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.
The conduction of electric charge in many biological processes, such as in the generation of the signal in the act of vision, occurs when protons are rapidly moved through an array of water molecules. This is a relay type event, where the oxygen atoms stay more or less where they are while an excess proton is attached to form a transient H3O+ hydrodium ion. This ion then gives up the extra proton to the next water along the chain. Similar "water wire" proton relays are central to the action of fuel cells and account for the positive charge flow needed when electrons are pushed through an external circuit to carry out useful work. The purpose of our research program is to create tools that can identify the pathways for charge flow when larger networks of water molecules form the medium for proton conduction. To this end, we have constructed a new type of apparatus that can select an exact number of water molecules around a compound, freeze the assembly into a rigid structure at low temperature (10 Kelvin), and then measure the frequencies at which all the chemical bonds vibrate. This information is then analyzed with high level theoretical methods to reveal which molecules are active in transport. The end result is a picture of how different water network shapes control the flow and direction of charge movement. Good examples illustrating these principles can be found in our report of water network-mediated processing of primary ions in Earth’s ionosphere, Relph et al., Science 327, 308 (2010) and more recent applications to catalysis in Garand et al., Science, 335, 694 (2012).
