Bond Dissociation Triggering Molecular Disorder in Amorphous H2O

Structure and Ultrafast X-ray Diffraction of the Hydrated Metaphosphate

We study the pathway of metaphosphate hydration when a metaphosphate anion is dissolved in liquid water with an explicit water model. For this purpose, we propose a sequential Monte Carlo algorithm incorporated with the ab initio quantum mechanics/molecular mechanics (QM/MM) method, which can reduce the amount of ab initio QM/MM sampling while retaining the accuracy of the simulation. We demonstrate the numerical calculation of the standard enthalpy change for the successive addition reaction PO3-·2H2O + H2O ⇌ PO3-·3H2O in the liquid phase, which helps to clarify the hydration pathway of the metaphosphate. With the obtained hydrated structure of the metaphosphate anion, we perform ab initio calculations for its relaxation dynamics upon vibrational excitation and characterize the energy transfer process in solution with simulated ultrafast X-ray diffraction signals, which can be experimentally implemented with X-ray free-electron lasers.

Exploring Structures and Dynamics of Molecular Assemblies: Ultrafast Time-Resolved Electron Diffraction Measurements

ConspectusMolecular assemblies have been widely applied to functional soft materials in a variety of fields. Liquid crystal is one of the representative molecular soft materials in which weak intermolecular interactions induce its dynamic molecular behavior under external stimuli, such as electric and magnetic fields, photoirradiation, and thermal treatment. It is important to understand molecular behavior and motion in the liquid-crystalline (LC) states at the picosecond level for further functionalization of liquid crystals and molecular assembled materials. For investigation of assembled structures of the materials on the nanometer scale, X-ray diffraction (XRD) measurements have been a powerful tool. Despite the dynamic nature of the assembled materials, however, time resolution of XRD is limited to millisecond due to the response speed of the detector, which hampered real-time observation of the dynamics of the molecular assembly. For further understanding of the dynamic behavior of functional molecules and improvement of performance for their applications, the insights of faster dynamics on the micro-, nano-, pico-, and even femtosecond time scales are required. In this context, the interdisciplinary approaches of the emerging fields of materials chemistry and ultrafast science will open up new aspects of molecular science and technology. These approaches may lead to more effective design of new functional materials, which enables us to control molecular behaviors and motions.The development of ultrashort pulsed X-ray and electron sources has resulted in the visualization of the key structural dynamics on the femto- to picosecond time scale not only in isolated molecules but also in assembled molecules, such as in the LC, crystal, and amorphous phases. We focus on ultrafast phenomena in molecular assemblies induced by photoexcitation. Ultrafast time-resolved electron diffraction measurements are sensitive to the molecular periodicity under photoexcitation, and thus the methodologies directly provide the ultrafast photoinduced molecular dynamic arrangements.In this Account, we describe ultrafast structural dynamics of molecules in the LC phases observed by time-resolved electron diffraction measurements. Photoinduced conformational changes of LC molecules is shown as the example, which is the first observation of LC molecule using time-resolved electron diffraction. It is important to understand the correlation between the conformational or configurational changes induced in a photoirradiated single molecule and the oriented collective motions of molecular assemblies induced by intermolecular interaction. We also show observation of collective motions of azobenzene LC molecules. The collective motions are initiated from photoreaction in a single molecule and are subsequently amplified by the steric interaction with its neighboring molecules.One remaining challenge is to create the platform of materials and sample preparations for time-resolved electron diffraction experiments, which can only be achieved by the interdisciplinary fusion of the fields of materials chemistry and ultrafast science. Time-resolved electron diffraction is a powerful tool for structural investigation of molecular materials with a dynamic nature, whose adaptability goes beyond that of more complex assemblies of carbon nanomaterials. This methodology will extend the possibility to investigate motions of a variety of molecular self-assemblies on a larger scale, for example, to understand responses of biomolecular assemblies and intermolecular chemical reactions.

Structure retrieval in liquid-phase electron scattering

Electron scattering on liquid samples has been enabled recently by the development of ultrathin liquid sheet technologies. The data treatment of liquid-phase electron scattering has been mostly reliant on methodologies developed for gas electron diffraction, in which theoretical inputs and empirical fittings are often needed to account for the atomic form factor and remove the inelastic scattering background. In this work, we present an alternative data treatment method that is able to retrieve the radial distribution of all the charged particle pairs without the need of either theoretical inputs or empirical fittings. The merits of this new method are illustrated through the retrieval of real-space molecular structure from experimental electron scattering patterns of liquid water, carbon tetrachloride, chloroform, and dichloromethane.

Liquid-phase mega-electron-volt ultrafast electron diffraction

The conversion of light into usable chemical and mechanical energy is pivotal to several biological and chemical processes, many of which occur in solution. To understand the structure-function relationships mediating these processes, a technique with high spatial and temporal resolutions is required. Here, we report on the design and commissioning of a liquid-phase mega-electron-volt (MeV) ultrafast electron diffraction instrument for the study of structural dynamics in solution. Limitations posed by the shallow penetration depth of electrons and the resulting information loss due to multiple scattering and the technical challenge of delivering liquids to vacuum were overcome through the use of MeV electrons and a gas-accelerated thin liquid sheet jet. To demonstrate the capabilities of this instrument, the structure of water and its network were resolved up to the 3 rd hydration shell with a spatial resolution of 0.6 Å; preliminary time-resolved experiments demonstrated a temporal resolution of 200 fs.