This dissertation describes experiments that were performed to study the spectroscopy and thermochemistry of organic radicals. The major focus concerns two peroxyl radicals, phenylperoxl and allylperoxyl. These molecules have important implications in the fields of combustion and atmospheric chemistry. The minor focus deals with ortho-benzyne, a diradical central to the thermal decomposition of aromatic hydrocarbons in combustion.

Two techniques were used in the laboratory to study the aforementioned radicals: Photoionization Time of Flight Mass Spectrometry and Matrix Isolation Fourier Transform Infrared Spectroscopy. The first part of this thesis describes these experimental techniques, addressing the theories and instrumentation central to their operation.

The second part of this manuscript details the infrared spectroscopy used to characterize two peroxyl radicals. The first of these is the phenylperoxyl radical (C₆H₅OO·). Gas phase pyrolysis of a suitable precursor was carried out in a hyperthermal nozzle to create a clean source of phenyl radicals. The phenyl radicals were deposited along with oxygen molecules and argon atoms on a cryogenically frozen matrix host window. Phenylperoxyl radicals were created through reaction of the phenyl radicals with oxygen in the matrix. IR spectroscopy was used as a probe to determine the vibrational energies for these radicals trapped in argon. This provides spectroscopic information about the ground state of these radicals to compliment the excited state spectroscopy that currently exists in the literature. The vibrational energies determined here should aid in the modeling of reaction schemes that include phenylperoxyl as a key intermediate, especially in combustion reactions.

Allylperoxyl radicals (CH₂=CH-CH₂-OO·) were also created and isolated in a similar fashion as the phenylperoxyl experiments. Suitable allyl radical precursors were pyrolysed in a hyperthermal nozzle. The resultant allyl radicals were co-deposited with oxygen molecules and argon atoms on a cryogenic window to form ground state matrix isolated allylperoxyl radicals. Electronic structure calculations performed with density functional theory on this floppy molecule show five stable conformers that are very similar in energy. The IR spectra support this, showing evidence for different conformers isolated in the matrix. The allyperoxyl radical represents a simple subgroup of more complex organic peroxyl radicals involved in the atmospheric processing of biogenic volatile organic compounds.

The third and final part of this thesis examines the thermal decomposition pathways of the diradical ortho-benzyne (o-C ₆H₄) in a hyperthermal nozzle. Suitable precursors were used to create clean sources of this diradical, followed by monitoring products upon heating past the threshold temperature for its formation. Products were monitored by the photoionization time of flight mass spectrometry technique. With the aid of chemical ionization mass spectrometry and matrix isolation infrared spectroscopy experiments, as well as a detailed coupled cluster theoretical investigation, evidence for a retro-Diels-Alder fragmentation of ortho-benzyne was discovered. This diradical can be formed from thermal breakdown of aromatic hydrocarbons in flames and is therefore central to the combustion chemistry of those species.