ISSN:
1600-5740
Quelle:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Thema:
Chemie und Pharmazie
,
Geologie und Paläontologie
,
Physik
Notizen:
2,3-Diketopiperazine (2,3-piperazinedione), C4H6N2O2, crystallizes in the monoclinic space group P21/c with a = 5.941 (3), b = 10.080 (3), c = 8.282 (2) Å, β = 95.87 (3)°, V = 493.4 (5) Å3, Z = 4, Dx = 1.536 Mg m−3 and Mr = 114.1. The six-membered ring adopts a skew-boat conformation with Q = 0.467 (3) Å, θ = 64.6 (3)° and φ = 269.8 (4)°. The C2 symmetry, which typifies the free molecule, is broken by the formation of two intermolecular N—H...O bonds involving only one of the C=O groups of the 2,3-diketopiperazine molecule. The intensity data typical for a time series are summarized in traditional statistics by B = σ2(B) = ΣBi and R = σ2(R) = ΣRi, where B is the background and R the raw intensity. Exploitation of the same data using Bayesian methodology leads to similar values for B and R, but the variances for these signals are significantly smaller. This reduction in variance is dictated by the length N of the time series. With 25 observations in each hkl time series we arrive at variances that are 25% of their classical values. So, a measuring strategy in which a single observation with a slow scan at speed 1/N is preferred above a series of N observations at speed 1 produces the worst possible benefit within a fixed time frame. In complete contrast to our expectation, structure refinements reveal that that the standard deviation of an observation of unit weight, S, will converge towards its ideal unit value only when we decrease the accuracy of the data set. This unpleasant behaviour points to a serious discrepancy between accuracy and precision. Our reflection intensities are systematically wrong, because we underestimate the wavelength dispersion. On our CAD-4 equipment a sealed Mo tube and the standard graphite monochromator produce an incident beam with Δλ/λ = 14%. As a consequence, the observed background intensities are not representative of the real background. The size of the background error depends on the nearby Bragg intensity. The magnitude of the signal pollution contaminating the local background is ∼2% of the maximum Bragg intensity found at λ = 0.71 Å.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1107/S0108768198004637
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