Understanding phenyloxenium ions: Spectroscopic properties, spin configurations and reactivities

Abstract

It is believed that understanding the reactive intermediates is essential to unveil mechanisms of synthetic chemistry and biological processes. In the library of reactive intermediates, oxenium ions are highly reactive with the formula R−O+. These reactive intermediates possess lone pairs of electrons on their central oxygen atoms, permitting different energetically accessible electronic configurations. More importantly, reactive intermediates with diverse electronic structures would very likely bear distinct reactivities, and thus various reaction pathways may occur. In general, intermediates with closed-shell singlet states usually react as electrophiles, while ions with triplet states show typical radical reaction pattern, such as hydrogen atom abstractions. Moreover, the chemical formula of oxenium ions suggest that these species are isoelectronic to the better understood nitrene intermediates but bear a formally positive charge on a hypovalent oxygen atom, which is substantially interesting considering the high electronegativity value of oxygen element. Although oxenium ions have been proposed as intermediates in numerous processes, relative studies about these ions themselves are surprisingly rare, largely because the isolation of oxenium ions is utterly difficult due to the extremely short lifetimes. To probe these exceedingly short-lived intermediates, a combination of theoretical calculation and experimental laser flash photolysis (LFP) has been applied. On one hand, LFP technology has allowed us to directly observe these reactive intermediates within femtosecond time scale in the aspects of the absorption spectroscopy, Raman spectroscopy, etc. On the other, owing to the advanced theoretical models, computational investigation helps us to validate and interpret the experimental spectroscopic data. Moreover, the theoretical approach also provides a close insight to the spin configurations of these reactive intermediates, guiding us to design oxenium ions with desired electronic configurations. In this manner, appropriate photo-precursors are strongly demanded for generating corresponding oxenium ions photochemically. My research focuses on synthetically developing new photoprecursors which can potentially produce corresponding oxenium ions possessing intriguing electronic structures. LFP studies perform direct observation of these transient species. By comparison to the theoretical predictions, each transient species collected by LFP shall be attributed correctly. Photo product studies are conducted to determine the reactivities of these reactive intermediate. I have also carried out my investigation in searching for anomalous electronic configurations of oxenium ions using computational methods. Oxenium ions with different electronic configurations exhibit distinct reactivities, which are potentially useful for future synthetic applications. More importantly, reactive intermediates bearing triplet states, if well stabilized, are capable of acting as practical magnetic materials.