Quasielastic neutron scattering study on the dynamical properties of an aromatic hydrogen bond

Im Rahmen der vorliegenden Dissertation wurden die dynamischen Eigenschaften von aromatischen Wasserstoffverbindungen am Beispiel eines typischen Vertreters, Ammonium Tetraphenylborat (ATPB), mit Hilfe von quasielastischer inkohärenter Neutronenstreuung (QENS) in einem breiten Temperaturbereich von 20K bis 350K untersucht. In ATPB ist ein Ammonium-Kation zwischen vier Phenylringen benachbarter Tetraphenylborat-Anionen eingeschlossen. Dabei dient jeder einzelne Phenylring als ein Wasserstoffbindungsempfänger für das Ammonium-Ion. Zur Reproduktion der QENS-Spektren wurden verschiedene Modelle für die lokalen Diffusionsbewegungen des Ammonium-Ions und des Phenylrings herangezogen. Die Bewegungen des Ammonium-Ions wurden als kontinuierlicher Übergang von quantenmechanischen Rotationen bei tiefen Temperaturen zu klassischen diffusiven Rotationsbewegungen bei hohen Temperaturen interpretiert. Im Temperaturbereich von 67K bis 350K rotieren die Ammonium-Ionen um eine C2- oder C3-Achse. Die Korrelationszeit variiert für diese Bewegung von 12.5ps bei 350K bis 1253ps bei 67K. Als Aktivierungsenergie der Rotationen des Ammonium-Ions wurde 3.2kJ/mol ermittelt. Dies zeigt, dass die Energiebarriere für eine Rotation der Ammonium-Ionen extrem niedrig ist. Bei Temperaturen unterhalb von 67K wurden quantenmechanische Rotationen der Ammonium-Ionen beobachtet. Darüber hinaus wurden im Rahmen dieser Studie Librations-Anregungszustände von Ammonium bei 4.3meV und 8.2meV identifiziert. Bei Temperaturen oberhalb von 200K zeigten sich auf der gemessenen Zeitskala lokale diffusive Orientierungsbewegungen der Phenylringe. Diese Bewegungen werden als Librationen aus der Ebene der Phenylringe heraus bewertet. Umorientierungswinkel und Sprung-Frequenz der Librationen der Phenylringe nehmen bei steigenden Temperaturen zu. Bei Raumtemperatur betrug der Umorientierungswinkel der Librationen der Phenylringe 7 Grade und die Korrelationszeit 150ps. Dieses Ergebnis lässt die Vorstellungen von aromatischen Wasserstoff-bindungen in einem neuen Licht erscheinen. Die aromatischen Wasserstoffbindungen sind extrem flexibel und ermöglichen sowohl dem Donator als auch dem Akzeptor Umorientierungen.In this work the dynamical properties of a prototypical example of the aromatic hydrogen bond, ammonium tetraphenylborate (ATPB), have been studied by quasielastic incoherent neutron scattering (QENS) in a broad temperature range from 20K up to 350K. In ATPB an ammonium cation is trapped between four phenyl rings of the two adjacent tetraphenylborate anions and each phenyl ring serves as a hydrogen bond acceptor for the ammonium ion. Models for local diffusive motions of reorientation of ammonium and the phenyl rings have been considered to reproduce the QENS spectra. The motions of the ammonium ions are explained as a continuous transition from quantum mechanical rotations at low temperatures to classical diffusive rotations at high temperatures. In a temperature range from 67K up to 350K classical diffusive rotations of the ammonium ions around a C2- or C3-axis were found. The correlation time for this motion varies from 12.5ps at 350K to 1253ps at 67K and the activation energy was determined to be 3.2kJ/mol. This result shows that the ammonium ions have to overcome an exceptionally low barrier to rotate. At temperatures below 67K quantum mechanical rotations of the ammonium ions were observed. Additionally, in this work excited librational states of ammonium were identified at 4.3meV and 8.2meV. At temperatures above 200K local diffusive motions of orientation of the phenyl rings were found within the experimental time-window. These motions of the phenyl rings are explained as librations out of the plane. The reorientation angle and the jump rate of the librations of the phenyl rings increase with increasing temperatures. At room temperature the librations of the phenyl rings were found to have a reorientation angle of 7 degrees and a correlation time of 150ps. This result sheds new light on the phenomenon of aromatic hydrogen bonds showing that they are extremely flexible and allow for reorientation of both, the donor and the acceptor

Characteristic length scales of the secondary relaxations in glass-forming glycerol

