Fachbeirat 2022

Isolating Fluorenyl and Cyclopentadienyl Cations at Cryogenic Temperatures in the Gas Phase for Vibrational Analysis

Not scheduled

20m

Speaker

Kim Greis (FHI MP Helden)

Description

Carbocations are elusive species that due to their short lifetimes cannot be commonly observed in the condensed phase. Cyclopentadienyl and fluorenyl cations are antiaromatic according to the Hückel-rule because their pi system is composed of 4n pi electrons. Hence, these species are extremely unstable and structural characterization is challenging. In the past decades, fluorenyl cations were stabilized with substituents and super acids leading to an increase in lifetime that allows structural characterization of the modified species. However, characterizing bare and unsubstituted species remains challenging. In the gas phase, these carbocations can be trapped and isolated using mass spectrometry-based techniques. Cyclopentadienyl and fluorenyl cations were generated from precursors carrying either a leaving group that can be cleaved or a carbonyl group that can be protonated during electrospray ionization followed by in-source fragmentation.

The carbocations are guided to a hexapole ion trap, where they are thermalized by collisions with helium buffer gas. Subsequently, a beam of superfluid helium nanodroplets, generated by expansion of helium into the vacuum by a pulsed Even-Lavie valve, passes through the ion trap. Here, the droplets pick up the ions and guide them to the interaction region, where the ion beam overlaps with an infrared beam of the Fritz Haber Institute free electron laser. The infrared beam is exciting vibrational modes of the ions, leading to evaporation of helium and subsequent release of the ions which is detected by a time-of-flight detector. Plotting the ion intensity as a function of the infrared wavenumber yields an infrared spectrum.

The infrared signatures of the cyclopentadienyl- and fluorenyl cations confirm in combination with computed harmonic frequencies derived from density functional theory calculations that intact ions are probed in their singlet states. This result is particularly surprising for the pentaphenylcyclopentadienyl cation, where theory predicts a triplet ground state. We were able to confirm that it is adopting a Jahn-Teller distorted dienyl-type structure. The results showcase the ability of the method to generate and trap elusive species for their structural characterization.