Correlation of the Charge Resonance Interaction with Cluster Conformations Probed by Electronic Spectroscopy of Dimer Radical Cations of CO and CS in a Cryogenic Ion Trap.

Radical cations of dimeric clusters of carbon dioxide/disulfide, [(CX)] (X = O and S), form strong intracluster bonds through charge resonance (CR) interactions. We herein performed electronic photodissociation spectroscopy of [(CX)] while regulating the temperature under ambient and cryogenic conditions using a quadrupole ion trap. Both ions exhibited broad band absorption in the near-infrared-visible light region; it is called the "CR band", as a measure of the strength of the CR interaction. Strikingly, this band underwent a noticeable blue shift upon cryogenic cooling for [(CS)] while not for [(CO)]. On the basis of quantum chemical calculations with a coupled cluster method, the band shift was attributed to the variations in the relative population of two energetically close conformers found for [(CS)]. This study highlights a strong correlation between CR interactions and conformation of the radical dimer cations, demonstrating the exceptional significance of cryogenic cooling in the chemistry of ionic molecular clusters.