Felix Fernandez-Alonso

Improved description of soft layered materials with van der Waals density functional theory

The accurate description of van der Waals forces within density functional theory is currently one of the most active areas of research in computational physics and chemistry. Here we report results on the structural and energetic properties of graphite and hexagonal boron nitride, two layered materials where interlayer binding is dominated by van der Waals forces. Results from several density functionals are reported, including the optimized Becke88 van der Waals (optB88-vdW) and the optimized PBE van der Waals (optPBE-vdW) (Klimeš et al 2010 J. Phys.: Condens. Matter 22 022201) functionals. Where comparison to experiment and higher-level theory is possible, the results obtained from the two new van der Waals density functionals are in good agreement. An analysis of the physical nature of the interlayer binding in both graphite and hexagonal boron nitride is also reported.

Nature of the bound states of molecular hydrogen in carbon nanohorns

The effects of confining molecular hydrogen within carbon nanohorns are studied via high-resolution quasielastic and inelastic neutron spectroscopies. Both sets of data are remarkably different from those obtained in bulk samples in the liquid and crystalline states. At temperatures where bulk hydrogen is liquid, the spectra of the confined sample show an elastic component indicating a significant proportion of immobile molecules as well as distinctly narrower quasielastic line widths and a strong distortion of the line shape of the para-->ortho rotational transition. The results show that hydrogen interacts far more strongly with such carbonous structures than it does to carbon nanotubes, suggesting that nanohorns and related nanostructures may offer significantly better prospects as lightweight media for hydrogen storage applications.

1. Introduction to Neutron Scattering

Abstract This introductory chapter provides a self-contained survey of the merits, strengths, and conceptual framework underpinning the use of neutrons as a probe of the structure and dynamics of materials. We also take the opportunity to illustrate important concepts using examples taken from the scientific literature, as well as establish the basic notation that will be used throughout, and listed in more detail in the List of Commonly Used Symbols.

Forward scattering in the H+D2→HD+D reaction: Comparison between experiment and theoretical predictions

We investigate the sensitivity of photoinitiated experiments to forward-scattering features by direct comparison of experimental angular distributions with quantum-mechanical calculations as well as by forward-convolution of theoretical and model center-of-mass differential cross sections. We find that the experimental sensitivity to forward-scattering angles depends on the instrumental velocity resolution as well as on the kinematics of the detected product channel. Explicit comparison is made between experimental HD(v′=1,2;j′) center-of-mass angular distributions at collision energies ≈1.6 eV (deduced from time-of-flight profiles using a single-laser, photolysis-probe approach) and quantum-mechanical calculations on the BKMP2 potential energy surface. The comparison takes into account the contributions from both slow and fast H atoms from the photolysis of HBr. We find that the contribution of the slow H atoms, which is the major source of experimental uncertainty, does not greatly affect the extraction...

Dynamics of a protein and its surrounding environment: A quasielastic neutron scattering study of myoglobin in water and glycerol mixtures

In this quasielastic neutron scattering (QENS) study we have investigated the relation between protein and solvent dynamics. Myoglobin in different water:glycerol mixtures has been studied in the temperature range of 260-320 K. In order to distinguish between solvent and protein dynamics we have measured protonated as well as partly deuterated samples. As commonly observed for bulk as well as for confined water, the dynamics of the surrounding solvent is well described by a jump diffusion model. The intermediate scattering function I(Q,t) from the protein (partly deuterated samples) was analyzed by fitting a single Kohlrausch-Williams-Watts (KWW) stretched exponential function to the data. However, due to the limited experimental time window, two different curve fitting approaches were used. The first one was performed with the assumption that I(Q,t) decays to zero at long times, i.e., it was assumed that all protein relaxations that are observed on the experimental time scale, as well as would be observed on longer time scales, can be described by a single KWW function. In the second approach we instead assumed that both the protein relaxation time tau(p) and the stretching parameter beta(KWW) were Q-independent, i.e., we assumed that the protein dynamics is dominated by more local motions. Advantages and disadvantages of both approaches are discussed. The first approach appears to work best at higher Q-values, indicating a power law relation of the Q-dependent protein dynamics for all samples and temperatures, whereas the second approach seems to work at lower Q-values, where the expected confined diffusion of hydrogen atoms in the protein gives the assumed Q-independent relaxation time. Independent of the chosen approach we find a significant correlation between the average relaxation time of the protein and the diffusion constant (or in this case the related relaxation time) of the solvent. However, the correlation is not perfect since the average relaxation time of the protein is more strongly dependent on the total amount of solvent than the diffusion constant of the solvent itself. Thus, the average relaxation time of the protein decreases not only with increasing solvent mobility, but also with increasing solvent content.

