Neutron Compton Scattering as a Probe of Hydrogen Bonded (and other) Systems

Reiter, George

关键词: quantum coherence;    momentum distribution;    protons;    water;   

DOI  :  10.2172/1000515
RP-ID  :  DOE/08ER46486-5
PID  :  OSTI ID: 1000515

美国|英语

来源: SciTech Connect

【 摘 要 】

One of the two major themes of the proposal was to study quantum coherence in stressed hydrogen bond networks. Our experiments on double wall carbon nanotubes and two versions of Nafion, together with earlier work on water confined in xerogel and in single wall carbon nanotubes demonstrate that water confined in dimensions on the order of 20 Angstroms is in a qualitatively different quantum ground state than bulk water. It cannot be described as a collection of molecules interacting only electrostatically. This has major implications for biology, where most of the water in the cell is confined to distances on that order, and for fuel cell development using Nafion like materials, where the conduction mechanism is expected to be strongly affected by the quantum state of the water. We have demonstrated its importance in the binding of water molecules to DNA. Protein folding experiments at several concentrations have been carried out and are presently being analyzed. Initial analysis shows strong temperature dependent effects on the proton momentum distribution. The theoretical work proposed has been completed, and complements the experiments by demonstrating that even in room temperature bulk water, the electron density overlap between donor and acceptor molecules in the hydrogen bond has a significant measurable effect on the momentum distribution of the protons. The electrons are distributed throughout the hydrogen bond network, not simply in localized molecules. We had proposed to look at low coverage experiments on MCM-41, and have done so, revealing the details of the interaction of water molecules with silanol groups. Related experiments, not proposed, on water layers on SnO2 and TiO2 powders have confirmed the strong deviations of the proton momentum distribution for water molecules adsorbed on these surfaces. These results are not yet published. Another major theme was to measure Born-Oppenheimer potentials in M3H(XO4)2 systems. We have done this for Rb3(SO4)2, observing a strongly temperature dependent potential, despite the absence of a phase transition, and have data not yet analyzed for Rb3(SeO4)2. The remaining samples proposed have not been done yet. Our priorities were to explore the quantum coherence in confined water and water on surfaces, and, due to the limitations of time on VESUVIO, the experiments on Nafion and the surface experiments displaced these. We hope to carry them out under the renewal proposal. A third theme was to confront the theory of the intensity deficit developed by the PI and P. Platzman with experiments on H2. This has been done, and, although the analysis is preliminary, and not yet published, it appears that the predictions of this theory are not born out, and that the intensity deficit is due to other sources. Finally, experiments on the simple tunneling system MnH0.07 have been carried out with larger samples. The results refute the earlier measurements, and defy theoretical expectations as to the temperature dependence and shape of the momentum distribution. This remains a puzzle.