Experimental and Theoretical Investigations of the Stability of Two Small Gaseous Dicarboxylate Dianions: Acetylene Dicarboxylate and Succinate

The stability of two small dicarboxylate dianions, (O 2 C−C⋮C−CO 2 ) 2 - (AD 2 - ) and (O 2 C−CH 2 CH 2 −CO 2 ) 2 - (SD 2 - ) 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 SD 2 - was very difficult to detect. Our ab initio calculation predicts that AD 2 - is adiabatically stable with respect to electron loss whereas SD 2 - is not. These calculations show that AD 2 - has a D 2 d 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. SD 2 - was found to have a C 2 h structure with a negative adiabatic electron binding energy (−0.086 eV). Our detailed theoretical analysis indicates that while the two excess charges in SD 2 - are completely localized on the terminal carboxylate groups, there is significant delocalization of the excess charges into the −CC− π-type orbitals in AD 2 - . The D 2 d structure of the AD 2 - dianion allows each carboxylate to interact with one of the two perpendicular π-type orbitals of the acetylene moiety, thus providing extra stabilization while minimizing the Coulomb repulsion.