Recent and future developments on TOSCA at ISIS

TOSCA is a high-resolution neutron spectrometer at the ISIS Pulsed Neutron and Muon Source. The instrument is optimised for broadband vibrational spectroscopy in the 0 – 4000 cm−1 region and it has been operational since 2000. This paper describes how the instrument has been progressively upgraded in the intervening years to enable new science. Future upgrades are outlined.

H/D Isotope Effects in Protein Thermal Denaturation: The Case of Bovine Serum Albumin

The present work investigates the effects of H/D isotopic substitution on the structural and thermodynamic stability of bovine serum albumin (BSA) in aqueous solution over the temperature range of 5-90 °C. Using far-ultraviolet circular dichroism, we have compared protein unfolding pathways in H(2)O and D(2)O. Our results show that BSA possesses similar conformations in H(2)O and D(2)O at temperatures below 50 °C but follows different unfolding pathways at higher temperatures. The presence of D(2)O retards the occurrence of irreversible thermal denaturation in BSA, as evidenced by a higher onset temperature of 58 °C, in contrast to 50 °C in H(2)O. D(2)O exhibits a protective effect on the domain structure during the early stages of domain denaturation. Following incubation at 90 °C over a period of minutes, D(2)O causes a rapid aggregation of BSA molecules. This behavior is not observed in H(2)O solutions. Meanwhile, H/D substitution does not influence the reversible structural transformation of the protein in a significant manner. Partly renatured BSA in H(2)O and D(2)O undergoes very similar reversible structural transformations during a second heating cycle.

Strong physisorption site for H2 in K-and Li-doped porous carbons

Molecular hydrogen adsorption between two Li, K-doped coronene molecules (taken as local environment of carbon microporous materials) is studied by first-principles DFT-B3LYP calculations. These cluster calculations are complemented with periodic DFT-LDA/GGA calculations on extended Li- and K-doped structures. In all cases, energy minimization calculations unravel that there is a stable adsorption site for molecular hydrogen in these Li- and K-doped sp(2) carbon structures with large adsorption energies. This is the direct consequence of the significant charge transfer from the doping agents on neighboring slab carbon atoms, which allows the coupling of the molecular H(2) polarizability with the resulting substrate electric field (polarization interaction) that in turn induces the stabilization of molecular hydrogen. These calculations also give an insight on the atomic configurations of interlayer species (H(2) and LiK) as the interlayer spacing increases. It can be shown that large positional changes correlate with electronic properties of interlayer species. The confined hydrogen molecule does not show any tendency for dissociation and adopts a position in the interlayer void that is deeply related to that of doping ions.

Polymers under extreme two-dimensional confinement: Poly(ethylene oxide) in graphite oxide

The confinement of poly(ethylene oxide) in graphite oxide is studied using a combination of diffraction, calorimetric, and spectroscopic methods. Polymer intercalation into subnanometer graphite oxide layers leads to the complete suppression of crystallization phenomena and dielectric α-relaxation processes, as well as a slowdown of β-relaxation modes. For the first time, high-resolution inelastic neutron scattering shows that poly(ethylene oxide) under these extreme confinement conditions adopts a planar zig-zag conformation, in no way resembling the characteristic 72 helical structure of the bulk crystal. The neutron data also shows a strong suppression of the COC and OCC bending modes and a distinct broadening of C–O torsional bands.

Simultaneous Neutron Scattering and Raman Scattering

The capability to make simultaneous neutron and Raman scattering measurements at temperatures between 1.5 and 450 K has been developed. The samples to be investigated are attached to one end of a custom-made center-stick suitable for insertion into a 100 mm-bore cryostat. The other end of the center-stick is fiber-optically coupled to a Renishawt® inVia Raman spectrometer incorporating a 300 mW Topticat® 785 nm wavelength stabilized diode laser. The final path for the laser beam is ∼1.3 m in vacuo within the center-stick followed by a focusing lens close to the sample. Raman scattering measurements with a resolution of 1 to 4 cm−1 can be made over a wide range (100–3200 cm−1) at the same time as a variety of different types of neutron scattering measurements. In this work we highlight the use of inelastic neutron scattering and neutron diffraction in conjunction with the Raman for studies of the globular protein lysozyme.