Infrared Spectroscopy of Molecular Radicals and Carbenes in Helium Droplets

Abstract

The helium droplet is an ideal environment to spectroscopically probe difficult to prepare molecular species, such as radicals, carbenes and ions. The quantum nature of helium at 0.4 K often results in molecular spectra that are sufficiently resolved to evoke an analysis of line shapes and fine-structure via rigorous “effective Hamiltonian” treatments. In this chapter, we will discuss general experimental methodologies and a few examples of successful attempts to efficiently dope helium droplets with organic molecular radicals or carbenes. In several cases, radical reactions have been carried out inside helium droplets via the sequential capture of reactive species, resulting in the kinetic trapping of reaction intermediates. Infrared laser spectroscopy has been used to probe the properties of these systems under either zero-field conditions or in the presence of externally applied, homogeneous electric or magnetic fields.

Hydrogen sharing between two nitroxyl radicals in the gas phase and other microsolvation effects on the infrared spectrum of a bulky hydroxylamine

“…39 This allows for the spectral resolution of subtle conformational isomerism and it provides upper bounds for the speed of degenerate hydrogen atom transfer from a hydroxylamine to its associated nitroxyl or aminoxyl radical. We believe that these are the largest neutral radical complexes which have been vibrationally characterised in the gas phase, so far 40 and they are clearly London dispersion stabilised.…”

Hydrogen sharing between two nitroxyl radicals in the gas phase and other microsolvation effects on the infrared spectrum of a bulky hydroxylamine

“…We believe o u r n a l N a me , [ y e a r ] , [ v o l . ] , that these are the largest neutral radical complexes which have been vibrationally characterised in the gas phase, so far 40 and they are clearly London dispersion stabilised.…”

Infrared Laser Stark Spectroscopy of Methyl Fluoride in 4He Nanodroplets

“…Spectra of helium solvated methyl fluoride were collected using two different HENDI spectrometers; one housed at UGA, [9] and the other at JMU [14–16] . The operating principle of them is similar, and we only briefly mention it here, and point the interested reader to review articles on the infrared spectroscopy of helium solvated neutral molecules for further details [9,17–26] . Helium nanodroplets consisting of a few thousand helium atoms (on average [27,28] ) were formed by expanding helium gas at high pressure and low temperature through a pinhole sized nozzle (diameter 5±1 μm) and into an evacuated chamber.…”