Experimental and theoretical investigations of the stability of two small gaseous dicarboxylate dianions: Acetylene dicarboxylate and succinate

The stability of two small dicarboxylate dianions, (O2C-C=C-CO2)(2-) (AD(2-)) and (O2C-CH2-CH2-CO2)(2-) (SD2-) has been studied in the gas phase by using electrospray mass spectrometry, photoelectron spectroscopy, and ab initio calculations. Despite their similar charge-separation and size, AD(2-) was observed abundantly, whereas SD2- was very difficult to detect. Our ab initio calculation predicts that AD(2-) is adiabatically stable with respect to electron loss whereas SD2- is not. These calculations show that AD(2-) has a D-2d structure with 0.418 eV adiabatic and 0.69 eV vertical electron binding energies, compared to the experimental values of 0.30 (0.10) and 0.60 (0.10) eV, respectively. SD2- was found to have a C-2h structure with a negative adiabatic electron binding energy (-0.086 eV). Our detailed theoretical analysis indicates that while the two excess charges in SD2- are completely localized on the terminal carboxylate groups; there is significant delocalization of the excess charges into the -CC- pi-type orbitals in AD(2-). The D-2d structure of the AD(2-) dianion allows each carboxylate to interact with one of the two perpendicular pi-type orbitals of the acetylene moiety, thus providing extra stabilization while minimizing the Coulomb repulsion.