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 us ing electrospray mass spectrometry, photoelectron spectroscopy, and ab init io calculations. Despite their similar charge-separation and size, AD(2-) w as observed abundantly, whereas SD2- was very difficult to detect. Our ab i nitio 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 theore tical analysis indicates that while the two excess charges in SD2- are comp letely localized on the terminal carboxylate groups; there is significant d elocalization 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 inte ract with one of the two perpendicular pi-type orbitals of the acetylene mo iety, thus providing extra stabilization while minimizing the Coulomb repul sion.