IR Spectroscopic Characterization of the Thermally Induced Isomerization in Carbon Disulfide Dimer Anions

ABSTRACT We report experimental vibrational spectra of thermalized carbon disulfide dimer anions, (CS2)2-,measured at ion trap temperatures from16 to 300K. Previous experiments showed evidence for several (CS2)2- isomers, whose relative abundance depends on the source conditions. We used infrared (IR) photodisso-ciation spectroscopy in the fingerprint region (550-1600 cm-1) of (CS2)2-thermalized in a temperature-controllable ion trap, in combinationwith simulated IR spectra derived from ab initio calculations, to identify the isomers present at various ion trap temperatures. The IR photodissociation spectra show character-istic signatures for at least three different isomers. Anions formed in the source are primarily trapped as high-energy ion-molecule complexes, inwhich the unpaired electron is localized on a single CS2 moiety. Thermal heating supplies sufficient energy to overcome the isomerization barriers and shifts the isomer population via a weakly bound isomer, in which the electron is delocalized over the complete complex, to lower-energy covalently bound structures. SECTION Kinetics, Spectroscop