Evidence from X-ray Diffraction

A considerable amount of information can be obtained about the location and bonding of the interlayer anions by XRD, although the detailed structure of the highly disordered hydrogen bonding network in the interlayer galleries is not amenable to study by this method. 

In the case of stearate ions intercalated in Mg/Al LDHs, the electron density shows a pronounced minimiun in the center of the interlayer region, which is consistent with the presence of a bilayer of guest molecules [151,152], The electron density distribution for an anionic azobenzene derivative intercalated in an Mg/Al LDH has also been reported [ 153]. 

Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25°C and w/c ratios of 0.45-0.65. XRD powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsum and other calcium sulphate phases are no longer detectable after, at most, 24 h, and tbe clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. 

The peaks of CH begin to appear within a few hours. Within a few days they dominate the XRD pattern. A diffuse band at 0.27-0.31 nm and a sharper one at 0.182 nm develop more slowly these are similar to the bands of C-S-H observed in CjS or P-C2S pastes (Fig. 5.6), but taking into account the amount of C-S-H likely to be present, they suggest an even lower degree of crystallinity than in the latter materials. 

The only peaks clearly attributable to AFm phases are usually those at about 0.288 nm (11.0), 0.166 nm (30.0) and the first- and second-order basal reflections corresponding to one or, usually, more layer thicknesses. Taking 

DTA curves for pastes of a typical Portland cement. Bensted and Varma 

Copeland et al. (C38) observed that the XRD patterns of many pastes included peaks of a hydrogarnet in addition to those of other hydrated aluminate phases, and obtained evidence that this had formed by hydrationr 

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Fusion, as an order-disorder transition, is the concept that fusion of a crystalline solid is essentially a change from the almost perfectly ordered solid state to a disordered liquid slate. The vacant spaces in the crystal lattice correspond lo the other component in the binary alloys, which undergo order-disorder transition in the pure form. Evidence from x-ray diffraction measurements indicates that short-range order is retained during fusion but long-range order is lost. 

The addition of NaOH gives Na[Ln(edta)]3H20. Evidence from X-ray diffraction, pH titration and ion migration studies suggests that in the [Ln(edta)]- ion the ethylenediaminetetra-acetate occupies only five coordination positions and possesses one uncomplexed —COO— group. 

Broadbent et al. [69] showed that ar-time curves for the decomposition of copper(II) oxalate (503 to 533 K) were sigmoidal and that data for the vacuum reaction fitted the Avrami-Erofeev equation with values of = 2.9 initially and later n = 3.5 ( , = 136 kJ mol ). Electron transfer was identified as the step controlling the reaction. There was no evidence from X-ray diffraction studies for the intervention of the Cu salt the orthorhombic structure was present until disappearance of the reactant and product copper metal was detected. However, many metal carboxylates, chilled after dehydration, yield anhydrous salts that are amorphous to X-rays or poorly crystalline, see, for example [70]. 

Resonance Raman spectroscopy carried out on carbon-specific, isotopically-labeled carotenoids can provide an important means to assign the Raman spectral bands and, upon incorporation of the carotenoid into RCs, can aid greatly in elucidating structural features not evident from X-ray diffraction studies and in understanding the effect of the protein on the vibrational activity of the carotenoid. Kok et al. (1994,1997) have examined the resonance Raman spectra of several C- and H-labeled spheroidenes incorporated into Rb. sphaeroides R26.1 RCs. 14- H, 15- H, 15 - 14,15 -2H and 15,15 - H... 

The X-ray diffraction pattern of DNA demonstrated the helical structure and the diameter. The combination of evidence from X-ray diffraction and chemical analysis led to the conclusion that the base pairing is complementary, meaning that adenine pairs with thymine and that guanine pairs with cytosine. Because complementary base pairing occurs along the entire double helix, the two chains are also referred to as complementary strands. By 1953, studies of the base composition of DNA from many species had already shown that, to within experimental... 

Direct measurements of cellulose orientation in fibers have yielded conflicting results. Evidence from x-ray diffraction studies of wood samples suggested that the cellulose crystallites are arranged randomly in the plane perpendicular to the fiber axis (30-33). Raman spectroscopic studies of cotton fibers dried under tenTTon, however, demonstrated that the methine C-H bonds are oriented preferentially perpendicular to the surface of the cell wall (34). Since the C-H bonds are perpendicular to the 002 plane, the orientation suggested by the Raman evidence differs from the alternating orientation in algal celluloses. 

It is evident from X-ray diffraction data that the base layers of DNA are, normally, making van der Waals contact with one another above and below, and that the distance between the centres of the atoms, in adjacent pairs on a strand, is 3.36 A. This means that another 3.36 A of space (6.72 Ain all) must be provided to admit the aminoacridine molecules, which have exactly the same thickness as those of the purine and pyrimidine bases. This space could be... 

Evidence from X-ray diffraction was interpreted to support this structure [49,50]. Later X-ray evidence seemed to indicate that tri and tetrasulfides have almost a linear arrangement (2) [51, 52] and more studies on the polymers and simple organic polysulfides support this view [53-57]. 

The human body contains on the order of 10 cells, most of which have complete copies of the DNA that originally was present in a single cell (the fertilized egg). When a cell divides, it makes complete copies of its DNA for each daughter cell. The ability of DNA to copy itself is related to its stracture, discovered in 1953 by James D. Watson and Francis H. C. Crick. Watson and Crick, aided by evidence from X-ray diffraction photos (see Section 11.10), determined that DNA exists as two complementary strands wound around each other in a double helix (Figure 21.19 ). The strands are antiparallel, so one runs 3 -> 5 while the other runs... 

While the orientations evident from x-ray diffraction are not singularly specific for each monolayer, the x-ray data clearly indicate that the orientation of the crystals depends upon the chemical nature of the terminal functional group of the monolayer. This result was also confirmed using grazing angle infrared reflection adsorption spectroscopy (IRRAS).(79) It is reasonable to suggest that this behavior is a consequence of interfacid interactions during the nucleation process. 

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