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The goal of the experimental research program was to separate and stabilize transitory intermediates and yields generated by prototype combustion reactions. The main method used was the Helium Nanodroplet Isolation (HENDI), which is a groundbreaking technology that uses liquid helium droplets to freeze out high-energy, highly stable structures of a reacting system. Consequently, it is possible to conduct infrared (IR) spectroscopic descriptions of products and intermediates that are generated from reactions involving molecular oxygen, hydrocarbon radicals as well as other small molecules that are pertinent to combustion reactions.
However, HENDI and spectroscopic analysis of various organic molecules can only be done following the thermal decomposition of the molecules in a constant, expansive pyrolysis source. During HENDI, helium droplets provide low temperatures that facilitate the characterization of various organic systems to greater details than those ever realized in the gas phase. Consequently, it has been possible to characterize large systems such as propyl radicals.
The only notable shortcoming of the HENDI technique is the accessibility of appropriate pyrolysis precursors. Therefore, the production of different hydrocarbon radicals is explained. For example, methyl and ethyl radicals were generated through the pyrolysis of peroxide precursors followed by separation and subsequent spectroscopic analysis in He droplets. On the other hand, the pyrolysis of di-vinyl sulfone was done to produce the vinyl radical, which was then entrapped in liquid He droplets. The resultant population was cooled at 0.4 K to the ortho or para rovibrational level, followed by the determination of IR spectra in the basic CH stretch region.
Recent investigations led to the production of gas-phase n-propyl and i-propyl radicals via the pyrolysis of n-butyl nitrite and i-butyl nitrite, in that order. A number of new, uncharacterized bands were noted in the IR spectrum between wavelengths of 2800 and 3150 cm. It was impossible to designate the spectra of n- and i-propyl radicals between 2800 and 2960 due to numerous anharmonic resonances.
However, the CH stretching modes have been seen at wavelengths exceeding 2960 cm, corresponded with anharmonic frequencies calculated using second-order vibrational perturbation theory. The pyrolysis of nitrite precursors was used to produce gas-phase cyclobutyl radicals.
The products formed in the reaction between the propargyl radical and oxygen within He droplets were evaluated using IR spectroscopy at a temperature of 0.4 K. The acetylenic-trans-propargyl peroxy radical was formed exclusively. Previously, the presence or magnitude of the barrier to the production of peroxy moieties was unknown. However, this reaction made it possible to explicate the outline of the entry conduit on the ground-state potential energy surface.
His droplets allowed fast cooling of the intermediates, which led to the conclusion that there is a very small barrier that cannot promote the formation of a kinetically favorable van der Waals complex between C3H3 and oxygen. The proposed reaction machinery suggested that the helium solvent led to more relaxation of closely spread out torsional levels than higher frequency vibrations, which permitted the system to enter the lowest energy conformational minimum.
The pyrolysis of glyoxylic acid led to the separation of hydroxycarbenes in He droplets. The subsequent IR spectrum revealed transitions, which matched the trans-conformation according to prior anharmonic frequency calculations. 2-hydroxy vinyl radical, which is an open-shell molecular complex, was produced by the electrophilic addition of the hydroxyl radical to the pi-bond of acetylene. This reaction led to the production of rovibrational Stark spectra, which were explained by a model Hamiltonian.
The ensuing experimental dipole moments from Stark spectroscopy served as valuable points of reference for electronic structure theory. It was expected that Stark spectroscopy would be used widely to explore the structural features of molecular radicals and other chemical entities in combustion chemistry.
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