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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 3928-3934 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: No reliable structural data have been reported on ice II under pressure, earlier work in the literature relating either to samples recovered to ambient pressure or the helium hydrate that is formed when helium is used as the pressurizing medium. We report structural refinements of helium-free ice II at three points in the phase's region of stability. The structural differences from the helium-affected structure are significant, and can be related to the mainly repulsive interaction between the helium and both the oxygen and hydrogen atoms of the ice framework. These repulsions explain, among other changes, the different behaviors of the a (expansion) and c (contraction) lattice parameters, and the change in compressibility on the inclusion of helium. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Although the early structural studies of liquids and amorphous systems used x-ray diffraction, the advent of high flux reactor sources led to neutron diffraction becoming the preferred technique. There are several advantages: high scattering vector information is not form factor limited and, where isotopes exist, the variation in scattering length allows more detailed structural information to be extracted in many cases. However, the development of synchrotron radiation sources has generated renewed interest in x-ray measurements. High intensity is available at wavelengths short enough to give reasonable access to the high scattering vector region and, in theory, the tunability can be used to maximize anomalous scattering effects and so allow element specific scattering to be recorded. Another use of the tunability is for largely removing the Compton scattering which can often dominate the total scattered intensity and which contains no structural information. This technique requires a choice of incident photon energy such that the x rays scattered without energy loss excite fluorescence in a metal foil in the detection system while most of the Compton scattering has insufficient energy to do so. Hence, recording the fluorescence intensity effectively gives a measurement of the elastic scattering only. Whereas early experiments were limited by the available x-ray emission lines of conventional sources, synchrotron radiation can be tuned so that the incident energy is much closer to the absorption edge of the metal foil thus maximizing the efficiency of the technique. We demonstrate the power of this method and suggest where its application may continue to develop interest in x-ray experiments.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 5438-5443 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Neutron diffraction involving seven isotope substitution experiments is used to extract the methyl hydrogen to the water hydrogen partial structure factor in a 2.0 m aqueous solution of tetramethylammonium chloride, in order to investigate the orientation of water molecules around the TMA ion. The correlation function of the TMA centers with water molecule centers is obtained and shows that the average distance is about 4.7 A(ring), in agreement with the result found previously using a nitrogen isotope substitution. The technique of spherical harmonic reconstruction is used to obtain orientational correlation functions between TMA ions and water molecules. The results show that the TMA ion has a predominantly "apolar'' character in water but that a range of water molecule orientations around the ion are possible. © 1995 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Europhysics news 28 (1997), S. 129-132 
    ISSN: 1432-1092
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1432-1017
    Keywords: Solvent structure ; hydrogen bond networks ; computer simulation ; B12 coenzyme crystals ; probability density
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract Both the ordered and disordered solvent networks of vitamin B12 coenzyme crystal hydrate have been generated by Monte Carlo simulation techniques. Several different potential functions have been use to model both water-water and water-solute (i.e., water-coenzyme) interactions. The results have been analysed in terms of the structural properties of the water networks, such as mean water oxygen and hydrogen positions, coordination of each water molecule, and maxima of probability density maps in all four asymmetric units of this crystal. The following results were found: (I) Within each asymmetric unit only one hydrogen bonding network was predicted although there were several hydrogen atom positions for any one solvent molecule (defined as maxima in probability density). (II) Reasonable agreement was obtained between predicted and experimental positions in the ordered solvent region, independent of the potential function used. (III) The positions of the calculated probability density maxima for the disordered channel region were different in different asymmetric units; this led to different simulated hydrogen bond networks which were not always consistent with the experimentally determined alternative (lower occupancy) sites. The results suggest that it is advisable to simulate more than one asymmetric unit if one wishes to look at disorder in the solvent regions. Probability density maps were qualitatively very useful for picturing these disordered regions. However, there were no significant differences between quantitative results predicted using either average atomic positions or maxima of the probability density distributions. Problems in quantifying agreement between experimental and predicted disordered solvent networks are discussed. The potential which included hydrogen atoms explicitly (EMPWI) seemed to give the best overall agreement, mainly because it was successful in predicting the unusually short hydrogen bonds which are found in this crystal.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 4878-4888 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Detailed structural studies using powder neutron diffraction have been carried out on ices III and IX on both under-pressure and recovered samples. The incomplete hydrogen ordering in recovered ice IX first observed by La Placa et al. is confirmed, and our studies suggest similar orderings for the under-pressure and recovered phases. Sample history was observed to influence the lattice constants of ice IX, an effect which could relate to hydrogen ordering. There is an indication that ice III is not fully disordered, a conclusion which would be consistent with independent thermodynamic measurements.
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  • 7
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Inelastic incoherent neutron scattering (IINS) cross sections have been measured for the recovered high-pressure phases of ice, ices II, V, VI, and IX (H2O) and ices II and IX (D2O) in the energy range from 2 to 150 meV at 20 K using the TFXA (time-focused crystal analyzer) neutron spectrometer on the ISIS pulsed neutron source at the Rutherford-Appleton Laboratory. The excellent resolution of the TFXA (less than 2%) covers the whole range of translational and librational modes for the ices investigated and enables us to identify the fine structures in their spectra. Hence a direct comparison between the amplitude-weighted phonon density of states derived from the experimental spectrum and from a lattice dynamic calculation is possible [E. Whalley and J. E. Bertie, J. Chem. Phys. 46, 1271 (1967); J. E. Bertie and E. Whalley, ibid. 40, 1646 (1964)]. The spectra from these polymorphic ices show many new features in the translational region, which are not visible using infared and Raman techniques. In the librational band region, the general features vary remarkably between phases. Thus, for instance, ice II, in which the protons are ordered, shows more structure in this region and has a sharper low-energy cutoff than do the other phases examined. The most significant observation is that the low-energy cutoff is characteristic of the phase. Relative to ice Ih, there is a shift of 4.3 meV for ice IX, of 8.8 meV for ice II, 8.6 meV for ice V, and 12.2 meV for ice VI.
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  • 8
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Biopolymers 22 (1983), S. 255-260 
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using direct difference ir and laser Raman spectroscopy, the sequential hydration of hen egg-white lysozyme was monitored. The ir data allowed us to identify some specific molecular hydration events that occur as water is added, whereas the Raman is interpreted in terms of conformational changes. The largest of these solvent-induced changes occurs below the hydration level at which activity commences.
    Additional Material: 3 Ill.
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  • 9
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Direct difference ir spectra are presented as a function of hydration for lysozyme and α-lactalbumin, and detailed sequential hydration molecular events identified. Despite the strong sequence homology between the two proteins, and their expected conformational similarity, the hydration behaviour of the polar groups is different for the two proteins. Using a Hill-type analysis, we conclude that the acid groups ionize and hydrate rapidly and noncooperatively in both proteins, consistent with the known (lysozyme) and postulated (α-lactalbumin) surface chemistry. The polar group hydration shows a clear cooperativity, which is quantitatively different in the two proteins. Complementary work suggests this cooperativity relates to a hydration-induced “loosening up” of the lysozyme conformation at about 55 mol water/mol protein. α-Lactalbumin appears to “open up” more easily for hydration than does lysozyme, consistent with its lower stability against thermal and acid denaturation.
    Additional Material: 9 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 257 (1975), S. 120-122 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] We thought it worthwhile to 'relax1 the '1968' (ref. 10) Bernal sphere-packing model under a realistic interatomic potential function to observe which of the resultant geometrical changes can occur fairly easily in an intially dense packing, as distinct from those which may need to be induced in ...
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