Chemical and Spectroscopic Studies of Nitrenium Ions and Related Species

本研究由化学系化学结构、动力学和机理-B计划资助，丹尼尔E。法尔维和他的学生来自马里兰州大学的化学和生物化学系，他们正在研究一类称为氮离子的新型分子物种的性质和行为。氮离子是一种高活性物质，只有两个键连接到氮上，并带有正电荷。 这与大多数含氮结构不同，其中氮通常具有三个键和中性电荷。在正常条件下，氮鎓离子只存在几分之一秒。然而，越来越明显的是，在化学合成、生物学和环境中的一些过程可能涉及氮鎓离子作为中间体。因此，要理解和预测这些反应的结果，就需要关于氮鎓离子性质的准确信息。法尔维教授和他的学生正在开发在明确的实验室条件下产生和观察氮鎓离子的方法。这些方法被用来进行详细的动力学和光谱测量典型的氮离子。将实验结果与最先进的计算机模型的预测进行比较，目的是改进和改进这些预测。然后，这些结果将提供给工程师和科学家，他们可以使用它们来改进制药和农业化学品的制造过程，预测环境中污染物的命运，并了解人体如何与药物和/或毒素相互作用。教授法尔维积极招募学生的多样化群体，包括妇女和学生从传统上代表性不足的群体参加这些调查。从事这项研究的学生获得实践经验，合成和表征新的化学物质，对反应速率和结果进行定量测量，并进行计算机建模。这些经验为他们在学术界，工业界和政府的职业生涯做好了准备。氮鎓离子是一种活性中间体，其特征在于具有一个二配位氮原子并带有一个形式正电荷。含氮化合物无处不在，因为这种元素是药物，农用化学品，高能材料和工业聚合物的组成部分。氮鎓离子是在含氮化合物被氧化和/或氮原子被好的离去基团取代的条件下可以形成的中间体。氮离子显示出广泛的反应性。简单的烷基氮鎓离子在单重态势能面上甚至不是局部最小值，而氨基取代的例子是无限稳定的，可以用作金属络合物中的刘易斯酸和配体。Falvey教授实验室的早期工作建立了产生芳基氮离子的光化学方法，为通过激光闪光光解和相关方法表征这些物质提供了一种手段。目前的努力将这些研究扩展到非芳基氮离子，包括烷基，环烷基和杂芳基氮离子。这些类别的氮鎓离子受到的关注要少得多，对它们的衰变反应或稳定性知之甚少。最近的一些计算表明，环丙基和环丁基氮离子将具有类似于相应的碳离子的非经典σ键。同样，磺酰氮鎓离子似乎具有高度扭曲的结构，使它们远不如直觉所暗示的亲电性。最后，有几种类型的杂芳基氮离子和乙烯基氮离子已被预测具有低能量三重态。该项目旨在解决有关氮离子结构和反应性的基本问题。本研究的方法包括光化学前体的合成、生成的氮鎓离子的稳定产物的分析、使用激光闪光光解的氮鎓离子的光谱和动力学表征、三重态氮鎓离子的低温电子自旋共振研究、该奖项反映了NSF的法定使命，并被认为值得支持通过使用基金会的知识价值和更广泛的影响审查标准进行评估。

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.