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  • 1
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this letter we report quantum mechanical integral and differential cross sections for the title reactions as calculated on a new ab initio potential energy surface. The calculations, all carried out in the reagents arrangement channel employing negative imaginary potentials, were done within the coupled-states approximation. The final vibrational state-to-state differential and integral cross sections were compared with experiment. Altogether, a very encouraging agreement was obtained. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of the F+HD reaction has been studied by means of quasiclassical trajectory calculations on an ab initio potential energy surface (PES) at several collision energies. At the collision energy of 85.9 meV and for the DF+H isotopic channel of the reaction, there is a remarkable agreement between calculated and experimental results, in both the center of mass (c.m.) differential cross sections (DCS) and in the simulation of the laboratory (LAB) time of flight (TOF) and angular distributions (AD). The good agreement also extends to the lower collision energy of 58.6 meV for this channel of the reaction. In contrast, the simulation of the LAB angular distributions for the HF+D channel shows strong discrepancies between theory and experiment at both collision energies, which can be traced back to the absence of a forward peak in the calculated c.m. DCS for HF(v'=3). Simulations made from QCT calculations on other PES with important HF(v'=3) forward scattering contributions also fail to reproduce the overall AD. The theoretical findings and especially the roles of translational energy and initial rotational momentum on the dynamics of this reaction are discussed in terms of the topology of the PES through the analysis of individual trajectories. © 1995 American Institute of Physics.
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  • 3
    ISSN: 1433-2965
    Keywords: Key words:Cortical shell – Fractal methods – High-resolution computed tomography (HR-CT) – Lumbar vertebrae – Spongiosa structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract: Different noninvasive techniques for the assessment of the individual fracture risk in osteoporosis are introduced, and the relation between structural properties of high-resolution computed tomography (HR-CT) images of vertebral bodies, their bone mineral density (BMD) and the fracture load is analyzed. In 24 unfractured lumbar vertebrae with different degrees of demineralization from six specimens, the trabecular and cortical BMD was determined using quantitative CT. A lateral X-ray image revealed the number of fractures in the entire spine. A structural analysis of spongy and cortical bone was performed based on the HR-CT images. In the spongiosa, the fractal dimension was calculated as a function of the threshold value. In the cortical shell, the maximum number of clusters of low BMD was determined at varying threshold values. After the CT measurements the vertebrae were excised and compressed until fractured. On the basis of the spongiosa BMD and the number of fractures, 3 cases were found to be severely osteoporotic; the other 3 cases showed osteopenia. The average fracture loads were determined as 3533 N for the non-osteoporotic cases (range 2602–5802 N) and 1725 N for the osteoporotic cases (range 1311–2490 N). The parameters were determined as follows: average spongiosa BMD 115.2 mg/ml (101.8–135.3 mg/ml) for the non-osteoporotic cases, 46.2 mg/ml (34.8–57.6 mg/ml) for the osteoporotic cases; average cortical BMD 285.1 mg/ml (216.4–361.9 mg/ml) for the non-osteoporotic cases, 136.1 mg/ml (142.5–215.2 mg/ml) for the osteoporotic cases; spongiosa structure: average 0.5 (range 0.32–0.75) for the non-osteoporotic cases, average 1.05 (range 0.87–1.24) for the osteoporotic cases; cortical structure: average 81 (range 55–104) for the non-osteoporotic cases), average 136 (range 102–159) for the osteoporotic cases. Single parameters (BMD and structure) and weighted sums of these parameters were correlated with the fracture load, resulting in correlation coefficients of r sBMD= 0.82 (spongiosa BMD), r cBMD= 0.82 (cortical BMD), r sStr=–0.75 (spongiosa structure) and r cStr=–0.86 (cortical structure). The weighted sum of cortical and spongiosa BMD resulted in r BMD= 0.86, of cortical and spongiosa structure in r Str=–0.86. A weighted combination of all four parameters correlates with the fracture load at r 4= 0.89, all correlations being statistically significant (p〈0.0001). The four individual parameters show only a slight overlap between non-osteoporotic and osteoporotic subjects. The high correlation of the cortical BMD and the structural parameter in cortical bone indicates the important contribution of the cortical shell to vertebral stability. A weighted sum of multiple parameters results in a higher correlation with the fracture load and does not show an overlap between the two groups. It is best suited to estimate the individual fracture risk. The presented methods are generally applicable in vivo; and allow an improvement of the diagnosis of osteoporosis compared with the measurement of the BMD alone.
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