We investigate the secondary relaxations and their link to the main structural relaxation in glass-forming liquids using glycerol as a model system. We analyze the incoherent neutron scattering signal dependence on the scattering momentum transfer, Q , in order to obtain the characteristic length scale for different secondary relaxations. Such a capability of neutron scattering makes it somewhat unique and highly complementary to the traditional techniques of glass physics, such as light scattering and broadband dielectric spectroscopy, which provide information on the time scale, but not the length scales, of relaxation processes. The choice of suitable neutron scattering techniques depends on the time scale of the relaxation of interest. We use neutron backscattering to identify the characteristic length scale of 0.7 Å for the faster secondary relaxation described in the framework of the mode-coupling theory (MCT). Neutron spin-echo is employed to probe the slower secondary relaxation of the excess wing type at a low temperature ( ∼ 1.13Tg . The characteristic length scale for this excess wing dynamics is approximately 4.7 Å. Besides the Q -dependence, the direct coupling of neutron scattering signal to density fluctuation makes this technique indispensable for measuring the length scale of the microscopic relaxation dynamics

Fractal diffusion in high temperature polymer electrolyte fuel cell membranes

© 2018 Author(s). The performance of fuel cells depends largely on the proton diffusion in the proton conducting membrane, the core of a fuel cell. High temperature polymer electrolyte fuel cells are based on a polymer membrane swollen with phosphoric acid as the electrolyte, where proton conduction takes place. We studied the proton diffusion in such membranes with neutron scattering techniques which are especially sensitive to the proton contribution. Time of flight spectroscopy and backscattering spectroscopy have been combined to cover a broad dynamic range. In order to selectively observe the diffusion of protons potentially contributing to the ion conductivity, two samples were prepared, where in one of the samples the phosphoric acid was used with hydrogen replaced by deuterium. The scattering data from the two samples were subtracted in a suitable way after measurement. Thereby subdiffusive behavior of the proton diffusion has been observed and interpreted in terms of a model of fractal diffusion. For this purpose, a scattering function for fractal diffusion has been developed. The fractal diffusion dimension dw and the Hausdorff dimension df have been determined on the length scales covered in the neutron scattering experiments

The role of oxygen vacancies on the vibrational motions of hydride ions in the oxyhydride of barium titanate

Perovskite-type oxyhydrides, BaTiO3-xHx, represent a novel class of hydride ion conducting materials of interest for several electrochemical applications, but fundamental questions surrounding the defect chemistry and hydride ion transport mechanism remain unclear. Here we report results from powder X-ray diffraction, thermal gravimetric analysis, nuclear magnetic resonance spectroscopy, inelastic neutron scattering (INS), and density functional theory (DFT) simulations on three metal hydride reduced BaTiO3 samples characterized by the simultaneous presence of hydride ions and oxygen vacancies. The INS spectra are characterized by two predominating bands at around 114 (ω⊥) and 128 (ω∥) meV, assigned as fundamental Ti-H vibrational modes perpendicular and parallel to the Ti-H-Ti bond direction, respectively, and four additional, weaker, bands at around 99 (ω1), 110 (ω2), 137 (ω3) and 145 (ω4) meV that originate from a range of different local structures associated with different configurations of the hydride ions and oxygen vacancies in the materials. Crucially, the combined analyses of INS and DFT data confirm the presence of both nearest and next-nearest neighbouring oxygen vacancies to the hydride ions. This supports previous findings from quasielastic neutron scattering experiments, that the hydride ion transport is governed by jump diffusion dynamics between neighbouring and next-nearest neighbouring hydride ion-oxygen vacancy local structures. Furthermore, the investigation of the momentum transfer dependence of the INS spectrum is used to derive the mean square displacement of the hydride ions, which is shown to be in excellent agreement with the calculations. Analysis of the mean square displacement confirms that the hydrogen vibrational motions are localized in nature and only very weakly affected by the dynamics of the surrounding perovskite structure. This insight motivates efforts to identify alternative host lattices that allow for a less localization of the hydride ions as a route to higher hydride ion conductivities

Quasielastic neutron scattering study of POSS ligand dynamics

Polyoligosilsesquioxanes are molecules having cage-like structures composed of silicon and oxygen. These molecules can have a wide variety of functional ligands attached to them. Depending on the nature of the ligand, interesting properties and applications are found. In this work we present results from quasielastic neutron scattering measurements of four different POSS molecules that illustrate the presence of strong coupling between the ligand dynamics and the POSS crystal structures

Quasielastic neutron scattering study of POSS ligand dynamics

Polyoligosilsesquioxanes are molecules having cage-like structures composed of silicon and oxygen. These molecules can have a wide variety of functional ligands attached to them. Depending on the nature of the ligand, interesting properties and applications are found. In this work we present results from quasielastic neutron scattering measurements of four different POSS molecules that illustrate the presence of strong coupling between the ligand dynamics and the POSS crystal structures