Crystal splitting

Sugar is ground up into powdered sugar. ( ) The crystal split into countless... 

Problem If one hammers on a sample of metal (lead or copper), the metal can be flattered to lead or copper leaf metals with the cubic closed structure are very ductile (see E5.4). If, on the other hand, one hammers on a rock salt sample, the crystal splits into tiny pieces or breaks apart, forming crystal plates. These properties can be explained through the structure the layers of ions in a crystal are moved when force is used similarly charged ions stand at opposite sides and are responsible for the repulsive effect of the crystal layers (see Fig. 5.14). 

Aided by spectroscopic data for a number of complexes, all having the same metal ion but different ligands, chemists calculated the crystal splitting for each ligand and established a spectrochemical series, which is a list of ligands arranged in order of their abilities to split the d orbital energies ... 

Thermal analysis, infrared. X-ray and iH NMR data were collected and interpreted as follows At about 390 K a sharp endothermic transition with most of the overall entropy-change indicates the transition into the condis phase of the paraffin chains. There is a second, small transition at about 408 K that may involve only a minor readjustment of the remaining crystal. At 7 K, finally the clearing is observed in a capillary melting point experiment. Only a minor endotherm corresponds to this transition to the isotropic phase. The low temperature phase is identified as fully ordered through X-ray diffraction, and the infrared spectrum shows the typical paraffinic crystal-splitting of the absorption lines. NMR-linewidth studies reveal at 77 K a second moment of about 27 that gradually narrows, and... 

The interaction between the transition dipoles of excited molecules in the crystal splits the original energy level into a band of levels. This is the exciton... 

Crystal Splitting energy (eV) Bands Temperature (K) Remarks... 

If the wave vector k forms an angle 0 0 or 90"" with the optical axis, the wave in the crystal splits into an ordinary beam (refractive index n = 2 = o) where the phase velocity is independent of 9, and an extraordinary wave (refractive index Uq) where and therefore the phase velocity does depend on the direction 6 (Fig. 6.1b). 

Salts of alkyl nitro compounds exist as ions in which there is extensive delocalisation and in which the C—N bond has considerable double bond character [71, 72]. The N—O bonds give antisymmetric and symmetric frequencies in the ranges 1315—1205 and 1175— 1040 cm" The bands are usually doubled due to crystal splitting [72]. The salts of aromatic nitro compounds have been less studied but Ezumi et al. [73] have given some data for nitrobenzene and nitropyridine which suggest that these absorb at rather higher frequencies. 

Fig. 5.10. Crystal splitting of the 730-720 cm" band, caused by the in-phase (CH2) rock vibration in crystalline long CH2 chain containing molecules. The unit cell shown has sections of two differently oriented CH2 chains, each performing the in-phase CH2 rock vibration. The bold-face CH2 groups all lie in one plane and the chain progression axis is more or less perpendicular to the page. The relative phase of the two differently oriented chains is different for the two unit cell modes. In the 730 cm" mode the two chains move their hydrogens toward and away from each other, and the close approach is shown. In the 720 cm" mode the two chains tend to stay out of each other s way.

In principle this crystal splitting effect occurs for every vibration when there is more than one molecule per unit cell but frequently the amount of splitting is too small to be noticed because of the weak coupling between nonbonded atoms in different molecules in the unit cell. 

Figure 3. CP/MAR spectrum (18) at 22.6 MHz of 1,4-bis(n-butylamlno)anthraquinone (II) showing splittings arising from the lack of molecular symmetry in the crystal. Splittings for C-5/C-8 and for C 13/C-17 are marked (and are 2.2 and 1.7 ppm, respectively), but others (in the carbonyl and methyl resonances for instance) are also visible.

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