Chemical and Spectroscopic Studies of Nitrenium Ions and Related Species

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In this project, funded by the Chemical Structure, Dynamics, and Mechanism-B Program of the Chemistry Division, Professor Daniel E. Falvey and his students from the Department of Chemistry and Biochemistry at the University of Maryland, College Park, are investigating the properties and behavior of a class of novel molecular species called nitrenium ions. Nitrenium ions are highly reactive substances with only two bonds to nitrogen and possessing a positive charge. This differs from most nitrogen-containing structures, where nitrogen typically has three bonds and a neutral charge. Under ordinary conditions, nitrenium ions exist for only small fractions of a second. However, it is becoming increasingly apparent that some processes in chemical synthesis, biology and the environment might involve nitrenium ions as intermediates. Understanding and predicting the outcomes of these reactions, therefore, requires accurate information on the properties of nitrenium ions. Professor Falvey and his students are developing methods for producing and observing nitrenium ions under well-defined laboratory conditions. These methods are used to carry out detailed kinetic and spectroscopic measurements on typical nitrenium ions. Experimental results are compared to predictions from state-of-the art computer models, with a goal of refining and improving these predictions. These results will then be available to engineers and scientists who can use them to improve pharmaceutical and agrochemical manufacturing processes, forecast the fate of pollutants in the environment, and understand how the human body interacts with drugs and/or toxins. Professor Falvey actively recruits a diverse group of students, including women and students from traditionally underrepresented groups to participate in these investigations. Students who are engaged in this research gain hands-on experience, synthesizing and characterizing novel chemical species, carrying out quantitative measurements on reaction rates and outcomes, and computer modeling. These experiences prepare them for careers in academia, industry and government. Nitrenium ions are reactive intermediates characterized by a dicoordinate nitrogen atom and bearing a formal positive charge. Nitrogen-containing compounds are ubiquitous, as this element is a component of pharmaceuticals, agrochemicals, high-energy materials, and industrial polymers. Nitrenium ions are intermediates that can form under conditions when nitrogen-containing compounds are oxidized, and/or when the nitrogen atom is substituted with good leaving groups. Nitrenium ions display a wide range of reactivities. Simple alkylnitrenium ions are not even local minima on the singlet potential energy surface, whereas amino-substituted examples are indefinitely stable and can employed as Lewis acids and ligands in metal complexes. Earlier work in Professor Falvey's lab established photochemical methods for the generation of arylnitrenium ions, providing a means for characterizing these species by laser flash photolysis and related methods. The current effort extends these studies to non-arylnitrenium ions, including alkyl, cycloalkyl and heteroarylnitrenium ions. These categories of nitrenium ions have received far less attention, and little is known about their decay reactions, or their stabilities. Some recent calculations indicate that cyclopropyl and cyclobutylnitrenium ions will have non-classical sigma bonding similar to the corresponding carbenium ions. Likewise, the sulfonylnitrenium ions appear to have highly distorted structures, making them far less electrophilic than intuition would suggest. Finally, there are several types of heteroarylnitrenium ions and vinylnitrenium ions that have been predicted to have low energy triplet states. This project aims to resolve fundamental questions about nitrenium ion structure and reactivity. Methodology in this research includes synthesis of photochemical precursors, analysis of stable products from the generated nitrenium ions, spectroscopic and kinetic characterization of nitrenium ions using laser flash photolysis, low temperature electron spin resonance studies of triplet nitrenium ions, and computation studies of nitrenium ion electronic structures and reaction mechanisms.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.