Reactions and Properties of Nitrogen-Based Reactive Intermediates

University Of Maryland, College Park, College Park MD

Investigators

Abstract

In this project, funded by the Chemical Mechanism, Function, and Properties Program of the Chemistry Division, Professor Daniel E. Falvey of the Department of Chemistry and Biochemistry at the University of Maryland, College Park will examine unconventionally bonded molecular ions that contain electron-deficient nitrogen atoms. These species, which include nitrenium ions and several related structures, have been shown to have interesting electronic and magnetic properties that could potentially be harnessed for important applications such as advanced energy storage solutions, quantum computing, novel digital memory devices, optoelectronic sensors, etc. However, the few examples that have been carefully examined are unstable, often existing for only fractions of a second, inhibiting any technological advances. An important goal of the project will be to learn what types of modifications to the molecular structures of these ions will stabilize them yet still preserve the desirable electronic and magnetic properties. The set of structures that prove to be less stable will be studied for a different set of applications. In this case their high reactivity can be used to chemically modify proteins in a way that will convert them into therapeutic or bioimaging agents. The undergraduate and graduate students that participate in this project will receive training and experience in computer modeling of molecular properties and reactions, organic synthesis, and advanced spectroscopic techniques for chemical analysis. Planned studies will focus on two families of electron-deficient nitrogen-based intermediates: nitrenium ions, which are characterized by an di-coordinate nitrogen atom that bears a formal positive charge, and azapyramidinium ions, which are non-classical cations that feature a square pyramidal array of atom, including one nitrogen atom. Several decades of experimental and theoretical studies of nitrenium ions have focused almost exclusively on the arylnitrenium ions–species where the electron deficient nitrogen is substituted with one or two carbocyclic aromatic rings. The latter have been shown to have very small singlet-triplet energy gaps. A recent experimental study demonstrated that appropriately substituted phenylnitrenium ions have a triplet ground state. However there has been little experimental exploration of heteroarylnitrenium ions– related structures where the electron deficient nitrogen is part of a (formally) aromatic ring and/or the electron deficient nitrogen has a heteroaromatic substituent. Preliminary calculations imply that the former have either triplet ground states or small singlet-triplet energy gaps. The latter appear to show singlet-triplet energy gaps that can be manipulated through simple acid-base chemistry. A combination of calculations, organic synthesis, laser flash photolysis experiments, and low-temperature EPR spectroscopy will be used to examine these species. There is very little information on azapyramidinium ions. These species have been predicted to exist by high-level calculations but have not been characterized experimentally. A recent computational study by Professor Falvey provides guidance that should lead to the first successful synthesis and characterization of such a species. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.