Direct observation and reactivity of oxenium ions
Oxenium ions are poorly understood reactive intermediates of the formula R-O+. This body of work is an accumulated computational and experimental investigation into understanding the electronic configuration, spectroscopic signatures, spin-selective reactivity, and lifetimes of these short-lived intermediates.
Chapter 1 overviews our attempts to find a photochemical precursor to oxenium ions, which ultimately led to the first direct detection of the short-lived parent phenyloxenium ion. Laser flash photolysis studies were performed on the phenyl hydroxylamine tetrafluoroborate salt to form the singlet phenyloxenium ion. These studies were performed by Jiadan Xue in the lab of Dr. David Lee Phillips. The lifetime for the phenyloxenium ion was found to be about 5 ns. Product studies from both thermolysis and photolysis with added traps suggest the formation of the phenyloxenium ion along with the phenoxy radical.
In order to better understand the effect of structure on the electronic configuration of oxenium ions, Chapter 2 provides an in depth study of heteroaryl oxenium ions by high-level multireference CASSCF/CASPT2 calculations. 4-pyridinyl, pyrizidinyl, and pyrimidyl oxenium ions all have singlet-triplet gaps (ΔEST) of less than -4 kcal/mol, showing significantly changed electronics from the parent phenyloxenium ion. This small gap suggests the possibility that triplet states as well as open shell singlet states could be chemically relevant to these intermediates.
In chapter 3, the synthesis of a photochemical precursor to the p-biphenylyl oxenium ion is described, and the ultrafast LFP experiments performed by Mingde Li in the lab of Dr. David Lee Phillips on this precursor suggest photolysis leads to the p-biphenylyl radical, as well as the p-biphenylyl oxenium ion as a closed shell singlet, open shell singlet, and a triplet state.
Chapter 4 describes the synthesis of a photoprecursor to the m-dimethylamino phenyloxenium ion, which is computed to have a triplet ground state (+12 kcal/mol, B3LYP/cc-PVTZ). Photolysis of this precursor was studied using product studies as well as by matrix isolation EPR studies. The results obtained by matrix isolated EPR provide evidence that the ground state of the m-dimethylamino phenyloxenium ion is a ground state triplet.