distinct anisotropy in the RCB involving different detachment channels is related to the different intramolecular Coulomb repulsions experienced by electrons localized on the carboxylates and the ring. Theoretical calculations were performed to obtain the equilibrium structures of both the dianions and the monoanions and to gain insight into the intramolecular electrostatic interactions. The two carboxylates in m- and p-BDC2− were shown to be in-plane with the ring whereas the strong Coulomb repulsion in o-BDC2− forces the carboxylate groups out of the plane of the ring. The theoretical results clearly show the localized nature of the excess charges on the carboxylates and help us understand the intramolecular Coulomb repulsions within the three dianions.