Speakers
Description
Gas-phase vibrational spectroscopy has been proven to provide a nearly ideal method for the investigation of ions, unperturbed by solvent effects. In addition, ions can be placed in helium nanodroplets as a matrix, where the interaction with the dopant ion is very weak. As the helium droplets are at very low temperatures (0.37 K), thermal broadening and therefore spectral congestion is significantly reduced. In the experiment, the ions that are embedded in a helium droplet are irradiated with a tuneable infrared free electron laser. Resonant absorption of photons can lead to an evaporation of helium, as the energy of the excitation gets dissipated through the molecule and the droplet, eventually leading to evaporation of helium. Detection of bare ions as a function of infrared wavelength then gives an infrared spectrum. This technique can be used to study a wide variety of molecular ions and ionic clusters, ranging from small carbocations through proton-bound complexes to large organic molecules and biomolecules.
Carbocations are playing an important role in many areas of chemistry. They can be found in the interstellar medium, as the availability of carbon and the long lifetime of gaseous ionic species in astrophysical environments causes a high abundance of ionised carbonaceous molecules, such as polyaromatic molecules and fullerenes.Furthermore, carbocations are intermediates in a variety of organic chemical reactions. Some of those can undergo facile chemical conversions, such as hydride shifts, which can occur due to the overlap of the empty p-orbital and a vicinal CH $\sigma$-orbital. Others can form three-center two-electron bonds and are therefore of a more fundamental interest. These so-called non-classical carbocations involve the delocalization of a chemical bond, which results in C-C and C-H bonds of fractional bond order.
Here we show a selection of infrared spectra of carbocations of interest in the spectral range of 800-1600~cm$^{-1}$ to investigate diverse processes such as hydride shifts or the formation of non-classical carbocations.
