Protonation and proton transfer studied with cluster-ion infrared spectroscopy

在这项由化学系化学结构、动力学和机制计划和国际科学与工程办公室资助的奖项中，来自佐治亚大学的邓肯教授将研究在气相中产生的质子化分子离子团簇，作为质子结合和质子传输过程的模型系统。这些过程在电化学、光合作用和氢燃料电池的运行中是核心的。关键分子上的质子结构和作为质子转移中间产物的桥联结构是本项目的重点。利用脉冲放电超音速喷嘴源，在分子束环境中低温产生一个质子与一个或多个小分子结合的络合物，从而可以研究不受环境扰动的这些体系的本征结构。质子化的络合物用质谱仪进行尺寸选择，并用红外激光光谱研究其结构。这项工作集中在小分子羰基，氨基酸和碳阳离子物种，研究了质子化产生的单体结构，它们的对称和不对称的质子结合二聚体单元，以及它们在水中逐步溶剂化的行为。利用新的红外激光器的扩展频率覆盖范围，测量了存在的每个分子单元的特征振动模式，包括发生质子振动的光谱的关键低频区域。质子化分子相对于自由分子的振动带的移动提供了电荷诱导效应的诊断，而质子的伸缩和弯曲振动提供了直接进入质子转移势能面的途径。这些实验提供了关于选定的质子转移反应的具体新的化学见解。他们还产生基准数据，与试图计算质子转移动力学的计算量子化学的结果进行比较。被称为碳正离子的质子化有机分子的振动特征测量可能对星际气体云的红外天文很有用。同样重要的是对本科生、研究生和博士后研究员进行现代物理化学方法、技术和仪器的培训。正在进行的合作包括一个非博士授予机构(肯纳索州立大学)和两个国际实验室(英国诺丁汉化学系和柏林弗里茨·哈伯研究所)。

In this award, funded by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, and the Office of International Science and Engineering, Professor Duncan frm the University of Georgia will study protonated molecular ion clusters produced in the gas phase as model systems for proton binding and proton transport processes. These processes are central in electrochemistry, in photosynthesis and in the operation of hydrogen fuel cells. The structures of protons attached to key molecules and the bridging structures that are intermediates in proton transfer are the focus of this project. Complexes containing a proton bound to one or more small molecules are produced at low temperature in a molecular beam environment with a pulsed-discharge supersonic nozzle source, so that the intrinsic structures of these systems unperturbed by their environment may be studied. Protonated complexes are size-selected with a mass spectrometer and their structures are investigated with infrared laser spectroscopy. This work focuses on small molecular carbonyls, amino acids and carbocation species, investigating the monomer structures resulting from protonation, their symmetric and asymmetric proton-bound dimer units, and their behavior upon stepwise solvation with water. Using the expanded frequency coverage of new infrared lasers, characteristic vibrational patterns are measured for each molecular unit present, including the critical low frequency region of the spectrum where proton vibrations occur. The shift in vibrational bands for protonated versus free molecules provides a diagnostic for charge-induction effects, while the proton stretching and bending vibrations provide direct access to the proton-transfer potential energy surface. These experiments provide specific new chemical insights about selected proton transfer reactions. They also produce benchmark data for comparison to the results of computational quantum chemistry which attempts to calculate proton transfer dynamics. Vibrational signatures measured for protonated organic molecules known as carbocations may be useful for infrared astronomy of interstellar gas clouds. Equally important is the training of undergraduates, graduate students and postdoctoral fellows in the methods, techniques and instrumentation of modern physical chemistry. Ongoing collaborations include a non-Ph.D. granting institution (Kennesaw State University) and two international labs (Chemistry Dept., Nottingham, UK and Fritz Haber Institute, Berlin).