Proton transport in water networks: Understanding cooperative effects in charge migration via isomer-selective vibrational spectroscopy of cold cluster ions

在这项由化学系实验物理化学项目资助的奖项中，耶鲁大学的马克·A·约翰逊教授将探索与水中过量质子相关的独特性质的分子水平相互作用。需要更好地了解水介导的质子传输，因为这一过程发生在许多科学和技术领域，那里只有少量的水分子可用。例如，从在生物系统中调节电信号的跨膜质子泵到在燃料电池中传导正电荷的质子交换膜。阻碍直接观测凝聚相中过剩质子的局部环境的长期障碍之一是，它的振动特征如此广泛，以至于掩盖了电荷在传输链的不同阶段是如何被容纳的分子水平的图像。约翰逊教授资助的这项研究的重点是通过利用气相中冷的、大小选定的水团簇的制备和光化学操作方面的最新进展来克服这一问题。这些方法包括Ar团簇介导的合成和使用多阶段质量选择的团簇离子的泵浦-探测振动光谱。具体目标是获得可以分析的精确光谱特征，以揭示电荷离域程度如何取决于氢键环境的不同网络拓扑。被选择来表达这些影响的目标系统来自于大气中自然发生的水团簇介导的反应，确实有望解决关于在电离层D区(80公里高度)观察到的环境NO+离子的去电离速率的长期难题。一种新型的测量方法将被开发出来，它能够测量质子转移的能垒，从而揭示控制质子迁移率的关键因素。在这项工作中探索的基础科学的广泛影响提供了一种自然的方式来吸引各级学生--从高中到博士生--的兴趣和参与，这些项目与对社会具有直接重要性的问题有关，如能源(就燃料电池而言)和环境(就水为媒介的大气化学而言)。已经参与这项研究的几名学生来自自然科学领域代表性不足的群体，包括耶鲁大学的本科生。

In this award funded by the Experimental Physical Chemistry Program of the Division of Chemistry, Professor Mark A. Johnson of Yale University will explore the molecular-level interactions responsible for the unique properties associated with an excess proton in water. Better understanding of water-mediated proton transport is needed because this process occurs in many fields of science and technology where only a small number of water molecules are available. Examples range from trans-membrane proton pumps that mediate electrical signaling in biological systems to proton exchange membranes that conduct positive charge in fuel cells. One of the long-standing roadblocks preventing direct observation of the local environment of an excess proton in the condensed phase is that its vibrational signature is so broad that it masks the molecular-level picture of how the charge is accommodated at various stages along the transport chain. The focus of the research funded by Prof. Johnson's award is to overcome this problem by exploiting very recent advances in the preparation and photochemical manipulation of cold, size-selected water clusters in the gas phase. These methods involve Ar-cluster mediated synthesis and pump-probe vibrational spectroscopy of cluster ions using multiple stages of mass selection. The specific goal is to obtain precise spectroscopic signatures that can be analyzed to reveal how the degree of charge delocalization depends on the different network topologies of the hydrogen-bonded environment. The target systems chosen to express these effects are derived from water cluster-mediated reactions that occur naturally in the atmosphere, and indeed promise to resolve a long-standing puzzle regarding the observed deionization rate of the ambient NO+ ion in the D region of the ionosphere (80 km altitude). A new type of measurement will be developed that is capable of measuring the energetic barriers to proton transfer, and thus expose the key factors controlling proton mobility. The wide ranging implications of the basic science explored in this effort provide a natural way to engage the interest and participation of students at all levels - high school through Ph.D. candidates - with projects that connect to issues of immediate importance to the society like energy (in the case of fuel cells) and the environment (in the case of water-mediated atmospheric chemistry). Several students already involved in this research are from under-represented groups in the physical sciences, including undergraduates at Yale University.